S A JV 2024b Annual Report AOC 50.pdf

FINAL

United States Army Corps of Engineers

New England District

2023 Annual Operations, Maintenance, and Monitoring

Report Area of Contamination 50

Former Fort Devens Army Installation

Devens, Massachusetts

Contract No. W912WJ-19-D-0014

Contract Delivery Order No. W912WJ-23-F-0055

July 2024

FINAL 2023 Annual Operations, Maintenance, and Monitoring Report

Area of Contamination 50, Former Fort Devens Army Installation i

2023 Annual Operations, Maintenance, and Monitoring Report

Area of Contamination 50

Former Fort Devens Army Installation

Devens, Massachusetts

July 2024

Prepared By: Prepared For:

SERES-Arcadis 8(a) JV 2, LLC United States Army Corps of Engineers, 669 Marina Drive, Suite B-7 New England District

Charleston, South Carolina 29492

Tel 843 216 8531

CERTIFICATION

I hereby certify that the enclosed Report, shown and marked in this submittal, is that proposed to be incorporated with Contract Number W912WJ-19-D-0014. This document was prepared in accordance with the United States Army Corps of Engineers (USACE) Scope of Work and is hereby submitted for

Government approval.

Reviewed By:

Andy Vitolins, PG

Project Manager

Heather Levesque, PMP

Deputy Project Manager

Received By:

Penelope Reddy

USACE Project Manager

NOTICE

The United States Department of Defense, Department of Army, funded wholly or in part the preparation of this document and work described herein under Contract No.

W912WJ-19-D-0014. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

ii

Contents Acronyms and Abbreviations ................................................................................................................................. v

1 Introduction

1.1 Site Description and History

1.2 Site Hydrogeology

1.3 Summary of Historical Groundwater Remediation

2 Long-Term Monitoring Event Methodology

2.1 Water Level Measurements

2.2 Groundwater Sampling and Analysis

2.3 Decontamination Procedures and Waste Management

3 Groundwater Monitoring Results

3.1 Monitoring Well Inspection and Maintenance

3.2 Groundwater Level Measurements

3.3 Groundwater Chemistry

3.3.1 Field Parameters

3.3.2 Volatile Organic Compounds

3.3.3 Geochemical Analytes

3.3.4 Inorganic Metals

3.4 Quality Assurance/Quality Control

3.5 Laboratory and Field Corrective Actions

4 Remedial System Update

5 Land-Use Control Monitoring

5.1 On-Site Inspections

5.2 LUC Interview

5.3 LUC Monitoring Summary

6 Summary and Conclusions

7 References iii

Tables Table 1 AOC 50 Interim Cleanup Levels (in text)

Table 2 Groundwater Level Data, Spring 2023

Table 3 Groundwater Level Data, Fall 2023

Table 4 Monitoring Wells Sampled in 2023

Table 5 Sample Preparation and Analysis Methods, Containers, Holding Times, and Preservation

Table 6 Groundwater Analytical Results, Spring 2023

Table 7 Groundwater Analytical Results, Fall 2023

Table 8 Area-Specific Summary of Cleanup Level Exceedances (in text)

Figures Figure 1 Site Location

Figure 2 Site Layout

Figure 3 Site Layout – Source Area

Figure 4 Groundwater Contour Map – Spring 2023

Figure 5 Groundwater Contour Map – Fall 2023

Figure 6 Tetrachloroethene Concentrations in Groundwater – Fall 2004

Figure 7 Tetrachloroethene Concentrations in Groundwater – Fall 2023

Figure 8 Cross-Section A-A’

Figure 9 CVOC and Biogeochemical Data – Fall 2023, Area 1

Figure 10 CVOC and Biogeochemical Data – Fall 2023, Areas 2 through 4

Figure 11 CVOC and Biogeochemical Data – Fall 2023, Area 5

Figure 12 Arsenic Concentrations in Groundwater – Fall 2023 iv

Appendices Appendix A Groundwater Monitoring Field Forms

Appendix B Laboratory Analytical Reports

Appendix C Summary of Quality Control Exceedances and Data Validation Reports

Appendix D Historical Groundwater Analytical Results

Appendix E CVOC Trend Graphs

Appendix F Arsenic Trend Graphs

Appendix G Annual Land-Use Control Plan Checklist, Reinforcement Letters, and Real Property Master

Plan

Appendix H Response to Comments v

Acronyms and Abbreviations µg/L microgram per liter

ABC® Anoxic BioChem®

ABC® Ole’ Anoxic BioChem® plus oleic acid

ABC® Ole’+ Anoxic BioChem® plus oleic acid plus zero valent iron

AFCEE Air Force Center for Environmental Excellence

AOC area of contamination

Arcadis Arcadis U.S., Inc.

Army United States Army

BCT Base Realignment and Closure Cleanup Team bgs below ground surface

BRAC Base Realignment and Closure

CCV continuing calibration verification cis-1,2-DCE cis-1,2-dichloroethene

CL cleanup level

COC contaminant of concern

CVOC chlorinated volatile organic compound

DO dissolved oxygen

DoD Department of Defense

DOE Department of Energy

DPT direct-push technology

DRFTA Devens Reserve Forces Training Area

ERD enhanced reductive dechlorination

ESTCP Environmental Security Technology Certification Program

Fort Devens Former Fort Devens Army Installation

ICV initial calibration verification

IWS in-well stripping

KGS KOMAN Government Solutions, LLC

LTM long-term monitoring

LTMMP Long-Term Monitoring and Maintenance Plan

LUC land-use control vi

MAAF Moore Army Airfield

MassDevelopment Massachusetts Development and Finance Agency mg/L milligram per liter

MiHPT membrane interface probe-hydraulic profiling tool

MS matrix spike

MSD matrix spike duplicate mV millivolt

NAVFAC Naval Facilities Engineering Service Center

O&M operation and maintenance

ORP oxidation-reduction potential

PCE tetrachloroethene

QAPP Quality Assurance Project Plan

Quality Systems Manual Department of Defense and Department of Energy Consolidated Quality Systems

Manual for Environmental Laboratories, Version 5.4

RAWP Remedial Action Work Plan

ROD Record of Decision

S-A JV SERES-Arcadis 8(a) Joint Venture 2, LLC site Area of Contamination 50

Sovereign Sovereign Consulting Inc.

