2011 08-19 flier-&-support-docs-on-ust-low-threat closure policy
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    2011 08-19 flier-&-support-docs-on-ust-low-threat closure policy 2011 08-19 flier-&-support-docs-on-ust-low-threat closure policy Document Transcript

    •   Regulatory Outreach Proposed Petroleum  Low‐Threat Closure Policy    BACKGROUND On July 19th, 2011, the nine‐member UST Low‐Threat Closure Policy Task  CALIFORNIA  Force presented its recommendations to the SWRCB. The SWRCB  ENVIRONMENTAL  encouraged the stakeholder group to hold outreach meetings to discuss  PROTECTION  technical and practical aspects of its recommend policy. At the request of  the SWRCB, we have arranged the following meeting schedule. All  AGENCY  interested parties are invited. We hope that you will attend one of these    sessions:  August 31, 2011, 1:30 P.M. September 16, 2011, 9:00 A.M. STATE WATER  San Francisco RWQCB   San Diego RWQCB  RESOURCES  1515 Clay Street, Suite 1400  9174 Sky Park Court, Suite 100  Oakland, CA 94612  San Diego, CA 92123  CONTROL BOARD  Contact: Steven Hill  SHill@waterboards.ca.gov  Contact: John Anderson    janderson@waterboards.ca.gov  (SWRCB)  September 15, 2011, 9:00 A.M.    Los Angeles RWQCB  September 23, 2011, 1:30 P.M.  320 W. 4th Street, Suite 200  Central Valley RWQCB  Los Angeles, CA 90013  11020 Sun Center Drive, Suite 200  Contact: Dr. Yue Rong yrong@waterboards.ca.gov  Rancho Cordova, CA 95670    Contact: Brian Newman  September 15, 2011, 2:30 P.M.  bnewman@waterboards.ca.gov Santa Ana RWQCB  3737 Main Street, Suite 500  Riverside, CA 92501‐3339  Contact: Kurt Berchtold  kberchtold@waterboards.ca.gov  CONTACT INFORMATION For additional information, questions or comments, please contact: Ravi Arulanantham, PhD. Barry Marcus, P.G.  Geosyntech Consultants  Sacramento County EMD  (510) 285‐2793  (916) 875‐8506  RArulanantham@Geosyntec.com  MarcusB@SacCounty.net  The complete proposed policy and technical justification documents are  available on the internet at the following website:  http://www.waterboards.ca.gov/water_issues/programs/ust/lt_cls_plcy .shtml 
    • Documents developed by the UST stakeholder group are listed below: Draft Low Threat UST Closure Policy - Final 7/14/11 Technical Justification for Direct Contact - Final 7/16/11Technical Justification for Groundwater Plume Lengths, etc - Final 7/12/11 Technical Justification for VI Pathway - Final 6/30/11
    • DRAFT Low-Threat UST Closure Policy 7-14-11PreambleThe State Water Resources Control Board (State Water Board) administers the petroleum UST(Underground Storage Tank) Cleanup Program, which was enacted by the Legislature in 1984 toprotect health, safety and the environment. The State Water Board also administers thepetroleum UST Cleanup Fund (Fund), which was enacted by the Legislature in 1989 to assistUST owners and operators in meeting federal financial responsibility requirements and toprovide reimbursement to those owners and operators for the high cost of cleaning upunauthorized releases caused by leaking USTs.The State Water Board believes it is in the best interest of the people of the State thatunauthorized releases be prevented and cleaned up to the extent practicable in a manner thatprotects human health, safety and the environment. The State Water Board also recognizes thatthe technical and economic resources available for environmental restoration are limited, andthat the highest priority for these resources must be the protection of human health andenvironmental receptors. Program experience has demonstrated the ability of remedialtechnologies to mitigate a substantial fraction of a petroleum contaminant mass with theinvestment of a reasonable level of effort. Experience has also shown that residual contaminantmass usually remains after the investment of reasonable effort, and that this mass is difficult tocompletely remove regardless of the level of additional effort and resources invested.It has been well-documented in the literature and through experience at individual UST releasesites that petroleum fuels naturally attenuate in the environment through adsorption, dispersion,dilution, volatilization, and biological degradation. This natural attenuation slows and limits themigration of dissolved petroleum plumes in groundwater. The biodegradation of petroleum, inparticular, distinguishes petroleum products from other hazardous substances commonly found atcommercial and industrial sites.The characteristics of UST releases and the California UST Program have been studiedextensively, with individual works including: a. Lawrence Livermore National Laboratory report (1995) b. SB1764 Committee report (1996) c. UST Cleanup Program Task Force report (2010) d. Cleanup Fund Task Force report (2010) e. Cleanup Fund audit (2010)In general, these studies have recommended establishing “low-threat case closure criteria” tomaximize the benefits to the people of the State of California through judicious application ofavailable resources.The purpose of this policy is the establishment of low-threat petroleum site closure criteria. Thepolicy is consistent with existing statutes, regulations, State Board precedential decisions andresolutions, and is intended to provide clear direction to responsible parties, their service 1
    • providers, and regulatory agencies. The policy seeks to increase UST cleanup processefficiency. A benefit of improved efficiency is the preservation of limited resources formitigation of releases posing a greater threat to human and environmental health.This policy is based in part upon the knowledge and experience gained from the last 25 years ofinvestigating and remediating unauthorized releases of petroleum from USTs. While this policydoes not specifically address other petroleum release scenarios such as pipelines or above groundstorage tanks, if a particular site with a different release scenario exhibits attributes similar tothose which this policy addresses, the criteria for closure evaluation of these non-UST sitesshould be similar to those in this policy.This policy is a state policy for water quality control and applies to all sites governed by Healthand Safety Code section 25296.10. The term “regulatory agencies” in this policy means theState Water Board, regional water boards and local agencies authorized to implement Health andSafety Code section 25296.10.Definitions: Unless expressly provided in this policy, the terms in this policy shall have thesame definitions provided in Chapter 6.7 of Division 20 of the Health and Safety Code andChapter 16 of Division 3 of Title 23 of the California Code of Regulations.Criteria for Low-Threat Case ClosureIn the absence of site-specific conditions that demonstrably increase the risk associated withresidual petroleum constituents, cases that meet the general and media-specific criteria describedin this policy do not pose a threat to human health, safety or the environment and are appropriatefor UST case closure pursuant to Health and Safety Code section 25296.10. Cases that meet thecriteria in this policy do not require further corrective action and shall be issued a uniformclosure letter consistent with Health and Safety Code section 25296.10. Periodically, or at therequest of the responsible party or party conducting the corrective action, the regulatory agencyshall conduct a review to determine whether the site meets the criteria contained in this policy.It is important to emphasize that the criteria described in this policy do not attempt to describethe conditions at all low-threat sites in the State. Regulatory agencies should issue a closureletter for a case that does not meet these criteria if the site is determined to be low-threat basedupon a site specific analysis.This policy recognizes that some petroleum-release sites may possess unique attributes and thatsome site specific conditions may make the application of policy criteria inappropriate. It isimpossible to completely capture those sets of attributes that may render a site ineligible forclosure based on this low-threat policy. This policy relies on the regulatory agency’s use of theconceptual site model to identify the special attributes that would require specific attention priorto the application of low-threat criteria. In these cases, it is the regulatory agency’sresponsibility to identify the conditions that make closure under the policy inappropriate. 2
    • General CriteriaGeneral criteria that must be satisfied by all candidate sites are listed as follows: a. The unauthorized release is located within the service area of a public water system; b. The unauthorized release consists only of petroleum; c. The unauthorized (“primary”) release from the UST system has been stopped; d. Free product has been removed to the maximum extent practicable; e. A conceptual site model has been developed; f. Secondary source removal has been addressed and g. Soil or groundwater has been tested for MTBE and results reported in accordance with Health and Safety Code section 25296.15.a. The unauthorized release is located within the service area of a public water systemThis policy is protective of existing water supply wells. New water supply wells are unlikely tobe installed in the shallow groundwater near former UST release sites. However, it is difficult topredict, on a statewide basis, where new wells will be installed, particularly in rural areas that areundergoing new development. This policy is limited to areas with available public drinkingwater supplies to reduce the likelihood that new wells in developing areas will be inadvertentlyimpacted by residual petroleum in groundwater. Case closure outside of areas with a publicwater supply should be evaluated based upon this policy and a site specific evaluation ofdeveloping water supplies in the area.b. The unauthorized release consists only of petroleumFor the purposes of this policy, petroleum is defined as crude oil, or any fraction thereof, whichis liquid at standard conditions of temperature and pressure, which means 60 degrees Fahrenheitand 14.7 pounds per square inch absolute, including the following substances: motor fuels, jetfuels, distillate fuel oils, residual fuel oils, lubricants, petroleum solvents and used oils, includingany additives and blending agents such as oxygenates contained in the formulation of thesubstances.c. The unauthorized release has been stoppedThe tank, pipe, or other appurtenant structure that released petroleum into the environment (i.e.the primary source) has been removed, repaired or replaced. It is not the intent of this policy toallow sites with ongoing leaks from the UST system to qualify for low-threat closure.d. Free product has been removed to the Maximum Extent PracticableAt petroleum unauthorized release sites where investigations indicate the presence of freeproduct, free product shall be removed to the maximum extent practicable. In meeting therequirements of this section: (a) Free product shall be removed in a manner that minimizes the spread of the unauthorized release into previously uncontaminated zones by using recovery and disposal techniques appropriate to the hydrogeologic conditions at the site, and that properly treats, discharges or disposes of recovery byproducts in compliance with applicable laws; (b) Abatement of free product migration shall be used as a minimum objective for the design of any free product removal system; (c) Flammable products shall be stored for disposal in a safe and competent manner to prevent fires or explosions. 3
    • e. A conceptual site model has been developedThe Conceptual Site Model (CSM) is a fundamental element of a comprehensive siteinvestigation. The CSM establishes the source and attributes of the unauthorized release,describes all affected media (including soil, groundwater, and soil vapor as appropriate),describes local geology, hydrogeology and other physical site characteristics that affectcontaminant environmental transport and fate, and identifies all confirmed and potentialcontaminant receptors (including water supply wells, surface water bodies, structures and theirinhabitants, etc.). The CSM is relied upon by practitioners as a guide for investigative designand data collection. Petroleum release sites in California occur in a wide variety ofhydrogeologic settings. As a result, contaminant fate and transport and mechanisms by whichreceptors may be impacted by contaminants vary greatly from location to location. Thereforethe CSM is dynamic and unique to each individual release site. All relevant site characteristicsidentified by the CSM should be assessed such that the nature, extent and mobility of the releasehave been established to determine conformance with applicable criteria in this policy.f. Secondary source removal has been addressed“Secondary source” is defined as petroleum-impacted soil or groundwater located at orimmediately beneath the point of release from the primary source. Unless site attributes preventsecondary source removal (e.g. physical or infrastructural constraints exist whose removal orrelocation would be technically or economically infeasible), petroleum-release sites are requiredto undergo secondary source removal to the extent practicable as described herein. “To theextent practicable” means implementing a cost-effective corrective action which removes ordestroys-in-place the most readily recoverable fraction of source-area mass. It is expected thatmost secondary mass removal efforts will be completed in one year or less. Followingremoval/destruction of the secondary source, additional removal and/or active remedial actionsshall not be required by regulatory agencies unless (1) necessary to abate a demonstrated threatto human health or (2) the groundwater plume does not meet the definition of low threat asdescribed in this policy.g. Soil and groundwater have been tested for MTBE and results reported in accordance with Health and Safety Code section 25296.15Health and Safety Code section 25296.15 prohibits closing a UST case unless the soil,groundwater, or both, as applicable have been tested for MTBE and the results of that testing areknown to the regional water board. The exception to this requirement is where a regulatoryagency determines that the UST that leaked has only contained diesel or jet fuel. Before closinga UST case pursuant to this policy, the requirements of section 25296.15, if applicable, shall besatisfied. 4
