PB95-963134
EPA/ESD/R10-94/106
March 1995
EPA Superfund
Explanation of Significant Difference
for the Record of Decision:
McChord Air Force Base,
Washrack Treatment Area,
Tacoma, WA
7/19/1994
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EXPLANATION OF SIGNIFICANT DIFFERENCE FOR
THE WASHRACK TREATMENT AREA OF MCCHORD AFB
INTRODUCTION
This document presents an Explanation of Significant Difference (ESD) from the
Record of Decision (ROD) for the Washrack Treatment Area (WT A) at McChord Air Force
Base. The ROD was signed by the U.S. Air Force; U.S. Environmental Protection Agency arid
the Washington State Department of Ecology.
Site Name and Location:
McChord Air Force Base, WT A, Site 54
Pierce County, Washington
The McChord Air Force Base (AFB) Washrack Treatment Area was listed on the
National Priorities List (NPL) in 1987 under the Comprehensive Environmental Response,
Compensation, and Liabilities Act of 1980 (CERCLA or Superfund) as amended by the
Superfund Amendments and Reauthorization Act of 1986 (SARA). The lead agency for the
WTA is the U.S. Air Force. The U. S. EPA, Region 10 and the Washington State Department of
Ecology (WDOE) signed a Federal Facility Agreement (FFA) under Section 120 of CERCLA on
August 23, 1989. All three agencies support the need for this ESD and participated jointly in the
decision and the preparation of this document.
This ESD, prepared in accordance with Section 117(c) of CERCLA and 40 CFR
300.435(c)(2)(i), is necessary to document the modifications to the selected remedy outlined in
the ROD. The differences between the selected remedy in the ROD and the ESD are described
below.
The selected remedy for the WTA addressed the potential risks posed by fuel-related
contaminants in the groundwater by reducing site contamination to levels that are protective of
human health and the environment. The major components of the selected remedy under
CERCLA included:
1.
Installation of one or more extraction trenches capable of capturing the
floating fuel in the unconfined aquifer.
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2.
Installation of on-site collection systems to contain fuel removed from
extraction trench.
3.
Monitoring of the groundwater and the floating-fuel extraction/collection
system during fuel removal activities to ensure that groundwater remediation
levels are achieved throughout the site.
4.
Construction of soil piles on a low permeability surface for bioremediation of
the contaminated soil excavated during trench construction.
5.
Backfilling excavated trench with remediated soil after fuel removal is
complete.
6.
Implementation of administrative and institutional controls, such as restrictive
covenants and McChord Air Force Base command directives, that supplement
engineering controls and minimize exposure to releases of hazardous
substances during remediation.
Additionally, section IV of the ROD and the Proposed Plan stated that the
groundwater will be monitored during" and following groundwater remediation activities to
evaluate the need for remediation of the residual fuel in the soil above the floating fuel layer.
Until the floating fuel is removed, it cannot be determined that the soil is acting as a secondary
source of contamination to the groundwater. If monitoring shows that subsurface soil
remediation is necessary, further investigation and development of alternatives will be required
at that time.
This ESD makes changes to the first two components of the selected remedy. During
design only one trench was installed and it was not necessary to install an on-site collection
system to contain the fuel. The 90-day pilot test conducted during remedial design, showed that
passive fuel recovery is not appropriate because the thickness of the floating fuel layer was
significantly less than anticipated. It was also determined during design that the subsurface soils
will not act as a major secondary source of contamination to the groundwater.
This and other relevant documents will become part of the administrative Record file
pursuant to Section 300.825(a)(2) of the National Oil and Hazardous Substances Pollution
Contingency Plan (NCP). Copies of this ESD and the Administrative Record are available to the
pubic at the specific information repositories listed below.
McChord AFB Library
Building no. 765
62 CSG/SSI
McChord AFB, W A 98438-1325
Pierce County Library
Lakewood Branch
6300 Wildaire Road S.W.
Lakewood, W A 98499
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I. Summary of Site History, Contamination Problems
McChord AFB occupies. an area of about 4,600 acres in Pierce County, Washington.
The WT A is within the north industrial portion of the base along the western portion of the
instrument runway and in the industrial and operational activity areas associated with aircraft
maintenance and flight operation (See Figure 1). The WTA is an area where airplanes were
washed and drained of fuel. The study area is defined by the Air Force as encompassing the
grassy area between Air Mobility Command (AMC) Ramp C and Ramp D and includes a
number of buildings. The C and D ramps are large paved parking areas for aircraft (See
Figure 2).
