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Prepared in cooperation with the New Hampshire Department of Environmental Services and the U.S. Environmental Protection Agency, Region 1

Effects of a Remedial System and its Operation on Volatile Organic Compound-Contaminated Ground Water, Operable Unit 1, Savage Municipal Well Superfund Site, Milford, New Hampshire, 1998-2004

By Philip T. Harte

U.S. Geological Survey Scientific Investigations Report 2006-5083


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The body of the report is available in PDF Format ( 3,390 KB)

Abstract

The Savage Municipal Well Superfund site in the Town of Milford, N.H., is underlain by a 0.5-square mile plume of volatile organic compounds (VOCs), mostly tetrachloroethylene (PCE). The plume occurs mostly within a highly transmissive sand and gravel layer, but also extends into underlying till and bedrock. The plume has been divided into two areas called Operable Unit 1 (OU1), which contains the primary source area, and Operable Unit 2 (OU2), which is defined as the extended plume area.

PCE concentrations in excess of 100,000 parts per billion (ppb) had been detected in the OU1 area in 1995, indicating a likely Dense Non-Aqueous Phase Liquid (DNAPL) source. In the fall of 1998, the New Hampshire Department of Environmental Services (NHDES) and the U.S. Environmental Protection Agency (USEPA) installed a remedial system in OU1 to contain and capture the dissolved VOC plume. The OU1 remedial system includes a low-permeability barrier wall that encircles the highest detected concentrations of PCE, and a series of injection and extraction wells to contain and remove contaminants. The barrier wall likely penetrates the full thickness of the sand and gravel; in most places, it also penetrates the full thickness of the underlying basal till and sits atop bedrock. Remedial injection and extraction wells have been operating since the spring of 1999 and include a series of interior (inside the barrier wall) injection and extractions wells and exterior (outside the barrier wall) injection and extraction wells. A recharge gallery outside the barrier wall receives the bulk of the treated water and reinjects it into the shallow aquifer.

From 1998 to 2004, PCE concentrations decreased by an average of 80 percent at most wells outside the barrier wall. This decrease indicates (1) the barrier wall and interior extraction effectively contained high PCE concentrations inside the wall, (2) other sources of PCE did not appear to be outside of the wall, and (3) ambient ground-water flow in conjunction with the exterior remedial wells effectively remediated most of the dissolved PCE plume outside the wall.

The overburden at middle depths (40 to 70 ft below land surface) downgradient from exterior extraction wells showed relatively slow decreases in PCE concentrations compared to other areas outside the barrier wall. Numerical simulation shows extraction caused the formation of a small downgradient slow-velocity zone. Because the ambient ground-water velocities are high (approximately 1 foot per day), temporary termination of extraction at the exterior wells may increase dilution downgradient from the exterior extraction wells. Extraction can also be optimized on the basis of seasonal hydrologic conditions to facilitate exterior well capture from upgradient areas outside of the barrier wall where PCE concentrations are highest.

Reductions in concentrations of PCE inside the barrier wall from 1998 to 2003 were minimal near suspected source areas, indicating that the operation of interior remedial wells had not been effective in remediating dissolved PCE or the DNAPL source. Capture of the dissolved PCE plume within the barrier wall by interior extraction wells could be enhanced if operation (injection rates) increased at underutilized interior injection wells, thereby increasing hydraulic gradients.

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TABLE OF CONTENTS

Abstract

Introduction

Purpose and Scope

Description of Study Area

Overview of the Remedial System and its Operation

Hydrogeology

Previous Investigations of Contaminant Sources and Previous Remediation Efforts

Effects of the Remedial System and its Operation on Ground-Water Quality and Flow

Hydrogeologic Responses

Ground-Water Quality

Water Chemistry

Volatile Organic Compound Trends

Effects of Barrier Construction and Trench Depths on Lateral Inflow

Simulation of the Remedial System and its Operation

Effects of Operational Variations on Ground-Water Flow and Solute Transport

Sensitivity of Inside Barrier Area to Lateral Inflow

Sensitivity of Inside Barrier Area to Upflow from Bedrock

Sensitivity of Outside Barrier Area on Capture Zones to External Extraction Wells

Effects of Transient Flow on Capture Zones to Extraction Wells

Alternative Operational Schemes to Accelerate Flushing of Contaminants

Evaluation of Effectiveness of Remediation

Summary and Conclusions

References Cited

Appendixes 1-4


 Appendix 1.
Information on Wells and Vertical Profile Points, Savage Municipal Well Superfund Site, Milford, N.H.
Appendix 2. Tetrachloroethylene Concentrations for Data Points Used in Figure 14, Operable Unit 1 (OU1), Savage Municipal Well Superfund Site, Milford, N.H.
Appendix 3. Information on Barrier-Trench Depths, Operable Unit 1 (OU1), Savage Municipal Well Superfund Site, Milford, N.H.
Appendix 4. Vertical Discretization for Model Row 91 of the Finite-Difference Grid, Savage Municipal Supply Well Superfund Site, Milford, N.H.

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For further information, contact:

 

Keith W. Robinson, Director

U.S. Geological Survey

New Hampshire-Vermont Water Science Center

361 Commerce Way

Pembroke, NH 03275

 

dc_nh@usgs.gov

 

(603) 226-7807

 

or visit our Web site at: http://nh.water.usgs.gov



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