PB99-963114
EPA541-R99-065
1999
EPA Superfimd
Explanation of Significant Difference
for the Record of Decision:
Commencement Bay
South Tacoma Channel
Tacoma, WA
9/29/1999
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Explanation of Significant Differences
South Tacoma Field
Record of Decision
Commencement Bay, South Tacoma Channel Superfund Site
August, 1999
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Explanation of Significant Differences for the South Tacoma Field Record of Decision
1. Introduction
This document presents an Explanation of Significant Differences (ESD) from the
Record of Decision (ROD) for South Tacoma Field, which was signed by the United
States Environmental Protection Agency in September 1994.
Site Name and Location:
Commencement Bay South Tacoma Channel Superfund Site
South Tacoma Field Operable Unit
Tacoma, Washington
Lead and Support Agencies:
The U.S. Environmental Protection Agency (EPA) is the lead agency on
this project. The Washington State Department of Ecology is involved as
a milestone reviewer according to the existing agreement between Ecology
and EPA regarding Superfund Sites in Washington state.
Statutory Citation for an ESD:
In Section 117(c) of the Comprehensive Environmental Response
Compensation, and Liability Act (CERCLA), provisions are made for addressing 'and
documenting changes to the selected remedy that occur after the ROD is signed. This
ESD documents the changes to the selected remedy in accordance with CERCLA Section
117.
The ROD for the South Tacoma Field identified air sparging and vapor extraction
to address groundwater contamination in the Pioneer Builder's Supply section of the site
In addition to source removal during underground storage tank (UST) excavations
groundwater contaminant levels have greatly decreased since the Remedial Investigation
m the early 90's. The responsible parties have investigated the area and found that the
contamination is isolated and the levels have dropped. Because of this, the responsible
parties asked EPA to re-evaluate the need to install the air sparging and vapor extraction
systems. EPA has determined that these systems are no longer necessary to address the
groundwater contamination at Pioneer. Initial calculations indicate that cleanup levels
should be reached within four years. The State of Washington has concurred with this
decision.
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Administrative Record:
This ESD and other relevant documents to this decision will become part of the
Administrative Record for the South Tacoma Field (STF) Site pursuant to Section
300.825(a)(2) of the National Contingency Plan and will be available to the public at the
following locations:
Tacoma Public Library
Main Library, Northwest Room
1102 Broadway
Tacoma, Washington 98402
U.S. Environmental Protection Agency
Region 10
Park Place Building
1200 Sixth Avenue, 7th Floor Records Center
Seattle, Washington 98101
H. Site History
The STF Site encompasses approximately 260 acres and has been used for a
variety of industrial and commercial purposes for more than 100 years. The area of the
site that is the subject of this ESD is property owned by Pioneer Builder's Supply.
Pioneer occupies roughly one acre of property located in the southwest section of the site.
This property lies within a large area that was historically called the South Tacoma Car
Shops. The Car Shops operated as a railroad manufacturing and repair facility from 1892
until 1974 for Burlington Northern Rail Road. The area was used for manufacturing,
repair, and maintenance of railroad equipment. Rail cars were also cleaned and
dismantled.
Currently, Pioneer Builder's Supply operates a distribution center for asphalt and
cedar roofing materials. In 1990, three old Burlington Northern underground storage
tanks (USTs) were found during Pioneer's parking lot expansion. Pioneer removed those
tanks. Contamination from these USTs was visible in soil and groundwater during tank
excavation. Soils that were visibly contaminated from the USTs were removed during
tank excavation. It was noted that soils were contaminated below groundwater. Soil
excavation did not occur below groundwater level.
As part of their business, Pioneer installed and used two underground storage
tanks (USTs) for about five years to store gasoline and diesel fuel. In 1991, Pioneer
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removed their underground storage tanks in order to install new, double-lined tanks. .
During that tank excavation in 1991, soils that had been visibly contaminated with
petroleum were removed. Reportedly, all visible contamination was removed.
Groundwater at Pioneer is found from 25 to 30 feet below ground surface.
Groundwater in the entire South Tacoma Field area changes flow direction when nearby
pumping wells are turned on. These wells are part of the City of Tacoma water supply
system that are only used during peak demand in the summer and are located about a half
mile southeast of the South Tacoma Field.
The remedy for the Pioneer addresses groundwater contamination associated with
the USTs. Air sparging and soil vapor extraction were selected to address the volatile
organic contamination in groundwater. Additionally, institutional controls were to be
implemented that would prevent property owners from installing wells and using
contaminated groundwater. Specifically, the contaminants of concern in groundwater
were 1,1,2-trichloroethane, naphthalene, benzene, toluene, ethylbenzene, xylene and total
petroleum hydrocarbons (TPH).
m. Description of Significant Differences and Basis
This BSD changes the selected remedy at Pioneer from air sparging and vapor
extraction to monitored natural attenuation and institutional controls. This change is
based on new information that was collected during groundwater investigations that were
conducted prior to remedial design.
During the remedial investigation, which was conducted in 1991, the potentially
responsible parties (PRPs) investigated the contamination associated with the USTs by
installing a monitoring well in the location of the contaminated soils and groundwater.
Sample results showed that the groundwater was contaminated with benzene,
ethylbenzene, 1,1,2-trichloroethane and naphthalene above the maximum contaminant
levels (MCLs). Therefore, the ROD set cleanup standards in groundwater for benzene,
toluene, ethylbenzene, xylene, 1,1,2-trichloroethane and TPH.
In 1997 and 1998, additional groundwater characterizations were conducted in
order to aid in remedial design for the air sparging and soil vapor extraction systems.
