Phased Sequential Soil Gas Investigation

938 views

Published on

Through the use of a passive and then active soil-gas investigation, the conceptual site model (CSM) and groundwater fate and transport model were revised using these high-resolution data sets. A significant savings in project costs and compression of the project schedule was realized through implementation of this phased soil gas investigation approach, which underscores its applicability and effectiveness in mapping VOC contamination in soil.

Published in: Technology
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
938
On SlideShare
0
From Embeds
0
Number of Embeds
17
Actions
Shares
0
Downloads
14
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Phased Sequential Soil Gas Investigation

  1. 1. Phased Sequential Soil Gas Investigations to Delineate a Soil VOC Source of Underlying Groundwater Contamination David Springer, P.G., Joachim Eberharter, P.G., Brian Dow, P.G., Harry O’Neill, and Craig Nathe Platform PresentationHistorical releases of trichloroethene (TCE) at a former missile launch facility known as SpaceLaunch Complex 3 West (SLC-3W) at Vandenberg Air Force Base (AFB) have impacted underlyinggroundwater and continue to source a contaminant plume estimated to occupy 265 acres. Previousinvestigative work suggested the presence of two distinct groundwater source areas centered onmonitoring wells with elevated TCE concentrations. Persistent increasing TCE concentration trendsin several site wells resulted in additional study into the potential presence of a continuing soil sourceof contamination.Due to the depth of groundwater (>200 feet below ground surface [bgs]) and the size of theinvestigation area, a conventional approach utilizing deep investigative soil borings was not cost-effective. Previous negative results from soil samples collected from borings installed in a suspectedsource area suggested that an alternative approach may prove more successful. Thus, a phasedinvestigative approach was devised, which featured an initial shallow passive soil gas (PSG) surveyusing an 80-point grid with implants installed at 100-foot spacing to address the area known to overlyTCE groundwater concentrations greater than 1,000 micrograms per liter (µg/L), and which includedall suspected source areas. An active soil gas (ASG) survey was then implemented to confirmpassive results and permit step-out sampling to further delineate TCE presence. Finally, based onthese initial data sets, locations were targeted for installation of deep nested soil vapor monitoringwells, designed to evaluate vertical contamination profiles and assess the provenance of TCE-impacted groundwater at various site wells.The conceptual site model (CSM) and groundwater fate and transport model were revised using thesedata sets. A significant savings in project costs and compression of the project schedule was realizedthrough implementation of this phased soil gas investigation approach, which underscores its applica-bility and effectiveness in mapping VOC contamination in soil.David Springer, P.G.Tetra Tech, Inc., 301 Mentor Drive, Suite A, Santa Barbara, CA 93111, USA, david.springer@tetratech.com,Telephone: (805) 681-3100, Fax: (805) 681-3108Joachim Eberharter, P.G.Tetra Tech, Inc., 301 Mentor Drive, Suite A, Santa Barbara, CA 93111, USA, joa-chim.eberharter@tetratech.com, Telephone: (805) 681-3100, Fax: (805) 681-3108Brian Dow, P.G.Tetra Tech, Inc., 301 Mentor Drive, Suite A, Santa Barbara, CA 93111, USA, brian.dow@tetratech.com, Tele-phone: (805) 681-3100, Fax: (805) 681-3108Harry O’NeillBeacon Environmental Services, Inc., 323 Williams Street, Bel Air, MD 21014, USA harry.oneill@beacon-usa.com, Telephone: (410) 838-8780, Fax: (410) 838-8740Craig NatheDepartment of the United States Air Force, 30 CCEVR, 1028 Iceland Avenue, Vandenberg AFB, CA 93437,USA, Amena.Atta@vandenberg.af.mil, Telephone: (805) 605-7249Presenting Author: David Springer
  2. 2. PHASED SEQUENTIAL SOIL GASINVESTIGATIONS TO DELINEATE A SOILVOC SOURCE OF UNDERLYINGGROUNDWATER CONTAMINATION,VANDENBERG AFB, USA17 March 2011Joachim Eberharter, P.G.Tetra Tech, Inc.David S. Springer, P.G.Tetra Tech, Inc.Craig NatheU.S. Air Force Integrity - Service - Excellence
