Record of Decision T-25 Area Ground Water (APR 2001).pdf

Record of Decision

T-25 Area Ground Water (Operable Unit 1) U.S. Army Soldier Systems Center (SSC) Natick, Massachusetts

Submitted to:

U.S. ARMY SOLDIER SYSTEMS CENTER

NATICK, MASSACHUSETTS

April 2001

U.S. Army Soldier Systems Center

Record of Decision

T-25 Area Ground Water (Operable Unit 1) U.S. Army Soldier Systems Center

Submitted to

U.S. Army Soldier Systems Center

Natick: Record of Decision Section No.: Table of Contents Date: April 2001

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Table of Contents

PART 1: DECLARATION FOR THE RECORD OF DECISION

1.1 Site Name and Location

1.2 Statement of Basis and Purpose

1.3 Assessment of the Site

1.4 Description of Selected Remedy

1.5 Statutory Determinations

1.6 Record of Decision Data Certification Checklist

1.7 Authorizing Signatures

PART 2: DECISION SUMMARY

2.1 Site Name, Location, and Description

2.2 Site History and Enforcement Activities

2.3 Community Participation

2.4 Scope and Role of Operable Unit or Response Action

2.5 Site Characteristics

2.6 Current and Potential Future Site and Resource Uses

2.7 Summary of Site Risks

2.7.1 Human Health Risk Assessment

2.7.2 Ecological Risk Assessment

2.8 Remediation Objectives

2.9 Development and Screening of Alternatives

2.10 Description of Alternatives

2.11 Summary of Comparative Analysis of Alternatives

2.12 Selected Remedy

2.13 Statutory Determinations

2.13.1 Protection of Human Health and the Environment

2.13.2 Compliance with Applicable or Relevant and Appropriate Requirements

2.13.3 Cost Effectiveness

2.13.4 Utilization of Permanent Solutions and Alternative Treatment Technologies to the Maximum Extent Practicable

2.13.5 Preference for Treatment as a Principal Element

2.13.6 Five-Year Review Requirements

2.14 Documentation of Significant Changes from Preferred Alternative of Proposed Plan

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PART 3: RESPONSIVENESS SUMMARY

PART 4: REFERENCES

List of Tables

Table 1: Chemicals of Concern and Cleanup Goals Table 2. Human Health Risk Assessment Sections Table 3: T-25 Area Ground Water Chemicals of Concern Table 4: Selected Chemicals of Concern and Estimated Risks at the T-25 Area ... 31 Table 5: Potentially Exposed Populations Considered in the Human Health Risk Assessment Table 6: Summary of Toxicity Values Table 7: Risk Results for Ground Water Chemicals of Concern – Upper Bound Adult Estimates Table 8: Remedial Alternative Cost Summary Table 9: Comparison of Alternatives with CERCLA Evaluation Criteria Table 10: Summary of Selected Remedy Costs Table 11a: Capital Costs – Selected Remedy Table 11b: Annual Operating Costs – Selected Remedy Table 12: Cleanup Levels for Chemicals of Concern Table 13: ARARs, Criteria, Advisories, and Guidance Table 14: Matrix of Cost and Effectiveness Data for Operable Unit Number 1 - T-25 Area Ground Water Alternatives

List of Figures

Figure 1: Site Location Map Figure 2: U.S. Army Soldier Systems Center Facility Map Figure 3: Conceptual Site Model Figure 4: Geologic Cross Section Figure 5: PCE and TCE Concentrations in B-Interval Ground Water Figure 6: Treatment System for T-25 Area Ground Water

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List of Appendices

Appendix A: Administrative Record Index

Appendix B: Memorandum of Agreement Between and Among the Town of Natick, Massachusetts and the United States Army Materiel Command, Soldier and Biological Chemical Command

Appendix C: Town of Natick Board of Health Ordinance

Appendix D: Preliminary Ground-Water Flow and Transport Modeling Results for the

T-25 Area at Soldier Systems Command – Letter Report

Appendix E: Concurrence Letter from Massachusetts Department of Environmental

Protection

Appendix F: Cooperative Agreement Between the U.S. Army Soldier Systems Center and the Town of Natick

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List of Acronyms and Abbreviations

ARAR Applicable or Relevant and Appropriate Requirements bgs Below Ground Surface CERCLA Comprehensive Environmental Response, Compensation, and Liability Act COC Chemical of Concern CSF Cancer Slope Factors DCE Dichloroethene EPA United States Environmental Protection Agency ERA Ecological Risk Assessment FFS Focused Feasibility Study GAC Granular Activated Carbon gpm Gallons Per Minute GW-1 Ground Water 1 HEAST Health Effects Assessment Summary Table HHRA Human Health Risk Assessment HI Hazard Index HQ Hazard Quotient IRIS Integrated Risk Information System MADEM Massachusetts Department of Environmental Management MADEP Massachusetts Department of Environmental Protection MCL Maximum Contaminant Level MCLG Maximum Contaminant Level Goal MCP Massachusetts Contingency Plan MNA Monitored Natural Attenuation msl Mean Sea Level NCP National Contingency Plan NPV Net Present Value O&M Operation and Maintenance OSWER Office of Solid Waste and Emergency Response PAH Polynuclear Aromatic Hydrocarbon PCB Polychlorinated Biphenyl PCE Perchloroethene or Tetrachloroethene POC Point of Compliance POL Petroleum, Oil, and Lubricant RAB Restoration Advisory Board RAO Remedial Action Objective RBC Risk-Based Concentration RCRA Resource Conservation and Recovery Act RfD Reference Dose RI Remedial Investigation

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List of Acronyms and Abbreviations (cont.)

