ARCADIS_2005_Remedial_Action_Work_Plan_AOC_50_Devens.pdf

FINAL

Remedial Action Work Plan, AOC 50, Devens, Massachusetts

Prepared For

GIFPR Remediation AOC 50 Devens, Massachusetts GSA Contract No. GS-1 OF-0266K Delivery Order No. DAKFI 1-01 -F-

June 2005

Charles Castelluccio Project Manager

Peter Milionis Project Director

FINAL

Remedial Action Work Plan, AOC 50, Devens Massachusetts

Prepared for:

GlFPR Remediation of AOC 50, Devens, Massachusetts GSA Contract No. GS-10F-0266K Delivery Order No. DAKFI 1-01 -F-0247

Prepared by:

ARCADIS G&M, I ~ c .

175 Cabot Street Suite 400 Lowell Massachusetts 01 854

Our Ref.:

Date:

June 2005

This document is intended only for the use of the individual or entity for which it was prepared and may contain information that is privileged, confidential, and exempt from disclosure under applicable law. Any dissemination, distribution, or copying of this document is strictly prohibited.

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FINAL RAWP

Table of Contents

I . 0 INTRODUCTION

1.1 PURPOSE

.................................................................. 1.2 SITE DESCRIPTION AND HISTORY 1

................................................................................... 1.2.1 Fueling Systems 2

1.2.2 Drywell. Tetrachloroethene Drum Storage Area. and Cesspool .. 3

2.0 RESULTS OF PRE-DESIGN INVESTIGATION WORK

2.1 PURPOSE

2.2 FINDINGS

...................................................................... 2.2. I Source Area ..................................................................................... 2.2.2 Mid-Plume Area 9

................................................................................. 2.2.3 Sentinel Well Area 9

2.3 CONCLUSIONS

.......................................................................................................... 2.3.1 Soil 10

2.3.2 Groundwater ...................................................................................... I 1

3.0 SELECTED REMEDIAL ACTION ALTERNATIVE

3.1 SUMMARY OF THE RATIONALE FOR SELECTION OF ALTERNATIVE 6

. 3 . I I Description of Alternative 6

3.1.2 Expected Outcome of Alternative 6

4.0 REMEDIAL ACTION DESCRIPTION

............................................................. 4.1 MOBILIZATIONISITE PREPARATION 17 4 . 1 . I Mobilization

................................................................................... 4.1.2 Site Preparation 18

4.2 SVE SYSTEM IMPLEMENTATION

............................................. 4.3 FULL-SCALE ERD SYSTEM CONSTRUCTION 19

4.3. I Injection Well Installation

...................................................... 4.3.2 Construction of Delivery System 20 ..................................................... ............. 4.3.3 Inorganics Contingency : 2 0

..................................................................... 4.4 IWS SYSTEM CONSTRUCTION 21

4.4.1 Well Installation

.......................................................................... 4.4.2 System Construction 2 2 ..................................................... 4.5 NORTH PLUME CONTINGENCY REMEDY 23

4.6 LUCRAWP

................................................................................................. 4.6.1 Purpose 23 . .

4.6.2 Performance Objectives

...................................................................... 4.6.3 . lmplemenfation Actions 29 ....................................................................... 4.7 WASTE MANAGEMENT PLAN 35

4.8 DEMOBILIZATION

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FINAL RAWP

Table of Contents

5.0 QUALITY CONTROL

5.1 CONSTRUCTION QAP ................................................................................ :.. 36

5.2 SAMPLING AND ANALYSIS PLAN

6.0 PROJECT SCHEDULE

7.0 PROJECT PERSONNEL

8.0 REFERENCES

TABLES

1. Summary of Pre-Design Well Installation, AOC 50, Devens, Massachusetts.

2. LUC Layering Table, AOC 50, Devens, Massachusetts.

FIGURES

1. Site Location, AOC 50, Devens, Massachusetts.

2. Site Plan, AOC 50, Devens, Massachusetts.

3. Pre-design Investigation Well Locations, AOC 50, Devens, Massachusetts.

4. Plume Delineation and Locations of Long-Term Monitoring Wells, AOC 50, Devens, Massachusetts.

5. Limits of Land-Use Controls, AOC 50, Devens, Massachusetts.

6. Project Schedule, AOC 50, Devens, Massachusetts.

APPENDICES

A. Pre-Design Investigation Logs

B. Summary of Laboratory Analytical Data

C. Construction Quality Assurance Plan

D. Licensed Site Professional Opinion

E. 1996 Lease of Furtherance of Conveyance

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AOC 50

Devens, Massachusetts

I .O INTRODUCTION

1 . Purpose

The purpose of this Remedial Action Work Plan (RAWP) is to provide a description of the planned tasks, methodologies, objectives, and site cleanup goals associated with remediation at Area of Contamination (AOC) 50, Ayer, Massachusetts (Site). The results of the pre-design investigation work performed in April and May 2003 are presented in this RAWP. Also included in this plan is the land use control (LUC) plan.

The proposed remediation work for AOC 50 includes the installation of the enhanced reductive dechlorination (with solubilized inorganics controls) and in-well stripping wells and system components for the treatment of tetrachloroethene (PCE) and other constituents of concern at the Site. In addition, the soil vapor extraction (SVE) system will be restarted to determine if additional PCE mass can be recovered fiom the vadose zone in the Source Area. The contingency remedies of chemical oxidation in the North Plume Area and iron injection at the downgradient end of the last ERD transect are also presented in this RAWP.

