Groundwater Monitoring Event 81 and 2020 GWETS Annual Report.pdf

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Mabbett & Associates, Inc. www.mabbett.com 105 Central Street, Suite 4100 Stoneham, MA 02180-1260 T. 781-275-6050 F: 781-954-5155 Massachusetts | Virginia | Rhode Island | New York

© 2022, Mabbett & Associates, Inc.

GROUNDWATER MONITORING

EVENT 81 AND 2020

GROUNDWATER EXTRACTION AND

TREATMENT SYSTEM

ANNUAL REPORT

U. S. Army Soldier Systems Center Natick, Massachusetts

Mabbett Project No. R2020031.000.002

March 7, 2022

Prepared For:

U.S. Army Soldier Systems Center 10 General Greene Avenue Natick, MA 02180

Contract No. GS-10F-0120T Order No. W911QY-20-F-0409

GROUNDWATER MONITORING EVENT 81 AND 2020 GWETS ANNUAL REPORT

U.S. Army Soldier Systems Center, Natick, MA

© 2022, Mabbett & Associates, Inc. Page i March 7, 2022 Project No.R2020031.002.003 Annual Report for 2020 Sampling Event 81 V3

ACKNOWLEDGEMENT

This Groundwater Monitoring Event 81 and 2020 Groundwater Extraction and Treatment System Annual Report has been prepared in accordance with the Performance Work Statement (PWS) attached to Contract No. GS-10F- 0120T, Order No. W911QY20F0409. This document is based on information available to Mabbett & Associates, Inc. (Mabbett®) at the time of the plan preparation. Should any of this information be incorrect, incomplete, or subject to change, Mabbett would wish to revise the report accordingly.

Michael L. Bloom, CPG Project Manager | Geologist

This report has been reviewed and approved by:

Stephen A. Vetere, PE, LSP, LEP Vice President of Operations

MLB/SAV:rsg

© 2022, Mabbett & Associates, Inc. Page ii March 7, 2022 Project No. R2020031.002.003 Annual Report for 2020 Sampling Event 81 V3

TABLE OF CONTENTS

SECTION PAGE

1.0 INTRODUCTION

1.1 Report Organization

1.2 Site Description and Background

1.3 Program Objectives

2.0 2020 GROUNDWATER MONITORING

2.1 Synoptic Groundwater Level Measurements

2.2 Groundwater Sampling

2.3 Laboratory Analyses

2.4 Data Validation

3.0 2020 GWETS OPERATIONS AND MAINTENANCE

3.1 Historical GWETS Modifications

3.2 2020 GWETS Operations and Maintenance

3.3 2020 Pumping Rates

3.4 2020 GWETS Operation and Maintenance Sampling Program

3.5 Laboratory Analyses

3.6 Data Validation

4.0 HYDROGEOLOGICAL ASSESSMENT

4.1 Site Hydrogeology

4.2 Groundwater Flow Direction

4.3 Evaluation of Vertical Hydraulic Gradients

4.4 Capture Zone Assessment

5.0 GROUNDWATER QUALITY ASSESSMENT

5.1 Data Usability Assessment

5.2 Long-Term Monitoring Program Results

5.2.1 Long-Term Monitoring Program Results – Event 80, Spring 2020 (Plexus)

5.2.2 Long-Term Monitoring Program Results – Event 81, Fall 2020 (Mabbett)

5.2.3 GWETS Sampling Results

5.2.4 Progress of Groundwater Cleanup

6.0 GWETS Efficiency Evaluation

6.1 Mass Removal

6.2 GWETS Removal Efficiency

6.3 Extraction Well Effectiveness

7.0 CONCLUSIONS

7.1 Status of Groundwater Plumes

7.2 GWETS Optimization

7.3 Scope of Sampling Changes for Event 82

7.4 Monitoring Well Network Maintenance Considerations

8.0 REFERENCES

© 2021, Mabbett & Associates, Inc. Page iii March 7, 2022 Project No. R2020031.000.002.003 Annual Report for 2020 Sampling Event 81 V3

TABLE OF CONTENTS (CONT.)