SU standard unit

SVE soil vapor extraction

TCE trichloroethene

TOC total organic carbon

USEPA United States Environmental Protection Agency

USFWS United States Fish & Wildlife Service

VC vinyl chloride

VOC volatile organic compound

1 Introduction SERES-Arcadis 8(a) Joint Venture 2, LLC (hereafter referred to as the S-A JV) prepared this 2023 Annual

Operations, Maintenance, and Monitoring Report for the United States Army Corps of Engineers – New England

District under Contract Number W912WJ-19-D-0014. This report details the operation and performance of ongoing remediation activities and groundwater monitoring results at Area of Contamination (AOC) 50 (the site) at the former Fort Devens Army Installation (Fort Devens) located in Devens, Massachusetts. Field activities were performed in accordance with the Long-Term Monitoring and Maintenance Plan (LTMMP; KOMAN Government

Solutions, LLC [KGS] 2017) and the Quality Assurance Project Plan (QAPP) for the Annual Long-Term Monitoring and Maintenance Program (S-A JV 2020a). The purpose of this report is to document the operation and maintenance (O&M) activities conducted at the site and to summarize the results of long-term monitoring (LTM) of groundwater conducted by the S-A JV in 2023.

1.1 Site Description and History

The site is located on the northeastern boundary of the former Moore Army Airfield (MAAF), within the former

North Post of the former Fort Devens, in the town of Ayer, Massachusetts (Figure 1). Fort Devens was identified for cessation of operations and closure under Public Law 101-510, the Defense Base Realignment and Closure

(BRAC) Act of 1990 and was officially closed in March 1996. A portion of the property formerly occupied by Fort

Devens was retained by the United States Army (Army) for reserve forces training and was renamed as the

Devens Reserve Forces Training Area (DRFTA). Areas not retained as part of the DRFTA were transferred to new owners (the Massachusetts Development and Finance Agency [MassDevelopment] and the United States

Fish & Wildlife Service [USFWS]) for reuse and redevelopment. In 2009, the DRFTA was renamed the United

States Army Garrison Fort Devens.

All but approximately 13.4 acres of the former MAAF (approximately 246 acres total) were transferred to

MassDevelopment for reuse in 1997; the Devens Army Installation retained approximately 9.1 acres of the former airfield for vehicle storage and maintenance and approximately 4.3 acres for remediation activities. Currently, the airfield is closed to aircraft traffic and is used by the Massachusetts State Police for training purposes and vehicle storage. The former MAAF is zoned for Special Use II and Innovation and Technology Business by

MassDevelopment. Under the Devens Reuse Plan (Vanasse Hangen Brustlin, Inc. 1994), Special Use II and

Innovation and Technology Business includes a broad range of industrial, light industrial, office, and research and development use. MassDevelopment is considering repairs and upgrades for the Massachusetts State Police’s use of the airfield, but the plans are still under development.

Sources of groundwater contamination at AOC 50 include two World War II fueling systems, a dry well formerly connected to the parachute shakeout tower, and the former tetrachloroethene (PCE) drum storage area; these sources are collectively referred to as the source area (aka, Area 1, displayed on Figures 2 and 3). The source area comprises less than two acres and surrounds Building 3803 (the former parachute shop), Building 3840 (the former parachute shakeout tower), Building 3824 (a gazebo), and Building 3801 (the former 10th Special Forces airplane parachute simulation building). The groundwater contamination of concern at this site is a chlorinated volatile organic compound (CVOC) plume that formerly extended from the source area (Area 1) approximately

3,000 feet downgradient southwest towards the Nashua River. Although the sources of contamination have been removed or taken out of commission, groundwater underlying the site contains residual concentrations of CVOCs, including PCE, and the degradation products of PCE: trichloroethene (TCE), cis-1,2-dichloroethene (cis-1,2-

DCE), and vinyl chloride (VC). The site layout with existing monitoring wells is shown on Figures 2 and 3.

The Record of Decision (ROD) for AOC 50 (United States Environmental Protection Agency [USEPA] 2004) identified the groundwater contaminants of concern (COCs) and their interim cleanup levels (CLs), as shown in

Table 1, below.

Table 1 AOC 50 Interim Cleanup Levels

Chemical CL (µg/L) Chemical CL (µg/L)

Arsenic 10 Lead 15

Benzene 5 Manganese 1,460

1,2-dichloroethane 5 Methylene chloride 5

1,1-dichloroethene 7 Nitrate 10,000 cis-1,2-DCE 70 PCE 5

1,2-dichloropropane 5 TCE 5

Iron 3,129 VC 2

Note:

µg/L = microgram per liter

Some of the contaminants were designated as COCs because they were present at concentrations above the applicable USEPA maximum contaminant level or state groundwater quality standard (i.e., cis-1,2-DCE, 1,2-dichloropropane, iron, methylene chloride, PCE, TCE, and VC). Benzene was designated as a COC due to past releases, cis-1,2-DCE and nitrate were identified as COCs due to their contribution to risk, and lead was identified as a COC due to its potential to pose ecological risks. In 2006, lead analysis was required as part of the Long-

Term Monitoring Plan (Arcadis Geraghty & Miller, Inc. 2006); however, as of 2008, lead monitoring is no longer a requirement (Arcadis 2008). Arsenic was designated as a COC because it may be solubilized (mobilized) by the selected remediation technology (enhanced reductive dechlorination [ERD]), even though it was not predicted to pose significant risks under baseline (i.e., oxic) conditions.

1.2 Site Hydrogeology

An unconfined groundwater aquifer is present within the overburden deposits at the site. Restrictions to vertical groundwater flow, such as silty clay layers, were noted to be present, but not prevalent or continuous, in soil borings drilled in the area during the remedial investigations.