    • Media-Specific CriteriaReleases from USTs can impact human health and the environment through contact with any orall of the following contaminated media: groundwater, surface water, soil, and soil vapor.Although this contact can occur through ingestion, dermal contact, or inhalation of the variousmedia, the most common drivers of health risk are ingestion of groundwater from drinking waterwells, inhalation of vapors accumulated in buildings, contact with near surface contaminatedsoil, and inhalation of vapors in the outdoor environment. To simplify implementation, thesemedia and pathways have been evaluated and the most common exposure scenarios have beencombined into three media-specific criteria: 1. Groundwater 2. Vapor Intrusion to Indoor Air 3. Direct Contact and Outdoor Air ExposureCandidate sites must satisfy all three of these media-specific criteria as described below.1. GroundwaterThis policy describes criteria on which to base a determination that risks to existing andanticipated future beneficial uses of groundwater have been mitigated or are de minimus,including cases that have not affected groundwater.State Water Board Resolution 92-49, Policies and Procedures for Investigation and Cleanup andAbatement of Discharges Under Water Code Section 13304 is a state policy for water qualitycontrol and applies to petroleum UST cases. Resolution 92-49 directs that water affected by anunauthorized release attain either background water quality or the best water quality that isreasonable if background water quality cannot be restored. Any alternative level of water qualityless stringent than background must be consistent with the maximum benefit to the people of thestate, not unreasonably affect current and anticipated beneficial use of affected water, and notresult in water quality less than that prescribed in the water quality control plan for the basinwithin which the site is located. Resolution No. 92-49 does not require that the requisite level ofwater quality be met at the time of case closure; it specifies compliance with cleanup goals andobjectives within a reasonable time frame.Water quality control plans (Basin Plans) generally establish “background” water quality as arestorative endpoint. This policy recognizes the regulatory authority of the Basin Plans butunderscores the flexibility contained in Resolution 92-49.It is a fundamental tenet of this low-threat closure policy that if the closure criteria described inthis policy are satisfied at a release site, water quality objectives will be attained through naturalattenuation within a reasonable time, prior to the need for use of any affected groundwater.If groundwater with a designated beneficial use is affected by an unauthorized release, to satisfythe media-specific criteria for groundwater, the contaminant plume that exceeds water qualityobjectives must be stable or decreasing in areal extent, and meet all of the additionalcharacteristics of one of the five classes of sites listed below. A plume that is “stable ordecreasing” is a contaminant mass that has expanded to its maximum extent: the distance fromthe release where attenuation exceeds migration. 5
    • (1) a. The contaminant plume that exceeds water quality objectives is less than 100 feet in length. b. There is no free product. c. The nearest existing water supply well and/or surface water body is greater than 250 feet from the defined plume boundary. (2) a. The contaminant plume that exceeds water quality objectives is less than 250 feet in length. b. The nearest existing water supply well and /or surface water body is greater than 1000 feet from the defined plume boundary. c. The dissolved concentration of benzene is less than 3000 μg/l and the dissolved concentration of MTBE is less than 1000 μg/l. (3) a. The contaminant plume that exceeds water quality objectives is less than 250 feet in length. b. Free product may be present below the site but does not extend off-site. c. The plume has been stable or decreasing for a minimum of five years. d. The nearest existing water supply well and/or surface water body is greater than 1000 feet from the defined plume boundary. e. The property owner is willing to accept a deed restriction if the regulatory agency requires a deed restriction as a condition of closure. (4) a. The contaminant plume that exceeds water quality objectives is less than 1000 feet in length. b. The nearest existing water supply well and/or surface water body is greater than 1000 feet from the defined plume boundary. c. The dissolved concentration of benzene is less than 1000 μg/l and the dissolved concentration of MTBE is less than 1000 μg/l. (5) a. An analysis of site specific conditions determines that the site under current and reasonably anticipated near-term future scenarios poses a low threat to human health and safety and to the environment and water quality objectives will be achieved within a reasonable time frame.Sites with Releases That Have Not Affected GroundwaterSites with soil that does not contain sufficient mobile constituents (leachate, vapors, or LNAPL)to cause groundwater to exceed the groundwater criteria in this policy shall be considered low-threat sites for the groundwater medium. Provided the general criteria and criteria for othermedia are also met, those sites are eligible for case closure.For older releases, the absence of current groundwater impact is often a good indication thatresidual concentrations present in the soil are not a source for groundwater pollution.2. Petroleum Vapor Intrusion to Indoor Air 6
    • Exposure to petroleum vapors migrating from soil or groundwater to indoor air may poseunacceptable human health risks. This policy describes conditions, including bioattenuationzones, which if met will assure that exposure to petroleum vapors in indoor air will not poseunacceptable health risks. In many petroleum release cases, potential human exposures tovapors are mitigated by bioattenuation processes as vapors migrate toward the ground surface.For the purposes of this section, the term “bioattenuation zone” means an area of soil withconditions that support biodegradation of petroleum hydrocarbon vapors.The low-threat vapor-intrusion criteria described below apply to release sites and impacted orpotentially impacted adjacent parcels when: (1) existing buildings are occupied or may bereasonably expected to be occupied in the future, or (2) buildings for human occupancy arereasonably expected to be constructed in the near future. Appendices 1 through 4 (attached)illustrate four potential exposure scenarios and describe characteristics and screening criteriaassociated with each scenario. Petroleum release sites shall satisfy the media-specific screeningcriteria for petroleum vapor intrusion to indoor air and be considered low-threat for the vapor-intrusion-to-indoor-air pathway if: a. Site-specific conditions at the release site satisfy all of the characteristics and screening criteria of scenarios 1 through 3 as applicable, or all of the characteristics and screening criteria of scenario 4 as applicable; or b. A site-specific risk assessment for the vapor intrusion pathway is conducted and demonstrates that human health is protected to the satisfaction of the regulatory agency.Exception: Exposures to petroleum vapors associated with historical fuel system releases arecomparatively insignificant relative to exposures from small surface spills and fugitive vaporreleases that typically occur at active fueling facilities. Therefore, satisfaction of the media-specific criteria for petroleum vapor intrusion to indoor air is not required at active commercialpetroleum fueling facilities, except in cases where release characteristics can be reasonablybelieved to pose an unacceptable health risk.3. Direct Contact and Outdoor Air ExposureThis policy describes conditions where direct contact with contaminated soil or inhalation ofcontaminants volatized to outdoor air poses an insignificant threat to human health. Releasesites where human exposure may occur satisfy the media-specific criteria for direct contact andoutdoor air exposure and shall be considered low-threat if they meet any of the following: a. Maximum concentrations of petroleum constituents in soil are less than or equal to those listed in Table 1 for the specified depth below ground surface; b. Maximum concentrations of petroleum constituents in soil are less than levels that a site specific risk assessment demonstrates will have no significant risk of adversely affecting human health; or 7
    • c. As a result of controlling exposure through the use of mitigation measures or through the use of institutional or engineering controls, the regulatory agency determines that the concentrations of petroleum constituents in soil will have no significant risk of adversely affecting human health. Table 1 Concentrations of Petroleum Constituents In Soil That Will Have No Significant Risk Of Adversely Affecting Human Health Depth PAH* Benzene Naphthalene (feet) (mg/kg) (mg/kg) (mg/kg) 0 to 5 2.3 13 0.038 5 to 10 100 1500 7.5 *Notes: Based on the seven carcinogenic PAHs as benzo(a)pyrene toxicity equivalent [BaPe]. The PAH screening level is only applicable where soil was affected by either waste oil and/or Bunker C fuel.Low-Threat Case ClosureCases that meet the general and media-specific criteria established in this policy satisfy the case-closure requirements of Health and Safety Code section 25296.10, including the requirement inState Water Board Resolution 92-49 that requires that cleanup goals and objectives be metwithin a reasonable time frame. If the site has been determined by the regulatory agency to meetthe criteria in this policy, the regulatory agency shall notify responsible parties that they areeligible for case closure and that the following items, if applicable, shall be completed prior tothe issuance of a uniform closure letter specified in Health and Safety Code section 25296.10.After completion of these items, the regulatory agency shall issue a uniform closure letter within30 days. a. Notification Requirements – Public water supply agencies with jurisdiction over the water impacted by the petroleum release, permitting agencies with authority over the land affected by the petroleum release, owners of the property, and the owners and occupants of all adjacent parcels and all parcels that are impacted by the unauthorized release shall be notified of the proposed case closure and provided a 30 day period to comment. The regulatory agency shall consider any comments received when determining if the case should be closed or if site specific conditions warrant otherwise. b. Monitoring Well Destruction – All wells and borings installed for the purpose of investigating, remediating, or monitoring the unauthorized release shall be properly destroyed prior to case closure unless a property owner certifies that they will keep and maintain the wells or borings in accordance with applicable local or state requirements. 8
    • c. Waste Removal – All waste piles, drums, debris and other investigation or remediation derived materials shall be removed from the site and properly managed in accordance with regulatory agency requirements.Closing CommentsThis concludes the Low-Threat UST Closure Policy. This policy is based on existing statutes,regulations and State Water Board resolutions. This policy clarifies aspects of prior guidanceand establishes criteria to be used by technical practitioners and all regulatory agencies inCalifornia. 9
    • Appendix 1 Scenario 1: Unweathered* LNAPL in Groundwater Required Characteristics of the Bioattenuation Zone Existing Building or Potential Future Construction Building Foundation TPH < 100 mg/kg throughout 30 depth 30 Unweathered LNAPLRequired Characteristics of the Bioattenuation Zone:1. The bioattenuation zone shall be a continuous zone that provides a separation of at least 30 feet vertically between the LNAPL ingroundwater and the foundation of existing or potential buildings; and2. Total TPH (TPH-g and TPH-d combined) are less than 100 mg/kg throughout the entire depth of the bioattenuation zone.*As used in this context, unweathered LNAPL is generally understood to mean petroleum product that has not been subjected tosignificant volitalization or solubilization, and therefore has not lost a significant portion of its volatile or soluble constituents (e.g.,comparable to recently dispensed fuel). Version date: July 11, 2011
    • Appendix 2 Scenario 2: Unweathered* LNAPL in Soil Required Characteristics of the Bioattenuation Zone Existing Building or Potential Future Construction 30 30 30 TPH < 100 mg/kg for 30 30 from foundation Unweathered LNAPL in soilRequired Characteristics of the Bioattenuation Zone:1. The bioattenuation zone shall be a continuous zone that provides a separation of at least 30 feet both laterally and verticallybetween the LNAPL in soil and the foundation of existing or potential buildings, and2. Total TPH (TPH-g and TPH-d combined) are less than 100 mg/kg throughout the entire depth of the bioattenuation zone.*As used in this context, unweathered LNAPL is generally understood to mean petroleum product that has not been subjected tosignificant volitalization or solubilization, and therefore has not lost a significant portion of its volatile or soluble constituents (e.g.,comparable to recently dispensed fuel). Version date:  July 11, 2011
    • Appendix 3 Scenario 3 - Dissolved Phase Benzene Concentrations Only in Groundwater (Low concentration groundwater scenarios with or without O2 measurements) Defining the Bioattenuation Zone Without Oxygen Measurements or Oxygen <4% Existing Building or Future Construction No O2 data or <4% TPH < 100 mg/kg 5 10 TPH < 100 mg/kg Benzene < 100 ug/L Benzene < 1000 ug/L Figure A Figure B Required Characteristics of Bioattenuation Zone For Sites Without Oxygen Measurements Figure A: 1) Where benzene concentrations are less than 100 ug/L, the bioattenuation zone: a) Shall be a continuous zone that provides a separation of at least 5 feet vertically between the dissolved phase Benzene  and the foundation of existing or potential buildings; and b) Contain Total TPH (TPH‐g and TPH‐d combined) less than 100 mg/kg throughout the entire depth of the bioattenuation  zone. Figure B: 1) Where benzene concentrations are greater than 100 ug/L but less than 1000 ug/L, the bioattenuation zone:  a) Shall be a continuous zone that provides a separation of at least 10 feet vertically between the dissolved phase Benzene  and the foundation of existing or potential buildings; and  b) Contain Total TPH (TPH‐g and TPH‐d combined) less than 100 mg/kg throughout the entire depth of the bioattenuation  zone Defining the Bioattenuation Zone With Oxygen ≥ 4% Existing Building or Future Construction With O2 data TPH < 100 O2 ≥ 4% mg/kg 5 Benzene < 1000 ug/L Figure C Required Characteristics of Bioattenuation Zone For Sites With Oxygen ≥ 4%Where benzene concentrations are less than 1000 ug/L, the bioattenuation zone:1. Shall be a continuous zone that provides a separation of least 5 feet vertically between the dissolved phase Benzene and thefoundation of existing or potential buildings; and2. Contain Total TPH (TPH-g and TPH-d combined) less than 100 mg/kg throughout the entire depth of the bioattenuation zone. Version date:  July 11, 2011