This RAWP has been prepared for the U.S. Army in accordance with the final Record of Decision (ROD) for AOC 50 (ARCADIS 2004a). The proposed actions will reduce identified current and future risks associated with PCE and other constituents of concern at AOC 50 to acceptable levels. The proposed remedy is expected to provide a permanent, long-term solution for the Site.

1.2 Site Description and History

AOC 50 is located on the northeastern boundary of the former Moore Army Airfield (MAAF), within the former North Post portion of Devens RFTA, Ayer, Massachusetts (Figure 1). The AOC 50 Source Area (Figure 2) comprises less than 2 acres and surrounds Buildings 3803 (the former parachute shop), 3840 (the former parachute shakeout tower), 3 824 (a gazebo), and 3 80 1 (the former 10' Special Forces airplane parachute simulation building). Sources of groundwater contamination within AOC 50 include two World War I1 fueling systems, a drywell, and the PCE drum storage area;

these sources are collectively referred to as the Source Area. Other potential sources of contamination may include a former cesspool and floor drain associated with Building 3840. Although these sources have been removed or decommissioned, groundwater underlying AOC 50 contains elevated concentrations of volatile organic compounds (VOCs) most notably PCE. The primary area of groundwater contamination at AOC 50 is referred to as the Southwest Plume, which extends fiom the Source Area approximately 3,000 feet downgradient to the Nashua River.

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AOC 50

Devens, Massachusetts

Fort Devens was established in 1917 as Camp Devens, a temporary training camp for soldiers from the New England area. In 1931, the camp became a permanent installation and was renamed Fort Devens. Throughout its history, Fort Devens served as a training and induction center for military personnel, and as a unit mobilization and demobilization site. All or portions of this function occurred during World Wars I and 11, the Korean and Vietnam conflicts, and operations Desert Shield and Desert Storm.

Fort Devens was identified for cessation of operations and closure under Public Law 101-5 10, the Defense Base Realignment and Closure (BRAC) Act of 1990, and was officially closed in September 1996. Portions of the property formerly occupied by Fort Devens were retained by the Army for reserve forces training and renamed the Devens RFTA. Areas not retained as part of the Devens RFTA were transferred to new owners for reuse and redevelopment.

All but approximately 13 acres of the former W (approximately 260 acres total) were transferred to Mass Development in 1997 for reuse. Currently, the airfield is closed to aircraft traffic and is used by the Massachusetts State Police for training and vehicle storage. The MAAF is zoned for Special Use 11 and Innovation and Technology Business. Under the Devens Reuse Plan (November 14, 1994), Special Use I1 includes a broad range of industrial, light industrial, office, and research and development uses. There are currently no plans for development of the MAAF, although the area can be developed if interested parties are identified. The Devens RFTA retained approximately 13 acres of the former airfield (including the AOC 50 Source Area) for vehicle storage and maintenance.

1.2.1 Fueling Systems

During World War 11, two fueling systems were used in the area subsequently designated AOC 50; one system was used for fueling aircraft and trucks (System A), and the other for fueling trucks (System B). These systems were not used for refueling operations after the late 1940s (Biang, et al., 1992). The two separate fueling systems were filled by gasoline shipments on a Boston & Maine Railroad spur (which no longer exists) located adjacent to Fueling System B (Figure 2).

Releases of fuel associated with incidental spills at the former aircraft fuel pits, mck-fill stands, and railroad fuel-delivery points were considered possible sources of contamination. Because the systems were approximately 50 years old, the underground storage tanks (USTs) were also considered possible continuing sources of releases. The potential for migration of contaminated groundwater to the Nashua River was a concern. At the time of the initial Site Investigation (SI) in 1992 (ABB, 1996), several fueling-system components were still visible in their original locations.

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AOC 50

Devens, Massachusetts

The Army removed all of these components in 1992. In addition, approximately 450 tons of contaminated soil was removed from under the oillwater separator, water-control pits, and three 25,000-gallon USTs. The excavation extended to a depth of approximately 18 ft below ground surface (bgs) due to the presence of water in the excavation. All excavations were backfilled to grade. Field screening results and post-excavation sample analyses are presented in the Remedial Investigation (RI) prepared by Harding Lawson Associates (HLA, 2000).

1.2.2 Drywell, Tetrachloroethene Drum Storage Area, and Cesspool

In 1969, Building 3840 was constructed and attached, via an enclosed walkway, to Building 3803. In addition, two large sinks and a janitors' room were added to Building 3803. The design drawings for Building 3840 indicate that a floor drain was constructed in the center of the concrete floor. This floor drain, the additional sinks in Building 3803, and the roof drains for Building 3840 were piped to a drywell located approximately 20 ft northeast of Building 3840 (Figure 2). The concrete drywell was approximately 5 ft in diameter and 8 ft deep, with an open bottom and a cover on the top. This drywell received wash water, rainwater, and PCE waste associated with parachute cleaning activities.

The drywell near Building 3840 and associated piping were removed for the Army by Roy F. Weston Corporation between November and December 1996 (Weston 1997).