Depth-to-groundwater measurements have been collected from a network of monitoring wells and sampling points routinely since 1997. In 2023, the average depth to groundwater ranged from approximately 10-12 feet below ground surface (bgs) in the upgradient (northern) portion of Area 1 to over 60 feet bgs in Area 5 (Tables 2 and 3). Hydraulic conductivities at the site range from one foot per day to more than 50 feet per day, with seepage velocities ranging from 0.024 to 1.19 feet per day (USEPA 2004). As shown on Figure 4 (Spring 2023) and

Figure 5 (Fall 2023), groundwater migrates from the source area southwest below the MAAF, and discharges to the Nashua River.

1.3 Summary of Historical Groundwater Remediation

The selected remedy for the site, as documented in the ROD (USEPA 2004), was Alternative 6: Soil Vapor

Extraction (SVE), ERD with solubilized inorganic controls, In-Well Stripping (IWS)/Oxic Bioremediation, Monitoring, Institutional Controls, and Five-Year Site Reviews. In addition, geochemical additives and in-situ chemical oxidation were included as contingencies to address inorganic compounds and CVOCs, respectively, if monitoring data indicate that implementation of these contingencies is warranted.

Portions of the CVOC plume are referred to as Areas 1 through 5 as indicated in Figure 2. These area designations are based on the locations of the initial five ERD injection well transects, beginning with Area 1 in the source area and ending with Area 5 to the southwest, upgradient of the Nashua River, with Areas 2 through 4 spanning the MAAF.

The SVE system was operated from September 2004 until November 2005, when operational monitoring indicated that the recoverable mass of CVOCs had been removed from the vadose zone (KGS 2020). The IWS system was operated in/near Area 5 from September 2004 until March 2013. The IWS system was shut down once it was clear that it was not needed to treat dissolved arsenic or to “polish” the CVOC plume (Sovereign

Consulting Inc. [Sovereign] 2013). In 2017, an assessment of the IWS system was performed and revealed corrosion of pumps and blowers. While the IWS system is not needed at this time, it is likely the central processing unit along with various pumps, blowers, and other system components would need to be replaced if there is need to reactivate it in the future. At the Base Realignment and Closure Cleanup Team (BCT) meeting on

January 18, 2018, the Army and USEPA agreed that the system could be repaired in the future if needed.

The first ERD injection event was conducted in October 2004, using molasses as the ERD substrate.

HydroGeoLogic, Inc. revised the ERD injection program in November 2008 by changing the substrate from molasses to Anoxic BioChem® (ABC®). Semi-annual injections of ABC® continued through 2014. In 2015, the substrate injection strategy was optimized based on review of the historical data and 2013 and 2014 source area investigation results (Sovereign 2015). Optimization included modification of the substrate used at some locations as well as use of direct push technology (DPT) for injection at some locations to supplement the injections that are performed in permanent wells. The July 2015 injection event used ABC® at some locations, and ABC® plus oleic acid (ABC® Ole’) and ABC® Ole’ plus zero valent iron (ABC® Ole’+), substrate mixtures that persist longer in the aquifer, at some locations. ABC® was injected into existing injection wells in Areas 4 and 5; ABC® Ole’ was injected into existing injection wells in Areas 1, 2, and 3; and ABC® Ole’+ was injected using DPT via Geoprobe® into targeted locations in Area 1. Additional injection events in July 2017, August to September 2018, and July to

August 2019 also used ABC® Ole’ and ABC® Ole’+, which were injected throughout the CVOC plume using injection wells and DPT injection points, respectively. The details of these injection events are presented in the

2017, 2018, and 2019 Annual O&M and Monitoring Reports (KGS 2018, 2019, 2020).

When the ROD was filed for the site in 2004 (USEPA 2004), it was estimated that ERD injections would be phased out after approximately 10 to 15 years. While some CL exceedances of PCE remain, the following has been observed as a result of performing ERD at the site since 2004:

The total area of exceedances of the CL for PCE has decreased in size, from an estimated 15.5 acres in Fall

2004 (Figure 6) to approximately 1.5 acres in Fall 2023 (Figure 7).

The maximum observed PCE concentration at source area well G6M-07-02X has decreased by three orders of magnitude, from 14,000 µg/L in 2014 to non-detect in Fall 2023.

The number of monitoring wells exceeding the CL for PCE has decreased from 35 of 41 wells (85%) in 2004 to 3 of 36 wells (8%) in Fall 2023.

2 Long-Term Monitoring Event Methodology The LTM program at the site was conducted by the S-A JV in Spring 2023 (June) and Fall 2023 (October and

November) in accordance with the most recent update of the LTMMP (KGS 2017).

2.1 Water Level Measurements

Before conducting groundwater sampling, static water level measurements were collected from site wells using an electronic water level meter. Water level measurements were collected from a total of 53 wells during the Spring

2023 monitoring event and 51 wells during the Fall 2023 monitoring event, with measurements recorded within one field day to the extent possible. Before gauging, the monitoring wells were allowed to equilibrate to atmospheric pressure. Water level elevations were calculated by subtracting depth-to-water measurements from the surveyed top-of-casing elevations and used to prepare potentiometric surface maps.

2.2 Groundwater Sampling and Analysis

Groundwater sampling was performed in accordance with the QAPP (S-A JV 2020a) and low-flow procedures documented in USEPA (2017) guidance. Groundwater samples from each well sampled were analyzed for VOCs.

In addition, a subset of wells were sampled for geochemical analytes, including total organic carbon (TOC), dissolved gases (methane, ethane, and ethene), alkalinity, nitrate, sulfide, sulfate, and dissolved metals

(manganese, iron, and arsenic). Dissolved metals samples were field-filtered using 0.45-micron filters before being containerized for laboratory analysis. The frequency, sampling locations, and analytical parameters for LTM sampling conducted in 2023 are summarized in Table 4. Information on the analytical methods, sample containers, preservatives, and holding times for the parameters sampled are presented in Table 5. Completed groundwater low-flow sampling forms and equipment calibration records for the 2023 groundwater monitoring events are presented in Appendix A.