    • Appendix 4 Scenario 4 - Direct Measurement of Soil Gas Concentrations Soil Gas Sampling Locations – No Bioattenuation Zone Existing Building Future Construction 5 5 Depth of Foundation b a Description of Soil Gas Sample Locations a - beneath or adjacent to building (soil gas sample shall be collected at least 5 deeper than the bottom of the building foundation) b - for future construction scenarios (soil gas sample shall be collected at least 5 below the ground surface) Soil Gas Sampling Locations – with Bioattenuation Zone Existing Building Future Construction TPH < 100 mg/kg TPH < 100 5 5 mg/kg O2 ≥ 4% at lower end of O2 ≥ 4% at lower end of zone zone Required Characteristics of Bioattenuation Zone Required data includes: petroleum concentrations in soil and soil gas, and oxygen concentrations. Measured concentrations of soil gases must be less than the screening values indicated in the table below for the applicable scenarios. 3 Soil Gas Screening Levels (ug/m ) With Bioattenuation Zone* No Bioattenuation Zone Residential Commercial Residential CommercialConstituent Soil Gas Concentration (µg/m 3) Soil Gas Concentration (µg/m 3)Benzene < 85,000  < 280,000           < 85           < 280Naphthalene < 93,000 < 310,000          < 93           < 310Notes:  *In order to use the screening levels with the bioattenuation zone, there must be: 1) 5 feet of soil between the soil vapor measurement and the building (or future building), 2) TPH (TPHg + TPHd) is less than 100 ppm (measured in at least two depths within the 5 foot zone), and 3) oxygen ≥ 4% measured at the bottom of the 5 foot bioattenuation zone. A 1000-fold bioattenuation of petroleum vapors is assumed for the bioattenuation zone. For the no bioattenuation zone, the screening criteria are the same as the California Human Health Screening Levels (CHHSLs). Version date:  July 11, 2011
    • Documents developed by the UST stakeholder group are listed below: Draft Low Threat UST Closure Policy - Final 7/14/11 Technical Justification for Direct Contact - Final 7/16/11Technical Justification for Groundwater Plume Lengths, etc - Final 7/12/11 Technical Justification for VI Pathway - Final 6/30/11
    •     Technical Justification for Soil Screening Levels for Direct Contact and  Outdoor Air Exposure Pathways      Table of Contents 1 EXECUTIVE SUMMARY .......................................................................................................................... 12 INTRODUCTION..................................................................................................................................... 23 CONCEPTUAL SITE MODEL.................................................................................................................... 34 DERIVATION OF SCREENING LEVELS ..................................................................................................... 55 RESULTS:  SOIL SCREENING LEVELS....................................................................................................... 76 DISCUSSION OF RESULTS ...................................................................................................................... 77 REFERENCES .......................................................................................................................................... 8Tables ............................................................................................................................................................ 9Figures......................................................................................................................................................... 16 1 EXECUTIVE SUMMARYSoil  Screening  Levels  have  been  proposed  to  be  used  in  conjunction  with  vapor  intrusion  criteria  and groundwater  criteria  for  identifying  sites  posing  a  low‐threat  to  human  health  and  the  environment.  That  is,  these  Soil  Screening  Levels  are  just  one  of  three  sets  of  criteria  that  should  be  evaluated  to determine if a site is low‐threat.   The  Soil  Screening  Levels  discussed  in  this  document  have  been  proposed  for  benzene,  naphthalene, and polyaromatic hydrocarbon (PAH) to define sites that are low‐threat with respect to “direct contact” with soil.  The exposure pathways considered in the site conceptual model are:  ingestion of soil, dermal contact with soil and inhalation of dust and volatile emissions from soil.  Note these exposure pathways are assumed to occur simultaneously, i.e. the screening levels are protective of the cumulative exposure from all four exposure pathways.     1   
    • These screening levels were derived using standard USEPA and Cal/EPA risk assessment equations.  The exposure  parameter  values,  chemical  toxicity  values,  and  chemical  fate  and  transport  properties  are based on standard values used in California.     Different screening levels have been developed for two soil horizons, one from 0 to 5 feet below ground surface (bgs), and one from 5 to 10 feet bgs. This document describes the technical background for the development of the direct contact screening levels.  Three exposure scenarios (types of receptors and land  use)  were  considered  and  the  screening  levels  for  each  soil  horizon  were  chosen  to  be  the  most conservative of the three scenarios. The soil screening level for “PAH” is appropriate to be compared with site concentrations for the total concentration  of  the  seven  carcinogenic  PAHs.    The  carcinogenic  PAHs  are:    benz[a]anthracene, benzo[a]pyrene,  benzo[b]fluoranthene,  benzo[k]fluoranthene,  chrysene,  dibenz(a,h)anthracene,  and indeno(1,2,3‐cd)pyrene. The  toxicity  value  used  for  the  entire  group  of  carcinogenic  hydrocarbons  is  California’s  Office  of Environmental Health Hazard Assessment (OEHHA) cancer potency value for benzo(a)pyrene.  This is a conservative assumption because the few PAHs that are more carcinogenic than benzo(a)pyrene are not commonly found in petroleum mixtures. 2 INTRODUCTIONThe  equations  used  to  develop  the  Soil  Screening  Levels  came  from  the  California  Environmental Protection Agency (Cal/EPA) OEHHA’s California Human Health Screening Levels (CHHSLs; OEHHA 2005).  Exposure  parameters  values  were  assumed  to  equal  the  defaults  values  used  in  OEHHA’s  California Human  Health  Screening  Levels  (CHHSLs;  OEHHA  2005).    The  Soil  Screening  Levels  presented  in  this document  are  conservative  because  the  assumptions  used  to  calculate  the  values  are  based  on  worst case exposure scenarios.   The CHHSLs for “direct contact with soil” pathways, do not include volatilization of chemicals from the soil to outdoor air.  For the Soil Screening Levels presented in this document a volatilization factor was added to the CHHSL equations in order to be conservative and was obtained from the American Society of  Testing  Material’s  (ASTM’s)  Standard  Guide  for  Risk‐Based  Corrective  Action  Applied  at  Petroleum Release Sites (ASTM 1996).  The ASTM volatilization factor used to calculate concentrations in outdoor air  considers  mass  balance.  The  volatilization  algorithm  commonly  used  in  USEPA  screening  level equations can greatly overestimate the amount of contaminant volatilizing into outdoor air for volatile chemicals  (OEHHA,  2005).    In  the  ASTM  volatilization  algorithm,  if  the  calculated  volatilization  rate depletes the source before the end of the exposure duration, then the volatilization rate is adjusted so that the total source mass is assumed to volatilize by the end of the exposure duration.  By using this mass‐balance check, it is ensured that the total amount volatilized does not exceed the total amount of contaminant in soil (which can happen with the USEPA volatilization algorithm).      2   
    • For  dermal  contact  with  soil,  ingestion  of  soil,  and  inhalation  of  dust  pathways,  the  exposure concentration in soil is assumed to be constant at the screening level for the entire exposure duration. 2.1 Screening Levels vs. RiskThese Soil Screening Levels represent concentrations that indicate that the site is a low‐threat risk for human  health;  they  cannot  be  used  to  estimate  site‐specific  risks.  Multiple  conservative  assumptions were made when developing these Soil Screening Levels.  Actual site risk is expected to be lower than the risk targets used to develop the screening levels.  For example, for residential sites, the receptor is assumed to come into contact with soil with concentrations at the screening level almost every day (350 days/year) for a total of 30 years. While most residential exposures would not be at the default levels used  in  this  analysis,  the  defaults  used  here  are  designed  to  be  protective  for  this  hypothetical “reasonable worst case” scenario. Site concentrations exceeding the screening levels do not indicate unacceptable human health risks with regards to these pathways; rather, an exceedance may indicate that a site‐specific evaluation of human health risk is warranted.      3 CONCEPTUAL SITE MODELThis section describes the exposure scenarios and receptors considered in the development of the Soil Screening Levels. 3.1 Exposure PathwaysThe Screening Levels consider four exposure pathways simultaneously:  • ingestion of soil,  • dermal contact with soil,  • inhalation of volatile soil emissions, and  • inhalation of particulate emissions. Ingestion of and dermal contact with soil are direct exposure pathways, i.e., the receptor is assumed to contact the soil directly and, therefore, the exposure point concentration is the actual concentration in soil.    For  the  inhalation  exposure  pathways,  the  exposure  medium  is  outdoor  air;  the  outdoor  air concentrations must be estimated using volatilization and particulate emission factors.    3.2 Receptors ConsideredSoil  Screening  levels  were  calculated  for  three  exposure  scenarios,  and  then  the  most  conservative screening level was chosen for the screening levels.  The exposure scenarios considered were:  • residential,    3   
    • • commercial/industrial, and   • workers in a utility trench or similar construction project.   It  is  assumed  that  all  four  of  the  exposure  pathways  (discussed  in  section  3.1)  are  potential  exposure pathways  for  each  of  the  three  types  of  receptors.  The  input  parameter  values  are  different  for  each receptor, however. For the residential exposure scenario, it is assumed that the receptor is a child for 6 years and then an adult for 24 years.  When calculating carcinogenic risk, the total intake of a chemical over a lifetime is used; therefore, the carcinogenic residential screening levels are protective of the combined child plus adult scenario.  For non‐carcinogenic health effects, the intake is not added over the exposure period.  In that case, the child is the more sensitive receptor, therefore the non‐carcinogenic screening levels are developed for a child receptor and are protective for the adult resident as well.  The  commercial/industrial  exposure  scenario  assumes  that  the  receptor  is  an  adult  and  works  in  an office  or  outdoors  at  the  site;  however,  the  adult  is  not  expected  to  be  digging  in  the  soil.  In  this scenario,  it  is  assumed  that  the  receptor  works  for  a  total  of  25  years  at  250  days/year  at  the  same location. It is likely that the direct contact exposure assumptions are very conservative for this exposure scenario. For the utility or construction worker, it is assumed that the worker may be working directly with  the impacted soil. In this exposure scenario, the exposure duration is assumed to be much shorter than in the  other  two  scenarios;  however,  the  chemical  intake  per  day  may  be  higher  due  to  increased incidental ingestion.   3.3 Depths to Which the Screening Levels ApplyTwo sets of screening levels were developed, based on depth of impacted soil:  one set applies to 0 to 5 feet  below  ground  surface  (bgs)  and  the  other  set  applies  to  5  to  10  feet  bgs.  The  screening  levels applying to soil at 0 to 5 feet bgs represent the lowest of the screening levels calculated for the resident, worker, and utility worker.  Screening levels for soil from 5 to 10 feet bgs represent the lower value of either  a  utility  trench/construction  worker  or  the  volatilization  to  outdoor  air  pathway  for  all  of  the receptors.  That is, the full depth of 0 to 10 feet is assumed to contribute to outdoor air concentrations for  all  scenarios.    Therefore,  the  screening  levels  for  both  soil  horizons  are  protective  of  inhalation  of volatile emissions. When  calculating  the  residential  screening  levels,  it  is  assumed  that  residents  may  come  into  contact with  the  soil  between  the  ground  surface  and  a  depth  of  5  feet  (“surface  soil”).    For  impacted  soil  at depths from 5 to 10 feet (a “swimming pool” or “septic system installation” scenario), it is assumed that the  potential  risk  posed  to  residents  by  direct  contact  would  be  small,  because  excavations  by  the homeowner  to  that  depth  would  be  rare  (exposure  frequency  and  duration  are  short),  most  of  the petroleum‐affected  soil  would  likely  be  removed  to  create  the  swimming  pool  or  septic  system,  and   4   
    • petroleum  constituents  in  soil  would  volatilize  and  biodegrade  very  quickly  if  the  affected  soil  was placed at the ground surface (i.e. the top few inches of soil).     For  commercial/industrial  receptors  it  is  assumed  that  commercial  workers  could  contact  the  soil  at depths between ground surface and 5 feet.  In the case of a utility trench or construction worker, it was assumed that direct contact (dermal and ingestion) with soils could occur at depths from 0 to 10 feet.   4 DERIVATION OF SCREENING LEVELSThis  section  describes  how  the  Soil  Screening  Levels  were  calculated.    Standard  equations  from  the OEHHA CHHSLs were used for everything except the volatilization term which was discussed in Section 2.  A target risk level of 1 × 10‐6 risk for carcinogens and a target hazard index of 1.0 for non‐carcinogens were assumed in all cases. 4.1 Equations Used4.1.1 Exposure EquationsThe  equations  used  to  develop  the  Soil  Screening  Levels  are  shown  in  Tables  1  through  3  and  the variable definitions are shown in Table 4.   4.1.2 Volatilization FactorAs mentioned previously, the CHHSLs do not include a volatilization factor (VF), i.e. they do not consider volatile  emissions  to  outdoor  air.    A  VF  was  included  in  the  Soil  Screening  Levels,  however  to  be conservative.    The  volatilization  factor  used  to  predict  outdoor  air  concentrations  due  to  volatilization from the soil is based on the ASTM guidance (1996). The assumptions in the ASTM volatilization factor algorithm (ASTM 1996) are:  • Dispersion  in  air  is  modeled  from  a  ground‐level  source.    It  is  assumed  that  the  air  in  the  outdoor air “box” is well‐mixed.  • The receptor is located onsite, directly over the impacted soil, 24 hours/day for the entire  exposure duration.  • A  long‐term  average  exposure–point  concentration  is  estimated  for  the  entire  exposure  duration. The conceptual model for volatile emissions and inhalation of outdoor air is shown in Figure 1.  Note the assumed  receptor  location  at  the  edge  of  the  downwind  side  of  the  source  (for  24  hours/day  for  the entire  exposure  duration)  is  the  most  conservative  location  that  could  be  used.  The  dispersion  of contaminant  in  the  air,  or  mixing,  is  limited  to  the  height  of  the  breathing  zone;  that  is,  vertical dispersion upwards as the air blows towards the receptor is not considered by the model.  This is one exposure scenario where  the actual  exposure assumed in the risk calculations would be impossible to achieve and the algorithm used to estimate the risk from volatile emission is very conservative.     5   