TABLES

TABLE 1-1 Primary and Secondary COCs and Cleanup Levels in NSSC Groundwater TABLE 2-1 Groundwater Level Measurements TABLE 2-2 Sampling and Analysis Program TABLE 2-3 Field Water Quality Measurements TABLE 3-1 Average Extraction Well Flow Rates for 2020 TABLE 3-2 2020 Flow Totals Calculations TABLE 3-3 GWETS Sampling and Analysis Schedule TABLE 4-1 Wells Sampled During Events 78 through 81 TABLE 4-2 Vertical Hydraulic Gradients TABLE 5-1 Summary of Analytes Detected in Groundwater Samples - T-25 Area TABLE 5-2 Summary of Analytes Detected in Groundwater Samples - T-25 Off-Post Area TABLE 5-3 Summary of Analytes Detected in Groundwater Samples - FPGS Area TABLE 5-4 Summary of Analytes Detected in Groundwater Samples - WSW Area TABLE 5-5 Summary of Analytes Detected in Groundwater Samples - Boiler Plant TABLE 5-6 Summary of Analytes Detected in Groundwater Samples - Buildings 22 and 36 TABLE 5-7 Summary of Analytes Detected in Groundwater Samples - Buildings 63, 2, and 45 Area TABLE 5-8 Monthly and Quarterly GWETS Analytical Results TABLE 7-1 Proposed Scope of Sampling for Event 82 - Spring 2021 TABLE 7-2 Historical Pesticide Results at MW-40BR and MW-168B-2

FIGURES

FIGURE 1-1 U.S. Army Natick Soldier Systems Center (NSSC) Site Locus Map FIGURE 1-2 U.S. Army Natick Soldier Systems Center (NSSC) Site Layout FIGURE 2-1 Interpreted Water Table Piezometric Surface - Event 81 FIGURE 2-2 Interpreted Deep Overburden Piezometric Surface - Event 81 FIGURE 3-1 Process Flow Diagram FIGURE 3-2 Groundwater Extraction Well Locations – 2020 FIGURE 3-3 1,4-Dioxane Pre-Treatment System Process and Instrumentation Diagram FIGURE 5-1 Distribution of PCE in Groundwater Event 80 FIGURE 5-2 Distribution of PCE in Groundwater Event 81 FIGURE 5-3 Distribution of TCE in Groundwater Event 80 FIGURE 5-4 Distribution of TCE in Groundwater Event 81 FIGURE 5-5 PCE and TCE Concentrations at MW-90B-4 FIGURE 5-6 PCE and TCE Concentrations at MW-94B-4 FIGURE 5-7 PCE and TCE Concentrations at MW-95B-4/MW-15B FIGURE 5-8 PCE and TCE Concentrations at MW-96B-4 FIGURE 5-9 PCE and TCE Concentrations at MW-39B-HP4 FIGURE 5-10 PCE and TCE Concentrations in EW-2 to EW-8 Combined Influent FIGURE 5-11 1,4-Dioxane Treatment Unit Influent-Effluent Concentrations FIGURE 5-12 T-25 Treatment Plant Influent Concentration FIGURE 5-13 Extraction Well Flow Rate Summary 2006-2020 FIGURE 5-14 Treatment Plant Influent Flow Rate 2006-2020

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FIGURE 6-1 Mass Balance for T-25 Groundwater Treatment System FIGURE 6-2 GWETS Monthly Mass Removal Rates FIGURE 6-3 Monthly Mass Removal Rates for PCE and TCE by Well FIGURE 6-4 Cumulative Mass of PCE and TCE Removed FIGURE 6-5 Cumulative Mass of 1,4-Dioxane Removed

APPENDICES

APPENDIX A Previous Monitoring Events APPENDIX B Field Data Sheets Event 81 APPENDIX C Comprehensive Analytical Results and Laboratory Reports Event 81 APPENDIX D Data Validation Report Event 81 APPENDIX E Plexus Spring 2020 Monitoring Event 80 Data Submittal APPENDIX F Contaminant Concentration Trend Graphs APPENDIX G Institutional Controls Certification

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LIST OF ACRONYMS

µg/L micrograms per liter bgs below ground surface cis-1,2-DCE cis-1,2-dichloroethene COCs Contaminants of concern DoD Department of Defense ECC Environmental Chemical Corporation ELAP Environmental Laboratory Accreditation Program ESD Explanation of Significant Differences (USEPA, 2013) ETA Engineering Technology Associates, Inc.

