PB97-963105
EPA/541/R-97/007
November 1997
EPA Superfund
Explanation of Significant Difference
for the Record of Decision:
South Municipal Water Well Supply
Peterborough, NH
2/3/1997
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DECLARATION FOR THE
EXPLANATION OF SIGNIFICANT DIFFERENCES
SITE NAME AND LOCATION
South Municipal Water Supply Well Superfund Site
Peterborough, New Hampshire
STATEMENT OF PURPOSE
This decision document sets forth the basis for the determination
to issue the attached Explanation of Significant Differences
(BSD) for the South Municipal Water Supply Well Superfund Site in
Peterborough, New Hampshire.
STATUTORY BASIS FOR ISSUANCE OF THE ESP
Under Section 117(c) of the Comprehensive Environmental Response,
Compensation, and Liability Act (CERCLA), 42 U.S.C. § 9617(c), if
the U.S. Environmental Protection Agency (EPA) determines that
the remedial action being undertaken at a Site differs
significantly from the Record of Decision (ROD) for that Site,
EPA shall publish an explanation of the significant differences
between the remedial action being undertaken and the remedial
action set forth in the ROD and the reasons such changes are
being made. Section 300.435 (c) of the National Contingency Plan
(NCP) , 40 C.F.R. § 300.435 (c), and EPA guidance (Office of Solid
Waste and Emergency Response [OSWER] Directive 9355.3-02),
indicate that an BSD, rather than a Record of Decision (ROD)
amendment, is appropriate where the changes being made to the
remedy are significant but do not fundamentally alter the overall
remedy with respect to scope, performance, or cost. Because the
adjustments to the ROD provided in the BSD are significant but do
not fundamentally alter the overall remedy for the Site with
respect to scope, performance, or cost, this BSD is properly
being issued.
In accordance with Section 300.435 (c) of the NCP, this BSD and
supporting documentation will become part of the Administrative
Record which is available for public review at both the EPA
Region I Record Center in Boston, Massachusetts and the
Peterborough Public Library in Peterborough, New Hampshire.
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OVERVIEW OF THE ESP
The 1989 ROD for the South Municipal Water Supply Well Superfund
Site (Site) specifies a remedy which addresses contamination of
ground water, soils, and wetland sediments. Between July, 1990,
and January, 1993, extensive pre-design investigations were
undertaken and the design finalized. As a result of having
obtained new, more detailed technical information during these
pre-design investigations, an BSD was issued on May 6, 1993,
which documented modifications to the remedy principally for air
emission controls and sediment excavation.
The ground water extraction and treatment system has been in
operation since March of 1994 and the vacuum extraction system
has been in operation since October of 1994. After reviewing
quarterly ground water sampling data over the past two years of
remedial actions and considering the changes in understanding
which have occurred since the ROD was issued concerning the
ability to restore ground water contaminated with dense non-
aqueous phase liquids (DNAPLs), EPA has determined that it is
technically impracticable from an engineering perspective to
restore that portion of the contaminated ground water affected by
DNAPLs to drinking water quality in a reasonable time frame.
Therefore, this BSD documents EPA's decision to waive certain
applicable or relevant and appropriate requirements (ARARs) for
ground water, Federal Drinking Water Standards. Because of the
determination of technical impracticability, three portions of
the remedy are modified by this BSD:
1. Groundwater Extraction and Treatment
Two aspects of the ground water extraction and treatment
component of the remedy will be modified.
Air Sparging - The ROD stated that it might be necessary to
implement technologies to enhance contaminant removal and to
address the presence of free phase solvents in the saturated
zone of the NHBB-area plume. Air sparging (in conjunction
with the soil vacuum extraction system) was the selected
technology. Because of technical problems encountered in
implementing the air sparging system, it will not be
operated so as to prevent further contamination problems.
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Ground Water Extraction - The ROD specified that the ground
water extraction system for the NHBB area would be designed
to create a hydraulic barrier between the NHBB area plume
and the rest of the aquifer. A time period of 19 to 32
years was estimated to be needed to restore that portion of
the aquifer to drinking water quality. Since ARARs are to
be waived, the pumping rates and extraction well
configuration will be changed to maintain the hydraulic
barrier between the NHBB plume area and the rest of the
aquifer, but not necessarily to restore the NHBB plume to
drinking water quality. Adjustments to the system will be
made to allow for the use of the South Well if the Town of
Peterborough elects to use it.
2. In-Situ Vacuum Extraction of Contaminated Soils
Since no soil contact threat was identified, the ROD
prescribed a vacuum extraction system (VES) to remediate
soils located near the corner of the NHBB facility solely to
allow attainment of ground water cleanup levels. Therefore,
since as described above, no air sparging will be employed
and the ground water ARARs will be waived, vacuum extraction
is no longer required and may be discontinued.
3. Long-Term Environmental Monitoring
Ground water monitoring (quality and water levels) will be
conducted to determine if the ground water within the
Technical Impracticability Waiver area is hydraulically
contained. Based on the analysis of the monitoring data,
adjustments to the extraction system will be made to ensure
that containment is maintained. This will be especially
important should the Town elect to re-use the South
Municipal Water Supply Well.
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4. Institutional Controls
In order to further ensure the protectiveness of the remedy,
a deed restriction will be placed upon the NHBB property
prohibiting extraction of the ground water for purposes
other than the remedial action unless the extracted ground
water meets or is treated to appropriate use standards and
the extraction of the ground water-does not adversely affect
the remedial action.
DECLARATION
For the foregoing reasons, by my signature below, I approve the
issuance of an Explanation of Significant Differences and
associated Technical Impracticability Waiver for the South
Municipal Water Supply Well Superfund Site in Peterborough, New
Hampshire, and the changes and conclusions stated therein.