2.3 Decontamination Procedures and Waste Management

In accordance with the QAPP (S-A JV 2020a), all non-disposable sampling and field equipment (such as bladder pumps and water level meters) were decontaminated between wells by using a non-phosphate detergent solution

(Alconox) and rinsing with distilled water. Dedicated bladders were used at each location.

Investigation-derived waste generated during groundwater sampling activities consisted of purge water from wells, equipment decontamination water, and disposable supplies such as nitrile gloves and paper towels. Purge water and equipment decontamination water were discharged back to the ground at the site of generation after sampling was completed, in accordance with the LTMMP (KGS 2017). Used disposable supplies were bagged and disposed of as general solid waste.

3 Groundwater Monitoring Results Sampling and analysis of chemical and geochemical parameters in groundwater was performed in accordance with the LTMMP (KGS 2017). Monitoring well inspection, water level elevation, and groundwater analytical results for the Spring 2023 (June) and Fall 2023 (October and November) monitoring events are discussed in the following subsections.

3.1 Monitoring Well Inspection and Maintenance

Before sampling, the following procedures were used to inspect the LTM wells at the site:

The monitoring well location was identified, and the well identification number was confirmed.

The wellhead was inspected for signs of damage and any damage was noted.

The monitoring well gripper seal was removed, and the wellhead was allowed to vent.

Water levels and total well depth readings were collected and compared to historical measurements.

Several wells had missing protective casing locks or missing bolts, which were replaced before completing the monitoring events.

During the Fall 2023 monitoring event, the dedicated bladder pump in well XSA-12-97X was not purging water and became lodged in the well casing upon removal. Numerous efforts were made via manual and mechanical methods to extract the pump but were unsuccessful. The S-A JV will assess whether an alternative monitoring well can be substituted for this location in 2024.

3.2 Groundwater Level Measurements

A summary of the groundwater level measurements and the corresponding groundwater elevation data are presented in Tables 2 and 3 for the Spring and Fall 2023 monitoring events, respectively. Groundwater elevation contours for Spring and Fall 2023 are presented on Figures 4 and Figure 5, respectively.

Consistent with previous monitoring events, the observed groundwater flow direction was toward the Nashua

River, from northeast to southwest. In general, groundwater elevations were higher in Fall 2023 compared to

Spring 2023. The groundwater hydraulic gradient between Area 1 well G6M 07-02X and downgradient well XSA-

12-96X (southwest edge of the CVOC plume downgradient of Area 5) was calculated to be 0.0029 feet per foot in

Spring 2023 and 0.0042 feet per foot in Fall 2023. In general, the horizontal hydraulic gradient is steeper between

Area 1 and Area 3, and the hydraulic gradient decreases downgradient of Area 3. Vertical gradients are indicated in some well pairs. Area 1 well pair G6M-96-22A and G6M-96-22B indicated a downward gradient, in an area of recharge, during the Spring 2023 monitoring event. These wells were not measured in Fall 2023 due to flooded ground around the wells. A slight downward gradient was noted in Area 1 well pair G6M-04-10A and G6M-04-10X in both the Spring and Fall 2023 monitoring events. A slight downward gradient was observed during the Spring

2023 monitoring event in Area 5 well pair G6M-04-06X and G6M-04-07X, with a slight upward gradient observed in this well pair during the Fall 2023 monitoring event.

3.3 Groundwater Chemistry

The Spring and Fall 2023 monitoring events consisted of sampling 10 wells and 37 wells, respectively. Thirty-six wells were sampled during the Fall 2023 monitoring event for VOC analysis; one additional well (G6M-97-28X) was sampled for dissolved metals only in Fall 2023. This differs from the 9 wells and 45 wells in Spring and Fall

2022, respectively, for the following reasons:

One additional monitoring well (XSA-12-97X, downgradient of Area 5) was added to the Spring 2023 monitoring event to assess increases in arsenic/iron/manganese concentrations observed in this well since the Fall 2020 monitoring event. The same monitoring well could not be sampled due to pump failure and then well blockage during the Fall 2023 monitoring event (Section 3.1) and was not sampled in Fall 2023.

Seven monitoring wells (G6M-02-12X, G6M-03-08X, G6M-13-03X, G6M-04-05X, G6M-04-11X, G6M-04-22X, and G6M-04-31X) are sampled on a biennial basis during the Fall events, and were not sampled in Fall 2023 in accordance with the LTMMP (KGS 2017).

The frequency, sampling locations, and analytical parameters for LTM sampling conducted in 2023 are summarized in Table 4. The Spring and Fall 2023 LTM analytical results and field parameters are presented in

Tables 6 and 7, respectively. The laboratory analytical reports for the 2023 monitoring events are included as

Appendix B. The data validation reports for the 2023 data are presented in Appendix C. Historical analytical results and groundwater field parameters, including the 2023 sampling results, are presented in Appendix D.

3.3.1 Field Parameters

The current well sampling program is detailed in the 2017 Long-Term Monitoring and Maintenance Plan prepared by KGS. The annual LTM sampling is conducted in the Fall to evaluate the extent of plume as part of the annual monitoring. Fewer wells are sampled in the Spring as this event is used to assess if injections are warranted. The

Spring 2023 field parameters were generally consistent with conditions in Fall 2023. Field parameter measurements collected during the Fall 2023 monitoring event are summarized below.

pH levels in groundwater were within the optimal pH range for ERD bioremediation (pH of 6 to 8 standard units [SU]) (Interstate Technology and Regulatory Council 2008) in all but two locations (Table 7): G6M-04-

02X (5.69 SU) in Area 3, and G6M-95-19X (5.53 SU) in Area 1. While below the optimal range in some wells, ERD is still capable of proceeding at pH values less than 6 SU.

Dissolved oxygen (DO) concentrations less than 1 milligram per liter (mg/L) indicate anoxic conditions, while negative oxidation-reduction potential (ORP) values indicate generally reducing conditions. In general, anoxic and/or reducing conditions were observed within and downgradient of the in situ reactive zones (areas/zones where substrate injections have occurred in the past) due to organic carbon addition, allowing for ERD of

CVOCs. Oxic or moderately reducing conditions are present outside of areas affected by ERD treatment.