ROD Record of Decision SARA Superfund Amendment and Reauthorization Act SSC United States Army Soldier Systems Center SVOC Semivolatile Organic Compound TAG Technical Assistance Grant TCE Trichloroethene TS Treatability Study UCL Upper Confidence Limit UV Ultraviolet VOC Volatile Organic Compound

Part: 1

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PART 1: DECLARATION FOR THE RECORD OF DECISION

1.1 Site Name and Location

Operable Unit 1 - T-25 Area Ground Water Natick Laboratory Army Research, Development, and Engineering Center National Priorities List Site U.S. Army Soldier Systems Center (SSC) Natick, Massachusetts CERCLIS FD No. MA121002063

1.2 Statement of Basis and Purpose

This decision document presents the U.S. Army’s Selected Remedy for Operable Unit 1 (OU-1), the T-25 Area ground water at the Natick Laboratory Army Research, Development, and Engineering Center National Priorities List Site, at the U.S. Army Soldier Systems Center (SSC) in Natick, Massachusetts (Site). It was developed in accordance with the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), as amended by the Superfund Amendments and Reauthorization Act of 1986 (SARA and to the extent practicable, the National Oil and Hazardous Substances Contingency Plan (NCP), 40 CFR Part 300 et seq., as amended;

as well as in accordance with the Department of the Army policy. The Assistant Chief of Staff for Installation Management or the Deputy Assistant Secretary of Army for Environmental, Safety and Occupational Health, and the Director of the U.S.

Environmental Protection Agency (EPA) New England Region have been delegated the authority to approve this Record of Decision (ROD). This decision is based on the Administrative Record that has been developed in accordance with Section 113(k) of CERCLA. The Administrative Record is available for public review at the SSC Environmental, Safety and Health Office, Building 4, Soldier Systems Center, Natick, Massachusetts. Information repositories are at the Natick Board of Health (Town Hall, 13 Central Street, Natick, MA 01760), the Massachusetts Department of Environmental Protection (MADEP) (Bureau of Waste Site Cleanup, Federal Facilities, 1 Winter Street, 7th Floor, Boston, MA 02108), and the Morse Institute (Reference Section, 14 East Central Street, Natick, MA 01760). The Administrative Record Index (Appendix A of this ROD) identifies each of the items considered during the selection of the remedial action.

The Commonwealth of Massachusetts concurs with the selected remedy.

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1.3 Assessment of the Site

The response action selected in this ROD is necessary to protect the public health or welfare or the environment from actual or threatened releases of hazardous substances into the environment.

1.4 Description of Selected Remedy

The selected remedial action addresses potential future exposure to contaminated ground water, the principal known threat at Operable Unit 1 the T-25 Area ground water. Several remedies were considered for this remedial action. The remedy selected consists of ground water extraction to contain the primary area of ground water contamination, followed by treatment using air stripping and carbon adsorption, with discharge of treated ground water to Lake Cochituate or other alternative uses approved by EPA and MADEP; institutional controls; monitored natural attenuation (MNA), and long-term monitoring. The remedy contains and treats the T-25 Area ground water contamination, and prevents potential future exposure to ground water with contaminant concentrations in excess of federal and more stringent state drinking water standards. The ground water extraction and treatment technology is one of EPA’s preferred approaches for treating dissolved volatile organic compound (VOC)-contaminated ground water. This technology was selected using the EPA's presumptive remedy guidance for streamlining remediation of CERCLA sites. The extraction and treatment portion of this alternative began operation in November 1997 as part of the Treatability Study (TS), and continues to operate.

The selected remedy includes the following major components:

• Ground water extraction and treatment

− Extraction of ground water from extraction wells − Treatment of the contaminated ground water in an air stripper to transfer VOCs to an off-gas stream (vapor effluent) − Secondary treatment of the air stripper's aqueous effluent by granular activated carbon (GAC) adsorption to reduce organic contaminants to concentrations less than established cleanup levels

− Treatment of the air –stripper’s vapor effluent containing the stripped volatile contaminants in a vapor-phase GAC adsorbent system

− Discharge of treated ground water to Lake Cochituate or other alternative uses approved by EPA and MADEP

• Long-term ground water monitoring including MNA parameters both on-facility and off-facility

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• Implementation of on-facility and off-facility institutional controls to restrict access to the contaminated aquifer. A town of Natick Board of Health ordinance will prohibit both the installation of new private drinking water wells and the use of existing private drinking water wells in a prescribed area around the facility, while the Army’s Master Plan for SSC will restrict the on-facility use of ground water. If the SSC property is transferred out of federal ownership, the United States will impose appropriate enforceable land-use restrictions through the inclusion of appropriate restrictions (e.g., restrictive covenants and/or easements) in all deeds or other transfer documents relating to that property.