3
~6ate "Linda M. Murphy, rl5ire$:tor
Office of Site Remediation and
Restoration
U.S.'E.P.A., Region I
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EXPLANATION OF SIGNIFICANT DIFFERENCES
SOUTH MUNICIPAL WATER SUPPLY WELL SUPERFUND SITE
PETERBOROUGH, NEW HAMPSHIRE
I. INTRODUCTION
A. Site Name and Location
Site Name: South Municipal Water Supply Well Superfund
Site
Site Location: Town of Peterborough, Hillsborough County,
New Hampshire
B. Lead and Support Agencies
Lead Agency: United States Environmental Protection Agency
(EPA)
Support Agency: New Hampshire Department of Environmental
Services (NHDES)
C. Legal Authority
Under Section 117 (c) of the Comprehensive Environmental Response,
Compensation, and Liability Act (CERCLA), 42 U.S.C. § 9617 (c),
Section 300.435 (c) of the National Contingency Plan (NCP),
40 C.F.R. § 300.435 (c), and EPA guidance (Office of Solid Waste
and Emergency Response (OSWER) Directive 9355.3-02), if EPA
determines that differences in the remedial action significantly
change but do not fundamentally alter the remedy selected in the
Record of Decision (ROD) with respect to scope, performance, or
cost, EPA shall publish an explanation of the significant
differences between the remedial action being undertaken and the
remedial action set forth in the ROD and the reasons such changes
are being made.
D. Summary of Circumstances Necessitating this Explanation
of Significant Differences
The 1989 ROD for the South Municipal Water Supply Well Superfund
Site (Site) specifies a remedy which addresses contamination of
ground water, soils, and wetland sediments. Between July, 1990,
and January, 1993, extensive pre-design investigations were
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Explanation of Significant Differences
South Municipal Water Supply Well Superfund Site
February 3, 1997 . Page 2
undertaken and the design finalized. As a result of having
obtained new, more detailed technical information during these
pre-design investigations, an BSD was issued on May 6, 1993,
which documented modifications to the remedy principally for air
emission controls and sediment excavation.
The ground water extraction and treatment system has been in
operation since March of 1994 and the vacuum extraction system
has been in operation since October of 1994. After reviewing
quarterly ground water sampling data over the past two years of
remedial actions and considering the changes in understanding
which have occurred since the ROD was issued concerning the
ability to restore ground water contaminated with dense non-
aqueous phase liquids (DNAPLs), EPA has determined that it is
technically impracticable from an engineering perspective to
restore that portion of the contaminated ground water affected by
DNAPLs to drinking water quality in a reasonable time frame.
Therefore, this BSD documents EPA's decision to waive certain
applicable or relevant and appropriate requirements (ARARs) for
ground water, Federal Drinking Water Standards.
E. Availability of Documents
This BSD and supporting documentation shall become part of the
Administrative Record for the Site. The BSD, supporting
documentation for the BSD, and the Administrative Record (the
index for the Administrative Record is attached) are available to
the public at the following locations and may be reviewed at the
times listed:
U.S. Environmental Protection Agency
Records Center
90 Canal Street
Boston, MA 02114
Weekdays from 10:00 a.m. to 1:00 p.m.,
and from 2:00 p.m. to 5:00 p.m.
Peterborough Public Library
Peterborough, New Hampshire
Monday through Thursday from 10:30 a.m. to 8:30 p.m., and
Friday through Saturday from 10:30 a.m. to 5:30 p.m.
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Explanation of Significant Differences
South Municipal Water Supply Nell Superfund Site
February 3, 1997 Page 3
II. SUMMARY OF SITE HISTORY, CONTAMINATION PROBLEMS. AND
SELECTED REMEDY
A. Site History and Contamination Problems
The South Municipal Water Supply Well Superfund Site is located
on Sharon Road, approximately two miles south of the center of
the Town of Peterborough in Hillsborough County, New Hampshire.
The South Municipal Water Supply Well was installed in 1952 and
provided water to the Town of Peterborough for nearly 30 years,
until it was closed in 1982.
The Site, located in a predominantly rural area, is situated in
the Contoocook River Valley, approximately 350 feet southwest of
the Contoocook River (see Figure 1). The New Hampshire Ball
Bearings, Inc. (NHBB) manufacturing facility, is located on
twenty-four acres of land about 1,200 feet northwest of the South
Well.
In October 1982, the New Hampshire Water Supply and Pollution
Control Commission (NHWSPCC) conducted routine sampling of water
supplies and found volatile organic compounds (VOCs) in a water
sample taken from the South Well. Due to the potential health
risks of consuming low levels of organic chemicals, NHWSPCC
recommended that the Town of Peterborough discontinue use of the
South Well. The well was shut down on December 2, 1982.
In September 1984, the Site was placed pn the National Priorities
List (NPL), EPA's list of hazardous waste sites eligible for
financing under Superfund. In 1985, EPA identified NHBB as a
potentially responsible party for the well's contamination and
requested that NHBB conduct the Remedial Investigation/
Feasibility Study (RI/FS) to determine the nature and extent of
site contamination and to evaluate approaches to site cleanup.
Under a Consent Order signed by both parties on July 22, 1986,
NHBB agreed to conduct the RI/FS under EPA supervision.