    • The ASTM VF is actually composed of two equations shown in Table 5:  one equation assumes an infinite source, and the other one equation includes a mass balance check to limit the volatilization term so that the amount volatilized cannot exceed the total amount of mass in the soil initially.  The VF is calculated using  both  equations  and  the  lower  of  the  two  volatilization  rates  is  used  for  the  VF  in  the  exposure equations. The default input values are shown in Table 6.  4.1.3 Particulate Emission FactorA particulate emission factor (PEF) is used to estimate the outdoor air concentrations due to chemicals airborne  on  particulates  (dust).    The  default  value  used  for  the  PEF  for  the  residential  and commercial/industrial scenarios is the default value used in the CHHSLs = (1.3 x 109) [(mg/kg)/(mg/m3)].  For  the  utility  trench  (construction)  worker,  a  PEF  value  of  1  x  106  [(mg/kg)/(mg/m3)]  was  used  (DTSC 2005). 4.2 Exposure Parameter Values UsedThe  CHHSLs  do  not  have  a  utility  trench/construction  worker  receptor,  so  the  default  exposure parameters  for  this  receptor  were  obtained  from  California  Department  of  Toxic  Substances  Control (DTSC)  Human  and  Ecological  Risk  Division  (HERD)  “Human  Health  Risk  Assessment  (HHRA)  Note Number 1” (DTSC 2005).  Table 4 shows the default values used for each parameter and provides the reference document where the value was obtained.    4.2.1 Ingestion of SoilReceptors  working  or  playing  outdoors  may  ingest  soil  through  incidental  contact  of  the  mouth  with hands  and  clothing.    For  the  residential  and  commercial  exposure  scenarios,  one  of  the  very conservative  assumptions  made  is  that  the  chemical  concentrations  remain  constant  over  time  in  the soil. In reality, this would not be the case for especially for volatile chemicals in the top few feet of soil, where  most  of  the  direct  contact  would  occur.    Benzene  is  highly  fugitive  in  surface  soil,  quickly depleting the upper soil depths.   4.2.2 Dermal Contact with SoilSome soil contaminants may be absorbed across the skin into the bloodstream. Absorption will depend upon the amount of soil in contact with the skin, the concentration of chemicals in soil, the skin surface area exposed, and the potential for the chemical to be absorbed across the skin.   4.2.3 Inhalation of Volatile and Particulate Emissions in Outdoor AirThe inhalation exposure route includes the inhalation of both volatile and particulate emissions.   The inhalation slope factors and non‐carcinogenic inhalation reference doses are shown in Table 7.   6   
    • 5 RESULTS: SOIL SCREENING LEVELSTable 8 (which is included here for convenience) shows the Soil Screening Levels.    Table 8:  Soil Screening Levels  Depth Benzene Naphthalene PAH (feet) (mg/kg) (mg/kg) (mg/kg) 0 to 5 2.3 13 0.038 5 to 10 100 1500 7.5 *Notes:  Based on the seven carcinogenic PAHs as benzo(a)pyrene toxicity equivalent [BaPe].  The PAH screening   level is only applicable where soil was affected by either waste oil and/or Bunker C fuel.  Table 9  shows  the  soil  screening  levels  calculated  for  each  exposure  scenario.  Note  that  the  lowest screening level was chosen for the two different soil depths to obtain the screening levels in Table 9.  Table 9:  Summary of Soil Screening Levels for Each Receptor  Subsurface Soil -- Volatilization Commercial/ Chemical Residential Utility only Industrial (for 5 to 10’ bgs) Residential Scenario mg/kg mg/kg mg/kg mg/kg Benzene 2.3 120 100 130 Naphthalene 13 45 1500 33,000 PAH 0.038 2.3 7.5 1 x 1066 DISCUSSION OF RESULTSThis  document  has  presented  Soil  Screening  Levels  to  be  used  to  identify  sites  that  are  low  threat  to human health risk for the direct contact pathways from impacted soil.  These Soil Screening Levels are designed  to  be  used  in  conjunction  with  the  Vapor  Intrusion  Criteria  and  Groundwater  Criteria  to determine if the site is a low‐threat from all exposure pathways.   Three  exposure  scenarios  were  originally  considered:  residential,  commercial/industrial,  and  a  utility trench/construction worker.  The final Soil Screening Levels were chosen as the lowest values for each receptor.  The equations used were based on the equations used by OEHHA in the development of the CHHSLs,  with  the  exception  of  the  volatilization  rate.    A  volatilization  rate  term  was  added  to  the  Soil Screening Level equations to be conservative.   7   
    • OEHHA  has  indicated  that  the  residential  exposure  scenario  is  protective  for  other  sensitive  uses  of  a site.  This means that these screening levels are also appropriate for other sensitive uses of the property (e.g., day‐care centers, hospitals, etc.) (Cal/EPA 2005). 7 REFERENCESAmerican Society for Testing and Materials (ASTM). 1996. Standard Guide to Risk‐Based Corrective  Action Applied at Petroleum Release Sites, ASTM E1739‐95, Philadelphia, PA.   DTSC (Department of Toxic Substances Control). 2005. Human and Ecological Risk Division (HERD).  Human Health Risk Assessment (HHRA) Note Number 1. Recommended DTSC Default Exposure  Factors for Use in Risk Assessment at California Military Facilities. OEHHA (Office of Environmental Health Hazard Assessment). 2005.  Human‐Exposure‐Based Screening  Numbers Developed to Aid Estimation of Cleanup Costs for Contaminated Soil, Integrated Risk  Assessment Branch, Office of Environmental Health Hazard Assessment. (Cal/EPA), January 2005  Revision.  Available at: http://www.oehha.ca.gov/risk/Sb32soils05.html  OEHHA (2009). OEHHA Cancer Potency Values as of July 21, 2009. SF RWQCB ESLs.  Regional Water Quality Control Board (RWQCB) Region 2 – San Francisco. 2008.  Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater. Interim  Final. May   USEPA. 1989. Risk Assessment Guide for Superfund (RAGS) Volume I Human Health Evaluation Manual  (Part A) EPA/540/1‐89/002, Office of Emergency and Remedial Response. December.     8   
    • TABLES Table 1:  Equations Used to Develop Soil Screening Levels for the Direct Contact Pathways  for a Residential Exposure Scenario  Carcinogenic – Residential  Age‐Adjusted Ingestion Rate    ⎡ ED × IRSc EDa × IRSa ⎤ IFSadj = ⎢ c + ⎥ ⎣ BWc BWa ⎦ Age‐Adjusted Dermal Contact Rate    ⎡ ED × SASc × AFc EDa × SAS a × AFa ⎤ SFS adj = ⎢ c + ⎥  ⎣ BW c BW a ⎦ Age‐Adjusted Inhalation Rate  ⎡ ED × InhR c EDa × InhR a × AFa ⎤ InFadj = ⎢ c + ⎥  ⎣ BW c BW a ⎦ Total  TR × ATCarc × 365 d yr Cres−risk =   ⎡⎡IFsadj × SFo ⎤ ⎡ SFSadj × ABS × SFo ⎤ ⎡ ⎛ 1 ⎞⎤ ⎤ EFr × ⎢⎢ ⎥ ×⎢ ⎥ × ⎢InFadj × SFi × ⎜ VFr + ⎜ ⎟⎥ ⎥ ⎢⎣ 1E6 mg kg ⎦ ⎣ ⎣ 1E6 mg kg ⎦ ⎢ ⎣ ⎝ PEFr ⎟⎥ ⎥ ⎠⎦ ⎦   Non‐Carcinogenic (Hazard) – Residential    THQ × BWc × 365 d yr C res - haz = ⎡⎛ 1 IRSc ⎞ ⎛ 1 SASc × AFc × ABS d ⎞ ⎛ 1 ⎛ 1 ⎞ ⎞⎤ ⎜ EFr × ED c × ⎢⎜ × 6 ⎟+⎜ ⎟ ⎜ RfDo × 6 ⎟+⎜ ⎜ ⎟ ⎟ ⎟ ⎜ RfD × InhR c ⎜ VFr + PEF ⎟ ⎟⎥ ⎢⎝ RfDo 10 mg kg ⎠ ⎝ ⎣ 10 mg kg ⎠ ⎝ i ⎝ r ⎠ ⎠⎥⎦    9   
    • Table 2:  Equations Used to Develop Soil Screening Levels for the Direct Contact Pathways  for a Commercial/Industrial Exposure Scenario  Carcinogenic – Commercial/Industrial (c/i)  TR × BWc / i × ATCarc × 365 d yr C c / i−risk = ⎡⎛ IRSc / i × SFo ⎞ ⎛ SASc / ij × AFc / i × ABS × SFo ⎞ ⎡ ⎛ 1 ⎞⎤ ⎤ ⎜ EFr × ⎢⎜ ⎟ ⎜ ⎟×⎜ ⎟ × ⎢InR c / i × SFi × ⎜ VFr + ⎟ ⎜ ⎟⎥ ⎥ PEFr ⎟⎥ ⎥ ⎢⎝ 1E6 mg kg ⎠ ⎝ ⎣ 1E6 mg kg ⎠ ⎢ ⎣ ⎝ ⎠⎦ ⎦ Non‐Carcinogenic – Commercial/Industrial    THQ × BWa/i × 365 d yr Cres - haz = ⎡⎛ 1 IRS ⎞ ⎛ 1 SASc / i × AF/ ic × ABSd ⎞ ⎛ 1 ⎛ 1 ⎞ ⎞⎤ ⎜ EFc/ir × EDc/i × ⎢⎜ × 6 c/i ⎟ + ⎜ ⎟ ⎜ RfDo × 6 ⎟+⎜ ⎜ ⎟ ⎟ ⎟ ⎜ RfD × InhRc / i ⎜ VFr + PEF ⎟ ⎟⎥ ⎢⎝ RfDo 10 mg kg ⎠ ⎝ ⎣ 10 mg kg ⎠ ⎝ i ⎝ r ⎠ ⎠⎥⎦   Table 3:  Equations Used to Develop Soil Screening Levels for the Direct Contact Pathways  for a Utility Trench Worker or Construction Exposure Scenario  Carcinogenic – Utility Trench Worker (ut)  TR × BWut × ATCarc × 365 d yr C uti−risk = ⎡⎛ IRSuti × SFo ⎞ ⎛ SASutj × AFut × ABS × SFo ⎞ ⎡ ⎛ 1 ⎞⎤ ⎤ ⎜ EFutr × ⎢⎜ ⎟×⎜ ⎟ ⎜ ⎟ × ⎢InR ut × SFi × ⎜ VFut + ⎜ ⎟⎥ ⎥ ⎢⎝ 1E6 mg kg ⎠ ⎝ ⎣ 1E6 mg kg ⎟ ⎠ ⎢ ⎣ ⎝ PEFutr ⎟⎥ ⎥ ⎠⎦ ⎦ Non‐Carcinogenic – Utility Trench Worker   THQ × BWut × 365 d yr C res- haz = ⎡⎛ 1 IRSut ⎞ ⎛ 1 SASut × AFut × ABS d ⎞ ⎛ 1 ⎛ 1 ⎞ ⎞⎤ EFut × ED uti ⎜ × ⎢⎜ × ⎟+⎜ × ⎟+⎜ ⎟ ⎜ RfD × InhR ut ⎜ VFut + PEF ⎟ ⎟⎥ ⎟ ⎢⎝ ⎣ RfDo 10 6 mg kg ⎟ ⎜ RfDo ⎠ ⎝ 10 6 mg kg ⎠ ⎝ i ⎜ ⎝ ⎟ ut ⎠ ⎠⎥⎦  10   
    •   Table 4:  Default Exposure Parameters  Variable  Parameter  Units  Value  Reference  Name  70 years by definition  Averaging time for carcinogens   ATcarc  years  70  (USEPA 1989)  Body weight, residential child  BWc  kg  15  OEHHA (2005)  Body weight, residential adult  BWa  kg  70  OEHHA (2005)  Body weight, commercial/industrial  BWc/i   kg  70  OEHHA (2005)  Body weight, utility worker  BWut   kg  70  DTSC HERD (2005)  Exposure duration, residential child  EDc   years  6  OEHHA (2005)  Exposure duration, residential adult  EDa   years  24  OEHHA (2005)  Exposure duration, commercial/industrial  EDc/i   years  25  OEHHA (2005)  DTSC HERD (2005)  Assumption is 1  Exposure duration, utility worker  EDut   years  1  month at 20 d/month,  therefore ED = 1  Exposure frequency, residential child  EFc   d/year  350  OEHHA (2005)  Exposure frequency, residential adult  EFa   d/year  350  OEHHA (2005)  Exposure frequency, commercial/industrial  EFc/i   d/year  250  OEHHA (2005)  DTSC HERD (2005),  Exposure frequency, utility worker  EFut   d/year  20  assumption is 1  month at 20 d/month  Soil ingestion rate, residential child  IRSc   mg/d  200  OEHHA (2005)  Soil ingestion rate, residential adult  IRSa   mg/d  100  OEHHA (2005)  Soil ingestion rate, commercial/industrial  IRSc/i   mg/d  100  OEHHA (2005)  Soil ingestion rate, utility worker  IRSut   mg/d  330  DTSC HERD (2005)  Soil to skin adherence factor, residential  AFc   mg/cm2  0.2  OEHHA (2005)  child  Soil to skin adherence factor, residential  AFa   mg/cm2  0.07  DTSC HERD (2005)  adult  Soil to skin adherence factor,  AFc/i   mg/cm2  0.2  OEHHA (2005)  commercial/industrial  Soil to skin adherence factor, utility worker  AFut   mg/cm2  0.8  DTSC HERD (2005)  Skin surface area exposed to soil,  SASc   cm2  2800  OEHHA (2005)  residential child  Skin surface area exposed to soil,  SASa   cm2  5700  DTSC HERD (2005)  residential adult  Skin surface area exposed to soil,  SASc/i   cm2  5700  DTSC HERD (2005)  commercial/industrial  Skin surface area exposed to soil, utility  SASut   cm2  5700  DTSC HERD (2005)  worker  Inhalation rate, residential child  InhRc   m3/day  10  OEHHA (2005)  3 Inhalation rate, residential adult  InhRa   m /day  20  OEHHA (2005)  
    •   Variable  Parameter  Units  Value  Reference  Name  Inhalation rate, commercial/industrial  InhRc/i   m3/day  14  OEHHA (2005)  3 Inhalation rate, utility worker  InhRut   m /day  20  DTSC HERD (2005)  ASTM (1996)   See  Averaging time for vapor flux  tau  sec  ‐ equals exposure  reference  duration in seconds  Particulate emission factor, residential and  PEFa  m3/kg  1.3 x 109  OEHHA (2005)  commercial/industrial  Particulate emission factor, utility worker  PEFut  m3/kg  1.0 x 106  DTSC HERD (2005)  Dermal absorption factor from soils  ABSd  unitless  See Table 7    Oral cancer slope factor  SFo   unitless  See Table 7     Inhalation cancer slope factor  SFi   unitless  See Table 7     Oral reference dose  RfDo   unitless  See Table 7     Inhalation reference dose  RfDi   unitless  See Table 7     Target hazard quotient   THQ  unitless  1  OEHHA (2005)  Target individual excess lifetime cancer risk  TR  unitless  1 x 10‐6  OEHHA (2005)  References:          ASTM (1996). American Society for Testing and Materials, Standard Guide to Risk‐Based Corrective Action  Applied at Petroleum Release Sites, ASTM E1739‐95, Philadelphia, PA.  DTSC HERD (2005). Department of Toxic Substances Control, Human and Ecological Risk Division (HERD).  Human Health Risk Assessment (HHRA) Note Number 1. Recommended DTSC Default Exposure Factors for  Use in Risk Assessment at California Military Facilities.  OEHHA (2005). Human‐Exposure‐Based Screening Numbers Developed to Aid Estimation of Cleanup Costs for  Contaminated Soil, Integrated Risk Assessment Branch, Office of Environmental Health Hazard Assessment.  (Cal/EPA).  USEPA. 1989.  Risk Assessment Guide for Superfund (RAGS) Volume I Human Health Evaluation Manual (Part  A) EPA/540/1‐89/002, Office of Emergency and Remedial Response. December 1989.        
    •     Table 5:  Equations Used to Estimate Volatilization and Particulate Emission Factors  Volatilization and Particulate Emission Factors  Effective Diffusion Coefficient (Deff)  ⎛ θ 10 / 3 ⎞ ⎛ 10 / 3 ⎞ Deff = Dair ⎜ a 2 ⎟ + Dwater 1 ⎜ θ W ⎟ ⎜ θ ⎝ T ⎟ ⎠ H ⎜ θT 2 ⎝ ⎟ ⎠ Volatilization Factor (VF)  Infinite source:              ⎡ (mg / m3 − air )⎤ 2 ⋅ W ⋅ ρ b D eff ⋅ H cm3 kg VF⎢ ⎥= × 10 3   ⎣ (mg / kg − soil)⎦ Uair ⋅ δ air π (θ w + FOC ⋅ K oc ⋅ ρ b + H ⋅ θ a )tau m3 g Mass‐balance considered:         ⎡ (mg / m 3 − air ) ⎤ W ⋅ ρb ⋅ d cm 3 kg VF ⎢ ⎥= × 10 3 ⎣ (mg / kg − soil) ⎦ Uair ⋅ δ air ⋅ tau m3 g   Calculate VF using both equations, then use the lower of the two values.  VFr :   Use tau = tauc + taur  VFc/i :   Use tau = tauc/i   VFut :   Use tau = tauut    
    •   Table 6: Default Volatilization and Soil‐Specific Parameters  Variable  Parameter  Units  Value  Reference  Name Fraction organic carbon in soil  FOC  g OC/g soil  0.01  ASTM (1996)  ASTM (1996)  Thickness of impacted soil  D  cm  305  (10 feet) Wind speed in outdoor air mixing zone  Uair  cm/s  225  ASTM (1996) Width of source area parallel to wind, or  W  cm  1500  ASTM (1996)  groundwater flow direction Outdoor air mixing zone height  δair  cm  200  ASTM (1996) Volumetric air content in vadose‐zone soils  ΘA  3 (cm )/(cm )  3 0.26  ASTM (1996) Total soil porosity  θ T  (cm3)/(cm3)  0.38  ASTM (1996) Volumetric water content in vadose‐zone  ΘW  (cm3)/(cm3)  0.12  ASTM (1996)  soils Soil bulk density  ρb  g/cm3  1.7  ASTM (1996) Averaging time for vapor flux, residential  ASTM (1996)  taur  s  7.57E8  adult  = EDr in sec Averaging time for vapor flux, residential  ASTM (1996)  tauc  s  1.89E8  child  = EDc in sec Averaging time for vapor flux,  ASTM (1996)  tauc/i  s  7.88E8  commercial/industrial  = EDc/i in sec  ASTM (1996) Averaging time for vapor flux, utility worker  tauut  s  3.15E7  = EDut in sec Effective diffusion coefficient in soil  Deff  cm2/s  Chem. specific  calculated  2Diffusion coefficient in air   Dair  cm /s  Chem. specific  See Table 7. Diffusion coefficient in water  Dwater  cm2/s  Chem. specific  See Table 7. Organic carbon‐water sorption coefficient  Koc  mL/g  Chem. specific  See Table 7. Henry’s Law coefficient  H  ‐  Chem. specific  See Table 7. References:         ASTM.  1996.  Standard Guide to Risk‐Based Corrective Action Applied at Petroleum Release Sites, ASTM  E1739‐95, Philadelphia, PA.    