The resulting excavation was approximately 9.5-ft deep and covered an area approximately 21 ft by 30 ft, equating to approximately 225 cubic yards (cy) of soil (in-place). Details regarding the removal activities are documented in a September 1997 report titled Removal Action Report; Dry Well, Cesspool, and Fuel Oil Underground Storage Tank; Area of Contamination (AOC) 50, Moore Army Air Field, Devens, M A (Weston, 1997).

In addition to the removal of the drywell, a 750-gallon fuel storage UST associated with the Building 3840 heating system was also removed. In connection with the tank removal, approximately 787 gallons of oil, water, and residual sludge were recovered from the tank and approximately 25 cy of contaminated soil were excavated. Solid and liquid wastes generated during removal of the drywell and he1 storage UST were taken off-site for proper treatment and disposal.

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AOC 50

Devens, Massachusetts

1.2.2.2 Tetrachloroethene Drum Storage Area

A PCE drum storage area east of Building 3801 was identified during field investigation activities completed in 1992. Historical records and interviews with former Fort Devens personnel indicate this area was used to store single drum quantities of PCE (HLA, 2000a). The PCE was used by Army personnel in Buildings 3803 and 3840 for spot cleaning of parachutes. Parachute cleaning was performed only as needed to maintain the integrity of the parachute material. Unused PCE was either reused or may have been washed down into the drywell system associated with Buildings 3803 and 3840. This information was supported by a review of the historic hazardous waste manifests, which did not include the removal of waste chlorinated solvents from AOC 50 (Mott, 1997). The use of this area for drum storage was discontinued in 1992. The length of time or total number of drums stored in this area of AOC 50 is unknown.

Based on the results of various field investigations, PCE was detected in vadose zone soils beneath the former drum storage area and was likely contributing to PCE impacts in groundwater. An interim removal action for PCE-contaminated soil at the former drum storage area was planned and implemented as a source-control measure while additional investigation activities were conducted across the Site. An in-situ soil vapor extraction (SVE) system was installed adjacent to the former drum storage area in December 1993 and January 1994. Five soil vapor extraction wells (SVE-1 through SVE-5) were installed, one in the center of the presumed PCE source and four on the periphery (Figure 2).

Operation of the SVE system began in February 1994 and continued through July 1996. Operation & Maintenance (O&M) data collected between February 1994 and July 1996 indicated that approximately 240 pounds (approximately 18 gallons) of PCE were successfully recovered in the vapor phase. Details regarding the installation, operation, and performance of the SVE system between February 1994 and July 1996 are documented in a November 1996 report titled Summary Report, SVE Monitoring, AOC 50 (ABB, 1996b).

The SVE system was operated again for brief periods in December 1998, May and June 1999, and October and November 1999. The brief periods of SVE system operation after the 1996 shut down were conducted to evaluate the concentration of PCE in the soil vapor, under equilibrium conditions. In general, recovered vapor concentrations were either below the detection limits of a photoionization detector (PID), or after a brief peak observed when the system was restarted, quickly attenuated

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AOC 50

Devens, Massachusetts within minutes. No appreciable mass of PCE was recovered during the brief periods of SVE operation between 1998 and 1999.

1.2.2.3 Cesspool

A cesspool associated with the bathroom in Building 3803 was identified on the site drawings; it appears to be the only septic system structure for either building. The concrete and rubble cesspool was approximately 10 ft in diameter and 9 ft deep with an open bottom and a cover on the top. The drywell and cesspool were investigated as potential contaminant sources for various volatile contaminants, including PCE detected in soil and groundwater during previous investigations.

The cesspool was removed concurrent with the drywell and UST associated with Building 3840. During the cesspool removal activities, a total of 25 cy of soil, sludge, and concrete were excavated and taken offsite for treatment and disposal.

2.0 RESULTS OF PRE-DESIGN INVESTIGATION WORK

The pre-design investigation activities are a key component of the Selected Remedy (Alternative 6) for the Site. Over the past 18 months extensive field investigations have been conducted at AOC 50 to further assess the nature and extent of PCE impacts at AOC 50.

2.1 Purpose

The purpose of the Pre-Design Investigation was to collect additional soil and groundwater quality, and hydrogeologic data to further define the extent of groundwater contamination at AOC 50. The installation of permanent monitoring andlor injection wells and soil and groundwater sampling activities were conducted at specific locations in the Source Area, Mid-Plume Area and Sentinel Well Area to provide supplemental data to support the remedial design.

2.2 Findings

This work involved the advancement of eleven soil borings via hollow stem augerldrive and wash drilling techniques in three specific areas of the Site: (1) Source Area, (2) Mid-Plume Area, and (3) Sentinel Well Area. Permanent monitoring wells andlor enhanced reductive dechlorination (ERD) injection wells were installed as part of this field investigation work. One SVE well was also installed as part of this work.

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AOC 50

Devens, Massachusetts

During the advancement of soil borings, subsurface samples were collected and screened for total VOCs using a photoionization detector (PID) and logged by an on-site ARCADIS scientist. Subsurface soil samples were initially collected in the Source Area at continuous two-foot intervals using split-spoon samplers until the water table was encountered. Once the water table was reached soil samples were collected at five-foot intervals to the desired total depth of the boring.