In Area 1, anoxic conditions (i.e., DO less than 1 mg/L) were observed at eight of 11 wells sampled, with

DO ranging from 0.27 to 0.85 mg/L in those eight wells. Negative ORP was recorded at all 11 wells sampled, with ORP ranging from -151 to -0.2 millivolts (mV).

In Area 2, anoxic conditions were observed at two of four wells sampled, with DO concentrations of 0.13 and 0.80 mg/L. Negative ORP was recorded at three of the four wells sampled, with ORP ranging from -160 to -78.8 mV in those three wells.

In Area 3, anoxic and reducing conditions were observed in all three wells sampled, with DO ranging from

0.13 to 0.20 mg/L and ORP ranging from -107 to -41.8 mV in those three wells.

In Area 4, anoxic conditions were observed in all four wells sampled, with DO ranging from 0.13 to 0.64 mg/L. Negative ORP was recorded at three of the four wells sampled, with ORP ranging from -160 to -98.4 mV in those three wells.

In Area 5, anoxic conditions were observed in eight of the 14 wells sampled, with DO ranging from 0.04 to

0.96 mg/L at those eight wells. Negative ORP was recorded at 10 of the 14 wells sampled, with ORP ranging from -217 to -30.9 mV in those 10 wells.

Adjacent to the Nashua River, the one monitoring well sampled (G6M-04-14X) had anoxic and mildly reducing conditions (0.26 mg/L DO and ORP value of -25.3 mV).

3.3.2 Volatile Organic Compounds

Groundwater samples were collected from 10 wells during the Spring 2023 monitoring event and 36 wells during the Fall 2023 monitoring event for VOC analysis; one additional well (G6M-97-28X) was sampled for dissolved metals only in Fall 2023. Benzene, PCE, TCE, cis-1,2-DCE, and VC were the only VOCs detected above the established AOC 50 CLs during the Spring and Fall 2023 monitoring events. Concentrations of these CVOCs in

Spring and Fall 2023 are shown in Tables 6 and 7, respectively. Historical concentrations are included in

Appendix D and concentration trends at select monitoring wells are provided in Appendix E. PCE results from the

Fall 2023 monitoring event and the associated PCE plume are shown on Figure 7 (plan view) and Figure 8 (cross-section view). VOC results from the Spring and Fall 2023 monitoring events are discussed in the following subsections; Fall monitoring results are described in greater detail than Spring results because more wells were sampled in Fall (37 wells) than Spring (10 wells).

3.3.2.1 Spring 2023

Results of the Spring 2023 sampling event indicate that the injections performed in 2019 and prior years continue to have the intended impact on CVOC concentrations at the targeted wells. Historically, locations where PCE was greater than 100 µg/L had been targeted for ERD treatment. As shown in Table 6, all PCE detections were below the historical target level of 100 µg/L. Two of 10 sampled locations had PCE detections greater than the CL; the two samples were collected from well G6M-04-02X in Area 3 (26.0 J µg/L) and well G6M-13-05X in Area 1

(39 µg/L). Benzene was detected above the CL of 5 µg/L in one of 10 sampled locations at a concentration of

5.5 µg/L (well XSA-12-97X in Area 5), and TCE and VC were detected above their respective CLs in four of 10 sampled locations in Areas 1, 3, 4, and 5. Table 6 summarizes these detections.

3.3.2.2 Fall 2023

Similar to Spring 2023, the Fall 2023 groundwater monitoring results indicate that the injections performed in 2019 and prior years continue to have the intended impact on CVOC concentrations within the targeted areas.

Analytical results are included in Table 7 and on Figure 9 (Area 1), Figure 10 (Areas 2, 3, and 4), and Figure 11

(Area 5). Figure 7 includes the results for PCE and depicts the associated area where PCE exceeds the CL.

Area-specific summaries of locations that exceeded the CL for PCE, TCE, cis-1,2-DCE, and VC in Fall 2023 are included in Table 8, below.

A summary of results for each CVOC is provided below:

PCE was detected above the CL of 5 µg/L at three of 36 sampled locations, compared to four of the 44 sampled locations during the Fall 2022 monitoring event. Three sampled locations with CL exceedances also exceeded the CL in Fall 2022 (wells G6M-04-02X, G6M-07-01X, and G6M-13-05X). The maximum detection of PCE in Fall 2023 was 11 µg/L at well G6M-07-01X (Area 2).

TCE was detected above the CL of 5 µg/L at five of 36 sampled locations, compared to four of 44 locations during the Fall 2022 monitoring event. Three sampled locations with CL exceedances also exceeded the CL in Fall 2022 (wells G6M-02-01X, G6M-04-02X, and G6M-04-07X). The maximum detection of TCE in Fall

2023 was 26 µg/L at well G6M-04-02X (Area 3).

cis-1,2-DCE was detected above the CL of 70 µg/L at one of 36 sampled locations (compared to two of

44 locations in the Fall 2022 monitoring event). The sampled location with a CL exceedance also exceeded the CL in Fall 2022 (well G6M-04-07X). The maximum detection of cis-1,2-DCE in Fall 2023 was 74 µg/L at well G6M-04-07X (Area 5).

VC was detected above the CL of 2 µg/L at five of 36 sampled locations, compared to 7 of 44 locations in the

Fall 2022 monitoring event. Four sampled locations with CL exceedances also exceeded the CL in Fall 2022

(wells G6M-04-02X, G6M-04-07X, G6M-13-05X, and XSA-12-96X). The maximum detection of VC in Fall

2023 was 42 µg/L at well G6M-13-05X (Area 1).

When compared to historical CVOC concentrations at the site, these results continue to indicate significant improvements to groundwater quality because of ERD remediation performed to date.