    •   Table 7:  Chemical Parameter Values  Chemical Parameters1 Units Benzene Naphthalene PAH1 Reference Henry’s Law constant - 0.23 0.018 1.9E-5 SF RWQCB ESLs Organic carbon partition mL/g 58.9 1500 5.9E+6 SF RWQCB ESLs coefficient Diffusion coefficient in air cm2/s 0.090 0.060 ND SF RWQCB ESLs Diffusion coefficient in cm2/s 9.8E-6 8.4E-6 ND SF RWQCB ESLs water Toxicity Parameters Oral slope factor (SFo) 1/(mg/kg-d) 0.1 ND 12 OEHHA (2009) Inhalation slope factor 1/(mg/kg-d) 0.1 0.12 3.9 OEHHA (2009) (SFi) Oral reference dose mg/kg-d 0.004 0.020 0.030 SF RWQCB ESLs (RfDo) Inhalation reference dose mg/kg-d 0.0086 8.6E-4 0.030 SF RWQCB ESLs (RfDi) Dermal absorption factor - ND 0.13 0.13 SF RWQCB ESLs from soil ND = No Data    SF RWQCB ESLs.  Regional Water Quality Control Board (RWQCB) Region 2 – San Francisco. 2008. Screening for  Environmental Concerns at Sites with Contaminated Soil and Groundwater. Interim Final. May    OEHHA (2009). OEHHA Cancer Potency Values as of July 21, 2009.  1  The chemical properties for benzo(a)pyrene were used as a surrogate in developing screening levels for the  “PAH” group.   Table 8:  Soil Screening Levels  Depth Benzene Naphthalene PAH (feet) (mg/kg) (mg/kg) (mg/kg) 0 to 5 2.3 13 0.038 5 to 10 100 1500 7.5 *Notes:  Based on the seven carcinogenic PAHs as benzo(a)pyrene toxicity equivalent [BaPe].    The PAH screening level is only applicable where soil is affected by either waste oil and/or Bunker C fuel.   Table 9:  Summary of Soil Screening Levels for Each Receptor  Subsurface Soil -- Volatilization Commercial/ Chemical Residential Utility only Industrial (for 5 to 10’ bgs) Residential Scenario mg/kg mg/kg mg/kg mg/kg Benzene 2.3 120 100 130 Naphthalene 13 45 1500 33,000 PAH 0.038 2.3 7.5 1 x 106 
    •   FIGURES   Figure 1.  Conceptual Site Model for the Soil Screening Levels.  Trench/Utility Commercial Residential Worker Exposure Exposure Media Routes Impacted Soil Surface Ingestion from 0 to 5 Soil feet bgs (0 to 5’ bgs) Dermal Contact Dust Emissions Outdoor Inhalation Air Volatilization Impacted Soil from 5 to 10 Subsurface Ingestion feet bgs Soil (5 to 10’ bgs) Dermal Contact Exposure pathway considered in the development of the Soil Screening Criteria Exposure route is considered potentially complete 
    •   Figure 2.  Schematic for the ASTM Volatilization Factor.    volatile and particulate Exposure point emissions in outdoor air. location for volatile 15 feet and particulate emissions et fe 15 Wind Direction (towards receptor 24 hours/day) Surface soil (0 to 5 feet bgs) Impacted Soil: Overall thickness -- uniform concentration, of source = 10 -- from 0 to 10’ bgs feet -- 15’ wide by 15’ long (areally) (for volatilization) Subsurface soil (5 to 10 feet bgs) 
    • Documents developed by the UST stakeholder group are listed below: Draft Low Threat UST Closure Policy - Final 7/14/11 Technical Justification for Direct Contact - Final 7/16/11Technical Justification for Groundwater Plume Lengths, etc - Final 7/12/11 Technical Justification for VI Pathway - Final 6/30/11
    • Technical Justification for Groundwater Plume Lengths, IndicatorConstituents, Concentrations, and Buffer Distances (Separation Distances) to ReceptorsThe purpose of this document is to provide technical justification for the four classes of low-threat groundwater plumes that are described in the Groundwater section of the Low-Threat USTClosure Policy (the Policy). The fifth plume class is a site-specific evaluation.The Policy Stakeholder Group chose benzene, MTBE, and TPHg as adequate indicatorconstituents for the groundwater plume lengths discussed in the Policy. The technicaljustification for using these three constituents, discussed in more detail below, relies heavily onthe facts that (1) benzene has the highest toxicity of the soluble petroleum constituents, (2)MTBE typically has the longest plume lengths, and (3) TPHg represents the additional dissolvedhydrocarbons that may be present resulting from a typical petroleum release. Although TPHd isnot used to describe plume lengths (largely because the hydrocarbons in the TPHd carbon rangeare of low solubility), other technical considerations associated with the use of TPHd data arediscussed below.Benzene and MTBE are used in research studies as key indicator constituents for the threat(human health risk and nuisance) posed by groundwater plumes from petroleum releases because(1) benzene has the highest toxicity of the soluble petroleum constituents, and (2) MTBEtypically has the longest plume lengths and has a low secondary MCL (taste and odor thresholdof 5 micrograms/liter [ug/l]).Several significant multi-site studies of groundwater plume lengths from petroleum release siteshave been conducted across the U.S. since the mid-1990s. These studies included sites whereremediation had been performed and sites where no active remediation had been performed.Most of these studies focused on benzene plumes (e.g., Rice, et al. 1995; Rice et al. 1997;Busheck et al. 1996; Mace, et al. 1997; Groundwater Services, Inc. 1997; API 1998); threestudied benzene and oxygenate plumes (including MTBE) (Dahlen et al. 2004; Shih et al. 2004;Kamath et al. in press). Most of these plume studies are further discussed in detail in the Fateand Transport chapter of the California LUFT Manual.In summary for all of these multi-site studies, the average benzene plume length was less than200 feet and 90% of the benzene plumes were less than 400 feet long. The peer-reviewed studyby Shih et al. (2004) of plume lengths at 500 UST sites in the Los Angeles area is widely reliedupon as representative of current knowledge of plume lengths at UST sites in California. Resultsfor benzene, MTBE and TPHg from Shih et al. (2004) are as follows:
    • Constituent Average Plume 90th Percentile Plume Maximum Plume (and plume limit Length Length Length concentration) (feet) (feet) (feet) Benzene (5 ug/l) 198 350 554 MTBE (5 ug/l) 317 545 1,046 TPHg (100 ug/l) 248 413 855Data are from Shih et al. (2004). Plume lengths were measured from the source area.Although the California MCL for benzene is 1 ug/l, Shih et al. (2004) used a plume limitconcentration of 5 ug/l because of statistical uncertainty with concentrations too close to thelaboratory reporting limit. The benzene plume lengths at a 1 ug/l concentration limit would beexpected to be slightly longer than those shown here.Ruiz-Aguilar et al. (2003) studied UST sites in the Midwest with releases of ethanol-amendedgasoline (10% ethanol by volume) and found that benzene plume lengths may increase by 40%to 70% due to the addition of ethanol in gasoline (replacing MTBE). Ethanol is preferentiallybiodegraded over the benzene, which results in a longer benzene plume. However, the Policyaddresses this potential for expansion of the plume lengths by adding safety factors of 100% to400%.It is well documented that, due to effective solubility, the hydrocarbons that will dissolve atmeasurable amounts into groundwater from a petroleum fuel release (including gasoline,kerosene, jet fuel, diesel or heavier fuels) are limited to primarily the very small aliphatics (lessthan C7) and the C14 or smaller aromatics (e.g., Shiu et al. 1990; Coleman et al. 1984). The C15and larger hydrocarbons have very low effective solubilities and are not found in the dissolvedphase of a petroleum fuel release. The carbon range of the potential dissolved hydrocarbons(less than or equal to C14) is largely covered by the TPHg carbon range (approximately C5 toC12). Therefore, TPHg should be sufficient to represent the dissolved hydrocarbons that may bepresent in addition to benzene and MTBE from virtually any type of product release. TPHd wasnot included as an indicator constituent for groundwater plume length because the vast majorityof the TPHd carbon range (approximately C12 to C22) is higher than the carbon range for thepossible dissolved hydrocarbons (less than or equal to C14). Oxygenates other than MTBE werenot included as indicator constituents because Shih et al. (2004) documented that MTBE had thelongest plume length of any of the oxygenates (MTBE, TBA, DIPE, TAME, ETBE) at anypercentile, and Kamath et al. (in press) found that TBA plumes were comparable in length toMTBE plumes. Therefore, MTBE can be used as a conservative indicator for the otheroxygenates including TBA.For groundwater samples analyzed for TPHd for comparison to Water Quality Objectives(WQOs), a silica gel cleanup (SGC) should be included for the following reasons. It is wellknown that the TPHd analysis (Method 8015B) is not specific to hydrocarbons unless a SGC isused; otherwise the reported TPHd concentration can include polar non-hydrocarbon compoundsin addition to the hydrocarbons that may be present in a water sample (e.g., Zemo and Foote
    • 2003). These polar compounds can be from various sources, including metabolites frombiodegradation of petroleum (primarily alcohols and organic acids, with possible phenols,aldehydes and ketones). At sites with biodegrading petroleum, the majority of the organics beingmeasured as “TPHd” (without SGC) can be polar compounds and not dissolved hydrocarbons.WQOs for diesel-range petroleum hydrocarbons for health risk or taste and odor concerns arebased on the properties of the dissolved hydrocarbons assumed to be present and not on theproperties of the polar compounds. For example, the health-based ESL for TPHd is based on theassumption that 100% of the TPH has a toxicity equivalent to the C11 to C22 aromatics, and thetaste and odor value for TPHd is based on the dissolved phase of fresh diesel/kerosene (whichwould be primarily the C14 and smaller aromatics) (SFRWQCB 2008). The San Francisco BayRWQCB recognized that reported TPHd concentrations may include polar compounds andissued a guidance memorandum recommending that SGC be routinely used so that “…..decisions could be made based on analytical data that represents dissolved petroleum.”(SFRWQCB 1999). Only the hydrocarbon component of the TPHd concentration should becompared to the TPHd WQOs, and thus SGC is necessary to separate the hydrocarbons from thepolar compounds in a groundwater sample prior to analysis. It is well established that a SGCdoes not remove the dissolved hydrocarbons in a sample (e.g., Lundegard and Sweeney 2004).Further, the potential for removal of hydrocarbons by a SGC is always monitored as part of theroutine laboratory quality assurance reporting where lab control samples are spiked with ahydrocarbon (surrogate), are subjected to a SGC, and recovery of the surrogate is measured andmust be within acceptable ranges.The four classes of stabilized plume lengths and buffer distances from the plume edge to theclosest water supply well or surface water (receptors) that are defined as “low threat” in thePolicy are initially based upon the plume lengths from the studies cited above, but also are basedon additional safety factors that the Stakeholder Group considered applicable to be protective ina state-wide policy document. For example, based on the plume studies, a total separationdistance from the source area to the receptor of about 500 feet should be protective for 90% ofplumes from UST sites, and a total separation distance from the source area to the receptor ofabout 1,000 feet should be protective for virtually all plumes from UST sites. Conversely, the“low-threat classes” require a known maximum stabilized plume length (which reducesuncertainty as to how long the plume might become in the future), and include additional safetyfactors and concentration limits developed by the Stakeholder Group.Stakeholder Group participants also recognize and acknowledge that this Policy is consistentwith other State and local practices regarding impacts to groundwater caused by otheranthropogenic releases. For example, State and local agencies establish required separationdistances or “setbacks” between water supply wells and septic system leach fields (typically 100feet), and sanitary sewers (typically 50 feet; [DWR 1981]).The Stakeholder Group acknowledges that the biodegradation/natural attenuation of petroleumhydrocarbon and oxygenate plumes has been documented by many researchers since the 1990s.
    • All of this work shows that biodegradation/natural attenuation of petroleum hydrocarbons andMTBE occurs under both aerobic and anaerobic conditions, but the rate ofdegradation/attenuation depends on the individual constituent and the plume geochemicalconditions. The maximum concentrations for benzene and MTBE specified in the low-threatclasses below are expected to biodegrade/naturally attenuate to WQOs within approximately 10to 30 years, based on commonly-accepted rate constants for typical plume conditions andcalculations (e.g., Wilson 2003; USEPA 2002). A time period of multiple decades or longer toreach WQOs has been determined to be “reasonable” for plumes of limited extent in existingState Water Board closure orders for UST sites (e.g., Order WQ 98-04 [Matthew Walker]).TBA is a byproduct of biodegradation of MTBE, and TBA concentrations can build uptemporarily in the anaerobic portion of a plume. With respect to the natural attenuation of TBA,Kamath et al. (in press) recently studied benzene, MTBE and TBA plumes at 48 UST sites (30sites in California) and found that (1) most (68%) of the TBA plumes were stable or decreasingin size, and (2) in the stabilized plumes, the median attenuation rate for TBA was similar to therates for MTBE and benzene. These findings indicate that TBA should not pose a significantthreat to groundwater resources, and are consistent with the finding from Williams (in press) thatTBA and MTBE have been detected in only a very limited number of public drinking watersupply wells in California between 1996 and 2010. The average annual detection frequencies atany concentration and at concentrations greater than the WQO (12 ug/l for TBA and 5 ug/l forMTBE), through 2010 are: 1.4% and 0.2% for TBA, respectively, and 1.6% and 0.8% forMTBE, respectively (Williams, in press).The following paragraphs present and discuss the key rationales for low-threat plume lengths,maximum concentrations, and separation distances for each low-threat class. Note that thespecified concentrations are maximums, and would likely occur in only a few wells; the averageconcentrations in the plume would be lower. Note also that these groundwater plume classcriteria (concentrations, plume lengths and separation distances) are only one component of theoverall evaluation of site conditions that must be satisfied to be considered for closure as a low-threat site under the Policy.Class 1: The “short” stabilized plume length (<100 feet) is indicative of a small or depletedsource and/or very high natural attenuation rate. The 250 feet distance to a receptor from theedge of the plume represents an additional 250% “plume length” safety factor in the event thatsome additional unanticipated plume migration was to occur.Class 2: The “moderate” stabilized plume length (<250 feet) approximates the average benzeneplume length from the cited studies. The maximum concentrations of benzene (3,000 ug/l) andMTBE (1,000 ug/l) are conservative indicators that a free product source is not present. Theseconcentrations are approximately 10% and 0.02%, respectively, of the typical effective solubilityof benzene and MTBE in unweathered gasoline. These concentrations are expected tobiodegrade/naturally attenuate to WQOs within a reasonable time frame. The potential for vapor
    • intrusion from impacted groundwater must be evaluated separately as per the vapor intrusionsection of the Policy. The 1,000 feet distance to the receptor from the edge of the plume is anadditional 400% “plume length” safety factor in the event that some additional unanticipatedplume migration was to occur. Also note that California Health and Safety Code §25292.5requires that UST owners and operators implement enhanced leak detection for all USTs within1,000 feet of a drinking water well. In establishing the 1,000 feet separation requirement thelegislature acknowledged that 1,000 feet was a sufficient distance to establish a protectivesetback between operating petroleum USTs and drinking water wells in the event of anunauthorized release.Class 3: The “moderate” stabilized plume length (<250 feet) approximates the average benzeneplume length from the cited studies. The on-site free product and/or high dissolvedconcentrations in the plume remaining after source removal to the extent practicable (as per theGeneral Criteria in the Policy) require five years of monitoring to validate plume stability/naturalattenuation (i.e., to confirm that the rate of natural attenuation exceeds the rate of NAPLdissolution and dissolved-phase migration). The potential for vapor intrusion from free productor impacted groundwater must be evaluated separately as per the vapor intrusion section of thePolicy. The 1,000 feet distance to the receptor from the edge of the plume is an additional 400%“plume length” safety factor in the event that some additional unanticipated plume migration wasto occur, and is consistent with H&S Code §25292.5 as discussed above.Class 4: The “long” stabilized plume length (<1,000 feet) approximates the maximum MTBEplume length from Shih et al. (2004). The maximum benzene and MTBE source areaconcentrations (1,000 ug/l each) in the stable plume are expected to biodegrade/naturallyattenuate to WQOs within a reasonable time frame. The maximum benzene concentration wouldnot pose a vapor intrusion risk over the extent of the plume (assuming that five feet ofbioreactive vadose zone is available over the extent of the plume; see justification for vaporintrusion screening criteria for details). The 1,000 feet distance to the receptor from the edge ofthe plume is an additional 100% “plume length” safety factor in the event that some additionalunanticipated plume migration was to occur, and is consistent with H&S Code §25292.5 asdiscussed above.Notes on Free Product RemovalState regulation (CCR Title 23, Division 3, Chapter 16, Section 2655) requires that “responsibleparties“…. remove free product to the maximum extent practicable, as determined by the localagency…” (Section 2655a) “…. in a manner that minimizes the spread of contamination intopreviously uncontaminated zones”… (Section 2655b), and that “[a]batement of free productmigration shall be the predominant objective in the design of the free product removal system”(Section 2655c). Over the years there has been debate on the meaning of the terms “freeproduct” and “maximum extent practicable”. Product (light non-aqueous phase liquid [LNAPL])can exist in three conditions in the subsurface: residual or immobile LNAPL (LNAPL that is
    • trapped in the soil pore spaces by capillary forces and is not mobile), mobile LNAPL (enoughLNAPL is present in the soil pore spaces to overcome capillary forces so that the LNAPL canmove) and migrating LNAPL (mobile LNAPL that is migrating because of a driving head).“Residual LNAPL”, “mobile LNAPL” and “migrating LNAPL” are described in detail in severalpeer-reviewed technical documents, including the 2009 Interstate Technology RegulatoryCouncil (ITRC) Technical/Regulatory Guidance “Evaluating LNAPL Remedial Technologies forAchieving Project Goals”. Given the predominant objective of abatement of migration, the term“free product” in the State regulation is primarily equivalent to “migrating LNAPL” (which is asubset of “mobile LNAPL”), and secondarily equivalent to “mobile LNAPL”. Whether LNAPLis mobile (and therefore could potentially migrate) or not is usually tested by observing rechargeof LNAPL after removing LNAPL from a monitoring well. Whether LNAPL is migrating or notis tested by monitoring the extent of the LNAPL body (usually using the apparent productthickness in monitoring wells) at a certain water level elevation over time. If the extent at thatwater level elevation does not expand, then the LNAPL is not migrating. Therefore, LNAPLmust be removed to the point that its migration is stopped, and the LNAPL extent is stable.Further removal of non-migrating but mobile LNAPL is required to the extent practicable at thediscretion of the local agency. Removal of mobile LNAPL from the subsurface is technicallycomplicated, and the definition of “extent practicable” is based on site-specific factors andincludes a combination of objectives for the LNAPL removal (such as whether the mobileLNAPL is a significant “source” of dissolved constituents to groundwater or volatile constituentsto soil vapor, or whether there is a high likelihood that hydrogeologic conditions would changesignificantly in the future which may allow the mobile LNAPL to migrate) and technicallimitations. The typical objectives for LNAPL removal, technologies for LNAPL removal andtechnical limitations of LNAPL removal are discussed in several peer-reviewed technicaldocuments including the 2009 ITRC Guidance (see especially Section 4 “Considerations/FactorsAffecting LNAPL Remedial Objectives and Remedial Technology Selection”, Table 4.1[Example Performance Metrics], Table 5-1 [Overview of LNAPL Remedial Technologies], andTable 6-1 [Preliminary Screening Matrix]).ReferencesAmerican Petroleum Institute (API), 1998. Characteristics of dissolved petroleum hydrocarbon plumes, Results from four studies. API Soil/Groundwater Technical Task Force, Vers. 1.1. December.Buscheck, T.E., D.C. Wickland, and L.L. Kuehne, 1996. Multiple lines of evidence to demonstrate natural attenuation of petroleum hydrocarbons. Proceedings of the 1996 Petroleum Hydrocarbon and Organic Chemicals in Groundwater Conference. NGWA/API. Westerville, OH.Coleman, W.E., J.W. Munch, R.P. Streicher, P. Ringhand, and F. Kopfler, 1984. The identification and measurement of components in gasoline, kerosene and No. 2 fuel oil that partition into the aqueous phase after mixing. Arch. Environ. Contam. Toxicol. 13: 171-178.