The WT A is designated by the Air Force as Installation Restoration Program (IRP)
Sites 54 and 60. The former washrack designated as Site 54 was located on a paved apron used
for aircraft washing and maintenance and was reported to have received solvents, alkaline-based
detergents, paint removers, and corrosion-removing compounds (Ebasco 1992). However, a
remedial investigation (RI) did not discover significant contamination at Site 54. At the request
of the WDOE, two wells downgradient of the former leach pits will be monitored, via
semi-annual sampling to insure that the low level of contamination at Site 54 is not migrating off
site.
Site 60 of the WT A is defined as the area including a former leach pit and former
storm drainage infiltration ditches located near the current jet engine test cell facility. In 1983,
floating fuel was detected in wells northwest of the washrack and former leach pit locations
during a Phase II IRP investigation.
In 1984 the EP A nominated Site 54 for inclusion on the NPL. It was placed on the
NPL in 1987. During scoping of the RI, however, Site 60 and its associated fuel were included
in the investigation due to the potential for commingled plumes with Site 54. Currently, the
floating fuel near Site 60 is the primary focus of the remedial efforts at the WT A.
The Remedial Investigation (RI) (1992) characterized the nature and extent of
contamination in the groundwater, soil, surface water and sediments. The RI also evaluated the
nature and extent of the floating fuel layer within the site. The Human Health Risk Assessment
(1992) and the Ecological Risk Assessment (1991) evaluated potential effects of the
contamination on human health aIid the environment. The Feasibility Study (FS) (1992)
evaluated alternatives for remediation of contamination.
Results from the RI and the Baseline Risk Assessment indicate that no CERCLA
remedial action is necessary for soil, surface water, or sediments to ensure protection of human
health or the environment. However, the remedial investigation did indicate benzene
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contamination of the shallow unconfmed aquifer as a result of the floating fuel. The benzene
concentrations slightly exceeded health-based levels and/or maximum contaminant level
(MCLs).
The Human Health Risk Assessment considered the risks to human health from
exposure to contaminants in soils, surface water, sediment, and air for current and on- and off-
site populations, which included off-site residents and recreational visitors, and onsite long and
short-term workers. The risk assessment identified no unacceptable risks to human health from
exposure to soils. According to the risk assessment conducted at this site, an excess lifetime
cancer risk was in the range of 1 x 10-5.
The risk assessment also concluded that groundwater does not pose any risk to current
onsite residents because the drinking water on base is pumped from deeper groundwater aquifers
which are safe for drinking. Off- site residents are not at risk because the contamination has not
been shown to migrate off site. The risk to groundwater in a future residential scenario as the
result of benzene contamination is 1.29 x 10-5. This risk range could warrant consideration for
no further action. However, the Air Force, EP A and the WDOE proposed to address the
groundwater because benzene was detected at levels slightly above the maximum contaminant
levels (MCLs) of 5 parts per billion.
The selected alternative for removal of the floating fuel layers was Passive Fuel. .
Removal\Treatment of Fuel combined with Institutional Controls and Monitoring. This remedy
would allow for the recovery of fuel without generating groundwater that would require
treatment. Implementation of this remedy was projected to reduce the baseline risks from 1.29 x
10-5 to residual risks of 8.5 x 10-6.
The Feasibility Study (FS) also evaluated five remedial alternatives for the fuel
contaminated soils above the floating fuel. The FS evaluated: 1) No Action, 2) Institutional
Controls, 3) Containment, 4) In Situ Bioremediation of soil, 5) ExcavationfTreatment/Disposal
of the Soil. The No Action alternative was recommended as the best alternative for the soil
above the floating fuel in the proposed plan. It was concluded, that until the floating fuel is
removed, it cannot be determined if the soil above the fuel layer is acting as a secondary source
of groundwater contamination. If monitoring showed that subsurface soil remediation is
necessary, further investigation and development of alternatives would be required at that time.
II. Description of the Significant Differences and the Basis for those Differences
The rational for writing the ESD is that the original remedial action specified in the ROD
(September 30, 1992) - passive removal of the floating fuel from the water table to permanently
remove the hydrocarbons - has been implemented to the physical extent that is possible. Upon
implementation of the remedial action specified in the ROD, it was found that the volume of
floating fuel was overestimated. Recent investigations performed in a 90-day pilot test revealed
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the relatively small volume of fuel. In the ROD, the floating fuel was identified as underlying an
area approximately 300,000 ft2. Originally, the thickness of the layer was estimated to range
from 0.065 to 0.58 feet. During design the fuel layer was determined to be less than 0.1 feet; and
it is primarily an oily emulsion rather than a true immiscible liquid.