Because of the changing groundwater motion at South Tacoma Field, the groundwater
investigation was conducted using a type of hydropunch technology in order to allow
proper placement of permanent groundwater monitoring wells. A StrataProbe was used
to punch 10 sample points. Based on that reconnaissance, four additional monitoring
wells were placed at Pioneer. Those wells (NMW 8A, NMW 9A, NMW 10A and NMW
11A) in addition to the one installed during the initial investigation (NMW 1 A) have been
sampled over the past year on four different occasions. This reconnaissance also
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indicated that the contamination was isolated in one location at Pioneer and did not
indicate a mobile contaminant plume.
The sample results from the most recent sampling events indicate that
contaminant levels are decreasing. For example, the original well, NMW 1 A showed
^991 r^v i w T^ 23°rCTOSramS P" Hter (Ug/1> durin* the investigations
in 1991. In May 1997 the benzene levels were not detected at 1 ug/I. In August 1997
benzene was at 1 1 micrograms per liter. In December 1997 benzene was at 9 8
rnicrograms per liter and in March- 1998 it was at 6.2 micrograms per liter. Since 1991
benzene levels appear to have dropped by an order of magnitude.
Groundwater samples were analyzed for all volatile contaminants using EPA
method 8020 and for TPH using Washington State analytical method WTPH-G and
WTPH-D. Results for all sampling can be found in the Groundwater Monitoring Report
for Pioneer Builder's Supply, July 1998. Of the original contaminants of concern
identified m the ROD, only benzene, ethylbenzene, naphthalene and TPH have currently
been detected above the cleanup levels from the ROD. Benzene has been detected in two
wells at concentrations of 1 1, 10, 6, 14, 41 and 38 ug/1 over the past four sampling
wTn 2 «m 1?ThP ^ **?«""* is 5 "&• Ethylbenzene has been detected in one
well at 800 ug/1. The cleanup level for ethylbenzene is 700 ug/I. Naphthalene has been
P^T Wf St f ° and 34° Ug/L Th£ CIeanUP level for ^Phthalene is 320 ug/1.
™ ^ f teCted ln °ne We" at 16' 17 md 17> md 2'8 milligrams per liter
. The cleanup level for TPH is lmg/1.
n*rt- m?D^°n thCSe reSUltS> ^ contractor mat was h^ed by the potentially responsible-
parties (PRPs) proposed that EPA re-evaluate the need to perform air sparging and soil
vapor extraction at Pioneer Builder's Supply in October, 1998. The contractor suggested
that contaminant levels are decreasing due to natural attenuation caused by microbial
degradation EPA has reviewed the.data and requested that the PRPs set up a monitoring
plan that will evaluate whether or not some sort of natural attenuation of the contaminants
ol concern is occurring. A work plan was submitted in November, 1998 to EPA EPA
5' ***** ** PR?S inCOrp°rated ^d Ae fimd work Plan wa* approved
T ^Jh& W°rk Plan pr°P°ses additional monitoring for natural attenuation indicators
In addition to monitoring the contaminants of concern, samples will be tested for pH« '
temperature; specific conductance; dissolved oxygen; nitrate+nitrite and ammonia; sulfate
and sulfide; ferrous iron; total, carbonate, and bicarbonate alkalinity; total dissolved
solids; and hardness. Currently, it appears that contaminant levels are decreasing It is
likely that levels are decreasing due to source removal and natural attenuation. When the
USTs were excavated, contaminated soils were removed. It is believed that the USTs
were the source of soil and groundwater contamination at Pioneer. Since the tank and soil
removals, contaminant concentrations in groundwater have decreased. By monitoring for
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natural attenuation indicators as well as contaminant concentrations, it is expected that a
more complete picture of attenuation can be realized.
The cleanup standards for the site"remain as identified in the ROD. The PRPs'
contractors believe that some sort of attenuation will occur and that the site will achieve
those goals within four years. Air sparging and soil vapor extraction systems are not cost-
effective m treating the current levels and quantity of contamination at the Site. The air
sparging and vapor extraction systems were estimated to cost $456,000 to install and
$807,000 per year to operate. It was expected that cleanup would be achieved in two
years and the total cost of the project was estimated to be $ 1,263,000.
At this point, it is EPA's position that additional monitoring is necessary to
evaluate this natural attenuation. Further, it is also EPA's position that air sparging and
soil vapor extraction are not necessary since the contaminant concentrations have
decreased to their current levels and appear to be on a downward trend. The current
information does not suggest that aggressive measures are necessary to achieve the
cleanup goals identified in the ROD.
Since waste will be left in place at South Tacoma Field, EPA will conduct a five-
year review in 2003. At that time, EPA can determine whether conditions have changed
at the site and if there is new information to indicate that the groundwater cleanup levels
will not be met.
As the lead regulatory oversight agency at the South Tacoma Field, EPA is
responsible for consulting with the Washington State Department of Ecology (Ecology).
At the initial PRP proposal, Ecology was contacted and the suggested changes were
discussed. Ecology agreed with the monitoring strategy and deferred evaluation to EPA.
IV. Affirmation of the Statutory Determinations
The proposed changes to the selected remedy satisfy the provisions of CERCLA
Section 121 since they are still protective of human health and the environment, comply
with Federal and State requirements identified in the original ROD as Applicable or
Relevant and Appropriate Requirements, and is cost-effective. In addition, the revised
remedy utilizes permanent solutions and alternative treatment technologies to the
maximum extent practical for this site.
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Approved:
'Michael F. Gearheard', Director
Environmental Cleanup Office
Date
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