  3. 3. Overview  Site Location and Background  Overview of Project Objectives  Rationale Behind Phased Investigative Approach  Scope and Methods / Results  Findings & Conclusions  Questions? Integrity - Service - Excellence 2
  4. 4. General Location Map Source: Google Earth 2010 Integrity - Service - Excellence 3
  5. 5. Background – Site Map Collectively, Sites 5, 6, and 7 are referred to as IRP Site 5 Cluster Site 7 Bear Creek Pond Site 6 SLC-3W Site 5 SLC-3E Integrity - Service - Excellence 4
  6. 6. Background Typical launch facility infrastructure and operations. TCE used to clean rocket engines. Historical release resulted in 90 acre groundwater TCE plume. Photo: U.S. Air Force Photo: U.S. Air Force Integrity - Service - Excellence 5
  7. 7. Background Photo showing SLC-3W, with Building 770, deluge channel, retention basin, and drilling at location 6-NSG-3B, looking northwest. Integrity - Service - Excellence 6
  8. 8. Background – Hydrogeology Integrity - Service - Excellence 7
  9. 9. Background – Previously Known Extent of Impacts OBSOLETE Integrity - Service - Excellence 8
  10. 10. Objectives of Phased Investigation Confirm/refute presence of potential persistent soil impacts sourcing groundwater contamination  Previous investigative work suggested presence of two distinct groundwater source areas centered on monitoring wells with elevated TCE concentrations (5-MW-15 and 5-MW-18)  Increasing TCE concentration trends in groundwater monitoring wells suggested possible persistent soil sources Delineate and assess soil impacts at source area and at previously identified groundwater hot spots Incorporate findings into updated HHERA, CSM, and numerical groundwater model to support the FS Integrity - Service - Excellence 9
  11. 11. Rationale for Phased Investigation Considerations included: (1) large investigative area; (2) depth to groundwater; and (3) negative results from previous soil sampling in suspected source area Initial passive soil gas survey designed to focus investigation early on  Target area approximately 18 acres in size Follow-up active soil gas investigation designed to confirm or refute PSG survey results and further delineate TCE impacts  Provide soil gas concentration data Areas identified during previous surveys targeted for deep nested soil gas well installations and sampling (two phases)  Create vertical soil gas concentration profiles Consistent with Triad approach featuring initial high data density followed by high data quality Integrity - Service - Excellence 10
  12. 12. Passive Soil Gas Survey – Scope and Methods PSG survey conducted within 18 acre area overlying known extent of TCE in groundwater >1,000 µg/L Target area included suspected source area (deluge channel, retention basin, well 6-MW-3) and wells 5-MW-15 and 5-MW-18 80 passive samplers containing hydrophobic adsorbent material 100-ft grid spacing Photo: Beacon Environmental Services, Inc. Integrity - Service - Excellence 11
  13. 13. Passive Soil Gas Survey – Scope and Methods Integrity - Service - Excellence 12
  14. 14. Passive Soil Gas Survey - Results 5-MW-18  Passive samplers used to evaluate mass flux distribution  Results in units of nanograms  Prominent TCE hotspot centered at inside elbow of deluge channel  Secondary hotspot identified approximately 250 ft to the west  Results ruled out significant contamination in remaining initial 18 acre target area 5-MW-15 6-MW-3 Integrity - Service - Excellence 13
  15. 15. Active Soil Gas Survey – Scope and Methods Confirm/refute results of PSG survey Assess TCE concentrations at source area and distal locations Installed 34 double-nested soil gas probes at 17 locations at nominal depths of 10 and 20 ft bgs using DPT On-site mobile laboratory provided real-time data Photo showing active soil gas survey at SLC-3W, looking north. Integrity - Service - Excellence 14
  16. 16. Active Soil Gas Survey – Results 10 ft bgs 20 ft bgs Integrity - Service - Excellence 15