• MNA

• Five-year reviews until cleanup levels are attained

• Army support of a portion of the operation and maintenance of the air stripping system at the town of Natick’s Springvale Treatment Plant

The selected remedy for Operable Unit 1 addresses the ground water associated with the T-25 Area, which contains elevated concentrations of the principal threat contaminants (PCE and TCE) and other secondary contaminants. The remedy will use ground water extraction to contain and clean up the T-25 Area ground water, prevent contamination from reaching downgradient receptors, pull back some off-facility contamination, and treat (with air stripping and carbon adsorption) the extracted ground water and air streams to established cleanup levels prior to discharge. Natural attenuation processes will also be actively monitored to ensure that cleanup goals are being met for any contamination outside the influence of the treatment system, and any residual contamination within the T-25 Area remaining after the treatment system is shut down due to diminishing returns. Further, institutional controls will prohibit anyone from using both the contaminated on-facility and off-facility ground water.

To further protect the drinking water of the town of Natick, the Army will support a portion of the operation and maintenance of the air stripping system at the town’s Springvale Treatment Plant. Agreements between the Army and the Town of Natick were developed through negotiations between the Town and the Army, and are presented in Appendices B and F. The agreements include several provisions including:

1) the Army will provide the Town with a one-time payment of $3.1 million, 2) agreements by the Town to continue operation of the Springvale Treatment Plant, and the Army to continue operation of source area containment of contaminated ground water at the T-25 Area site, and 3) agreement by the Town of Natick to impose institutional controls in the area for the full time of the cleanup. These agreements allow the selected remedy to avoid building redundant systems and to leverage existing systems already in place.

Other areas at SSC, in addition to Operable Unit 1, either currently under investigation or previously addressed under separate CERCLA actions include: T-25 Area Storage Area (past soil removal action), T-25 Area Storm Water Outfall Area, Former Proposed Gymnasium Site, Boiler Plant Site, the SSC Water Supply Wells Site, and the Main

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Storm Water Outfall. Field investigations for many of these other study areas at SSC have already been completed, however, at this time, remedial activities have not been planned. Operable Unit 1 is the second response action for the NPL Site and addresses the ground water contamination associated with the T-25 Area. The first response action at the NPL Site addressed an isolated area of pesticide-contaminated soil from a small Storage Area within the T-25 Area, that posed an ecological risk. Approximately 1,000 cubic yards of contaminated soil was removed under a separate removal action.

No current soil source area for the observed ground water contamination (PCE and TCE) associated with the T-25 Area was found during the investigations. Therefore, no action for soils is required under this ground water ROD. The remaining soils associated with the T-25 Area do not pose an unacceptable incremental human health or ecological risk. However, some small areas of soil associated with other operable units at the NPL Site are under study for a possible limited soil removal action.

1.5 Statutory Determinations

The selected remedy is protective of human health and the environment, complies with Federal and State requirements that are applicable or relevant and appropriate (ARAR) to the remedial action, and is cost effective. The remedy utilizes permanent solutions and alternative treatment technologies to the maximum extent practicable.

This remedy also satisfies the statutory preference for treatment as a principal element of the remedy (i.e., reduce the toxicity, mobility, or volume of materials comprising principal threats through treatment).

Because this remedy will result in hazardous substances remaining on-facility above levels that allow for unlimited use and unrestricted exposure (for a period of time estimated at about 27 years), a review will be conducted within five years after initiation of remedial action, and every five years hence until cleanup goals are attained, to ensure that the remedy continues to provide adequate protection of human health and the environment.

1.6 Record of Decision Data Certification Checklist

The following information is included in the Decision Summary section (Section 2) of this ROD.

• Chemicals of concern (COCs) and their respective concentrations

• Baseline risk presented by COCs

• Cleanup levels established for COCs and the basis for the levels

• Current and future land and ground water use assumptions used in the baseline risk assessment and ROD

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• Land and ground water use that will be available at the site as a result of the Selected Remedy

• Estimated capital, operation and maintenance (O&M), and total present worth costs; discount rate; and the number of years over which the remedy cost estimates are projected

• Decisive factor(s) that led to selecting the remedy (i.e., describe how the Selected Remedy provides the best balance of tradeoffs with respect to the balancing and modifying criteria)

Additional information can be found in the Administrative Record file for this site.

1.7 Authorizing Signatures

1.7.1 U.S. Department of the Army Authorizing Signatures

The foregoing represents the selection of a remedial action by the U.S. Department of the Army and the U.S. Environmental Protection Agency, with the concurrence of the Commonwealth of Massachusetts Department of Environmental Protection.

Concur and recommend for immediate implementation:

Raymond J. Fatz Deputy Assistant Secretary of Army (Environment, Safety and Occupational Health)

OASA (I&E)

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1.7.2 U.S. Environmental Protection Agency Authorizing Signatures The foregoing represents the selection of a remedial action by the U.S. Department of the Army and the U.S. Environmental Protection Agency, with the concurrence of the Commonwealth of Massachusetts Department of Environmental Protection.

Concur and recommend for immediate implementation:

Patricia L. Meaney Director Office of Site Remediation and Restoration U.S. Environmental Protection Agency New England Region

Part: 2

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PART 2: DECISION SUMMARY

2.1 Site Name, Location, and Description

Site Name: T-25 Area Ground Water at Natick Laboratory Army Research, Development, and Engineering Center National Priorities List Site Site Location: U.S. Army Soldier Systems Center (SSC), Natick, Massachusetts

CERCLIS ID: MA121002063

Lead Entity: U.S. Army Site Type: Active facility

The T-25 Area site is one of several sites at the Natick Laboratory Army Research, Development, and Engineering Center National Priorities List Site (Site). The T-25 Area is located approximately 17 miles west-southwest of Boston at the U.S. Army Soldier Systems Center (SSC) in Natick, Massachusetts. SSC is an active research facility, which occupies a small peninsula extending from the eastern shoreline of the South Pond of Lake Cochituate, and encompasses approximately 78 acres. Lake Cochituate is made up of three connected ponds (North, Middle, and South Ponds). The land use surrounding SSC includes residential, commercial/retail, and light industrial areas. The facility is located approximately 2,500 feet southeast of the Springvale Municipal Water Supply Well Field (Springvale Well Field). A site location map is provided in Figure 1 and the SSC facility map is provided in Figure 2.