The Remedial Investigation (RI) field studies for the RI/FS began
in August 1986 and were completed in March 1989. The major
conclusions of the Remedial Investigation (RI) were:
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Explanation of Significant Differences
South Municipal Hater Supply Hell Superfund Site
February 3, 1997 Page 4
high concentrations of VOC contamination exist in ground
water at the Site, originating from past releases at the
NHBB facility,
VOC contamination in the soils continues to contribute to
ground water contamination, and
Polychlorinated Biphenyls (PCBs) and Polycyclic Aromatic
Hydrocarbons (PAHs) were found in a small unnamed brook that
crosses the NHBB property and are dispersed through the
wetlands area east and northeast of the NHBB facility.
NHBB, in accordance with the Consent Order, conducted a
Feasibility Study (FS) which developed and evaluated remedial
alternatives to address the contamination at the Site. The FS
was completed in July, 1989, and formed the basis of EPA's
Proposed Plan released that same month. Subsequently the ROD was
issued September 27, 1989. In July of 1990, EPA issued a
Unilateral Order to NHBB and its parent.corporation, Minebea Co.,
LTD., to perform the Remedial Design and Remedial Action
prescribed in the ROD. As a result of the pre-design studies
performed pursuant to that Order, EPA modified the Record of
Decision through the issuance of an BSD on May 6, 1993. NHBB has
completed the sediment remedy and is continuing to operate and
maintain the soils and ground water remedial elements of the
remedy.
B. Summary of the Selected Remedy
After evaluating the feasible alternatives presented in the FS,
EPA selected a remedy, documented in the ROD and modified by the
May, 1993, BSD, consisting of six components to address soil,
sediment, and ground water contamination at the Site:
1. In-Situ Vacuum Extraction of Contaminated Soils
2. Excavation and/or Dredging with Dewatering of
Contaminated Sediments and Off-Site Disposal
The May, 1993, BSD determined that only excavation
was appropriate and that a small area of
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Explanation of Significant Differences
South Municipal Water Supply Well Superfund Site
February 3, 1997 Page 5
contaminated sediments would be left in place and
monitored to confirm the limited area involved.
3. Wetlands Restoration
4. Ground water Extraction and Treatment with Air
Stripping and Carbon Columns for Air Emission
Control
The May, 1993, BSD documented the decision:
1) to remove the requirement for air emission
control, 2) to use air sparging to attempt to
enhance DNAPL removal, and 3) to allow natural
attenuation of a small portion of the leading edge
of the contaminated plume.
5. Long-Term Environmental Monitoring
6. Institutional Controls, Including Restrictions on
Use of the South Municipal Water Supply Well
Currently, the vacuum extraction and the ground water extraction
and treatment portions of the remedy are operating. The
contaminated sediments have been removed, the wetlands have been
regraded and replanted, and the restoration efforts are being
monitored for effectiveness. The Town of Peterborough, using
information supplied by NHBB, has enacted zoning changes
prohibiting use of contaminated ground water. Discussions have
been held with the Town concerning the potential for reuse of the
South Well. However, there are currently no plans to use the
aquifer for public water supply.
C. Technical Impracticability Evaluation
An evaluation of contaminant source information and ground water
data (especially that collected during the past two years of the
cleanup) has been conducted. A complete discussion of the
factors leading to the technical impracticability waiver
determination is attached. Two conclusions can be drawn from the
evaluation: first, DNAPLs are present at the Site and second,
while the exact locations of these DNAPLs are not known,
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Explanation of Significant Differences
South Municipal Hater Supply Hell Superfund Site
February 3, 1997 Page 6
information suggests that more than one location of DNAPLs exists
in the sub-surface soils and/or aquifer. Using this information,
a calculation using the existing pumping rates and configuration
which takes into account the presence of DNAPLs estimates an
average time to remediate the portion of the aquifer which is
west of Route 202 of 108 years, rather than the estimate in the
ROD of a maximum of thirty-two years.
As part of this process, several remedial alternatives were
investigated to determine if a more reasonable ground water
cleanup time frame could be achieved. These have included:
excavation, DNAPL pumping, in-situ biodegradation, containment
with barrier walls, treatment with permeable reaction walls,
solidification, stabilization and/or in-situ vitrification, soil
flushing and natural attenuation. None would result in faster or
more efficient cleanup times. Several would, in fact, be less
effective.
The ROD set cleanup levels for the following VOCs based upon
Federal Maximum Contaminant Levels (MCLs): tetrachloroethylene,
trichloroethylene, 1,1,1-trichloroethane, 1,1-dichloroethylene,
toluene, and vinyl chloride. In accordance with Section
121(d)(4)(C) of CERCLA, EPA may select a remedial action that
does not attain an ARAR if compliance with the ARAR is
technically impracticable from an engineering perspective. When
the ROD was issued in 1989, EPA did not issue a waiver of these
ARARs because EPA believed, based on data available at that
time, that ground water ARARS would be attained within a time
period of decades. EPA's reevaluation, based on recent data,
indicates that the ARARs listed above may not be attainable for
108 years or more, because of hydrogeologic and contaminant
related factors that limit the effectiveness of ground water
remediation at the Site. Therefore, EPA is waiving ARARs,
Federal Drinking Water Standards, for that portion of the aquifer
which is currently being affected by DNAPLs (see Figure 2).
Furthermore, the cleanup level for 1,1-dichloroethane which was
based on a State Health Advisory will be waived in the same area.
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Explanation of Significant Differences
South Municipal Water Supply Hell Superfund Site
February 3, 1997 Page 7
DESCRIPTION OF SIGNIFICANT DIFFERENCES
Based upon the technical impracticability evaluation and related
waiver of ground water ARARs and cleanup levels, modifications to
three components of the remedy are warranted. A description of
these modifications, by component, follows:
Ground Water Extraction and Treatment
Two aspects of the ground water extraction and treatment
component of the remedy will be modified.