In addition, split-spoon samples were collected from borings advanced in the Mid- Plume Area and Sentinel Well Area at five-foot intervals below the water table in areas where the soils were not previously logged.

The approximate locations of the soil borings and permanent wells are shown on Figure 3. Table 1 presents a summary of the predesign investigation field program.

The table presents the boringlwell designation, purpose of installation, total depth, sampling description, well screen interval, PID range, analytical results, and general lithology.

The following sections provide a summary of the findings of the pre-design investigation for each specific area (i.e. Source Area, Mid-Plume, and Sentinel Well Area).

2.2.1 Source Area

2.2.7.7 Former Drywell Area

Two soil borings were advanced in the vicinity of the former drywell. These borings provide data to evaluate the potential for contamination in the intermediate and deep zones of the overburden aquifer at these locations.

Well G6M-03-01X was installed on the north side of the former drywell.

Continuous soil samples were collected to the water table and at five-foot intervals from the water table to refusal (approximately 76 ft below ground surface [bgs]). No PID readings were recorded above background in this well. The lithology consisted primarily of fine sand with a trace of coarse sand and silt. Unstratified till was encountered at 71 ft bgs. No soil samples were analyzed in the laboratory from this boring. PCE was detected in groundwater at a concentration of 4,100 micrograms per liter (pg/L) from the screened interval (50-70 ft bgs).

Well G6M-03-02X was installed on the south side of the former drywell.

Continuous soil samples were collected using split-spoon samplers to the

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AOC 50

Devens, Massachusetts water table and at five-foot intervals from the water table to approximately 46 ft bgs. No PID readings were recorded above background in this well.

The lithology consisted of fine sands with minor amounts of silt and clay.

A clayey silt layer was encountered at 44 ft bgs. PCE was not detected from laboratory analysis of the soil sample collected fiom 6 to 8 ft bgs.

PCE was detected in groundwater at a concentration of 1,300 yg1L fiom the screened interval (28 to 43 ft bgs).

A third boring was planned near the former dry well area. The boring was to be located in the former drain inside Building 3840 to determine if a potential source of PCE exists in this area. The boring could not be drilled at this time due to building access problems.

2.2.1.2 Former Drum Sforage Area

Three soil borings were advanced on the east, north, and south sides of the former drum storage area.

Well G6M-03-03X was installed on the south side of the drum storage area to assess groundwater quality in the deeper portion of the overburden aquifer. Continuous soil samples were collected to the water table and at five-foot intervals fiom the water table to approximately 91 ft bgs. The lithology consisted of very fine-to-fine grained sands with minor interfingering silt and clay lenses. Iron staining was common. A gray till was encountered at approximately 87 ft bgs. The highest PID reading was recorded in organic matter from the 0 to 2 ft interval and the soil sample from the 4 to 6 ft interval which was analyzed in the laboratory, had a PCE concentration of 1,600 yglkg. The groundwater sample collected from the screened interval (75 to 85 ft bgs) did not detect PCE above the laboratory detection limit (less than 2 pa).

Well G6M-03-04X was installed on the north side of the drum storage area in the shallow overburden to be used as a potential ERD injection point. Continuous soil samples were collected to the water table and thereafter at five-foot intervals to 30 ft bgs. The lithology consisted of poorly sorted very fine to coarse sand with some silt and clay lenses throughout the boring. The highest PID reading was recorded from the 14 to 16 ft interval. The groundwater sample collected fiom the screened interval (15 to 30 ft bgs) had a PCE concentration of 9,500 p a .

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AOC 50

Devens, Massachusetts

One SVE well (SVE-6) was installed on the east side of the drum storage area near Well G6M-94-18X. Continuous soil samples were collected to a total depth of 14 ft bgs. A maximum PID readings of 1.2 parts per million by volume (ppm,) was recorded at 4 to 6 ft bgs. The lithology consisted of very fine-to-fine sands with minor amounts of silt. PCE was detected at a concentration of 100 pglkg in the soil sample collected fiom 6 to 8 ft bgs.

SVE-6 is screened from 3 to 13 ft bgs.

2.2.1.3 Former Cesspool Area

Two soil borings were advanced in the vicinity of the former cesspool area.

Temporary Well SB-03-05A was installed on the north side of the former cesspool area in the shallow overburden to determine groundwater impact at the water table. Continuous soil samples were collected to 12 ft bgs.

The lithology consisted of very fine-to-fine sand with some silt. No PID readings were recorded in soil above background levels. The groundwater sample collected from the screened interval (8 to 18 ft bgs) did not detect PCE above the laboratory detection limit (less than 2 pg/L).

Well G6M-03-05X was installed on the south side of the former cesspool to serve as a possible monitoringIERD injection point. Continuous soil samples were collected to 10 ft bgs and at 5-foot intervals to a total depth of 45 ft bgs (refusal) and the boring was advanced to 51 ft bgs. The lithology consisted of very fine to coarse sand with some silt and iron staining. Bedrock was encountered at 46 ft bgs. No PID readings were recorded above background levels. The soil sample collected fiom 8 to 10 ft bgs did not detect PCE above the laboratory detection limit (19 pglkg).

The groundwater sample collected fiom the screened interval (35 to 45 ft bgs) detected PCE at a concentration of 6.1 p a .