FPGS Former Proposed Gymnasium Site ft foot/feet gpm gallons per minute GWETS Groundwater Extraction and Treatment System ICF ICF Consulting, Inc.

LDC Laboratory Data Consultants, Inc.

LGAC Liquid-phase granular activated carbon LL Lake level LTM Long Term Monitoring LTMP Long Term Monitoring Plan MassDEP Massachusetts Department of Environmental Protection MCLs Maximum Contaminant Levels MCP Massachusetts Contingency Plan MDL Method detection limit MTBE Methyl tert-butyl ether ND Non-detect NES National Environmental Systems NSSC Natick Soldier Systems Center O&M Operation & Maintenance PCE Tetrachloroethene PTS Pre-Treatment System PVC Polyvinyl chloride QA Quality Assurance QAPP Quality Assurance Project Plan QC Quality Control RAOs Remedial Action Objectives RI Remedial Investigation ROD Record of Decision SDG Sample delivery group SI Site Investigation SIM Selective ion monitoring TCE trichloroethene trans-1,2-DCE trans-1,2-dichlorothene USACE United States Army Corps of Engineers USEPA United States Environmental Protection Agency VGAC Vapor-phase granular activated carbon VOCs Volatile organic compound(s) WSW Water Supply Well

© 2022, Mabbett & Associates, Inc. 1 March 7, 2022

1.0 INTRODUCTION

This report summarizes the activities and presents the results of the Fall 2020 semi-annual groundwater monitoring events (Event 80 and 81) for the U.S. Army Natick Soldier Systems Center (NSSC) in Natick, Massachusetts. The report also summarizes the operation and maintenance (O&M) of the Groundwater Extraction and Treatment System (GWETS) for the annual period of January through December 2020. This report has been prepared in accordance with the:

Draft Final Long-Term Monitoring Plan (Environmental Chemical Corporation [ECC], 2009), amended

December 2009.

T-25 Area Ground Water Treatment System Operations and Maintenance (O&M) Manual (ICF Consulting

Inc. [ICF], 2004).

Draft O&M Manual – OU-1 T-25 Area, Buildings 22 and 36 Area, and the Buildings 63, 2, and 45 Area

Groundwater Treatment System and 1,4-dioxane Treatment System (ECC, 2010).

This report summarizes the field sampling tasks, sampling procedures, and GWETS O&M activities, optimization, and performance evaluation. The report also presents the analytical data from the GWETS performance monitoring and samples collected during Events 80 and 81 which were conducted in 2020. The analytical results are reported with historical data trends to determine the overall effectiveness of the GWETS and support optimization recommendations.

1.1 Report Organization

This report is organized into eight sections:

Section 1.0 (this section) includes this introduction, a Site description, and discussion of program objectives.

Section 2.0 provides a summary of groundwater monitoring activities completed in 2020.

Section 3.0 provides a summary of GWETS O&M activities completed in 2020.

Section 4.0 discusses the hydrogeologic conditions observed at the Site through analysis of groundwater monitoring data.

Section 5.0 provides a comparison of groundwater sampling data and GWETS operational data to remedial action objectives for the remedy.

Section 6.0 presents an evaluation of the overall efficiency and rate of contaminant mass removal achieved by operation of the GWETS.

Section 7.0 presents a summary level of conclusions.

Section 8.0 provides a listing of references used throughout this report.

Tables and figures are referenced throughout this document and are included in their own sections at the end of this report. Six appendices are included with this document and provide additional detail for the data collection and evaluation summarized in this report:

Appendix A lists previous monitoring reports dating back to 1993.

Appendix B presents the field and calibration data sheets for Event 81.

Appendix C presents a table of analytical results and the laboratory data reports for Event 81.

Appendix D presents the data validation reports for Event 81.

Appendix E includes the Spring 2020 Groundwater Monitoring Event 80 Data Submittal (Plexus, 2020).