Air Sparging - The ROD stated that it might be necessary to
implement technologies to enhance contaminant removal and to
address the presence of free phase solvents in the saturated zone
of the NHBB-area plume. In order to determine the advisability
of this enhancement, pilot tests were conducted during pre-design
investigations, which indicated that air sparging (forcing
ambient air into the ground water) would strip significant
amounts of VOCs from the ground water. The VOCs would travel
with the ambient air upward through the soils and be captured by
the vacuum extraction system. It was believed that this
technique could be effective for as many as fifteen years.
Therefore, this technology was identified in the May, 1993, BSD
to be implemented as an enhancement to the pump-and-treat
technology. However, after initiation of ground water pumping,
water levels in several of the air sparging wells fell below the
bottom of the well screens, thus preventing their use. If the
wells had been drilled deeper, less pervious materials would have
been penetrated which would inhibit collection of the VOC-
contaminated air by the vacuum extraction system. New areas of
the aquifer and sub-surface soils would have been contaminated.
In addition, VOC levels were rising rapidly in a bedrock well
located near the corner of the NHBB facility possibly due to the
mobilization of DNAPLs through the extraction of ground water.
Thus, the air sparging system was never operated and will not be
operated so as to prevent further contamination problems.
Ground Water Extraction - The ROD specified that the ground
water extraction system for the NHBB area would be designed to
create a hydraulic barrier between the NHBB area plume and the
rest of the aquifer. A time period of 19 to 32 years was
estimated to be needed to restore that portion of the aquifer to
drinking water quality. Since ARARs are to be waived, the
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Explanation of Significant Differences
South Municipal Water Supply Well Superfund Site
February 3, 1997 Page 8
pumping rates and extraction well configuration will be changed
to maintain the hydraulic barrier between the waiver area and the
rest of the aquifer, but not necessarily to restore the waiver
area to drinking water quality. Adjustments to the system will
be made to allow for the use of the South Well if the Town of
Peterborough elects to use it.
In-Situ Vacuum Extraction of Contaminated Soils
The ROD prescribed a vacuum extraction system (VES) to remediate
soils located near the corner of the NHBB facility in order to
attain ground water cleanup levels. Two years of operation were
estimated to reach the soil cleanup levels. However, since the
May, 1993, BSD indicated that air sparging would occur for up to
fifteen years and since the VES would be needed to capture the
resulting VOC-laden vapors stripped from the ground water during
the air sparging process, the soil cleanup levels would not be
expected to be achieved until approximately two years after the
air sparging system stopped operating. However, soil vacuum
extraction was proposed solely to allow for the remediation of
the ground water, since no soil contact threat was identified.
Therefore, since as described above, no air sparging will be
employed and the ground water ARARs will be waived, vacuum
extraction is no longer required and may be discontinued.
Long-Term Environmental Monitoring
Ground water monitoring (quality and water levels) will be
conducted to determine if the ground water within the Technical
Impracticability Waiver area is hydraulically contained. Based
on the analysis of the monitoring data, adjustments to the
extraction system will be made to ensure that containment is
maintained. This will be especially important should the Town
elect to re-use the South Municipal Water Supply Well.
Institutional Controls
In order to further ensure the protectiveness of the remedy, a
deed restriction will be placed upon the NHBB property
prohibiting extraction of the ground water for purposes other
than the remedial action unless the extracted ground water meets
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Explanation of Significant Differences
South Municipal Water Supply Hell Superfund Site
February 3, 1997 ^ Page 9
or is treated to appropriate water use and/or disposal standards
in effect at the time of extraction and the extraction of the
ground water does not adversely affect the remedial action.
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All of the proposed modifications embodied in this BSD will
protect human health and the environment, will comply with all
applicable or relevant and appropriate Federal and State
requirements except those waived due to technical
impracticability, will provide for a long-term and permanent
remedy for the Site, will reduce the mobility, toxicity, and
volume of contaminants at the Site to a similar degree as the
remedy outlined in the ROD, and will pose the same short-term
risks as the remedy contained in the ROD. The proposed
modifications to the remedy will reduce certain remedial costs by
approximately $3.5 million over thirty years.
IV. SUPPORT AGENCY COMMENTS
The State of New Hampshire has participated with EPA in
developing the changes to the remedy presented in the ROD and
May, 1993, BSD which are described herein. The State is in
agreement with the changes.
V. STATUTORY DETERMINATIONS
EPA has determined that the selected remedy specified in the ROD,
with the above-described changes, remains protective of human
health and the environment, complies with Federal and State
requirements that are applicable or relevant and appropriate to
this remedial action, and is cost-effective. In addition, the
revised remedy utilizes permanent solutions and alternative
treatment technologies to the maximum extent practicable for this
Site.
VI. PUBLIC PARTICIPATION
This BSD was available for thiry-day public comment period which
ended December 23, 1996. Comments received have been summarized
and the Agency's response to those comments is attached to this
ESD in a Responsiveness Summary.
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Explanation of Significant Difference*
South Municipal Water Supply Hall Superfund Site
February 3, 1997
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I
N
figure
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Explanation of Significant Differences
South Municipal Water Supply Hell Superfund Site
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NEW HAMI SHIRE
ALL BEARINGS. I
APPROXIMATE Tl WAIVER
BOUNDARY
FIGURE 2
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Attachment to Explanation of Significant Differences for the
South Municipal Water Supply Well Superfund Site
Technical Impracticability Evaluation
/. Specific ARARs
The September 27,1989 ROD set Target Ground Water Cleanup Levels at Federal Maximum
Contaminant Levels (MCLs) and, absent an MCL, at a proposed MCL, and at a New Hampshire
Health Advisory. Thus, the ARARs for the Site in effect at the time of the ROD are: 1,1,1-
trichloroethane - 200 ppb, trichloroethylene - 5 ppb, 1,1-dichloroethylene - 7 ppb, toluene - 2000
ppb, vinyl chloride - 2 ppb. The target level for tetrachloroethylene was based upon a proposed
MCL while the target level for 1,1-dichloroethane was based upon the New Hampshire
Department of Public Health Service consumption advisory for water supplies, since no MCL
had been promulgated. The requirement to meet these cleanup levels is waived along with the
others.