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AOC 50

Devens, Massachusetts

2.2.2 Mid-Plume Area

One soil boring was advanced in the Mid-Plume Area in the proximity of existing Well G6M-92-07X.

Well G6M-03-07X was installed in the Mid-Plume Area to better delineate the extent of PCE impact in this area. Soil samples were collected at 5-foot intervals from 59 to 101 ft bgs. The lithology consisted of fine to medium grained sand with a lense of fine sand and silt at 69 to 71 ft bgs and silt and clay stringers at 84 ft bgs. The highest PID readings was recorded at 84 to 86 ft bgs. The groundwater sample collected from the screened interval (80 to 90 f t bgs) detected PCE at a concentration of 1,200 yg/L.

2.2.3 Sentinel Well Area

Three soil borings were advanced between the Nashua River and the existing ERD injection well transect (IW-1 through IW-5). These wells were installed laterally across the groundwater plume to further delineate the extent of the PCE plume and will be used to monitor the presence of solubilized inorganics.

Well G6M-03-08X was installed at the southeast end of the Sentinel Well transect. Soil samples were generally collected at 5-foot intervals fiom the water table to total depth (59 to 151 ft bgs). The lithology consisted of fine to medium grained sand with trace of gravel and coarse sand with a greater percentage of silt with depth. The highest PID reading was at 129- 13 1 ft bgs. The groundwater sample collected from the screened interval (125 to 140 ft bgs) detected PCE at a concentration of 750 pgL.

Well G6M-03-09X was installed at the central portion of the Sentinel Well transect (between Wells G6M-03-08X and G6M-03 - 1 OX). Soil samples were generally collected at 5-foot intervals from 124 to 151 ft bgs. The lithology consisted of fine to medium sand with some coarse sand and silt.

The highest PID reading in soil was between 139 and 141 ft bgs. The soil sample collected from the 59 to 61 ft bgs interval did not detect PCE above the laboratory detection limit (less than 20 p.g/kg). The groundwater sample collected fiom the screened interval (125 to 140 ft bgs) did not detect PCE above the laboratory detection limit (less than 2 clgn).

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AOC 50

Devens, Massachusetts

Well G6M-03-10X was installed at the northwest end of the Sentinel Well transect. Soil samples were generally collected at 5-foot intervals from 59 to 141 ft bgs. The lithology consisted of fine to medium sand with some silt and trace of coarse grained sand. A fine sandy silt was detected below 139 ft bgs. The highest PID reading in soil was measured between 64 to 66 ft bgs. The groundwater sample collected from the screened interval (120 to 135 ft bgs) detected PCE at a concentration of 15 pa.

The sample core and well construction logs are presented in Appendix A. A summary of the laboratory analytical data is provided in Appendix B.

The wells installed as part of the pre-design investigation may be used for groundwater monitoring or as ERD injection wells based on the data collected to date.

2.3 Conclusions

2.3.1 Soil

The lithologic data collected as part of the pre-design investigation indicate that the soil in the area of the former drum storage area is more variable than other parts of the Site and includes more silt and clay stringers interbedded with fine to coarse sand. The silt and clay stringers are more pronounced at about 40 ft bgs and appear to have effectively restricted the downward migration of PCE. The till was encountered at approximately 87 ft bgs in the former drum storage area. The lithology in the former dry well and cesspool area are more continuous sands with lesser amounts of silt. The till was encountered at approximately 70 ft bgs in the former dry well area and bedrock was encountered at approximately 45 ft bgs in the former cesspool area.

The lithologic data collected in the Mid-Plume Area (G6M-03-07X) indicate a fining downward sequence with silt and clay stringers encountered at approximately 84 to 85 ft bgs. This is also the interval where the highest PID reading -was recorded. The lithologic data in the Sentinel Well Area also indicate a general fining downward sequence with more silt encountered with depth and subsequent reductions in PID readings.

The soil analytical data collected as part of the pre-design investigation indicate that there may be limited residual PCE in the vadose zone or capillary fringe at the water table in the former drum storage area. This can be addressed through the reactivation of the SVE system and implementation of ERD in this area. There is no evidence to indicate that there is residual PCE impact in vadose zone soil that needs to be remediated at any other area of the Site.

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AOC 50

Devens, Massachusetts

There was no evidence of dense nonaqueous phase liquid (DNAPL) observed in any soil sample collected as part of the pre-design investigation work or other work performed at the Site.

2.3.2 Groundwater

The new groundwater analytical data have been incorporated into the revised PCE plume map and presented on Figure 4. The most significant findings from the pre-design investigation are that PCE impact in groundwater is limited to less than 40 ft bgs in the former drum storage area and concentration of PCE in the cesspool area is close to the MCL of 5 ppb. The groundwater in the former dry well area is impacted by PCE from the water table to 70 ft bgs with higher concentrations present at depth above a till layer. The concentrations of PCE detected in groundwater in the Source Area are not indicative of DNAPL.

The PCE analytical data in groundwater at the mid-plume location (Well G6M-03- 07X) indicate that the extent and distribution of PCE is clearly delineated in this area with impact restricted to less than 90 ft bgs. The PCE analytical data in groundwater in the Sentinel Well Area indicate that the extent of impact is limited to less than 140 ft bgs and the plume is narrower than originally mapped; the extent of PCE in groundwater is shown on Figure 4. Wells G6M-03-09X and G6M- 03-10X are both outside of the center line of the PCE plume.