  17. 17. Deep Soil Gas Investigation – Scope and Methods Advanced four borings through nominally 230 ft of vadose zone to groundwater using HSA technology Continuously logged and screened soils (PID), collected soil samples for environmental and soil physical properties testing, and collected grab groundwater samples Installed four quintuple-nested soil gas monitoring arrays (20 probes total) in two phases  6-inch woven stainless steel vapor probe inlets attached to ¼-inch Nylaflow tubing terminated by Luer valves  Array assembled around central 1” PVC pipe while being lowered inside augers  Probes carefully emplaced with construction materials (bentonite hydrated in lifts, sand, etc.) while augers meticulously raised to prevent formation collapse and ensure proper seal between probes Minimum 3 system volumes purged days prior to sampling Integrity - Service - Excellence 16
  18. 18. Deep Soil Gas Investigation – Scope and Methods  Samples collected with 50-ml glass syringe and analyzed by on-site mobile laboratory  Purge volume tests  Confirmation samples collected in Summa canisters for TO-15 analysis Photo showing system purge prior to sampling Integrity - Service - Excellence 17
  19. 19. Deep Soil Gas Investigation – Scope and Methods Photo showing installation of well 6-NSG-1, as tubing isunspooled and attached onto central PVC pipe, looking northwest. Integrity - Service - Excellence 18
  20. 20. Deep Soil Gas Investigation – Results Primary purpose of nested arrays was to delineate vertical TCE impacts in source area and determine provenance of TCE impacts to groundwater at wells 5-MW-15 and 5-MW-18 Hypothesized three possible profiles, each indicative of a different scenario explaining cause of soil gas impacts  Off-gassing from groundwater according to Henry’s and Fick’s laws  Shallow soil impacts coupled with off-gassing from groundwater  Mass loading from soil impacts throughout vertical profile of vadose zone Integrity - Service - Excellence 19
  21. 21. Deep Soil Gas Investigation – Results Integrity - Service - Excellence 20
  22. 22. Deep Soil Gas Investigation – Results Investigation found deep perched groundwater zone beneath source area at deluge channel; high TCE impacts (8,000 µg/L); underlying groundwater impacts approx. 5,000 µg/L, consistent with concentrations in wells 5-MW-15 and 5-MW-18 Vertical soil gas profiles at 6-NSG-1 and 6-NSG-4 (collocated with 5-MW-18 and 5-MW-15, respectively) consistent with off-gassing from groundwater; ruled out as soil source areas; impacted groundwater from these locations interpreted to have originated from source area beneath deluge channel (near 6-MW-3) Vertical soil gas profile at 6-NSG-2 shows mixed source of soil gas impacts an no mass loading to groundwater from shallow vadose zone Vertical soil gas profile at 6-NSG-3B (source area) shows continuous soil source throughout upper vadose zone and mass loading to underlying perched groundwater; soil impacts to lower vadose zone (beneath perching clay) likely due to off-gassing from underlying groundwater Integrity - Service - Excellence 21
  23. 23. Deep Soil Gas Investigation – Results 6-NSG-1 6-NSG-4 Integrity - Service - Excellence 22
  24. 24. Deep Soil Gas Investigation – Results 6-NSG-2 6-NSG-3B Integrity - Service - Excellence 23
  25. 25. Deep Soil Gas Investigation – Results Integrity - Service - Excellence 24
  26. 26. Deep Soil Gas Investigation – Results Integrity - Service - Excellence 25
  27. 27. Findings & Conclusions – Updated CSM Integrity - Service - Excellence 26
  28. 28. Findings & Conclusions Initial PSG survey quickly focused investigation, ultimately providing more robust data set from area of interest Results of subsequent ASG survey demonstrated viability and utility of PSG surveys for preliminary screening purposes Successful installation of 200-ft, quintuple-nested soil gas monitoring well arrays and resulting informative data set demonstrated feasibility and utility of this approach to evaluate provenance of groundwater and extent of soil contamination Use of soil gas matrix demonstrated as cost-effective alternative to soil matrix sampling when evaluating soil source area, due to (1) greater spatial reach with fewer sample locations and (2) repeatability of sampling regimen Approach facilitated regulator buy-in, expedited project schedule, and reduced project costs allowing for bonus SVE pilot study at no extra cost to the client Integrity - Service - Excellence 27
  29. 29. Questions? Joachim Eberharter, P.G. Tetra Tech, Inc. joachim.eberharter@tetratech.com David Springer, P.G. Tetra Tech, Inc. david.springer@tetratech.com Integrity - Service - Excellence 28

×