2. Spatial area over which the TI Waiver will apply
As shown on Figure 2, attached to the BSD, the waiver will apply to the New Hampshire Ball
Bearings, Inc. (NHBB) property from fifty feet west of the centerline of Route 202 and to the
north of a line running from the entrance to the parking lot to just south of well EM-107. This
area will entirely encompass what was referred to in the ROD as the NHBB area plume. The
waiver will include both the overburden aquifer and the bedrock aquifer beneath it. Little
contamination has been found in the bedrock except at well GZ-4R. The affect of the waiver will
likely be a reduced pumping rate and no extraction occurring near GZ-4R. This may result in
lessening the increase in the contaminant concentrations in the bedrock as DNAPL pools will be
under less hydrologic stress. However, the possibility of remediating the bedrock aquifer in a
reasonable time frame if contamination does migrate into it is as problematic as remediating the
overburden aquifer.
3. Site conceptual model
The conceptual model of the site is described in the ROD and summarized here as modified by
information obtained after the issuance of the ROD.
The overburden aquifer behaves differently than that of the bedrock, but a hydraulic connection
exists between the two. The overburden aquifer is semi-confined to unconfined. The bedrock
aquifer behaves as a leaky confined aquifer. Aquifer parameters for the overburden aquifer are
highly variable throughout the site. Vertical hydraulic conductivity is generally low relative to
radial hydraulic conductivity indicating that lateral rather than vertical flow predominates.
The average flow direction is east-northeast in the vicinity of the NHBB plant and changes to a
northerly direction at the Contoocook River, paralleling the river. The flow in the bedrock is
generally upward throughout the Site.
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The major constituents of the contamination plume are 1,1,1-trichloroethane, trichloroethylene,
and tetrachloroethylene. Interviews and historical investigations indicate that these solvents were
used by NHBB as pure products or mixtures. These investigations also indicate that these and
other compounds were released to the environment through:
in-house releases subsequently washed out floor drains or sinks to outfalls,
or washed out facility doors, and
exterior releases through the draining of a truck-mounted waste solvent
tank.
Ground water data from the latest sampling event (February, 1996) indicate highest VOC levels
at the northeast corner of the NHBB facility (PCE - 21,000 ppb), and at the tumble sump area
(TCA-1300).
The strongest evidence of DNAPL in ground water is near the northeast corner of the building
(Outfall 003 A), near the southwest corner of the building (the tumble area sump and related drain
and/or the aboveground 1,1,1-TCA tank), and beneath the building near the GZ-7 well cluster
(cracks in the floor and/or subgrade drains). An area near the warehouse and maintenance garage
(where the tank trailers are reported to have been emptied) and another area along the south wall
of the building (where the tank trailers were apparently stationed) are also strongly suspected of
containing DNAPL. Of these, the source area near the northeast corner of the building appears to
contain the greatest mass and/or largest distribution of DNAPL. An area beneath the wetlands
and former locations of Outfall 002 are also suspected to contain DNAPL.
4. Restoration potential
The potential for restoring ground water to drinking water quality is dependent upon several
factors:
a. As mentioned above, DNAPL sources occur throughout the aquifer on the NHBB
property. One has been positively identified (the northeast corner of the building) while
the others have been inferred from knowledge of disposal practices and evaluation of
water quality data. The current well configuration and pumping scenario have isolated
the DNAPL source areas from the portion of the aquifer identified in the ROD as the
"dilute plume area." Based upon ground water modeling and aquifer characteristics,
these source areas can continue to be hydrologically isolated or contained so as to allow
full restoration of the aquifer outside of the waiver area and to allow use of the South
Well.
b. Remedial systems installed at the site for contaminant source control and remediation
of ground-water include: a seven well ground water extraction system and air stripping
treatment plant, a ten well soil vacuum extraction (SVE) system, and a ground water
sparging system.
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The ground water extraction system has been in near-continuous operation since
March 12,1994. The total flow rate has averaged just below 600 gpm for over two years
and the system has removed approximately 4,000 pounds of VOCs from ground water as
of July, 1996. Reductions in VOC concentrations for most of the wells in the dilute
plume area and within much of EX-4's capture zone have been large, with several of the
formerly contaminated wells (e.g., the EM-3 couplet, MW-5B and EMH-2U) consistently
having concentrations below Cleanup Levels. In some locations east of the parking lot
the plume has shrunk in width up to fifty percent. VOC reductions observed in the dilute
plume area indicate both that EX-5 A and EX-7 are effective in removing contaminants
from the aquifer and that upgradient wells, as mentioned above, are successfully
providing hydraulic containment of source areas.
VOC concentrations near the building have shown both significant increases and
decreases. In general, significant increases have been seen in lower wells near the
northeast corner of the building (including GZ-4R and GZH-4L) and the central portion
of the building (GZ-7L and GZH-7M). Most of the wells showing decreases hi VOCs
have shallow screens. It is probable that decreases in VOC concentrations in shallow
wells (and increases hi deep wells) are related at least in part to relatively shallow
contamination being transported to deeper portions of the aquifer via downward flow
induced by deeply screened, partially penetrating wells. Concerns also exist that VOC
concentration increases seen in bedrock well GZ-4R may be related to mobilization and
downward migration of DNAPL into bedrock, caused by drawdown created by EX-1. In
general, the size of the plume near the building has changed little since pumping began.