3.0 SELECTED REMEDIAL ACTION ALTERNATIVE

The selected remedy for AOC 50 is Alternative 6: Soil Vapor Extraction, Enhanced Reductive Dechlorination (with inorganics controls), In-Well StrippingIAerobic Bioremediation, Monitoring, and Institutional Controls. The following sections summarize the selection rationale, a description of remedial components, and expected outcome for Alternative 6. Changes in the selected remedy may occur as a result of new information and data collected during the design of the alternative.

3.1 Summary of the Rationale for Selection of Alternative 6

Alternative 6 provides the best balance among the candidate alternatives for AOC 50.

Alternative 6 is protective of human health under current and anticipated fiture land use scenarios. Existing and proposed institutional controls will prevent unrestricted use. Alternative 6 is also protective of the environment, attains ARARs, offers long-term and short-term effectiveness, and is readily implementable at a reasonable cost.

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AOC 50

Devens, Massachusetts

3.1 .I Description of Alternative 6

Alternative 6 includes multiple components to reduce potential human-health and ecological risks associated with groundwater at AOC 50. The principal components of Alternative 6 consist of the following:

Soil Vapor Extraction (SVE) in the Source Area;

Enhanced Reductive Dechlorination (ERD) throughout the Site (with solubilized inorganics controls);

In-Well Stripping (IWS) along the downgradient portion of the Southwest Plume;

Chemical oxidation in the North Plume (contingency);

Geochemical adjustments including iron injection (contingency);

Long-term monitoring;

Institutional Controls (ICs); and, Five-Year Site Reviews.

A description of the components of Remedial Alternative 6 and other related activities is provided below.

Pre-Design Investigation Activities - Over the past 18 months, the AJXI~ has undertaken extensive field investigation at AOC 50 to further assess the nature and extent of PCE impacts at AOC 50. A pilot test of the ERD technology was completed between December 2001 and July 2002, the results of which were documented in a report incorporated into the Final Feasibility (FS) Study (ARCADIS 2002a).

Additional investigation activities have been conducted to support the remedial design (RD). A summary of the investigation activities is presented in Section 2 of this report.

Additional activities will include the installation and testing of IWS and the testing of the SVE system. Work plans will be submitted for review prior to initiating testing.

Application of SVE in the Source Area - Based on the results of pre-design investigation, the existing SVE system formerly operated in the Source Area at AOC 50 will be refurbished and tested as part of the preferred alternative. The system will apply vacuum to wells completed within the unsaturated soils, capturing VOC mass in the vapor phase as soil gases are withdrawn. The soil gases extracted from the subsurface will be treated, if needed with activated carbon prior to being discharged to the atmosphere. The air emissions will be monitored according to Massachusetts Policy WSC 94-150. Operation of the SVE system in the Source Area will provide indirect

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Devens, Massachusetts remediation of groundwater impacts, if recoverable chlorinated VOC (CVOC) mass is present. Specifically, the capture of adsorbed phase mass potentially present in the vadose zone soils will be removed as a continuing source for groundwater contamination. One new SVE well was installed in the Source Area to supplement the existing SVE well network.

Enhanced Reductive Dechlorination (ERD) Implementation - This technology is implemented in-situ by stimulating microbial activity and significantly increasing rates of CVOC degradation. The microbial activity is stimulated through the injection of an organic carbon substrate. The areas in which this substrate is delivered become anaerobic and reducing due to the uptake of available electron acceptors to support respiration of the microbes promoting the environment required for the ERD process to take place. The preferred remedy will involve the installation of multiple injection wells in a series of transects or other spacing pattern to optimize the effectiveness of the system. A dilute solution of potable water and the organic carbon substrate (molasses or other) will be periodically injected into the formation through these wells to drive the groundwater environment to anaerobic and reducing conditions. The exact locations, spacing, and completion details of the injection wells/transects are specified in the RD. To optimize the design and further reduce the remedy duration, the design reflects the most up to date groundwater quality data and flow modeling. Following treatment of CVOCs to levels approaching Maximum Contaminant Levels (MCLs), a period of monitoring will occur prior to discontinuing injection of the organic carbon substrate to ensure that CVOC rebound does not occur.

Solubilized Inorganics Controls

As outlined in the Final Feasibility Study (ARCADIS 2002a) and confirmed during the ERD pilot test, inorganics including iron, manganese and arsenic are solubilized within the reducing zones created by ERD technology. Inorganics solubilized within the reducing IRZs are not expected to migrate beyond the boundary of reducing conditions, and are not expected to persist once the prevailing aerobic groundwater environment is restored. Outside of the zone of reducing conditions (i.e., under the naturally aerobic conditions present in the groundwater at AOC 50) the inorganic constituents will be oxidized and subsequently immobilized through precipitation and/or adsorption. However, it is recognized that a subsequent phase of remediation will be implemented should groundwater monitoring indicate that the inorganics have not attained remediation goals.

After the ERD remedy is completed within sections of the plume and injection transects are phased out (which is expected to be approximately 10 to 15 years based

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Devens, Massachusetts on the groundwater modeling prepared in the Feasibility Study (FS) and updated as part of the 60% remedial design), the inorganic data collected during the long-term monitoring will be evaluated to assess that adequate restoration of natural aerobic conditions and re-precipitation of inorganics have been achieved. If warranted, the re-precipitation of inorganics will be expedited through manipulation of aquifer chemistry or application of more effective treatment technologies along the length of the plume utilizing existing ERD injection wells as transects are phased out following the treatment of VOCs.