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Appendix F includes concentration trend graphs for selected monitoring wells.

1.2 Site Description and Background

NSSC is located approximately 17 miles west-southwest of Boston in the Town of Natick, Massachusetts. NSSC is a research facility that occupies approximately 74 acres on a peninsula on the eastern shore of the South Pond of Lake Cochituate (Figures 1-1 and 1-2).

NSSC, formerly called the Quartermaster Research and Engineering Command, and subsequently the U.S. Army Natick Research and Development Command, has been a permanent U.S. Army installation since October 1954.

NSSC’s mission includes research and development in food engineering, food service, clothing, equipment, materials engineering, and aero-mechanical engineering.

Previous investigations at NSSC have focused on characterizing physical conditions and contaminant distribution at six locations or sites within NSSC. These sites consist of the following as shown on Figure 1 2):

T-25 Area NSSC Water Supply Well (WSW) Site Former Proposed Gymnasium Site (FPGS) Boiler Plant Area Buildings 22 and 36 Area Buildings 63, 2, and 45 Area

The Record of Decision (ROD) for the Site dated April 2001 (USEPA, 2001) specifies the following remedial action objectives (RAOs):

Prevent contamination in the groundwater, above federal and state drinking water standards, from migrating outside of the T-25 Area toward off-facility receptors.

Prevent any potential exposure to groundwater beneath the T-25 Area and off-facility with contaminant concentrations in excess of federal and state drinking water standards.

Restore aquifer to drinking water standards within a reasonable time frame.

Monitor potential future migration of groundwater contamination to verify that elevated concentrations decrease over time.

In addition to the 2001 ROD, an Explanation of Significant Differences (ESD) was signed in 2013 (USEPA, 2013).

The ESD addresses groundwater from the entire Site (not only the T-25 Area) and the additional water volume associated with the other portions of the Site, periodic slug volume removals from the MW114B-2 and the Army Research Institute of Environmental Medicine (ARIEM) Building Areas and the treatment of that water, removing the air stripper from the treatment train, and establishment of revised cleanup goals for two contaminants of concern (1,4-dioxane and manganese).

To achieve these objectives, a GWETS was installed in 1997 and a secondary 1,4-dioxane treatment system was installed in 2008. The system has been operational since. This report compiles the results from periodic sampling of both groundwater from the Site and the treatment system.

A list of previous monitoring events is presented in Appendix A. Primary and secondary contaminants of concern

© 2022, Mabbett & Associates, Inc. 3 March 7, 2022

(COCs) and groundwater cleanup levels specified in the ROD and ESD are listed in Table 1-1.

1.3 Program Objectives

The objectives of the groundwater monitoring program include:

Monitoring the nature and extent of groundwater contamination at NSSC and the surrounding area.

Evaluating trends in contaminant concentrations over time.

Characterizing the local and regional groundwater flow conditions.

Assessing the effectiveness of the GWETS.

Collecting data for use in designing possible future remedial actions.

The objectives of the GWETS monitoring include:

Continuous evaluation of GWETS efficiency and effectiveness.

Providing information for optimization of the remedy.

Monthly collection of water samples at different stages of the GWETS and at discharge to evaluate the efficiency of different components of the GWETS, and to document discharge concentrations.

Measure effectiveness of the GWETS by calculating the total mass of contaminants removed over time.

© 2022, Mabbett & Associates, Inc. 4 March 7, 2022

2.0 2020 GROUNDWATER MONITORING

This section summarizes the sampling and analysis program for the Event 81 groundwater monitoring event and the 2020 GWETS sampling events. Sampling and GWETS operation activities for the Event 81 groundwater monitoring program and the 2020 GWETS sampling program were performed in accordance with the Draft Final Long-Term Monitoring Plan (LTMP) for the U.S. Army Natick Soldier Systems Center (ECC, 2009); the Draft Operations and Maintenance Manual – OU-1 T-25 Area, Buildings 22 and 36 Area and the Buildings 63, 2, and 45 Area and the 1,4-dioxane Pre-Treatment System (PTS; ECC, 2010); and the Final UFP-Quality Assurance Project Plan (QAPP) for Environmental Remediation Services and U.S. Army Soldier Systems Center (Plexus, 2017a).