A test trench was installed at the WT A to gauge the ability for fuel recovery and fuel
was not observed in the trench. Passive recovery was found to be limited to two wells where
recovery is occurring on a regular basis (semi-monthly) and will continue until fuel fails to
recharge in recoverable quantities. Currently, only a fraction of a gallon per month per well of
non-aqueous phase liquid (NAPL) is being recovered from two wells (CR-01 and CR-02) as
shown in Figures 3. Results also show that groundwater contaminant concentrations
immediately downgradient of the fuel layer do not exceed the remedial action objectives, which
are:
Contaminant
Benzene
Toluene
Ethylbenzene
Xylene
TPH
Lead
Remediation Level
5ppb
1000ppb
700ppb
1O,OOOppb
1000ppb
11 ppb
Basis
MCL
MCL
MCL
MCL
MTCA Method A
Background
Groundwater sampled directly under the fuel contains hydrocarbon constituents which
slightly exceed the remedial action objectives of benzene but it must be clarified that this sample
was taken directly under (within inches) of the fuel layer and that samples a few more inches
away from the fuel layer and samples downgradient in the groundwater do not exceed the
remedial action objectives.
It was also determined in the pilot study that natural biodegradation and volatilization
is occurring by a healthy population of hydfocarbon degrading bacteria in the soils. Stimulation
of the growth of petroleum-degrading bacteria by bioventing appears feasible only in the upper
unsaturated portion of the smear zone. However, bioventing would not be immediately
successful in the deeper smear zone soils because they are saturated most of the year and contain
concentrations of volatile hydrocarbons which are toxic to bacteria.
In order for bioventing to be feasible in the lower smear zone, dewatering and vapor
extraction would be necessary. Dewatering is impractical, in that dewatering would increase the
depth of the smear zone at the site. An estimation of the capital cost for bioventing with air
sparging is $1.73 million. Operation and maintenance costs are estimated at $1.109 million for
five years. This costly remedial action is not deemed warranted given the limited risks, due to
the subsurface soils acting as a secondary source of contamination.
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The Air Force, EP A and WDOE agreed that a combination of natural attenuation with
long-term groundwater monitoring is appropriate, given the low risk the soils present. Long term
monitoring will provide data to demonstrate the progress of natural attenuation to decide whether
any additional remedial actions would be necessary in the future.
Sampling procedures to do this and to insure that one is collecting samples in the
groundwater and not from the hydrocarbon layer on top of the water table will be followed. The
use of specific monitor wells, installation of any additional wells, water quality parameters,
sampling frequencies and duration and compliance boundaries will be negotiated separately and
incorporated into the sampling plan for the WT A.
III. Affirmation of the Statutory Determinations
The modified remedy continues to satisfy the requirements of CERCLA section 121.
Considering the new information gathered in the 90-day pilot study performed by the
U. S. Air Force's 'contractor, all parties believe that the remedy remains protective of human
health and the enVironment, complies with federal and state requirements that were identified in
the ROD as applicable or relevant and appropriate to this remedial action at the time the original
ROD was signed and is also cost-effective with regard to the risk imposed. In addition, the
revised remedy utilizes permanent solutions and treatment technology to the maximum extent
practicable for this site. . .
IV. Public Participation
Notice has been issued that the contents of the Administrative Record File are available for
public review and comment as previously stated at both the McChord AFB and Lakewood Public
libraries. This Explanation of Significant Difference (ESD) will become part of the
Administrative Record File (NCP 3oo.825(a)(2)). This ESD has been published and a notice
placed in the Tacoma Tribune (Tacoma) and the Lakewood Journal (Lakewood) notifying the
public that the ESD and content of the Administrative Record are available for public review. In
addition, the Record of Decision, the Remedial Investigation and Feasibility Study, as well as,
the final pilot study, NAPL Recovery Test/Design Consideration, McChord AFB,
Washrackffreatment Area, are available at the information repositories.
Implementation of this action will begin approximately 30 days after issuance of this ESD. This
ESD does not represent a fundamental change in scope or purpose of the original ROD. Hence, a
formal comment period will not be conducted. Consistent with the NCP Section 300.435
(c)(2)(i), this ESD has been placed into the following papers:
Tacoma Tribune (Tacoma) and the Lakewood Journal (Lakewood)
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The public is encouraged to review this ESD and other relevant documentation in the
Administrative Record and provide comments to any of the agencies involved. Additional
information may be requested within 14 days of the notice of issuance for this ESD by
contacting:
Odette Hughes
McChord AFB
Environmental Management Flight, Public Relations
555 A Street
McChord AFB, W A 98438-1325
(206) 984-3913
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Washrackffreatment Area Pilot Recovery Study
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site Name:
INITIAL
NAME
DATE
EXPLANATION OF SIGNIFICANT DIFFERENCES
Concurrence
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