In-Well Stripping CIWS) Well Transect - Alternative 6 includes the installation of groundwater IWS wells in the downgradient portion of the Southwest Plume, upgradient of the Nashua River. The inlet (lower) screen interval of the IWS wells will be positioned to intercept the zone of highest CVOC concentrations, with the recharge (upper) screen interval positioned at the upper limit of the impacted zone (to prevent cross-contamination of unimpacted zones). The lower screen will also intercept the zone of highest potential solubilized inorganic compounds should this condition present itself. The IWS system will strip VOCs from the groundwater and will create aerobic conditions conducive to the precipitation of solubilized inorganic compounds. The location, spacing, and completion details of the IWS wells are specified in the RD.

Sentinel Groundwater Monitoring Wells - Monitoring wells will be placed in strategic locations between the Nashua River and the most downgradient ERD injection transect to serve as Sentinel Wells. The Sentinel Well network will consist of a series wells installed approximately 400 ft from the most downgradient ERD injection transect.

These wells will be located horizontally and vertically across the plume to monitor groundwater quality including the possible presence of solubilized inorganics. The number of wells required to adequately monitor the residual plume and solubilized inorganics are discussed in Section 4.6.

- Long-term monitoring will be performed to evaluate performance of the remedy and to confirm that concentrations of constituents of concern are reduced to remediation goals. During the initial phases of implementation, monitoring will be conducted more frequently. As the progress of the remedy is established, monitoring frequency andlor number of monitoring analytes will be reduced. Samples will primarily be analyzed for VOCs, with additional analyses including dissolved metals (arsenic, iron, lead, and manganese), nitrate, sulfate/sulfide, total organic carbon (TOC), methane, and dissolved gases (ethane and ethene), as appropriate. Field parameters (e.g., OW, pH, dissolved'oxygen, conductivity, turbidity, and temperature) will also be collected during sampling. Details of the monitoring are outlined in a .

long-term monitoring plan (LTMP) in Section 4.6 of this report.

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Institutional Controls - ICs will be implemented in each area of the plume (i.e., North, Source Area, and Southwest), shown on Figure 5, through formal negotiations during the preparation of this document and the remedial design with the different entities that own the properties overlying these areas. ICs are necessary to restrict land and groundwater use at the Site to prevent unacceptable risk for the duration of the remedy.

The ICs to be implemented in each area are outlined in the Land Use Control Plan in Section 4.7 of this report.

5-Year Site Reviews - Under CERCLA 121c, any remedial action that results in contaminants remaining on-site at concentrations greater than those allowing unrestricted use must be reviewed at least once every 5 years. During 5-year site reviews, an assessment is made of whether the implemented remedy continues to be protective of human health and the environment or whether the implementation of additional remedial action is appropriate. Additional details regarding the five-year review are in Section 4.7.3.6 of this report.

CONTINGENCIES

North Plume

As outlined in the FS, the primary method of groundwater remediation for the low levels of CVOCs observed in the North Plume area will be the application of ERD in the AOC 50 Source Area. The application of ERD will reduce the concentrations of CVOCs in the Source Area, thus limiting the potential for possible future migration of CVOCs off-site to the north. Groundwater analytical data fiom January 2004 indicates that only one well on the Merrimack Warehouse property (G6M-96-24B) exceeds the EPA primary drinking water standard of 5 ug/L. The PCE concentration detected in this well in January 2004 was 11 u&. This is currently the only well in the North Plume to have detectable concentrations of PCE. The proposed contingency remedy associated with the North Plume will consist of two components:

Monitoring Promam - Selected monitoring wells in the North Plume will be monitored on an annual basis for the presence of CVOCs and inorganics when ERD is implemented in the Source Area. Well G6M-96-24B will be sampled on a quarterly basis as noted in Section 4.6.

Remedv Implementation - In the event that PCE or its daughter products exceed their respective MCLs in the North Plume one year after ERD implementation in the Source Area, a direct application of in-situ chemical oxidation will be utilized to treat the CVOCs in the North Plume. The

G:W-PRJCTS\Fort DevensWOC SO\Reports\Rernedial Action WP\RAW\FinaRtinai RAWP 06-2805.doc

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Devens, Massachusetts treatments would continue periodically (i.e., annually), if needed based on groundwater monitoring results.

Inorganic Compounds

As outlined in the Final FS (ARCADIS 2002a), inorganic compounds such as iron, manganese or arsenic can be solubilized within the reducing zones created by ERD technology. Inorganic compounds solubilized within the reduced zone are not expected to migrate beyond the boundary of reducing conditions. Oxidizing aerobic groundwater conditions will be restored through a combination of natural processes and by aeration at the IWS wells. Outside of the zone of reducing conditions (i.e., under the naturally aerobic conditions present in the groundwater at AOC 50) and in the area of the IWS wells, it is expected the inorganic constituents will be oxidized and subsequently immobilized through precipitation and/or adsorption processes. The dominant process is expected to be co-precipitation with oxidized iron. Despite this expectation, it is recognized that a contingency must be available should groundwater monitoring indicate that there is an iron deficiency in the IWS treatment area (i.e., towards the Nashua River) that may preclude the effective immobilization of dissolved arsenic. The proposed contingency remedy associated with inorganic compounds will consist of two components:

Monitoring Promam - The monitoring of the sentinel wells will be conducted on a regular basis to detect a deficiency of iron in the system and allow time for Remedy Implementation. The specific details of the monitoring program associated with the contingency remedy will be outlined in the LTMP (Section 4.6).