Table 8 Area-Specific Summary of Cleanup Level Exceedances

Area Number of Locations

Sampled for VOCs

Number of Locations

Exceeding the CL for PCE, TCE, cis-1,2-DCE, and/or VC

Wells and Compounds that

Exceeded the Interim CL

1 11 2

G6M-13-05X: PCE, VC

G6M-04-10A: TCE, VC

2 4 2

G6M-02-01X: TCE

G6M-07-01X: PCE

3 3 1 G6M-04-02X: PCE, TCE, VC

4 3 0 None

5 14 2 G6M-04-07X: cis-1,2-DCE, TCE, VC

XSA-12-96X: TCE, VC

Nashua River

1 0 None

3.3.2.3 Concentration Trends

Trend graphs showing PCE and associated daughter products (TCE, cis-1,2-DCE, VC, and ethene) for 14 key performance monitoring wells are included as Appendix E. These wells are representative of conditions at different locations within the CVOC plume and help illustrate the effects of the ERD injection program. Long-term

CVOC trends support the conclusion that the July/August 2019 ERD injections continue to be effective. PCE degradation products are detected throughout the extent of the CVOC plume, and concentrations appear to be generally decreasing or stable in most wells. CVOC trends are described below.

Area 1

At monitoring well G6M-07-02X, located within the 2019 DPT injection area, PCE and TCE concentrations have decreased from their historical maximums of 14,000 µg/L (Fall 2014) and 190 µg/L (Fall 2012), respectively, to non-detect for each parameter in Fall 2023. All CVOCs have been below their applicable CLs at this well since Fall 2021 (Appendix E, Figure E-1).

At monitoring well G6M-04-10A, located adjacent to the 2019 DPT injections, PCE concentrations have decreased substantially from 6,100 µg/L in Spring 2006 to non-detect in Fall 2023. PCE and/or TCE concentrations have occasionally exceeded CLs during recent monitoring events (Spring 2019, Fall 2019, Fall

2020, and Fall 2023). TCE (9.4 µg/L) and VC (3.0 µg/L) were detected above their applicable CLs in Fall 2023 after five non-detect sampling events between Spring 2021 and Spring 2023 (Appendix E, Figure E-2).

At monitoring well G6M-04-09X, located outside and down/cross gradient to the 2019 DPT injection area, PCE concentrations have decreased by over three orders of magnitude, from 7,400 µg/L in Spring 2004 to non-detect in Fall 2023. Similar to well G6M-04-10A, PCE and/or TCE have occasionally exceeded CLs recently (Fall 2020 and Spring 2021), but all CVOCs have been below CLs since Fall 2021 (Appendix E, Figure E-3).

At monitoring well G6M-13-05X, located outside and cross gradient to the area influenced by 2019 injections, PCE concentrations have decreased by approximately two orders of magnitude, from 1,700 µg/L in Fall 2014 to 10 µg/L in Fall 2023. At this well, PCE and TCE concentrations have fluctuated recently, albeit at relatively low levels. From Spring 2021 to Fall 2023, PCE concentrations have ranged from 8.0 to 50 µg/L, while TCE concentrations have ranged from 2.4 to 18 µg/L (Appendix E, Figure E-4). The PCE exceedance shown on

Figure 7 at this monitoring well is bounded by monitoring wells with detections below the CL of 5 µg/L and non-detect concentrations.

Monitoring well G6M-96-25B, sampled most recently in Fall 2012 (2 µg/L), located approximately 155 feet west of G6M-13-05X;

Monitoring well G6M-04-11X, sampled most recently in Fall 2022 (non-detect), located approximately

120 feet southwest of G6M-13-05X; and

Monitoring well G6M-04-22X, sampled in Fall 2022 (non-detect), located approximately 150 feet south of

G6M-13-05X.

Area 2

At monitoring well G6M-02-01X, PCE concentrations have decreased by over two orders of magnitude, from

1,900 µg/L in Fall 2007 to non-detect in Fall 2023. PCE, TCE, and VC concentrations have fluctuated at this location recently (at relatively low levels), with concentrations slightly greater than CLs observed in Fall 2020, Fall 2022, and Fall 2023 (Appendix E, Figure E-5). TCE was the only CVOC to exceed the CL in Fall 2023

(6.3 µg/L).

At monitoring well G6M-07-01X, PCE concentrations have declined from 50 µg/L in Fall 2010 to 11 µg/L in

Fall 2023. TCE has not exceeded the CL since Fall 2009, and cis-1,2-DCE and VC have never exceeded their applicable CLs (Appendix E, Figure E-6).

Area 3

At monitoring well G6M-04-02X, located within the 2019 DPT injection area, PCE and TCE concentrations have decreased from their historical maximums of 1,900 µg/L (Fall 2004) and 170 µg/L (Fall 2006), respectively, to 9.9 and 26 µg/L in Fall 2023, respectively. However, PCE and TCE concentrations have increased compared to Spring 2020. Daughter products cis-1,2-DCE and, to a lesser extent, VC have been generated by reductive dechlorination (Appendix E, Figure E-7). In Fall 2023, cis-1,2-DCE was detected at concentration of 46 µg/L which is below the CL of 70 µg/L, and VC was detected at a concentration of 8.1 µg/L which is above the CL of 2 µg/L.

At monitoring well G6M-03-07X, located downgradient of the area influenced by 2019 injections, CVOC concentrations have remained below applicable CLs in 23 of 26 monitoring events completed since Spring 2010, including every event since Fall 2020 (Appendix E, Figure E-8).

Area 4

At monitoring well G6M-13-02X, located hydraulically downgradient of permanent wells used in the 2019 injections, PCE and TCE concentrations have decreased from their historical maximums of 210 µg/L (Fall 2014) and 54 µg/L (Summer 2014), respectively, to non-detect and 3.3 µg/L in Fall 2023. All CVOCs were below their applicable CLs in Fall 2023 (Appendix E, Figure E-9).

Area 5

At monitoring well G6M-13-04X, located immediately downgradient of the 2019 injection area, CVOC concentrations have declined since 2014 and have been below applicable CLs since Spring 2020 (Appendix E, Figure E-10).