VOC concentrations measured in EX-1 have stabilized over the last year of pumping.
Similar stabilization (but at lower concentrations) may be occurring at EXH-3 and EX-9.
At least in the case of EX-1 the cause of the stabilization is attributed to the nearby
presence of DNAPL. Evidence also points to the DNAPL being present within the
capture zones of EXH-3 and EX-9.
The vacuum extraction system consists often vacuum extraction (VX) wells screened at
various depths within the vadose zone near the northeast corner of the building. The
system was started in October 1994, and has been roughly fifty percent operational. The
primary cause of system shutdowns has been elevated water table conditions during wet
months (primarily during the spring) and during periods when the ground water
extraction system has been shut down for maintenance.
Total VOC concentrations for seven of the VX wells have been reduced by a factor of
three or greater since the first two months of operation. However, none of the wells have
contained VOC concentrations below detection limits for extended periods (i.e., months).
It is anticipated that it will be difficult or impossible to obtain clean samples of extracted
soil gas due to contribution via volatilization off of contaminated ground water.
Between October 1994 and 1996, roughly 4,200 pounds of VOCs were removed from
contaminated soils. The average VOC removal rate was approximately 9.3 pounds per
day.
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While the SVE system has been effective in achieving contaminant mass removal in
unsaturated soils and has the potential to remove a source for ground water
contamination, it is not expected to have a significant bearing on the time frame for
aquifer restoration since DNAPL beneath the water'table is not influenced by vacuum
extraction.
Due to the fact that the water table has been drawn to below the well screens for many of
the sparging wells throughout most of the year, but primarily out of concern that sparging
could induce uncontrolled mobilization of DNAPL, sparging has not been conducted at
the Site. As ground water monitoring data have indicated the probable presence of
DNAPL below the screens of the sparging wells and at several other locations at the Site,
it was also recognized that sparging would ultimately not have an effect on the timeframe
required to remove the DNAPL in its entirety.
c. An estimation of time required for complete dissolution of DNAPL can be made for a
portion of the aquifer near the northwest corner of the building that contains DNAPL and
where relatively intensive subsurface investigations have been conducted. The
dissolution time (t) was calculated as follows using tetrachloroethylene as the
representative contaminant:
t =
Where: m = the mass of DNAPL (20,400 grams/m3),
Vj= the average interstitial ground water velocity (0.23 m/day),
n = the effective porosity in the aquifer (.15),
Cw = the dissolved DNAPL concentration in ground water (15 mg/1), and
A = the cross-sectional area of DNAPL (m2).
The time for dissolution of tetrachloroethylene to a concentration of 15 mg/1 was
calculated to be 108 years. Remediation to ARARs would be longer still. The
dissolution calculation is very sensitive to the mass of DNAPL per unit volume of
aquifer. It is probable that much of the aquifer containing DNAPL has a lower mass per
volume value than that used hi the above calculation, and the dissolution time could be
reduced to tens of years. It is equally likely that the aquifer contains pools of DNAPL
where the mass per volume is higher than the value used above. It is these areas which
will ultimately dictate the time required to clean up the aquifer, and it is therefore
considered probable that close to source areas the aquifer will not be restored in a
reasonable time frame.
d. Based on a literature review, several remedial technologies have been identified that
have been used in attempts to restore ground water at DNAPL sites. Each technology is
briefly described below, and an evaluation of its ability to attain Cleanup Levels within a
reasonable time frame is examined within the context of Site hydrogeology and
contaminant distribution.
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Excavation - In order to be effective, excavation would require that virtually all
DNAPL at the Site be excavated. Finding all of the DNAPL, if possible, would
be extraordinarily expensive, and invasive investigative measures could cause
migration of the DNAPL to deeper portions of the aquifer. As DNAPL probably
exists beneath the building, excavation would require disruption of facility
operations. The depth of DNAPL penetration in some portions of the Site is such
that extremely expensive deep excavation would be required, if even possible;
excavation of DNAPL in the bedrock would certainly not be possible. Finally,
treatment or disposal costs for excavated media would be unacceptably expensive.
Pumping DNAPL - When pools or lenses of DNAPL can be located, it is possible
to recover small quantities of free product by pumping from extraction wells or
trenches. As the DNAPL saturation thickness decreases, the efficiency of a
DNAPL recovery system generally decreases, and over pumping of a product
recovery well or trench may result in truncation of the DNAPL layer and
significantly reduce the formation transmissivity to DNAPL flow. Also,
significant residual DNAPL (between 10% and 90% of the initial volume) will
always remain after the removal of mobile DNAPL requiring further
treatment/removal methods. All of the mobile DNAPL would need to be found in
order to make removal by pumping marginally effective. The process of detecting
the DNAPL could cause mobilization of DNAPL and it is unlikely it could be
accomplished. Pumping of DNAPL is therefore not considered a practical
remedial technology for the Site.