The T-25 Area, named because Building T-25 is located there, is a 15.6-acre rectangular area in the northwestern portion of the SSC facility. Most of the T-25 Area ground surface is covered by buildings or asphalt. Many of the buildings are temporary.

The area is ringed by an unpaved road on an embankment approximately 10 feet above the base of the site. The embankment rises an additional 10 feet above the dirt road and is topped by a chain-linked fence, which abuts residential properties. The only open, uncovered areas include a baseball field for employee use located in the northwest corner of the site, and the unpaved perimeter road and embankment. The T-25 Area is bounded to the west, north, and east by residential properties; it is bounded to the south by the rest of the SSC facility.

A more complete description of the facility and the T-25 Area can be found in the Phase II T-25 Area Remedial Investigation Report (Arthur D. Little, Inc., 1998) on pages 1-2 through 1-19.

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2.2 Site History and Enforcement Activities

Site History SSC has previously been called the Quartermaster Research and Engineering Command; the U.S. Army Natick Research and Development Command; the U.S. Army Natick Research, Development, and Engineering Center; and the Soldier Systems Command. SSC has been a permanent Army installation since October 1954.

Its mission includes research and development activities in food engineering, food science, clothing, equipment, and materials engineering, and aero-mechanical engineering.

The SSC location was selected in 1949 for development of a research facility from 340 possible sites in 40 states by a special site-selection committee appointed by the Secretary of Defense. The town of Natick was selected because of its proximity to the nation's capital and to one of the nation's largest scientific and educational communities. Approval for construction of the facility was granted in October 1949 and construction was completed in October 1954. Much of the topography at the facility was modified during its construction and development.

The T-25 Area buildings are built upon a filled and regraded former gravel pit in the northern portion of SSC that was owned and operated by the town of Natick. The gravel pit was regraded prior to development. The T-25 Area is almost completely paved with the exception of the baseball field and the unpaved perimeter road and embankment. The location of the T-25 Area is illustrated in Figure 2.

Present and past operations within the T-25 Area include: quarrying; indoor and outdoor storage of bulk items, wastes, petroleum, solvents, antifreeze, pesticides, and Freon 113; warehouse operations (shipping and receiving); laboratory research activities including the testing of petroleum, oil, and lubricant (POL) pumping equipment, refrigeration units, and various types of fuel in engines; clothing and textile research; drop-testing; waste incineration; and garage operations including spray painting, vehicle maintenance, insect and rodent control, metal parts and brush cleaning, battery charging, silk screening, and rubber adhesive thinning.

Perchlorethylene (PCE) and trichloroethene (TCE) have been used in the past at the facility for metal parts washing in the garage (Building 14), for dry cleaning in the mobile dry cleaning machines in the T-25 Area, fabric treatment in Building 5 (located just to the south of the T-25 Area), and in relatively smaller amounts in the various laboratories within the facility (in Buildings 4, 7, and 8). TCE was used in the largest volumes in the climatic control chamber (Building 2) located in the south-central portion of the facility, where it was used in a cooling brine solution, and stored in three

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1,000-gallon tanks. The climatic control chambers have been remodeled and TCE has been replaced with diethyl benzene (Dowtherm J). PCE and TCE have been used at the facility from the time it was constructed (1954) to the present, in volumes that have steadily decreased over time (presently less than 10 gallons per year for the entire facility). Prior to the facility's construction, the T-25 Area was a town-owned gravel pit. PCE/TCE use during this time is unknown. A more complete description of the facility and the T-25 Area can be found in the Phase II T-25 Area Remedial Investigation Report (Arthur D. Little, Inc., 1998) on pages 1-2 through 1-19.

Federal and State Investigations A series of investigations have been conducted at SSC and the T-25 Area. A description of the investigations listed below are presented in the Final Phase II Remedial Investigation Report: T-25 Area at the U.S. Army Soldier Systems Command (SSCOM), Natick, Massachusetts (Arthur D. Little, Inc. 1998).

• 1980 Installation Assessment - U.S. Army Toxic and Hazardous Materials Agency.

1980. Installation Assessment of U.S. Army Natick Research and Development Command. Report No. 170. May.

• 1989 and 1990 Soil Gas Surveys - Northeast Research Institute, Inc. 1990. Phase II Soil Gas Survey Conducted at the U.S. Army Research, Development, and Engineering Center in Natick, Massachusetts. April.

• 1991 Expanded Site Investigation - Dames & Moore, Inc. 1991. Expanded Site Inspection (ESI) of Natick Research, Development and Engineering Center.

• 1991 Underground Storage Tank Removal Data – summarized in Argonne National Laboratory. 1993. Master Environmental Plan for the U.S. Army Natick Research, Development and Engineering Center, Natick, Massachusetts. January.