At monitoring well G6M-97-05B, located along the centerline of the CVOC plume and in an area treated via DPT injections in 2019, the previously increasing PCE concentration has decreased from 180 µg/L in Spring 2019 (i.e., prior to the 2019 injections) to non-detect since Fall 2021. The TCE concentration at this location decreased from 22 µg/L in Spring 2019 to 0.98 µg/L in Fall 2023, while cis-DCE and VC concentrations initially increased due to reductive dechlorination but were below their respective CLs in Fall 2023 (Appendix E, Figure E-11).

At downgradient monitoring well XSA-12-96X, PCE concentrations continue to follow a long-term decreasing trend. While injections are not performed adjacent to this well, the effect of treatment upgradient is apparent in the PCE concentration trend at this location, decreasing from 120 µg/L in Fall 2012 to 4.8 µg/L in Fall 2023.

TCE, cis-1,2-DCE, and VC have been declining notably since Fall 2019; however, slight increases in concentrations of TCE and VC were noted in the Fall 2023 data when compared to Fall 2022. TCE (7.7 µg/L) and VC (7.9 J µg/L) were the only CVOCs detected at concentrations greater than CLs in Fall 2023 (Appendix E, Figure E-12).

At downgradient monitoring well XSA-12-97X, PCE and TCE concentrations have been non-detects during the last four annual sampling events. While injections are not performed adjacent to this well, the effect of treatment upgradient is apparent, with decreasing PCE and TCE concentrations since Fall 2019 (Appendix E, Figure E-13).

At downgradient monitoring well G6M-04-07X, PCE concentrations have decreased by approximately three orders of magnitude, from 1,100 µg/L in Winter 2004 to 4.7 µg/L in Fall 2023. While injections have not been performed adjacent to this well, the decreasing PCE concentrations and increase of daughter products TCE, cis-1,2-DCE, and VC since Fall 2012 is occurring due to reductive dechlorination (Appendix E, Figure E-14).

The three daughter products exceeded the CL in Fall 2023.

3.3.3 Geochemical Analytes

As shown in Table 5, groundwater samples were submitted for analysis of dissolved gases (methane, ethane, and ethene), TOC, alkalinity, nitrate, sulfide, and sulfate for select wells during the Spring and Fall 2023 monitoring events, in accordance with the LTMMP (KGS 2017). These general chemistry parameters were analyzed to document the geochemistry of the aquifer, which can then be used to interpret whether conditions are suitable for reductive dechlorination (or, in the case of ethane and ethene as further evidence of reductive dechlorination).

The key geochemical parameters for ERD include the following:

Following depletion of oxygen, alternate terminal electron acceptors are used by microorganisms, including nitrate and sulfate, which reduce to nitrite and sulfide, respectively. Concentrations of these anions are generally low across the site and, therefore, are not discussed further in this report.

Methane is the result of metabolic reactions involving the reduction of alternate terminal electron acceptor carbon dioxide. Methane concentrations greater than 500 µg/L are indicative of favorable reducing conditions

(Air Force Center for Environmental Excellence [AFCEE], Naval Facilities Engineering Service Center

[NAVFAC], and Environmental Security Technology Certification Program [ESTCP] 2004). High dissolved methane concentrations are generally indicative of an ongoing reducing zone where bacteria continue to metabolize organic carbon. Dissolved methane can be used to infer proximity to locations that are conducive to promoting ERD.

The presence of ethene/ethane indicates the microbes required for complete reductive dechlorination are active and present within site groundwater and indicate that chlorinated ethenes are being completely degraded.

TOC is an indicator of natural organic carbon present at a site during baseline characterization and is an indicator of substrate distribution and persistence during performance monitoring.

A total of 15 monitoring wells were sampled for all general chemistry parameters during the Fall 2023 monitoring event. A summary of the results is provided below, and general chemistry parameter values are presented in

Table 7 and on Figures 9 through 11.

3.3.3.1 Dissolved Gases

The presence of methane throughout the CVOC plume at the site indicates that conditions in the aquifer support continued reductive dechlorination of CVOCs. Methane concentrations were above 500 µg/L in 13 of 15 monitoring wells sampled and were highly elevated (defined as above 10,000 µg/L [AFCEE, NAVFAC, and

ESTCP 2004]) in 11 of 15 monitoring wells. Methane concentrations were generally highest in wells within or adjacent to the 2019 injections (e.g., G6M-03-07X and G6M-97-05B). These elevated methane concentrations indicate conditions are sufficiently reducing to allow for continued reductive dechlorination to occur in site groundwater.

Ethene concentrations were above detection limits at 10 of 15 monitoring wells and ethane concentrations were above detection limits at 13 of 15 monitoring wells. Ethene and/or ethane were most prevalent within and immediately downgradient of injection areas. The presence of ethene and ethane throughout the site indicate the microbes required for complete reductive dechlorination are active and present within site groundwater and indicate that PCE is being completely degraded.

3.3.3.2 Organic Carbon Distribution

Concentrations of TOC measured in and downgradient of the ERD injection areas indicate that carbon is available in the aquifer to continue supporting reductive dechlorination of CVOCs at the site. TOC concentrations at wells not influenced by previous ERD injections have historically been below 10 mg/L, and typically below 5 mg/L;

therefore, any concentration greater than 10 mg/L is considered elevated above background.

Monitoring well G6M-07-02X (Area 1) located within 2019 injection area exhibited highly elevated TOC concentrations (greater than 100 mg/L) in Fall 2022. The Fall 2023 TOC concentration was 95 mg/L.

Three monitoring wells contained moderately elevated TOC concentrations (between 18 and 52 mg/L), including two wells located within or adjacent/downgradient of the 2019 injection areas (G6M-04-10A in Area

1, G6M-03-07X in Area 3) and one well located in Area 1 to the west of where injections were most recently performed in 2018 (G6M-13-06X).

The organic carbon reagents injected in 2017, 2018, and 2019 are semi-soluble substrates designed to provide both a soluble and readily available carbon substrate (via lactate and/or alcohols) as well as a less soluble portion

(vegetable oils and fatty acid esters) that more slowly release organic carbon over a period of years. Monitoring data indicate that the desired slow dissolution of TOC from the injected substrate persists and continues to promote reductive dechlorination of CVOCs in groundwater at the locations discussed above.