    • Dahlen, P.R., M. Matsumura, E.J. Henry, and P.C. Johnson, 2004. Impacts to Groundwater Resources in Arizona from Leaking Underground Storage Tanks (LUSTs). http://www.eas.asu.edu/civil/Environmental/Groundwater.htm. Groundwater Services, Inc. 1997. Florida RBCA Planning Study. www.GSI-net.comITRC (Interstate Technology & Regulatory Council). 2009. Evaluating LNAPL Remedial Technologies for Achieving Project Goals. LNAPL-2. Washington, D.C.: Interstate Technology & Regulatory Council, LNAPLs Team. www.itrcweb.org.Kamath, R., J.A. Connor, T.E. McHugh, A. Nemir, M.P. Lee and A.J. Ryan, in press. Use of long-term monitoring data to evaluate benzene, MTBE and TBA plume behavior in groundwater at retail gasoline sites. Journal of Environmental Engineering. (Accepted for publication on June 15, 2011)Lundegard, P.D. and R.E. Sweeney, 2004. Total petroleum hydrocarbons in groundwater: Evaluation of nondissolved and nonhydrocarbon fractions. Environmental Forensics, Vol 5: 85-95.Mace, R.E., R.S. Fisher, D.M. Welch, and S.P. Parra, 1997. Extent, mass, and duration of hydrocarbon plumes from leaking petroleum storage tank sites in Texas. Bureau of Economic Geology, Geological Circular 97-1.Rice, D.W., R.D. Grose, J.C. Michaelsen, B.P. Dooher, D.H. MacQueen, S.J. Cullen, W.E. Kastenberg, L.G. Everett, M.A. Marino, 1995. California leaking underground fuel tank (LUFT) historical case analyses. Lawrence Livermore National Laboratory (LLNL). UCRL- AR-122207. November.Rice, D.W., B.P. Dooher, S.J. Cullen, L.G. Everett, W.E. Kastenberg, and R.C. Ragaini, 1997. Response to USEPA comments on the LLNL/UC LUFT cleanup recommendations and California historical case analysis. LLNL. UCRL-AR-125912. January.Ruiz-Aguilar, G.M.L., K. O’Reilly, and P.J.J. Alvarez, 2003. A comparison of benzene and toluene plume lengths for sites contaminated with regular vs. ethanol-amended gasoline. Ground Water Monitoring & Remediation, Vol. 23, No. 1: 48-53.San Francisco Regional Water Quality Control Board (SFRWQCB), 2008. Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater. Interim Final, May.SFRWQCB, 1999. Memorandum: Use of silica gel cleanup for extractable TPH analysis. February.Shih, T., Y. Rong, T. Harmon, and M. Suffet, 2004. Evaluation of the impact of fuel hydrocarbons and oxygenates on groundwater resources. Environmental Science & Technology. Vol. 38, No. 1: 42-48.Shiu, W.Y., M. Bobra, A.M. Bobra, A. Maijanen, L. Suntio, and D. Mackay, 1990. The water solubility of crude oils and petroleum products. Oil and Chem. Poll. Vol. 7, No. 1, 57-84.USEPA, 2002. Ground Water Issue: Calculation and use of first-order rate constants for monitored natural attenuation studies. EPA/540/S-02/500. November.Williams, P.R.D., in press. MTBE in California’s public drinking water wells: Have past predictions come true? Environmental Forensics. (Accepted for publication on June 4, 2011)
    • Wilson, J.T., 2003. Fate and Transport of MTBE and Other Gasoline Components. Chapter 3 in MTBE Remediation Handbook, E.E. Moyer and P.T. Kostecki (editors). Amherst Scientific Publishers, Amherst, MA.Zemo, D.A. and G.R. Foote, 2003. The technical case for eliminating the use of the TPH analysis in assessing and regulating dissolved petroleum hydrocarbons in ground water. Ground Water Monitoring & Remediation, Vol. 23, No. 3: 95-104.
    • Documents developed by the UST stakeholder group are listed below: Draft Low Threat UST Closure Policy - Final 7/14/11 Technical Justification for Direct Contact - Final 7/16/11Technical Justification for Groundwater Plume Lengths, etc - Final 7/12/11 Technical Justification for VI Pathway - Final 6/30/11
    • Technical Justification for Low‐Threat   Closure Scenarios for Petroleum Vapor Intrusion Pathway 1 EXECUTIVE SUMMARYFor petroleum‐related volatile organic compounds (VOCs) at retail sites, current risk‐based screening levels (such as the California Human Health Screening Levels [CHHSLs]) for evaluating risk from vapor intrusion at retail sites are extremely conservative.  This conservatism is caused by excluding biodegradation in site screening and often drives further unnecessary site evaluation.  Recent models and field studies show that bioattenuation of petroleum hydrocarbons at retail sites is significant.  Petroleum VOCs (such as benzene, toluene, ethylbenzene and xylenes (BTEX)) concentrations can attenuate by 4 to 6 orders of magnitude within short vertical distances (e.g., < 2 m) in the unsaturated zone.  The VOC attenuation increases by an additional order of magnitude (or more) if transport across the building foundation to indoor air is also considered.  The sharp decrease in petroleum VOC concentrations within a short vertical distance of the unsaturated zone is amenable to use of exclusion distances as a site‐screening methodology for vapor intrusion.  Exclusion distances are defined as source (VOCs in soil or groundwater)‐receptor (building) separation distances beyond which the risk of vapor intrusion is negligible.  Exclusion distance criteria can be broadly defined for two types of sources:  low‐concentration and high‐concentration sources which are defined below. Recent modeling studies and evaluations of field (soil‐gas) data from numerous retail sites and sampling locations demonstrate that biodegradation is sufficient to limit the potential for vapor intrusion at sites with “low concentration” hydrocarbon sources.  For example, there is less than a 5% probability that benzene concentrations in soil gas would exceed a conservative screening level of 100 ug/m3 at a distance of 5 feet above the source.  (Note the CHHSL for benzene in soil gas is 83 ug/m3.)  The attenuation is predicted to increase with lateral displacement of the source from the building foundation. Vapor intrusion risks are thus expected to be rare to non‐existent at sites with low‐concentration sources.    At sites with “high concentration” volatile sources (unweathered residual LNAPL in soil and/or unweathered free‐phase LNAPL on groundwater), transport modeling shows that hydrocarbons will attenuate in the unsaturated zone by approximately 6 orders of magnitude within 7 m (~20 ft) at sites.  This result is achieved assuming reasonable approximations for source type and biodegradation rate.   Analysis of soil‐gas data collected from many retail sites with LNAPL sources indicate that the distance required to attenuate soil vapor concentrations to below typical screening levels are on the order of 8 – 13 ft.  As with “low‐concentration” sources (weathered residual LNAPL in soil and/or dissolved concentrations in groundwater), the bioattenuation is more significant for LNAPL sources separated laterally from building foundations (i.e. the soil gas concentrations would attenuate in even shorter distances).     The Stakeholder Group has proposed screening criteria for four basic scenarios that can be used to identify low‐threat closure scenarios for vapor intrusion (VI).  The purpose of this technical document is    1  Version date:  June 30, 2011 
    • to outline the intent of the Stakeholder Group for use of these screening criteria and to provide justification for the four scenarios below. These scenarios are:  Scenario 1:  Unweathered LNAPL on groundwater  Scenario 2:  Unweathered LNAPL in soil  Scenario 3:  Dissolved phase benzene concentrations in groundwater  Scenario 4:  Direct measurement of soil gas concentrations  For each of these scenarios, screening criteria have been proposed that, if met, would identify the site as a low‐threat to human health from the vapor intrusion pathway.   It is important that the current state of the science as described herein be used to develop rational, technically defensible, approaches to address these potential vapor intrusion risk scenarios.   In addition, many of the cited exclusion criteria are based on analysis of soil‐gas data collected from retail sites.  The screening criteria may therefore not be applicable for non‐retail (e.g., pipeline, manufacturing, and terminal) sites where significantly larger volume petroleum hydrocarbon releases may have occurred.  If conditions at non‐retail sites are significantly different than would be encountered at a typical retail site, they should be evaluated on a site‐specific basis.  The materials referenced in this technical justification are consistent with the technical material being used to develop guidance by US EPA’s Office of Underground Storage Tanks (OUST)’s Task Force on Petroleum Vapor Intrusion.  2 INTRODUCTIONBiodegradation is the most critical process governing the potential for vapor intrusion at petroleum release sites.  The significance of biodegradation depends largely on the demand for oxygen (O2) and its availability.  Key factors that affect the O2 demand/availability include source strength/type (e.g., LNAPL or dissolved phase), source location (i.e., above or below the capillary zone), soil type (DeVaull, 2007), variable and/or high soil‐moisture saturation, building foundation type/size (Patterson and Davis, 2009; DeVaull (in press) and surface cover.   At sites with “low‐concentration” sources (weathered residual in soil and/or dissolved concentrations in groundwater), the significance of biodegradation is most notable because biodegradation conditions in the unsaturated zone generally remain aerobic.  At these sites, O2 availability in the unsaturated zone generally exceeds O2 demand resulting from biodegradation.  Biodegradation under aerobic conditions has been shown to be rapid resulting in the development of sharp attenuation fronts where BTEX concentrations decrease by several orders of magnitude over relatively short (e.g. <1 m) vertical distances (Fischer et al., 1996; Lahvis et al., 1999; DeVaull, 2007; Davis, 2009; and Hartman, 2010).  The hydrocarbon reaction fronts (the point at which most of the degradation is occurring) tend to develop very near the water table at sites with dissolved‐phase only sources in groundwater (e.g., benzene concentrations < 15 mg/L).  At these sites, effects of soil type, building foundation and surface cover will tend to be limited.  Evidence to support these assertions exists both in the theory (modeling) (DeVaull, 2007, Abreu et. al. 2009, API, 2009) and in the field (Lahvis and Baehr, 1996; API, 2009; Davis, 2009).     2  Version date:  June 30, 2011 
    • Further attenuation is predicted for dissolved‐phase sources displaced laterally from the building foundation (Abreu and Johnson, 2005).    At sites with LNAPL on the groundwater, biodegradation can also be quite notable.  Exclusion distances  for benzene and total petroleum hydrocarbons (TPH) determined from analysis of soil‐gas data primarily collected at retail sites have been estimated to be in the range of 8 to 15 feet (Davis, 2009; Hartman, 2010; Lahvis, 2011)1.  The greater exclusion distance for LNAPL sites compared to dissolved‐phase sites is in part related to the added demand for O2 (noted above) for LNAPL sources and the tendency for LNAPL sources to be distributed above the capillary zone.  For dissolved phase sources in groundwater, the capillary zone has been documented as an active zone for VOC attenuation (Lahvis and Baehr, 1996).  Results from the analysis of the Davis (2010) soil‐gas database are consistent with other large field studies (Lahvis, 2011).  As noted above, the significance of bioattenuation is largely dependent on source type.  Differentiation of residual‐phase LNAPL (high concentration) sources from dissolved‐phase (low concentration) sources can, however, be difficult.  The following general rules of thumb could be used as indicators of residual‐phase  LNAPL sources in groundwater or in soil:  Presence of LNAPL  Direct evidence:  • sites with current or historical evidence of LNAPL in soil or LNAPL at the water table as  evidenced in wells  Indirect evidence:  • chemicals of concern (COCs) approaching (> 0.2) effective solubilities (Bruce et al., 1991) in  groundwater (e.g., benzene > 3 mg/L ; total benzene, toluene, ethylbenzene and xylenes  (BTEX) > 20 mg/L; TPH diesel range organics (DRO) > 5 mg/L) and in soil (TPH gasoline range  organics (GRO) > 100 ‐ 200 mg/kg(2); TPH DRO > 10 ‐ 50 mg/kg) (see ASTM, 2006, Alaska DEC,  2011)3  • TPH vapor readings from a photo‐ionization detector (PID) of > 1,000 ppm (recent gasoline  release sites), > 100 ppm (recent diesel/historic gasoline release sites), and > 10 ppm  (historic diesel sites) (Alaska DEC, 2011).  Note weathered LNAPL typically has a significant  reduction in the VOC content and therefore represents less of a concern for vapor intrusion. The following rules‐of‐thumb for can be used to determine whether LNAPL is a concern for vapor intrusion risk:  Differentiating between Weathered and Unweathered LNAPL  • For groundwater impacted by LNAPL or where groundwater is in proximity to LNAPL,   effective solubility is a key indicator for whether the LNAPL is depleted of VOCs.  For                                                             1  It is important to note, that the soil‐gas data were collected primarily at retail sites.  Approximately 16% of the soil‐gas  sampling locations were directly below building foundations (i.e., sub‐slab). 2  TPH (GRO) between 100 to 200 mg/kg may indicate there may be a slight amount of LNAPL.  TPH (GRO) less than 100 mg/kg is  a good indication that there is no LNAPL present. 3  The primary driver for vapor intrusion is benzene.  For petroleum‐based fuels other than gasoline, benzene is not found at  levels that would cause a vapor intrusion problem.    3  Version date:  June 30, 2011 
    • example, benzene’s effective solubility is approximately 18 mg/L, if it constitutes 1% of  gasoline.  Therefore benzene concentrations < 1 mg/L are reasonable indicators that the  LNAPL is weathered (depleted of VOCs).    • For soil sources, TPH (GRO) < 100 mg/kg is a good indication that there is a small or low  concentration (VOC) source. Naphthalene is currently considered a carcinogen via the inhalation exposure route and since it is also volatile, it can be considered a potential risk driver.   The exclusion criteria defined for benzene are assumed to be conservative for naphthalene, which is also highly susceptible to biodegradation (Anderson et al., 2008; GSI, 2010).  Naphthalene also has a much lower solubility value and Henry’s Law coefficient (compared to benzene), thereby limiting the amount of naphthalene available to volatilize into the gas phase.  For these reasons, the screening criteria described here, while developed for benzene, should also be protective of naphthalene vapor intrusion.         3 TECHNICAL BACKGROUND – Discussion of Biodegradation EffectsThis section will present the results of model studies and field data that support the assumptions made in the vapor intrusion exclusion criteria.  First, the results found at “low‐concentration source” cases will be discussed followed by “high‐concentration source” cases. Lastly, it is important to note that once the groundwater concentrations are below effective solubility, the actual hydrocarbon concentrations in groundwater are not necessarily good predictors of vapor intrusion risk.  Field site observations show that dissolved‐phase hydrocarbon concentrations in shallow groundwater and soil gas concentrations overlying the water table are poorly correlated (Lahvis, 2011).  The poor correlation at dissolved‐phase only sites can be attributed to the inability to accurately measure hydrocarbon concentrations at the water table and to the considerable bioattenuation of hydrocarbon vapors between the water‐table source and the soil‐gas measurement location.  At LNAPL (residual‐phase) sites, soil‐gas concentrations are also poorly correlated with groundwater concentrations because LNAPL sources are typically present above the water table.  For these reasons, it is recommended to focus the development of screening criteria solely on the basis of source type (LNAPL and groundwater) rather than source (groundwater) concentration.  3.1 Low-Concentration Sources (weathered residual in soil and/or dissolved concentrations in groundwater)For purposes of this technical justification, low concentration sources at hydrocarbon sites are defined as dissolved‐phase concentrations.  Low concentration sources will therefore be composed primarily of the more soluble (aromatic) LNAPL constituents, benzene, toluene, ethylbenzene, xylenes, and naphthalene.  Of these constituents, benzene is the primary risk driver for vapor intrusion because of its relatively higher toxicity and vapor migration potential.  Note, weathered LNAPL can behave like low‐concentration sources because the LNAPL may be depleted of volatile chemicals.    4  Version date:  June 30, 2011 