2.1 Synoptic Groundwater Level Measurements

Synoptic water level measurements (i.e., measurements taken within a short time span) were collected on November 2 and 3, 2020. At the time of measurements on November 2, 2020, the team was unaware that the system was down and wells were not pumping. The GWETS experienced a shutdown on October 30, 2020 and alarms were not received by the GWETS operator. Non-pumping conditions were measured until the system was restarted at 0900 on November 3, 2020. A piezometric water level summary, including the measured water levels, is presented in Table 2-1 and on Figures 2-1 and 2-2 for shallow and deep wells, respectively. Water level measurements were made from the reference point (e.g., cut notch), where present, or from the rim of the polyvinyl chloride (PVC) well riser pipes.

Depth-to-water measurements were made using a hand-held electric conductivity water level indicator. Water level measurements were used in conjunction with monitoring well elevation survey data to calculate groundwater elevations at each of the locations measured, as shown in Table 2-1. Survey data were provided by Plexus, and are cited as ICF 1997-1998 Remedial Investigation (RI) survey, ICF survey of the extraction wells (MW- 94B-4, MW-95B-4, and MW-96B-4) installed in October 2002, MACTEC Engineering and Consulting, Inc.’s survey of the more recently installed wells and benchmarks (1997-to present RI and Site Investigation [SI] surveys), and Plexus Optimization Investigation (Plexus 2017b). Riser elevation changes, resulting from the installation of dedicated bladder pumps, were incorporated into Table 2-1.

A hydrogeologic assessment, based on the groundwater elevation data from November 2020 and results of previous investigations and modeling, is provided in Section 4.0 of this report.

2.2 Groundwater Sampling

During Event 81, groundwater samples were collected from 52 wells, including five T-25 Area extraction wells (MW-39B-HP4, MW-90B-4, MW-94B-4, MW-95B-4, and MW-96B-4); extraction wells EW-2 through EW-8 in the Buildings 22 and 36 and Buildings 63, 2, and 45 Areas; and extraction well MW-40BR at the Boiler Plant Area. The groundwater sampling effort was conducted at NSSC during the period between November 3 and November 13, 2020. Groundwater samples were submitted to ALS Environmental in Middletown, Pennsylvania, for laboratory analysis. Appendix B contains the Event 81 data sheets for groundwater sampling and the daily instrument calibration logs.

With the exception of active extraction well locations, monitoring locations were sampled in accordance with the United States Environmental Protection Agency (USEPA) Region 1 Low Stress (low-flow) Purging and Sampling Procedure for the Collection of Groundwater Samples for Monitoring Wells (USEPA, 2017) using dedicated and non-dedicated stainless steel and Teflon bladder pumps or non-dedicated peristaltic pumps. At the operating extraction wells, samples were collected directly from a sample port in the discharge line from each well.

Table 2-2 displays which monitoring wells were sampled with dedicated pumps. Each dedicated pump intake is

© 2022, Mabbett & Associates, Inc. 5 March 7, 2022 located at or below the center of the saturated well screen interval. The pumps remain in place, suspended by small diameter cable from the wellhead assembly. The use of dedicated bladder pumps enhances the effectiveness of low-flow sampling by minimizing disturbance of the water column prior to sampling.

Sampling procedures are described in the Final UFP-QAPP for Environmental Remediation Services and U.S. Army Soldier Systems Center (Plexus, 2017a). Purge water and decontamination fluids were containerized and transported to Building 94, where they were subsequently treated in the GWETS. Field water quality parameters and other purge data from November 2020 are summarized in Table 2-3.

2.3 Laboratory Analyses

The sampling and analysis program for groundwater is presented in Table 2-2. Groundwater samples were analyzed by ALS Environmental Laboratory (ALS) of Middletown, Pennsylvania. ALS is a Department of Defense (DoD) Environmental Laboratory Accreditation Program (ELAP) certified laboratory. In accordance with the analytical schedule specified in Table 2-2, groundwater samples were submitted for laboratory analyses of:

Volatile organic compounds (VOCs) by USEPA SW 846 Method 8260C.

