Historical TCE releases at a missile launch facility contaminated underlying groundwater. A phased soil gas investigation was used to delineate the soil VOC source. An initial passive soil gas survey identified two hotspots. Follow up active soil gas sampling confirmed results and further delineated impacts. Deep nested soil gas wells profiled vertical contamination and showed one area had a continuous soil source through the vadose zone mass loading groundwater. This approach successfully mapped the soil contamination source.

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 Presentation
Historical releases of trichloroethene (TCE) at a former missile launch facility known as Space
Launch Complex 3 West (SLC-3W) at Vandenberg Air Force Base (AFB) have impacted underlying
groundwater and continue to source a contaminant plume estimated to occupy 265 acres. Previous
investigative work suggested the presence of two distinct groundwater source areas centered on
monitoring wells with elevated TCE concentrations. Persistent increasing TCE concentration trends
in several site wells resulted in additional study into the potential presence of a continuing soil source
of contamination.
Due to the depth of groundwater (>200 feet below ground surface [bgs]) and the size of the
investigation 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 suspected
source area suggested that an alternative approach may prove more successful. Thus, a phased
investigative approach was devised, which featured an initial shallow passive soil gas (PSG) survey
using an 80-point grid with implants installed at 100-foot spacing to address the area known to overly
TCE groundwater concentrations greater than 1,000 micrograms per liter (µg/L), and which included
all suspected source areas. An active soil gas (ASG) survey was then implemented to confirm
passive results and permit step-out sampling to further delineate TCE presence. Finally, based on
these initial data sets, locations were targeted for installation of deep nested soil vapor monitoring
wells, 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 these
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 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) 681-3108
Joachim 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) 681-3108
Brian 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) 681-3108
Harry O’Neill
Beacon Environmental Services, Inc., 323 Williams Street, Bel Air, MD 21014, USA harry.oneill@beacon-
usa.com, Telephone: (410) 838-8780, Fax: (410) 838-8740
Craig Nathe
Department of the United States Air Force, 30 CCEVR, 1028 Iceland Avenue, Vandenberg AFB, CA 93437,
USA, Amena.Atta@vandenberg.af.mil, Telephone: (805) 605-7249
Presenting Author: David Springer

2. PHASED SEQUENTIAL SOIL GAS
INVESTIGATIONS TO DELINEATE A SOIL
VOC SOURCE OF UNDERLYING
GROUNDWATER CONTAMINATION,
VANDENBERG AFB, USA
17 March 2011
Joachim Eberharter, P.G.
Tetra Tech, Inc.
David S. Springer, P.G.
Tetra Tech, Inc.
Craig Nathe
U.S. Air Force
Integrity - Service - Excellence

3. Overview
 Site Location and Background
 Overview of Project Objectives
 Rationale Behind Phased Investigative Approach
 Scope and Methods / Results
 Findings & Conclusions
 Questions?
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4. General Location Map
Source: Google Earth 2010
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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
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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
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7. Background
Photo showing SLC-3W, with Building 770, deluge channel, retention
basin, and drilling at location 6-NSG-3B, looking northwest.
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8. Background – Hydrogeology
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9. Background – Previously Known Extent of Impacts
OBSOLETE
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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
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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
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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.
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13. Passive Soil Gas Survey – Scope and Methods
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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
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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.
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16. Active Soil Gas Survey – Results
10 ft bgs 20 ft bgs
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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
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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
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19. Deep Soil Gas Investigation – Scope and Methods
Photo showing installation of well 6-NSG-1, as tubing is
unspooled and attached onto central PVC pipe, looking northwest.
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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
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21. Deep Soil Gas Investigation – Results
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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
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23. Deep Soil Gas Investigation – Results
6-NSG-1 6-NSG-4
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24. Deep Soil Gas Investigation – Results
6-NSG-2 6-NSG-3B
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25. Deep Soil Gas Investigation – Results
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26. Deep Soil Gas Investigation – Results
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27. Findings & Conclusions – Updated CSM
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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
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29. Questions?
 Joachim Eberharter, P.G.
Tetra Tech, Inc.
joachim.eberharter@tetratech.com
 David Springer, P.G.
Tetra Tech, Inc.
david.springer@tetratech.com
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