• 1992 Expanded Site Investigation Addendum - U.S. Army Toxic and Hazardous Materials Agency. 1992. Addendum Expanded Site Inspection (ESI) of Natick Research, Development and Engineering Center.

• 1993 Master Environmental Plan - Argonne National Laboratory. 1993. Master Environmental Plan for the U.S. Army Natick Research, Development and Engineering Center, Natick, Massachusetts. January.

• 1992 U.S. Army Environmental Center Investigation – Installation and Sampling of 16 Ground Water Monitoring Wells. Results reported in the Phase I Report below.

• 1993/1994 U.S. Army Environmental Center Phase I Remedial Investigation/Feasibility Study- Arthur D. Little, Inc. 1996b. Final Phase I Remedial Investigation Report, T-25 Area at the U.S. Army Natick Research, Development, and Engineering Center (Natick), Natick, Massachusetts. August.

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• Storage Area Investigation – Arthur D. Little, Inc. 1996a. Action Memorandum -

Storage Area. June.

• 1995/1996 U.S. Army Environmental Center Phase II Remedial

Investigation/Feasibility Study – Arthur D. Little, Inc. 1998. Final Phase II Remedial Investigation Report: T-25 Area at the U.S. Army Soldier Systems Command (SSCOM), Natick, Massachusetts December.

Also, the Final Focused Feasibility Study/Treatability Study Report: T-25 Area at the U.S. Army Soldier Systems Center (SSC), Natick, Massachusetts was completed in September 1999.

History of CERCLA Enforcement Activities In May 1993, SSC, then known as the Natick Laboratory Army Research, Development, and Engineering Center was proposed for inclusion on the NPL. It was officially added to the NPL in May 1994 as a result of ground water contamination found at the T-25 Area and its location relative to the town of Natick Springvale Municipal Water Supply Well Field. The Phase I T-25 Area RI was completed in August 1996 and the Phase II T-25 Area RI was completed in December 1998. A Focused Feasibility Study/Treatability Study (FFS/TS) for the T-25 Area ground water was completed in Fall of 1999. The Proposed Plan detailing the Army’s preferred remedial alternative for the T-25 Area ground water was presented to the public in September of 1999.

2.3 Community Participation

Throughout the NPL Site's history, community concern and involvement has been high.

The Army has kept the community and other interested parties apprised of the NPL Site activities through informational meetings, fact sheets, open houses, press releases, public meetings, Information Repositories, and phone, e-mail, letter, and personal access to Army representatives.

A community relations plan for SSC was prepared in December 1995 and updated in July 1996. The plan outlined a program to 1) educate and inform all interested parties about remedial activities, any potential impacts posed by the site, enforcement activities, and opportunities for public involvement; 2) listen to concerns of organized groups, individuals, and town officials regarding proposed work plans, schedules, remedial activities, and health and environmental concerns; and 3) address community concerns in a timely manner and incorporate public concerns, to the extent possible, in selecting, designing, and implementing the remedial actions.

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The public has been involved in a number of different ways. They are kept informed through regular newsletters and open houses, and they have direct involvement as a result of a Restoration Advisory Board (RAB). A RAB was formed for SSC in 1995, with members from the public at large (including SSC neighbors and town representatives), SSC facility staff, and federal and state regulators. Since its inception there have been monthly meetings. The purpose of the RAB is not only informational, but also to actively solicit input from local stakeholders to ensure that any cleanup actions address community concerns. The RAB has allowed the community easy access to the remediation process, kept the community informed, and given them the opportunity to make recommendations that affect the community. Members of the RAB reviewed the planning and report documents, and provided valuable comments and insights on all phases of the process, from investigation through cleanup. In addition, the Army continues to provide the RAB with a list of potential response actions that could be taken at various locations at SSC, and asks for the RAB members to rank the potential actions in terms of importance. The rankings are taken into account by the Army when determining budgetary decisions for the upcoming years.

As part of the CERCLA process, a Technical Assistance Grant (TAG) was made available to a local community group, the Lakewood Association, for use in hiring an independent consultant to provide technical review/evaluation of activities at SSC. The Lakewood Association hired a consultant (Environmental Insight) in 1998, and since then, the consultant has reviewed and commented on a number of planning and report documents produced in association with the T-25 Area.

In March 1993, the Army made the administrative record available for public review.

The documents in the administrative record have been available to the public since 1992. The administrative record is maintained at SSC, and at information repositories at the Morse Institute (Natick Public Library), the Natick Board of Health, and the Massachusetts Department of Environmental Protection (MADEP). The Army published a notice and brief analysis of the Proposed Plan in the MetroWest Daily News on August 23, 1999, September 22, 1999, and September 23, 1999, and the Natick Tab on August 24, 1999 and September 21, 1999, and made the plan available to the public at the information repositories. On September 9, 1999, the Army held an Informational Meeting to discuss the results of the Remedial Investigation and the cleanup alternatives presented in the Focused Feasibility Study and to present the Army’s Proposed Plan. Also during this meeting, the Army answered questions from the public.

The community TAG consultant also presented their comments at the informational meeting. On September 23, 1999, the Army held a Public Hearing to discuss the Proposed Plan and to accept any oral comments. From August 23 to October 1, 1999, the Army held a 30-day public comment period to accept public comment on the alternatives presented in the Focused Feasibility Study and the Proposed Plan and on any other documents previously released to the public.