    • 3.1.1 Model StudiesResults from detailed numeric (3‐dimensional) models (see Figure 3 from API below) indicate that complete attenuation of the hydrocarbons (approximately 10 orders of magnitude) is predicted between a relatively low concentration source (< 10 mg/L total hydrocarbon in soil gas) and indoor air where the source is separated from the receptor by > 3 meters (see API, 2009; Abreu et al., 2009)4.  Note, the “hydrocarbon” modeled in these studies was assumed to have the same fate and transport properties as benzene.  In addition, the simulations are based on assuming biodegradation takes place only in the aerobic portion of the unsaturated zone (i.e., when O2 concentrations exceed 1%).  An aerobic biodegradation rate of 0.79 hr‐1 is assumed for the hydrocarbon (benzene) based on a  mean of published rates (DeVaull, 2007).   Note, while a degradation rate of 0.75 hr‐1 may seem high, the model only allows degradation in the regions where there is enough O2 to support it.  The model cutoff for allowing degradation was 1% O2.   A 10 mg/L benzene vapor source is consistent with a dissolved‐phase source of  benzene (or BTEX) of around 40 mg/L assuming  equilibrium partitioning between soil gas and groundwater and a Henry’s law coefficient of 0.25 for benzene (or BTEX).  The attenuation with distance is increased for the latter condition because diffusion of the hydrocarbons will tend to be vertically upwards (toward the soil surface) rather than laterally towards the receptor.   Hence, there is little potential for vapor intrusion to occur at sites with dissolved‐phase sources separated laterally from building foundations. The following two figures from API (2009) show hydrocarbon and O2 profiles predicted by transport modeling  for  low‐concentration vapor sources varying between 0.1 mg/L hydrocarbon (0.4 mg/L dissolved‐phase equivalent) and 10 mg/L  hydrocarbon (40 mg/L dissolved‐phase equivalent) and  two different foundation configurations, basement and slab, respectively.  Note, the “hydrocarbon” modeled in these studies was again assumed to have the same fate and transport properties as benzene.  The source concentration was assumed to be equal to the combined concentrations of all of the BTEX.  This approach was used because it was conservative to consider the increased O2 demand from the additional VOCs present (all of the BTEX).  Therefore, these modeling study results can be considered conservative for benzene.                                                             4 A 10 mg/L hydrocarbon soil gas source would equate to a ~40 mg/L source of BTEX in groundwater assuming a vapor/aqueous phase partition coefficient of around 0.25 (Morrison, 1999) assuming the source were dissolved.    5  Version date:  June 30, 2011 
    •        6  Version date:  June 30, 2011 
    • 3.1.2 Field DataA soil‐gas database has been developed by Robin Davis (Utah Department of Environmental Quality ‐ DEQ).  The database was compiled from numerous retail, distribution, and manufacturing sites across several states, including California.  The soil‐gas data were collected from locations on and off‐site.  Approximately 16% of the soil‐gas data were collected directly below building foundations (i.e., subslab). The data from retail sites are being used to support the development of new state (see http://www.swrcb.ca.gov/ust/luft_manual.shtml ) and federal (US EPA OUST) vapor intrusion guidance.  Analyses of the soil‐gas data are described in Davis (2009) and Hartman (2010).  The data analyses support the model results discussed in the previous section.  The analyses indicate that “dissolved‐phase” sources < 6 mg/L benzene in groundwater (or ~24,000,000 ug/m3 vapor phase equivalent5) are completely attenuated within distances of 5 ft. or less (see figure below from Davis, 2009).       It is important to note, however, that the Davis (2009) analyses of thickness of clean overlying soil required to attenuate benzene vapors (or “exclusion distance”) did not rigorously screen out potential residual LNAPL sources above the water table.  These sites pose a similar risk for vapor intrusion as sites                                                             5 3 3 Assuming a Henry’s Law coefficient of 0.25 cm /cm for benzene.   7  Version date:  June 30, 2011 
    • with free‐phase LNAPL on groundwater (i.e., sites where LNAPL is observed in monitoring wells).  The analysis shown in Figure 5 also includes data from “non‐retail” locations.  It is also important to note that the Davis (2009) results imply that the vapor intrusion risk is dependent on the source concentration in groundwater.  Again, this dependence has not been observed at other sites and is not recommended to be used in developing groundwater concentration‐based exclusion distances.   A slightly different analysis of the “retail‐only” data from the Robin Davis database by Lahvis (2011) shows that benzene will be bioattenuated below a relatively conservative soil‐gas screening level of 100 ug/m3 within 5 ft of the water table6.  The analysis focused on identified sources of benzene in groundwater and filtered out sites with either direct evidence of LNAPL (current, historical) or indirect evidence of LNAPL (soil‐gas measurements collected near potential sources (i.e., locations within 25 ft of USTs and dispensers), and also screened out sites with benzene concentrations in groundwater > 15 mg/L or BTEX > 75 mg/L).  The vast majority (84%) of the soil‐gas measurements were taken from sites with source concentrations of benzene in groundwater ranging from 0.1 mg/L (100 ug/L) to 15 mg/L.    Figure from Lavis (2011)  10000 24  SITES BENZENE 60 LOCATIONS SOIL‐GAS CONCENTRATION  (ug/m3) 151 SAMPLES 1000 50% 45% GROUNDWATER 40% CONCENTRATIONS 37% PERCENTAGE OF SITES 30% 20% 8% 8% 100 10% 0% 2% 10 Measured 1 Non Detect 95th Percentile 0.1 0 10 20 30 40 50 DISTANCE ABOVE WATER TABLE (ft)                                                                  6 3 This value represents the attenuation between a benzene source in groundwater up to 15 mg/L (or 7,500,000 ug/m in soil-gas) 3 and a conservative soil-gas screening level concentration of 100 ug/m . This concentration is representative of a screening-level 3 concentration in soil gas (assuming an indoor air risk-based concentration of 2 ug/m and a slab attenuation factor of 0.02).   8  Version date:  June 30, 2011 
    • From a probability standpoint, the results from the scatter plot can be defined as follows (Lahvis, 2011):   Figure from Lahvis (2011)  100% 95% PROBABILITY (%) 90% BENZENE 85% < 50 ug/m3 < 100 ug/m3 80% includes NON DETECTS (@ 1/2 detection limit) 75% 0 5 10 15 20 DISTANCE ABOVE WATER TABLE (ft)    The probability of having benzene vapor concentrations near the receptor that exceed a conservative screening level (i.e., 100 ug/m3) at dissolved‐phase (retail) sites is less than 5%.  The water table would have to essentially be in contact with a building foundation for there to be a potential concern for vapor intrusion at low concentration sites.    3.1.3 Summary of Low Concentration SourcesIn summary, field data from retail sites shows that for low concentration (e.g., dissolved‐phase only) sources, benzene will be attenuated to below screening levels within 5 ft above the water table.  Vapor intrusion risks would be rare to non‐existent at these retail sites provided the water table does not come in contact with the building foundation.  3.2 High-Concentration Sources (unweathered residual in soil and/or free- phase LNAPL on groundwater)3.2.1 Model StudiesAs shown in the attached figure from Abreu et al. (2009), hydrocarbons are predicted to completely attenuate in the unsaturated zone above an LNAPL source within ~ 7m of the source.  Again, the model simulations use benzene as a surrogate for all of the TPH present.  A mean biodegradation rate of 0.79 hr‐1 was again assumed (DeVaull, 2007) in model regions where the O2 level was sufficiently high enough to support aerobic biodegradation.        9  Version date:  June 30, 2011 
    •   Again, the attenuation is expected to increase for NAPL sources displaced laterally from the basement foundation (see Abreu and Johnson, 2005).   3.2.2 Field DataA more recent analysis of the soil‐gas database by Davis (2010) indicates that the model predicted bioattenuation is conservative.  Exclusion distances of only 8 ft. were found to be sufficient to attenuate LNAPL sources.  This analysis takes into account residual LNAPL sources in the unsaturated zone (see the following figure from Davis (2010)).        10  Version date:  June 30, 2011 
    •   Method 2Davis (2010) for Figure from Results LNAPL & Soil Sources Benzene: 48 exterior/near-slab + TPH: 17 exterior/near-slab + 22 sub-slab = 70 total 18 sub-slab = 35 total Benzene SV Sample Event over LNAPL & Soil Sources TPH SV Sample Event over LNAPL & Soil Sources Near-Slab Multi-Depth, Sub-Slab Near-Slab Multi-Depth, Sub-Slab 10 Thickness of Clean Soil Overlying LNAPL 10 Thickness of Clean Soil Overlying LNAPL Required to Attenuate Vapors, feet 9 Required to Attenuate Vapors, feet 9 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 0 0 ~8 ft CLEAN overlying soil attenuates vapors associated with LNAPL/Soil Sources  Lahvis (2011) has interpreted the soil‐gas database compiled by Davis slightly differently.  The next figure shows benzene concentrations in soil gas from retail‐only sites plotted as a function of distance above the water table ) (see following figure):    11  Version date:  June 30, 2011 
    • Figure from Lavis (2011)  10000000 BENZENE SOIL GAS CONCENTRATION (ug/m3) 62  SITES 1000000 218 LOCATIONS 503 SAMPLES 100000 10000 1000 100 10 measured 1 non detect 0.1 0 10 20 30 40 50 DISTANCE ABOVE SOURCE (ft)   As shown, benzene concentrations in soil gas generally attenuate by more than 4 orders of magnitude with at a source separation distance of > 12 ft from the source at LNAPL sites.  The attenuation is most significant at distances > 12 ft above the source.  A statistical analysis of these data shows a > 95% probability of encountering benzene concentration below 100 ug/m3 in soil gas at distances >~ 13 ft above the source.     Figure from Lahvis (2011)  100% 95% BENZENE 90% 85% PROBABILITY (%) 80% 75% 70% < 50 ug/m3 < 100 ug/m3 65% 60% includes NON DETECTS  55% (@ 1/2 detection level) 50% 0 5 10 15 20 25 30 DISTANCE ABOVE WATER TABLE (ft)        12  Version date:  June 30, 2011 
    • The lateral separation exclusion distances would be expected to be less than the vertical exclusion distances for the reasons previously explained.  3.2.2 SummaryMost recent field data analyses indicate that 8 to 13 ft of clean soil (soil with no LNAPL present) are sufficient to limit the risk for vapor intrusion at sites with LNAPL sources in either soil or groundwater.     3.3 Technical Background ConclusionsLow‐concentration sources have been shown to attenuate up to 6 orders of magnitude in the unsaturated zone within short vertical distances (e.g., < 5 ft) due to biodegradation.  Biodegradation is sufficient to essentially eliminate these sites from further vapor intrusion consideration.   At sites with unweathered LNAPL sources (“high‐concentration sources”), 8 to 13 ft of clean soil are required to fully attenuate the hydrocarbon vapors.  The attenuation due to biodegradation would be equally or more significant for LNAPL sources separated laterally from building foundations (i.e. a shorter distance would be required for attenuation). It is important that the current state of the science as described here be used in the development of more rational, technically defensible, approaches to vapor intrusion risk assessment. 4 THE FOUR LOW-THREAT VAPOR INTRUSION SCREENING SCENARIOSThe Stakeholder Group that was assembled by the Cal‐EPA/SWRCB examined the available current and relevant scientific information and recommends the following low‐threat guideline to manage the petroleum vapor intrusion pathway incorporating additional safety factors to protect human health in a state‐wide policy document.   The Stakeholder Group developed four basic scenarios for decision‐making purposes and they are respectively:  Scenario 1:  Unweathered LNAPL on groundwater  Scenario 2:  Unweathered LNAPL in soil  Scenario 3:  Dissolved phase benzene concentrations in groundwater  Scenario 4:  Direct measurement of soil gas concentrations  Scenarios 1 and 2 are essentially “high‐concentration sources”, while scenarios 3 and 4 are “low‐concentration sources”.  The following section details the specific justification(s) for each of the sets of exclusion criteria outlined in these four scenarios.  Benzene is assumed to be the primary risk driver for vapor intrusion from petroleum hydrocarbon sites.  Although naphthalene is not present in gasoline at levels as high as typical benzene levels, and is potentially present at very low concentrations (mass fraction of 0.0026) in diesel (TPHCWG, 1998), it is another volatile carcinogenic chemical, and could potentially be considered as an additional risk driver. Also, naphthalene has similar (if not, higher (GSI, 2010)) degradation rates as benzene and much lower aqueous solubility.  The discussions below focus on benzene attenuation.    13  Version date:  June 30, 2011 
    • 4.1 Scenario 1: Unweathered LNAPL on Groundwater ‐ 30 ft vertical bioattenuation zone between a unweathered LNAPL (residual or free‐phase)  source and a building foundation. The proposed 30 ft exclusion distance7 is conservative based on:  • Model theory shows full attenuation within 7 m (~ 21 ft) of the source assuming reasonable  approximations of the biodegradation rate (see figures below from Abreu et al., 2009).    Figure from Abreu et al. (2009)                                                                 7  The top of the residual‐phase source can generally be assumed to be consistent with the historic high water‐table elevation.    14  Version date:  June 30, 2011 