3.3.4 Inorganic Metals

Groundwater samples from both 2023 monitoring events were submitted for dissolved metals (arsenic, iron, and manganese) analysis (Table 5). Results for Spring 2023 and Fall 2023 are included in Tables 6 and 7, respectively. Of the three dissolved metals sampled, arsenic is of primary interest. As noted in Section 1.1, arsenic was designated as a COC in the ROD (USEPA 2004) because it may be solubilized (mobilized) by the selected remediation technology (ERD), even though it was not predicted to pose significant risks under baseline conditions. Under the reducing conditions present at the site due to substrate injections, naturally occurring arsenic associated with iron and manganese oxyhydroxide minerals is typically released into solution due to reductive dissolution (USEPA 2007). For example, ferric iron present in the oxides is reduced to ferrous iron, which is more soluble. Similarly, solid manganese minerals (Mn4+) are reduced to more soluble manganese

(Mn2+). These dissolution processes also release arsenic associated with the minerals, and the iron, manganese, and arsenic can then be detected in dissolved metals analyses. This relationship is observed throughout site treatment areas and was an anticipated by-product of the selected remedial action.

Dissolved arsenic concentrations for the Fall 2023 monitoring event are presented on Figure 12, historical arsenic results are included in Appendix D, and historical arsenic trends at select wells throughout the plume are provided in Appendix F. Arsenic concentrations in groundwater exceeded the CL of 10 µg/L at 24 of 34 wells sampled for metals in Fall 2023. These wells are located within and downgradient of former injection locations, where geochemical conditions in groundwater are reducing.

The most noticeable increase in arsenic concentrations in recent years is at Area 5 monitoring well G6M-97-05B and downgradient monitoring well XSA-12-97X (Appendix F). These locations are within and downgradient of

2019 injections performed near G6M-97-05B (Figure 12). These increases point to the importance of balancing the desire to perform additional carbon injections to treat CVOCs with the desire to maintain groundwater quality in the downgradient area of the plume. The maximum concentration of dissolved arsenic detected in Fall 2023 was 680 µg/L at monitoring well G6M-02-11X (Area 5). In wells where conditions are primarily oxidizing and/or locations that are outside the areas where ERD has resulted in reducing geochemical conditions in the aquifer, the Fall 2023 dissolved arsenic concentrations were generally non-detect or below the CL of 10 µg/L (Figure 12).

Once ERD injection events are suspended, it will take time for the aquifer to return to conditions that will allow for re-attenuation of arsenic. Conditions were generally oxic (DO above 1 mg/L and positive ORP) before injections were initiated at the site and are expected to return to those conditions gradually once injections are suspended.

The dissolved arsenic and iron concentrations in site groundwater typically correlate with a greater than 100:1 ratio of iron to arsenic, indicating that both metals are being solubilized at the site. These metals re-precipitate when the groundwater migrates to oxic zones and when the groundwater within the treatment zone returns to oxidizing conditions. Under these conditions, arsenic attenuation occurs as iron oxides precipitate and the arsenic sorbs to and co-precipitates with the particulate iron.

3.4 Quality Assurance/Quality Control

Eurofins Savannah of Savannah, Georgia, was the primary contract laboratory used for the analysis of groundwater samples for the Spring and Fall 2023 monitoring events. This laboratory is compliant with the United

States Department of Defense (DoD) and Department of Energy (DOE) Consolidated Quality Systems Manual for

Environmental Laboratories, Version 5.4 (Quality Systems Manual; DoD and DOE 2021) under the DoD National

Environmental Laboratory Accreditation Program (certification #L2463, expires September 22, 2024) and holds current National Environmental Laboratory Accreditation Program accreditation for all applicable analytical methods.

As part of the 2023 monitoring events, quality assurance/quality control samples were collected to confirm that the data quality objectives were met for the field sampling and laboratory analysis. Field duplicate samples were collected during monitoring events to evaluate field precision at the rate of 10% (one per 10 samples). Matrix spike (MS)/matrix spike duplicate (MSD) sample pairs were also submitted at a rate of 5% (one per 20 samples) to evaluate matrix effects on analytical precision and accuracy. Trip blank samples were submitted with VOC samples to assess the potential for contamination during sampling and shipping.

Data validation was completed on all laboratory deliverables by Laboratory Data Consultants, Inc. of Carlsbad, California. The analytical results from the 2023 monitoring events were reviewed and evaluated for data acceptability in accordance with the USEPA Region 1 Environmental Data Review Program Guidance (USEPA

2018), the QAPP (S-A JV 2020a), and the DoD General Data Validation Guidelines (DoD 2019). The method requirements from the Quality Systems Manual (QSM) Version 5.4 (DoD and DOE 2021) and the USEPA Test

Methods for Evaluating Solid Waste, Physical/Chemical Methods, SW-846 (USEPA 2014) were also used as supplemental information.

The data validation reports for the 2023 monitoring events are presented in Appendix C; a summary of quality control exceedances noted during data validation is also included in Appendix C. Appropriate qualifiers, if needed, have been added to the 2023 analytical data in the project database and are included in Tables 6 and 7 and

Appendix C. Some values were J-qualified (analyte detected at estimated concentration) or UJ-qualified (analyte not detected, laboratory reporting limit is estimated) due to quality control exceedances. Sample results that were qualified as estimated (UJ/J) are usable with caution. The 2,2-dichloropropane results for six monitoring wells, one trip blank, and one field duplicate were qualified as rejected (R) due to gross exceedances in the continuing calibration verification results; these results are not usable. The data from the Spring and Fall 2023 monitoring events are acceptable for use, with the exception of the eight sample results that were rejected; a detailed summary is included in Appendix C.

3.5 Laboratory and Field Corrective Actions

There were no major laboratory or field corrective actions required in 2023. Laboratory analyses were performed in general compliance with the precision, accuracy, representativeness, completeness, comparability, and sensitivity requirements listed in the QAPP (S-A JV 2020a).