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2.4 Scope and Role of Operable Unit or Response Action

As with many Superfund sites, the problems at SSC are complex. A number of investigations have been completed or are currently ongoing at SSC. This ROD is being written for Operable Unit 1, the T-25 Area ground water. Other areas of SSC either currently under investigation or previously addressed under separate actions include:

Past response actions:

• T-25 Area Storage Area (past soil removal action)

Activities proposed in this ROD:

• Operable Unit 1: T-25 Area Ground Water

Future response plans:

• T-25 Area Storm Water Outfall Area

• Former Proposed Gymnasium Site

• Boiler Plant Site

• SSC Water Supply Wells Site

• Main Storm Water Outfall Area

The first response action at the NPL Site addressed an isolated area of pesticide-contaminated soil from a small Storage Area within the T-25 Area, that posed an ecological risk. Approximately 1,000 cubic yards of contaminated soil was removed under a separate removal action. No current soil source area for the observed ground water contamination (PCE and TCE) associated with the T-25 Area was found during the investigations. Therefore, no action for soils is required under this ground water ROD. The remaining soils associated with the T-25 Area do not pose an unacceptable incremental human health or ecological risk. However, some small areas of soil associated with other operable units on-facility are under study for a possible limited soil removal action.

Operable Unit 1, the subject of this ROD, addresses the ground water contamination associated with the T-25 Area. The concentrations of PCE and TCE, and other secondary chemicals of concern in the T-25 Area ground water exceed federal and more stringent state drinking water criteria and acceptable risk levels for a future ingestion of ground water use scenario. This operable unit is the second response action for the NPL Site and addresses the ground water contamination at the NPL Site through the extraction and treatment of ground water.

Field investigations for the other study areas at this NPL Site have been performed, however, at this time, remedial activities have not been planned.

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2.5 Site Characteristics

A summary of the site characteristics is provided below. A more detailed analysis of the site characteristics can be found in the Phase II T-25 Area Remedial Investigation Report (Arthur D. Little, Inc., 1998).

Conceptual Model The conceptual model for the site is illustrated on Figure 3. This model provides the basis for the risk assessments and response actions taken at the site. This ROD is focused on the T-25 Area ground water. Other media in the T-25 Area, including soil, air, and surface water, do not pose risks to human health and/or the environment.

Sediment near the T-25 Area outfall in Lake Cochituate is being addressed separately from the ground water.

Based on the volatile organic carbon (VOC) contamination found in ground water beneath the T-25 Area, estimated noncancer and cancer risks exceeded the EPA’s acceptable risk levels for future residential ground water ingestion and for dermal contact during future industrial use. These results indicate that ground water is a priority for cleanup.

Site Overview SSC in the town of Natick, Massachusetts, is a research facility occupying a small peninsula extending from the eastern shoreline of Lake Cochituate and is about 74 acres in size. One surface water body, Little Roundy Pond, is located within the northeastern corner of the facility. Wetlands surround Little Roundy Pond and skirt the shoreline of the Lake Cochituate.

The T-25 Area, named because Building T-25 is located there, is a 15.6-acre rectangular area located in the northwestern portion of the SSC facility. The T-25 Area is a former gravel pit, which was filled with soil and construction debris prior to the development of the overall T-25 Area in the 1950s. The current area consists of paved storage space with permanent and temporary shelters and a ball field.

Figure 3 U.S. Army SSC Conceptual Site Model for Contaminated Ground Water

Primary Sources

Preliminary Release

Mechanism

Secondary Sources

Secondary Release

Mechanism Pathway

Exposure Route

Area Residents

Facility Employees

Site Trespassers Terrestrial Aquatic

Spills Dust and/or

Volatile Emissions

Wind Ingestion

Inhalation Historical Releases

Dermal Contact

Sanitary/ Storm Sewer Soil Ingestion

Inhalation Dermal Contact

Infiltration/ Percolation

Ground Water Ingestion

Inhalation Dermal Contact

Plant Uptake

(e.g., garden vegetables)

Ingestion

Inhalation Dermal Contact

Storm Water Runoff

Surface Water and Sediments

Ingestion

Inhalation Dermal Contact

Notes:

Indicates that the exposure route for the receptor is likely.

All exposure pathways (ingestion, inhalation, dermal contact) were assessed for each pathway.

Receptor

Human Biota

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Site Geology The region surrounding SSC is underlain by unconsolidated sediments of glacial origin dominated by ice contact, till, and glaciofluvial and glaciolucastrine deposits. The regional surficial geology is complicated and heterogeneous due to the coalescing and overlapping of lacustrine deltas from the many ice fronts of the retreating glacier. The result of this depositional history is a complex distribution of stratigraphic horizons and laterally discontinuous beds of glacial deposits.

Overburden thickness in and around the T-25 Area ranges from approximately 155 feet at the northeast boundary (EB-42E-HP), to 199 feet at the southwest (EB-51E-HP) and northwestern (EB-208E-HP) boundaries. In general, overburden thickness increases to the west and north. The unconsolidated overburden deposits observed at SSC can be divided into six broad stratigraphic units listed generally in order of depth from ground surface : 1) sand and gravel, 2) fine-to-medium sand, 3) silty sand, 4) silt, 5) clayey silt,

6) glacial till. Bedrock at SSC is encountered at depths ranging from approximately 54 to 199 feet below ground surface (bgs), and elevations ranging from 34 feet below mean sea level (msl) to 107 feet above msl. The lithology of the bedrock at SSC is consistent with the regional bedrock descriptions, and consists primarily of Dedham Granodiorite and Diabase/Hornblende Gabbro Undivided. Bedrock cores collected at SSC displayed varying degrees of competency. A geologic cross section of the site is illustrated on Figure 4.