Remedy Implementation - Adjustments to the chemistry of the groundwater approaching the IWS system will be made as deemed necessary to facilitate the re-precipitation of arsenic to less mobile forms. Such adjustments may include but are not limited to the addition of ferrous iron. Geochemical adjustments would be performed on an as-needed basis to maintain the necessary aquifer conditions. Field parameter measurements and inorganic groundwater samples will be collected on a periodic basis to confirm the desired conditions, and the monitoring of the sentinel well network will be maintained to assure the success of the contingency remedy.

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3.1.2 Expected Outcome of Alternative 6

The primary expected outcomes of the selected remedy are that: a) the groundwater at the Site (including the Source Area, the Southwest Plume, and the North Plume) will no longer present an unacceptable risk to future workers or residents via potable water ingestion and inhalation; b) the Site will be suitable for unrestricted land use; and c) groundwater will be suitable for potable purposes. Approximately 23 years are estimated as the amount of time necessary to achieve the goals consistent with unrestricted land use and potable use of groundwater for the entire Site (as noted in the remedial design prepared by ARCADIS). Portions of the Site (e.g. North Plume) may achieve the goals in a shorter period of time. Abating the unacceptable risk to benthic invertebrates via direct contact from discharge of groundwater to porewater of the Nashua River is also an expected outcome of the selected remedy. The low to moderate potential ecological effects will be mitigated by the remedy and goals consistent with long-term protection of benthic invertebrates. Another expected outcome of the selected remedy is that redevelopment in specified areas will be able to proceed once the remedy is Operating Properly and Successfully.

4.0 REMEDIAL ACTION DESCRIPTION

The proposed remedial action for AOC 50 is Alternative 6 from the Proposed Plan and ROD: Soil Vapor Extraction, Enhanced Reductive Dechlorination (with inorganics controls), In-Well StrippingIAerobic Bioremediation, Monitoring, and Institutional Controls. A description of the remedial action is outline in Section 3.0. This section will provide more details into the approach and rationale for implementation of the major components of the remedy.

4.1 MobilizationlSite Preparation

4.1 .I Mobilization

There will likely be multiple mobilizations to the Site for installation of the remedial system. The major components of the system include the installation of ERD injection wells, IWS wells, and monitoring/contingency wells. The major construction components include the construction of the mixing and delivery system for the ERD reagent, installation of an electrical line and transformer in the IWS area, trenching of remedial lines, construction of the remedial building and components for the IWS system, and refurbishing the SVE system.

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The construction Quality Assurance Plan (QAP) will be discussed in Section 5 of this report and provides the controls necessary to assure quality construction of the remedial system. A copy of the construction QAP is included in Appendix C.

The revised Sampling and Analysis Plan (SAP) completed by ARCADIS (ARCADIS 2003a) details field sampling protocols and field screening and laboratory procedures for groundwater and soil sampling and waste characterization, if necessary. The SAP is discussed further in Section 5.0 on Quality Control. The revised SAP is a stand-alone document to be used with this RAWP and other pertinent documents during remedy implementation.

The Site Health and Safety Plan (HASP) completed by ARCADIS (ARCADIS 2004b) has been updated to include safety guidelines for the work to be performed during remedial system implementation and includes key personnel, personal protection, and construction safety. The HASP is a stand-alone document to be used with this R A W and other pertinent documents during remedy implementation.

4.1.2 Site Preparation

Prior to drilling and trenching activities, Dig-Safe clearance will be obtained and coordinated with local utilities and property owners to insure worker safety and reduce the likelihood of damage to existing utilities.

If applicable, appropriate work zones shall be established to include at a minimum;

exclusion, contamination reduction, and support zones. Construction cones and work signs will be posted as appropriate to prevent unauthorized access to work areas.

A drudroll-off staging area will be designated in the Source Area for soil and groundwater that needs to be containerized for off-site disposal. The soil and groundwater detected at the Site to date has been designated as non-hazardous.

Health and safety equipment, including photoionization detector, fire extinguishers, first aid kit, eye wash station, and mobile communications will be available on the Site.

Communications will be provided via mobile telephones.

Drilling rigs, backhoes, and delivery vehicles, or equivalent machinery will be mobilized to the Site for drilling, trenching, and construction activities. A decontamination pad is already in-place at the Site.

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4.2 SVE System Implementation

SVE implementation will involve a review of the existing system plans, performing testing on the equipment and system components to be sure the system is operating properly, replacing and repairing damaged components, testing of existing SVE wells, and connecting and testing the newly installed SVE well. The overall effectiveness of the system will be determined during the testing period and will be based on the amount of mass that is recovered.

4.3 Full-Scale ERD System Construction

The full-scale ERD system design is based on pilot test data collected between December 2001 and July 2002…