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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.
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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
I n t e g r i t y - S e r v i c e - E x c e l l e n c e
17 March 2011
Joachim Eberharter, P.G.
Tetra Tech, Inc.
David S. Springer, P.G.
Tetra Tech, Inc.
Craig Nathe
U.S. Air Force
PHASED SEQUENTIAL SOIL GAS
INVESTIGATIONS TO DELINEATE A SOIL
VOC SOURCE OF UNDERLYING
GROUNDWATER CONTAMINATION,
VANDENBERG AFB, USA
I n t e g r i t y - S e r v i c e - E x c e l l e n c e
Overview
Site Location and Background
Overview of Project Objectives
Rationale Behind Phased Investigative Approach
Scope and Methods / Results
Findings & Conclusions
Questions?
2
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 3
General Location Map
Source: Google Earth 2010
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 4
Background – Site Map
Site 6
SLC-3W Site 5
SLC-3E
Site 7
Bear Creek
Pond
Collectively,
Sites 5, 6, and 7
are referred to as
IRP Site 5 Cluster
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 5
Background
Typical launch facility infrastructure and operations.
TCE used to clean rocket engines.
Historical release resulted in 90 acre groundwater TCE plume.
Ph
oto
: U
.S.
Air
Fo
rce
Ph
oto
: U
.S.
Air
Fo
rce
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 6
Photo showing SLC-3W, with Building 770, deluge channel, retention
basin, and drilling at location 6-NSG-3B, looking northwest.
Background
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Background – Hydrogeology
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Background – Previously Known Extent of Impacts
OBSOLETE
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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
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 10
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
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 11
Passive Soil Gas Survey – Scope and Methods
Photo: Beacon Environmental Services, Inc.
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
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 12
Passive Soil Gas Survey – Scope and Methods
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 13
Passive Soil Gas Survey - Results
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-18
5-MW-156-MW-3
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 14
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.
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 15
Active Soil Gas Survey – Results
10 ft bgs 20 ft bgs
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 16
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
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 17
Deep Soil Gas Investigation – Scope and Methods
Photo showing system purge
prior to sampling
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
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 18
Photo showing installation of well 6-NSG-1, as tubing is
unspooled and attached onto central PVC pipe, looking northwest.
Deep Soil Gas Investigation – Scope and Methods
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 19
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
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 20
Deep Soil Gas Investigation – Results
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 21
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
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 22
Deep Soil Gas Investigation – Results
6-NSG-1 6-NSG-4
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Deep Soil Gas Investigation – Results
6-NSG-2 6-NSG-3B
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Deep Soil Gas Investigation – Results
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 25
Deep Soil Gas Investigation – Results
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 26
Findings & Conclusions – Updated CSM
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 27
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
Findings & Conclusions
I n t e g r i t y - S e r v i c e - E x c e l l e n c e 28
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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