The soils borings and monitoring wells at the site are described using an alphabetic identification system, which corresponds to absolute elevations above msl, or depths bgs relative to the T-25 Area. All wells/borings whose depths are generally less than 30 bgs within the T-25 Area and are screened across the top of the water table, are in the A-interval. B-interval wells/borings are screened at the target depth of the Springvale wells, and/or those wells that are set on top of the clayey silt layer with depths typically ranging from 30 to 65 feet bgs within the T-25 Area. C-interval wells/borings are typically encountered in or immediately below the clayey silt layer, and have screens set at least 10 feet in elevation lower than the Springvale wells. D-interval wells/borings are defined as being below the C-interval but above the top of bedrock. Any wells/borings whose screens are set in bedrock are in the E-interval.

Site Hydrogeology Ground water at the T-25 Area is generally encountered between 14 and 16 feet bgs.

The overburden aquifer at the T-25 Area can be characterized by two main hydrostratigraphic units: an upper unconfined gravel to silty sand unit [which includes the A- (less than 30 feet bgs in the T-25 Area) and B- (30 to 65 feet bgs in the T-25 Area) intervals] and a lower silty sand to clayey silt unit. The upper and lower portions of the aquifer are separated by a layer of light olive gray clayey silt. The upper portion

TEPS/Natick/ROD/ROD_Apr01.doc 12/21/04 19 of the aquifer (A- and B-intervals) was the focus of the Phase II investigation because it is where most of the PCE and TCE have been found in previous investigations. PCE and TCE concentrations up to approximately 1,000 to 2,000 µg/L have been found in ground water samples from the B-interval. PCE and TCE have generally not been detected at concentrations greater than 10 µg/L in any ground water samples collected from the shallow A-interval (water table). Field-screened ground water samples collected from depths below the B-interval (including the bedrock) were relatively free of PCE and TCE, with only sporadic detections at concentrations below 10 µg/L.

The geologic materials encountered in the T-25 Area are fairly heterogeneous with lenses of very fine material, and sands and gravels. As is commonly the case for glacial aquifers, ground water flow through the aquifer at the site was determined to be highly anisotropic with horizontal hydraulic conductivities higher than vertical hydraulic conductivities.

Ground water flows across the T-25 Area in the A-interval (less than 30 feet bgs) and B-interval (30 to 65 feet bgs) to the west-northwest with a downward vertical component, over a relatively gradual gradient. Ground water elevation and the magnitude of the horizontal gradient appear to fluctuate seasonally, though ground water flow direction generally remains constant. Long-term ground water elevation monitoring and the preliminary results from the ground water model indicate that a connection between Lake Cochituate and the aquifer exists.

The A- and B-intervals of the aquifer responded differently to aquifer stresses (e.g., pump tests and precipitation events) in the T-25 Area, suggesting they are weakly connected. Chemical data collected during the field screening and quarterly sampling programs also support this theory. PCE and TCE have generally not been detected at concentrations greater than 10 µg/L in any ground water samples collected from the A-interval. However, widespread and elevated concentrations of PCE (up to 2,000 µg/L) and TCE (up to 1,100 µg/L) have been detected in samples collected from the B-interval. The difference in response could also indicate a contrast in storage properties or differences between horizontal and vertical hydraulic conductivity of the A- and B-intervals.

The aquifer beneath the site has been classified by the state of Massachusetts as Groundwater 1 (GW-1) under the MCP, and was determined to be of high use and value. This classification is based on the proximity of the Interim Wellhead Protection Area for the Springvale Well Field, the existence of Potentially Productive Aquifers within the site, and the known regional ground water pathways that feed the Springvale Well Field.

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The ground water model developed for the site consists of a three-dimensional grid that is sufficiently large enough to represent the relationship among Lake Cochituate, the local ground water, and the town of Natick drinking water supply wells. Appendix D includes a letter report summarizing the preliminary 1999 ground water model assumptions and results. A more detailed ground water model report is expected to be available in the Winter/Spring 2000.

Surface and Subsurface Features SSC currently has two EPA-regulated underground storage tanks, one containing diesel and one containing gasoline. Both tanks have a 2,000-gallon capacity and currently meet 1998 specifications. SSC also has six consumptive-use 20,000-gallon heating oil (No. 6) tanks and one 10,000-gallon diethylbenzene (o,m,p mixture, 97 percent) tank, all of which are state regulated. SSC also has one 1,000-gallon underground propane tank.

Two oil/water separators are located in the T-25 Area for storm water drainage. A hazardous waste storage facility and a drum-holding area exist within the T-25 Area.

Other structures located in the area include temporary storage facilities for Army supplies and a warehouse. A small office building is also located in the T-25 Area.

There are no areas of archaeological or historical importance. A map of the T-25 Area is shown on Figure 2.