    • Figure from Abreu et al. (2009)      • Field soil‐gas data show full attenuation within 8 ft of the source (see figure, below, from R.  Davis (2010) – also published in Hartman (2010)).  Method 2Davis (2010) for Figure from Results LNAPL & Soil Sources Benzene: 48 exterior/near-slab + TPH: 17 exterior/near-slab + 22 sub-slab = 70 total 18 sub-slab = 35 total Benzene SV Sample Event over LNAPL & Soil Sources TPH SV Sample Event over LNAPL & Soil Sources Near-Slab Multi-Depth, Sub-Slab Near-Slab Multi-Depth, Sub-Slab 10 Thickness of Clean Soil Overlying LNAPL 10 Thickness of Clean Soil Overlying LNAPL Required to Attenuate Vapors, feet 9 Required to Attenuate Vapors, feet 9 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 0 0 ~8 ft CLEAN overlying soil attenuates vapors associated with LNAPL/Soil Sources   • Analysis of the same soil‐gas data by Lahvis (2011) that shows benzene is attenuated to  concentrations in soil gas < 100 ug/m3 (a conservative risk‐based screening level) at distances  more than 13 ft from a LNAPL (residual or free‐phase) source benzene (probability = 95%).   15  Version date:  June 30, 2011 
    • Figure from Lahvis (2011)  10000000 BENZENE SOIL GAS CONCENTRATION (ug/m3) 62  SITES 1000000 218 LOCATIONS 503 SAMPLES 100000 10000 1000 100 10 measured 1 non detect 0.1 0 10 20 30 40 50 DISTANCE ABOVE SOURCE (ft)     Figure from Lahvis (2011)    100% 95% BENZENE 90% 85% PROBABILITY (%) 80% 75% 70% < 50 ug/m3 < 100 ug/m3 65% 60% includes NON DETECTS  55% (@ 1/2 detection level) 50% 0 5 10 15 20 25 30 DISTANCE ABOVE WATER TABLE (ft)      16  Version date:  June 30, 2011 
    • 4.2 Scenario 2: Unweathered LNAPL in Soil ‐ 30 ft lateral and vertical separation distance between a unweathered LNAPL (residual or  free‐phase) source in soil and a building foundation. The same technical justification provided for Scenario 1 applies to Scenario 2.  The proposed 30 ft. lateral off‐set distance is even more conservative for sources displaced laterally as shown in the following figure from Abreu and Johnson (2005).  For example, an additional order of magnitude of attenuation is predicted for plume centerlines displaced 10 m (~30 ft).  The attenuation would be significantly greater (e.g., several orders of magnitude) in cases where the plume (dissolved‐phase) boundary was separated by 30 ft.  Figure from Abreu and Johnson (2005)   As discussed in the technical background section 3.2.2, 13 ft. is more than adequate to fully attenuate LNAPL sources in soil and groundwater, therefore assuming a 30’ separation is very conservative.    4.3 Scenario 3: Dissolved Phase Benzene Concentrations in Groundwater ‐ No Oxygen Measurements ‐ 5 ft. vertical separation distance between a dissolved‐phase  source < 100 ug/L benzene  and a building foundation;  10 ft. vertical exclusion distance  for a dissolved‐phase source < 1,000 ug/L benzene.  ‐ With Oxygen > 4% – 5 ft. vertical separation distance between a dissolved‐phase source <  1,000 ug/L and a building foundation.    17  Version date:  June 30, 2011 
    • These separation distances are conservative with respect to protecting human health based on the following:  • Model theory shows 9 orders of magnitude (i.e., complete) attenuation (for reasonable  approximations of the biodegradation rate) within a source/building separation distance of L=3  m (10 ft) for benzene vapor sources < 10 mg/L (or 40 mg/L dissolved phase concentration in  groundwater assuming Henry’s Law coefficient of 0.25) (see attached figure from Abreu et al.,  2009).  The attenuation is complete regardless of the dissolved‐phase concentration (up to ~ 40  mg/L benzene in groundwater) for sources located 3 meters or more from a building foundation.   The dissolved phase concentrations (especially) and required bioattenuation zone thickness  specified in this scenario are therefore very conservative.    Figure from Abreu et al. (2009)      • The attenuation is shown  to be complete within 2 m (6 ft.) for a soil gas source of benzene < 10  mg/L (or 40 mg/L dissolved phase concentration in groundwater assuming Henry’s Law  coefficient of 0.25) (see attached figure from API (2009)).      18  Version date:  June 30, 2011 
    • Figure from API (2009)   Figure 3 API  Figure 4 API      • Field soil‐gas data from Robin Davis collected at retail sites (Lahvis, 2011) that show the  proposed exclusion distances and groundwater concentrations are highly conservative.  The  data imply that the potential risk of vapor intrusion from dissolved‐phase sources (up to 15 mg/L  benzene in groundwater) is minimal unless groundwater is essentially in contact with the  building foundation.    19  Version date:  June 30, 2011 
    •   Figure from Lahvis (2011)    10000 24  SITES BENZENE 60 LOCATIONS SOIL‐GAS CONCENTRATION  (ug/m3) 151 SAMPLES 1000 50% 45% GROUNDWATER 40% CONCENTRATIONS 37% PERCENTAGE OF SITES 30% 20% 8% 8% 100 10% 0% 2% 10 Measured 1 Non Detect 95th Percentile 0.1 0 10 20 30 40 50 DISTANCE ABOVE WATER TABLE (ft)     • From a probability standpoint, the soil‐gas data show a > 95% probability of detecting benzene  in soil gas at concentrations < 100 ug/m3 @ dissolved‐phase sites; conversely, there is less than  a 5% probability that benzene soil gas concentrations will exceed 100 ug/m3 (a conservative risk‐ based screening number for soil gas, Lahvis (2011)).    Figure from Lahvis (2011)    100% 95% PROBABILITY (%) 90% BENZENE 85% < 50 ug/m3 < 100 ug/m3 80% includes NON DETECTS (@ 1/2 detection limit) 75% 0 5 10 15 20 DISTANCE ABOVE WATER TABLE (ft)  20  Version date:  June 30, 2011 
    • 4.4 Scenario 4: Direct Measurement of Soil Gas Concentrations ‐ Application of a bioattenuation (additional attenuation) factor of 1000x to risk‐based soil‐ gas criteria (i.e., vapor sources) located within 5 ft. of a building foundation.   • Model theory predicts that bioattenuation is significant for LNAPL sources provided vapor  concentrations are < 0.1 (1/10th) of a TPH vapor source of 100,000 ug/L (or 10,000,000 ug/m3).   Therefore the proposed vapor screening criteria of 5,000 ug/m3 for benzene is very  conservative.   (See the following figures from Abreu et al. 2009.)    Figure from Abreu et al. 2009        21  Version date:  June 30, 2011 
    • Figure from Abreu et al. 2009  • The 4% oxygen requirement in this scenario is also a very conservative level for  biodegradation to occur.  The numeric models used 1% as a conservative estimate.  5 REFERENCESAbreu, L.D., Ettinger, R. and T. McAlary , 2009,  Simulated soil vapor intrusion attenuation factors including biodegradation for petroleum hydrocarbons. Ground Water Mont. Rem. 29, 105–177. Abreu, L.D. and P.C. Johnson, 2005, Effect of vapor source, building separation and building construction on soil vapor intrusion as studied with a three‐dimensional numerical model, Environ. Sci. and Technol., 39, 4550‐4561. Abreu, L.D. and P.C. Johnson, 2006, Simulating the effect of aerobic biodegradation on soil vapor intrusion into buildings: Influence of degradation rate, source concentrations, Environ. Sci. and Technol., 40, 2304‐2315. Alaska DEC, 2011, Hydrocarbon Risk Calculator User Manual, prepared for Alaska Department of Environmental Conservation by Lawrence Acomb Geosphere, Inc., January 4, 2011 (http://www.dec.state.ak.us/spar/csp/guidance/hrc/HRC%20User%20Manual.pdf)     22  Version date:  June 30, 2011 
    • Andersen, R. G., Booth, E. C., Marr, Widdowson, M.A., Novak, J.T., 2008, Volatilization and biodegradation of naphthalene in the vadose zone impacted by phytoremediation, Environ. Sci. Technol., 42, 2575–2581. API, 2009, Simulating the Effect of Aerobic Biodegradation on Soil Vapor Intrusion into Buildings—Evaluation of Low Strength Sources Associated with Dissolved Gasoline Plumes, Publication No. 4775; American Petroleum Institute: Washington, D.C., April 2009, pp. 37.  ASTM E‐2531–06, 2006, Standard Guide for Development of Conceptual Site Models and Remediation Strategies for Light Nonaqueous‐Phase Liquids Released to the Subsurface, ASTM International, West Conshohocken, PA, 19428‐2959 USA  Bruce, L., Miller, T., and B. Hockman, 1991, Solubility versus equilibrium saturation of gasoline compounds: A method to estimate fuel/water partition coefficient using solubility or Koc, proceedings of National Ground Water Association Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Restoration, November 20‐22, 1991, Houston, Tx, 571 ‐582. Davis, R.V., 2009, Bioattenuation of petroleum hydrocarbon vapors in the subsurface update on recent studies and proposed screening criteria for the vapor‐intrusion pathway, LUSTLine Report 61, May 2009, New England Interstate Water Pollution Control Commission (NEIWPCC), pp. 11‐14. (http://www.neiwpcc.org). Davis, R., 2010, Evaluating the vapor intrusion pathway:  Subsurface petroleum hydrocarbons and recommended screening criteria, 22nd Annual US EPA National Tanks Conference, Boston, Massachusetts, September 20‐22, 2010. DeVaull, 2007, Indoor vapor intrusion with oxygen‐limited biodegradation for a subsurface gasoline source, Environ. Sci. Technol., 41, 3241‐3248. DeVaull, in press, Vapor intrusion from subsurface to indoor air: biodegradable petroleum vapors versus recalcitrant chemicals. Fischer,  D. and C. G. Uchrin, 1996, Laboratory simulation of VOC entry into residence basements from soil gas, Environ. Sci. Technol., 30, 2598‐2603. GSI Environmental Inc., 2010, BioVapor, A 1‐D Vapor Intrusion Model with Oxygen‐Limited Aerobic Biodegradation, User’s Manual, Published by American Petroleum Institute: Washington, D.C., April 2010.  Hartman, B., 2010, The vapor‐intrusion pathway: Petroleum hydrocarbon issues, LUSTLine Report 66, December 2010, New England Interstate Water Pollution Control Commission (NEIWPCC), pp. 11‐14. (http://www.neiwpcc.org). Interstate Technology and Regulatory Council, 2007, Vapor intrusion pathway: A practical guideline, Interstate Technology & Regulatory Council, Washington, D.C., January, 2007, pp. 74. Lahvis, M.A., 2011, Significance of biodegradation at petroleum hydrocarbon sites:  Implications for vapor intrusion guidance, Presentation to the Ministry of Environment British Columbia, June 1, 2011.    23  Version date:  June 30, 2011 
    • Lahvis, M.A., and G. E. DeVaull, 2010, Alternative screening methodology for vapor intrusion assessment at petroleum hydrocarbon release sites, 22nd Annual US EPA National Tanks Conference, Boston, Massachusetts, September 20‐22, 2010.  Lahvis, M.A., and A.L. Baehr, 1996, Estimating rates of aerobic hydrocarbon biodegradation by simulation of gas transport in the unsaturated zone: Water Resources Res., 32, 2231‐2249. Lahvis, M.A., Baehr, A.L., and R.J. Baker, 1999, Quantification of aerobic‐biodegradation and volatilization rates of gasoline hydrocarbons near the water table during natural‐attenuation conditions: Water Resources Res., 35, 753‐765. McHugh, T., Davis, R., DeVaull, G., Hopkins, H., Menatti, J., and T. Peargin, 2010, Evaluation of vapor attenuation at petroleum hydrocarbon sites: considerations for site screening and investigation, Soil and Sediment Contamination, 19:1–21, 2010. Morrison, R.D., 1999, Environmental Forensics: Principles and Applications, CRC Press. Potter, T. and K.E. Simmons. 1998.  Total Petroleum Hydrocarbon Criteria Working Group Series, Volume 2: Composition of Petroleum Mixtures.       24  Version date:  June 30, 2011 
    • Documents developed by the UST stakeholder group are listed below:TASK GROUP DISSENTING OPINION LETTER Jan 2010
    • January 4, 2010Members of the State Water Resources Board1001 I StreetP.O. Box 100Sacramento, CA 95812-0100Re: Resolution 2009-0042 - UST Cleanup Program Task Force, Minority OpinionDear Water Board Members:Thank you for the opportunity to participate in the Underground Storage Tank ProgramTask Force. The Task Force members have all invested a great deal of time andresources in preparing this report and many points of view were well represented. Asregulators who served on the task force we offer the comments below as a dissentingopinion to the UST Cleanup Program Task Force Report. The regulators who served onthe Task Force are concerned that the Task Force membership was dominated byresponsible parties (RPs), and consultants who work for RPs. Regulators comprisedapproximately ten percent to the Task Force. This imbalanced representation resulted inproposals that primarily reflect the views of the RPs and their consultants. Dialoguewithin the task force was not neutral and minority views received very little consideration.The group focused on closing cases over the protection of groundwater resources andhuman health. The following issues represent some, but not all of our concerns with theReport.Default site closure criteria are proposed which assume uniform hydrogeologicconditions. For example it is assumed that a safe distance from a source ofcontamination to a water protection well is 1,000 feet. This and any other default closurecriteria assumptions should be peer reviewed to demonstrate that the criteria areprotective in all cases. Protection of groundwater resources requires the considerationof site specific conditions and the application of scientific and engineering principles.Groundwater basins are a complex system of surface recharge areas, multiple aquifers,and discharge areas, all in hydraulic communication with each other and each requiringthe full measure of protection mandated by State law. With the State’s waterdependency based on an unstable supply of imported water, it is even more important toprotect local aquifer systems, many of which are currently being developed to providemore of the State’s water supply. The State’s continued growth and uncertain watersupply make the ability to project future land and water use uncertain. We areconcerned with the assumption that aquifers will not be used for centuries andconsequently, contaminated conditions will be allowed to persist.Closing cases based on an arbitrary age of a case should not be considered. To protectthe resource and human health, cases should be closed when the site meets therequired cleanup criteria. Low risk closures should be considered based on site specificconditions, including off-site impacts and planned changes in land-use. It is important toprotect groundwater aquifers so that current and future groundwater needs areprotected.
    • The recommendations and findings provided in the report should be based on peerreviewed scientific principles. We are concerned with many of the recommendations ofthe report including the ones cited above. Implementing sweeping change based onanecdotal evidence could put human health and environmental quality at unnecessaryrisk. Prior to making sweeping changes to the UST cleanup approach we recommendthat the Board direct a peer review process where evidence and experience isconsidered in a scientific manner. To do otherwise is to develop a scientificallyindefensible environmental policy for California that compromises groundwaterresources and human health.We agree with the goals of Task Force to revise and improve the UST Cleanup process.Going forward we support a similar process that involves balanced representation of allstake holder groups and utilizes independent experts.Brian NewmanKen WilliamsGerald O’Regan