In-Situ Biodegradation - Naturally occurring microbes can be used to degrade
hazardous chemicals in the subsurface. This treatment technology usually
involves enhancement of the natural processes to increase the rate of degradation
by delivery of oxygen, nutrients and/or specialized microorganisms to the
contaminated zone via wells. In general, chlorinated solvents are relatively
resistant to microbial degradation. Some chlorinated solvents degrade aerobically
and anaerobically (trichloroethylene) and others degrade strictly anaerobically
(tetrachloroethylene). Effective biodegradation at the Site would therefore require
both an anaerobic dechlorination step and an aerobic step in which a supplemental
carbon source is provided. Due to the toxicity of most DNAPLs and the lack of
essential nutrients, electron acceptors and other requirements for life hi the
DNAPL pool itself, the potential for biologically mediated degradation of
DNAPLs is limited. Biodegradation may be most appropriate as a polishing step
after methods that remove the majority of DNAPL. However, as no methods have
been identified to locate, much less remove DNAPL, bioremediation (enhanced or
otherwise) is not considered a practical means of achieving cleanup of the Site.
Barrier Walls - The lateral migration of contaminants in the saturated zone can be
impeded by the construction of low permeability, fine-grained barrier walls (i.e.,
slurry walls, concrete walls, sheet piling with grouted joints, etc.). These walls
should be keyed into a low permeability, capillary barrier beneath the contaminant
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(DNAPL) source to prevent vertical DNAPL migration. The walls are typically
installed as an enclosure around the contaminated zone. Non-enclosing walls are
typically installed downgradient of the contaminated zone, but require hydraulic
controls (i.e., ground water extraction wells placed on the upgradient side of the
barrier wall) to prevent contamination from'flowing around the ends of the wall.
Barrier walls would probably not be effective at the Site due to the absence of low
permeability deposits down to the depth that DNAPL extends (the walls would
need to be keyed into the crystalline bedrock). A barrier wall would require deep
installation (up to 100 feet) and would need to be nearly 2,500 feet in length to
encircle all areas that probably contain DNAPL, and would therefore be very
expensive and disruptive to the wetlands. Barrier walls would also not be effective
in decreasing the time necessary to dissolve DNAPL without employment of other
remedial technologies, which, hi turn, are dependent on detailed (and probably
unattainable) knowledge of the distribution of DNAPL.
Reactive Walls - Reaction walls are high permeability walls that improve water
quality by facilitating chemical or biological reactions in ground water passing
through the wall. An important factor hi the design and effectiveness of reaction
walls is the retention tune that contaminated ground water requires for a complete
reaction. In areas where flow velocities are relatively rapid, the barrier wall needs
to be thick or there needs to be auxiliary ground water controls (i.e., ground-water
extraction). The long-term effectiveness, cost and maintenance requirements for
reaction walls has not been thoroughly analyzed on the site scale. Further, the
technology would not promote more rapid dissolution of DNAPL and hence
would not provide for a shortened time frame for attainment of Cleanup Levels.
Finally, based on rapid buildup of iron bacteria slime on extraction wells at the
Site, there is concern that clogging would be a problem.
Immobilization - Subsurface contaminants can be contained or immobilized
through the processes of solidification, stabilization and/or in-situ vitrification. In
order to be effective, use of solidification and stabilization would require
knowledge of all portions of the aquifer containing DNAPL. Further, during
solidification/stabilization processes, the potential for migration of DNAPL would
be high. Perhaps to a lesser extent, the same can be said for vitrification.
Solidification/stabilization/vitrification methods are very expensive, and would
require significant disruption of facility operations, particularly to address
DNAPL under the building. For these reasons, solidification, stabilization and
vitrification are not considered to be practical remedial technologies for
restoration of the aquifer at the Site.
Soil Flushing - Soil flushing includes methods to flood the contaminated zone
with flushing solutions to sweep the contaminants to recovery wells, drains, or
high permeability reaction walls. Flushing methods applicable to DNAPLs
include chemically-enhanced displacement and dissolution and steam
displacement. The complex stratigraphy and uncertainties about the distribution
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of DNAPL would make it impossible to design effective DNAPL collection
systems at the Site. These factors also make delivery of dissolution agents into all
areas containing DNAPL problematic. Finally, the complex stratigraphy and
DNAPL distribution, expenses of installing and maintaining delivery systems, and
the expense of displacement/dissolution agents makes the remedial technologies
impractical for restoring ground water at the Site.
Natural Attenuation - Natural attenuation involves a passive approach to site
cleanup, utilizing the natural processes of in-situ biodegradation and the dilution
effects of ground water recharge and discharge. Costs for natural attenuation
would be significantly less than those estimated for the existing treatment system.
However, as gradients would be lower in a naturally attenuating system than in a
system subjected to ground-water extraction, dissolution of DNAPL would be
considerably slower. Natural attenuation would also likely result in a plume size
and location similar to that seen before initiation of pumping, and therefore, the
South Well could not be operated for extended periods. Finally, the dilute plume
area would remain contaminated for as long as DNAPL exists in the source areas
(probably for hundreds of years) so that ground water within the plume could not
be used.
5. Additional Considerations
The area being considered for the Technical Impracticability Waiver lies entirely on NHBB
property. Overlying land use is not compatible with domestic water supplies. The
manufacturing facility, parking lots and access roads, and a wetland are above the aquifer. The
original ROD foresaw the difficulties with remediating the heavily contaminated ground water
near the NHBB facility and the problems which that would cause with the timely remediation of
the down-gradient ground water. The concern was addressed by requiring that the two plume
areas be hydraulically separated from each other and from the South Well. This Technical
Impracticability Waiver will allow for the use of the South Well and for restoration of the ground
water outside of the waiver area while acknowledging the technical impracticabiliy of restoring
that portion of the aquifer which is impacted by DNAPLs..
6. Costs
Cost savings have been projected for the lower pumping rate and one-well extraction scenario
which may be possible as a result of the waiver of ARARs. The savings is the result of lower
O&M costs. The total present worth cost savings is approximately $3.5 million over the next
thirty years.