Sampling Strategy The field sampling portion of the Phase I T-25 Area Remedial Investigation (RI) was conducted from July 1993 until December 1993. The purpose of this investigation was to (1) identify the source(s) of contamination at the T-25 Area, (2) identify the nature and extent of contamination from the T-25 Area, (3) identify any potential sensitive receptors of the ground water contamination, (4) estimate the potential risk to human health and the environment posed by the ground water contamination at the site, and

(5) characterize the hydrogeology of the T-25 Area sufficiently for the design of a remedial system for the ground water in the area. Soil, ground water, surface water, and sediment samples were collected and analyzed to characterize contamination present, identify localized potential sources, and assess the likelihood and potential impact of off-facility contaminant migration.

The field sampling portion of the Phase II T-25 Area RI occurred from September 1995 until May 1996. During the Phase II RI, soil, ground water, surface water, and sediment samples were collected and analyzed to evaluate data gaps from the Phase I investigation. The analytical results were used to characterize contamination present, identify localized potential sources, and assess the likelihood and potential impact of off-facility contaminant migration. Both the Phase I and Phase II RIs involved a combination of a document review, interviews, a field-screening program, a hydrogeologic characterization, and a multimedia sampling program.

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A phased approach was used for ground water characterization for both RIs. The first phase included a fast-paced screening program to identify potential contaminant sources and to assess the nature and extent of the contamination. Field screening samples were collected both on site and off site from small-diameter wells, soil borings, and bedrock borings. The second phase included installing standard monitoring wells in areas needed to characterize the lateral or vertical extent of contamination or in contaminant “hot spots”. Standard monitoring wells were also installed to obtain additional information on upgradient water quality and hydrogeologic conditions.

This ROD addresses ground water contamination at the T-25 Area of SSC only. All other media will be addressed in separate actions or as separate operable units at SSC.

Known or Suspected Sources of Contamination Despite the exhaustive number of soil and ground water samples collected at the site, the present and previous investigations have not conclusively identified the source(s) of contamination to the ground water at the T-25 Area. Various activities throughout the T-25 Area over the past 40 years may have contributed minor contamination to the ground water at the site. These activities included metals parts washing, mobile dry cleaning, laboratory usage, and storage and transport of cooling brine containing TCE.

Contamination released during these activities may have contributed to the TCE and PCE ground water contamination found at the site. In addition, leaky sanitary or storm water sewer lines to the south of the T-25 Area may have contributed to ground water contamination, through the transport of materials/liquids improperly disposed of in laboratory sinks, catch basins, floor drains, and/or storm drains.

There is no evidence of a current, active contaminant source(s). If the source(s) was current, the contaminant distribution would be different. One would expect the shallow aquifer in the T-25 Area to contain high contaminant concentrations. If the release(s) was recent and to the ground surface, significant contaminant concentrations also would be found in the unsaturated zone, and shallow ground water in relatively close proximity to the areas of elevated concentration. This is especially true because the majority of the site is currently covered by asphalt, preventing surface water infiltration and percolation through the unsaturated zone and, therefore, preventing the leaching of contaminants from the soil. A large number of soil samples from the unsaturated zone, and ground water samples from the shallow portions of the aquifer, have been collected across the T-25 Area, specifically near suspected source areas. PCE and TCE have not been detected in any of the soil samples, and only low levels of PCE and TCE have been detected in shallow ground water samples. These results do not indicate a current or recent source, active or inactive.

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Types of Contamination and Affected Media The nature and extent of ground water contamination are summarized below. A detailed description of the nature and extent of contamination for all media can be found in the Phase II T-25 Area RI Report (Arthur D. Little, Inc., 1998).

On-Facility Ground Water Contaminants. Data from the Phase I and Phase II T-25 Area RI field screening programs, and from 20 rounds of quarterly ground water monitoring, indicate that VOCs are present in the ground water beneath the T-25 Area at SSC. The VOCs are limited primarily to TCE, PCE, and to a much lesser extent, cis- 1,2-dichloroethene (DCE). PCE and TCE are the only volatile organics listed as COCs.

The total mass of PCE and TCE associated with Operable Unit 1 was estimated in the ground water flow and contaminant transport model as 78 kilograms (HydroGeologic, Inc.). During the first two years of the Treatability Study, a total of approximately 20 kilograms of PCE and TCE have been removed from the aquifer.

The ground water model is draft and is subject to change, as more data become available. Any changes to the model are not anticipated to substantially affect the analysis presented in this ROD. Appendix D includes a letter summarizing the current ground water model results and its assumptions (e.g., treatment system and town of Natick water supply pumping rates, lake contribution factors).

The highest PCE and TCE concentrations have been found in the B-interval (30 to 65 feet bgs within the T-25 Area) above the clayey silt layer, primarily from the southwestern to north-central portions of the T-25 Area. PCE and TCE have generally not been detected at concentrations greater than 10 µg/L in any ground water samples collected from the A-interval (less than 30 feet bgs within the T-25 Area) or from below the clayey silt layer (i.e., the C- or D-intervals). [The C-interval is generally defined as being at least 10 feet below the bottom of the screen for the deepest Springvale well (73.4 feet above msl). The D-interval is defined as being below the C-interval but above bedrock]. TCE and PCE were not detected in any bedrock ground water samples (E-interval).

The highest PCE concentrations observed during quarterly monitoring (up to 2,000 µg/L during Round 4 in Summer 1994) were found in samples collected from the southwestern portion of the T-25 Area, immediately northeast of Building 20. PCE has been detected less frequently than TCE.