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RESPONSZVBHESSS SUMMARY FOR THE EXPLANATION OF SIGNIFICANT DIFFERENCES
FOR THE SOUTH MUNICIPAL HATER SUPPLY HELL SUPERFUND SZTE
February 3, 1997
PREFACE
The U.S. Environmental Protection Agency (EPA) held a public comment period, from
November 22, 1996, to December 23, 1996, to provide an opportunity for interested
parties to comment on the draft Explanation of Significant Differences (ESD) and
Technical Impracticability (TI) Waiver prepared for the South Municipal Hater Supply
Hell Superfund Site (the Site) in Peterborough, New Hampshire. The draft ESD
presented EPA's changes to the ground water remedy resulting from the determination
that it is technically impractical to restore all of the aquifer to drinking water
standards.
The purpose of this responsiveness summary is to identify major comments raised during
the public comment period and to provide EPA response to the comments. EPA has
considered all of the comments summarized in this document before issuing the final
ESD for the Site in Peterborough, New Hampshire.
I. OVERVZEH OF THE SIGNIFICANT CHANGES
EPA has determined that it is technically impracticable from an engineering
perspective to restore that portion of the contaminated ground water affected by
DNAPLs to drinking water quality in a reasonable time frame. Therefore, this ESD
documents EPA's decision to waive certain applicable or relevant and appropriate
requirements (ARARs) for ground water. Because of the determination of technical
impracticability, three portions of the remedy are modified by this ESD:
1. Groundwater Extraction and Treatment
Two aspects of the ground water extraction and treatment component of the remedy will
be modified.
Air Sparging - The ROD stated that it might be necessary to implement
technologies to enhance contaminant removal and to address the presence of free
phase solvents in the saturated zone of the NHBB-area plume. Air sparging (in
conjunction with the soil vacuum extraction system) was the selected technology.
Because of technical problems encountered in implementing the air sparging
system, it will not be operated so as to prevent further contamination problems.
Ground Hater Extraction - The ROD specified that the ground water extraction
system for the NHBB area would be designed to create a hydraulic barrier between
the NHBB area plume and the rest of the aquifer. A time period of 19 to 32
years was estimated to be needed to restore that portion of the aquifer to
drinking water quality. Since ARARs are to be waived, the pumping rates and
extraction well configuration will be changed to maintain the hydraulic barrier
between the NHBB plume area and the rest of the aquifer, but not necessarily to
restore the NHBB plume to drinking water quality. Adjustments to the system
will be made to allow for the use of the South Well if the Town of Peterborough
elects to use it.
2. In-Situ Vacuum Extraction of Contaminated Soils
Since no soil contact threat was identified, the ROD prescribed a vacuum
extraction system (VES) to remediate soils located near the corner of the NHBB
facility solely to allow attainment of ground water cleanup levels. Therefore,
since as described above, no air sparging will be employed and the ground water
ARARs will be waived, vacuum extraction is no longer required and may be
discontinued.
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3. Long-Term Environmental Monitoring
Ground water monitoring (quality and water levels) will be conducted to
determine if the ground water within the Technical Impracticability Waiver area
is hydraulically contained. Based on the analysis of the monitoring data,
adjustments to the extraction system will be made to ensure that containment is
maintained. This will be especially important should the Town elect to re-use
the South Municipal Water Supply Well.
4. Institutional Controls
In order to further ensure the protectiveness -of the remedy, a deed restriction
will be placed upon the NHBB property prohibiting extraction of the ground water
for purposes other than the remedial action unless the extracted ground water
meets or is treated to appropriate use standards and the extraction of the
ground water does not adversely affect the remedial action.
II. SUMMARY OF COMMENTS RECEIVED DURING THE PUBLIC COMMENT PERIOD AND EPA RESPONSES
This responsiveness summary addresses the comments received by EPA concerning the ESD
and TI Waiver for the Site. Only comments from the Potentially Responsible Party, New
Hampshire Ball Bearings, Inc. (NHBB), were received. Copies of the comments are also
available at the Peterborough Public Library and at the EPA Records Center at 90 Canal
Street, Boston, Massachusetts, as part of the Administrative Record.
The comments and EPA responses are presented below.
1. Comment: NHBB pointed out that the eastern boundary of the TI Waiver area should
extend to the eastern limit of the property (Lot 22 of the Peterborough R-3 Tax Map)
controlled by NHBB. This would place the eastern boundary fifty feet to the west of
the centerline of Route 202.
Response: This was EPA's intent. EPA has adjusted the boundary in the ESD and TI
evaluation.
2. Comment: NHBB notes that the discussion of the "NHBB plume area" and the "dilute
plume area" causes confusion since the TI Waiver area overlaps a portion of the dilute
plume area. The TI Evaluation indicates that the dilute plume area can be fully
restored, however.
Response: EPA acknowledges the confusion and has modified the text in the ESD and
in the TI Evaluation and included the "Figure 1" provided by NHBB which eliminates the
plume designations.
3 Comment: NHBB disagrees with that element of the institutional control which would
require treating any ground water extracted from the waiver area to drinking water
standards. NHBB maintains that no requirement is needed in the discussion of a deed
restriction, but if it were, treatment to drinking water standards would be
unreasonable if the water were not used for human consumption.
Response: The intent of the institutional control is to safeguard potential
future users of the ground water by emphasizing the existence of water unsuitable for
human consumption. However, the imposition of a requirement to treat to drinking
water standards regardless of the intended disposition of the extracted water is not
necessary. EPA has modified the standard to treat to "appropriate water use and/or
disposal standards in effect at the time of extraction."
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