1 ,4-dioxane by USEPA SW 846 Method 8270 in Selective lon Monitoring (SIM) mode.

Organochlorine pesticides by USEPA SW 846 Method 8081B.

Dissolved metals (including chromium, lead, manganese, nickel, thallium, and vanadium) by USEPA SW

846 Method 6020A, and mercury by EPA SW 846 Method 7470A.

Analytical results of groundwater samples collected during Event 81 are discussed in Section 5.0.

2.4 Data Validation

Laboratory Data Consultants, Inc. (LDC) completed data validation of the Event 81 groundwater data under Stage 2B & 4 guidelines using the project quality goals presented in the UFP-QAPP (Plexus, 2017a). The analyses were validated using the following documents, as applicable to respective analytical method:

Final Uniform Federal Policy - Quality Assurance Project Plan for Environmental Remediation Services at

U.S. Army Soldier Systems Center, Natick, Massachusetts; May 2017.

U.S. Department of Defense Quality Systems Manual for Environmental Laboratories, Version 5.3; 2019.

USEPA National Functional Guidelines for Inorganic Superfund Methods Data Review; January 2017a.

USEPA National Functional Guidelines for Organic Superfund Methods Data Review; January 2017b.

EPA SW 846, Third Edition, Test Methods for Evaluating Solid Waste (USEPA 2020).

A summary of the data validation actions and an evaluation of data usability is provided in Section 5.0.

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3.0 2020 GWETS OPERATIONS AND MAINTENANCE

The GWETS in Building 94 has been in operation since 1997 and currently consists of the following components, as illustrated on Figure 3-1.

Fourteen (13) groundwater extraction wells (5 offline) Coliform PTS 1,4-dioxane PTS Equalization tank Particulate filters Granulated activated carbon for primary treatment of groundwater (liquid-phase granular activated carbon [LGAC]).

The GWETS was designed to primarily extract and treat PCE and TCE in groundwater and secondarily heavy metals and pesticides.

Extraction wells MW-90B-4, MW-96B-4, EW-2, EW-5, and MW-40BR were not operating during the Event 81 sampling event. These wells were sampled using peristaltic pumps and low flow groundwater sampling methodologies, apart from MW-40BR which was turned on temporarily and sampled via its dedicated sampling port. Additionally, EW-6 was sampled via peristaltic pump and low flow methodologies as no sample port was installed on the extraction well when it was replaced in summer 2020.

The following extraction wells (Table 3-1, Figure 3-2) were in operation throughout the 2020 calendar year:

T-25 Area: MW-94B-4, MW-95B-4, and MW-39B-HP4 Buildings 22 and 36 Area: EW-6, EW-7, and EW-8 Buildings 63, 2, and 45 Area: EW-3 and EW-4.

Boiler Plant Area: None.

3.1 Historical GWETS Modifications

Since its original operation, several modifications have been made to the GWETS, as cited in the most recent Plexus Annual Report (Plexus, 2020) and summarized below.

2007. Upgrades were made to the GWETS to accommodate the additional influent contributed from new extraction wells installed at the Buildings 22 and 36 Area and the Buildings 63, 2, and 45 Area. The additional extraction wells were installed in 2006 and 2007 to provide containment of groundwater plumes with PCE or TCE.

2008. An ex situ wellhead PTS was installed to treat the groundwater from the extraction wells in the Buildings 63, 2, and 45 Area to remove 1,4-dioxane, which was found to be present in a portion of the solvent plume in this area. The 1,4-dioxane PTS consists of a Fenton’s Reagent Advanced Oxidation Process, as illustrated on Figure 3-

3. After treatment for 1,4-dioxane, this water is pumped to the T-25 Area GWETS for additional treatment.

2009. Monitoring well MW-40B-2 (at the Boiler Plant Area) was over-drilled and an extraction well was installed into the borehole (MW-40BR) to address the dieldrin concentrations identified at this location.

2010. Monitoring well MW-39B-HP4 (in the northwest corner of T-25 Area) was converted to an extraction well

© 2022, Mabbett & Associates, Inc. 7 March 7, 2022 to enhance capture of the PCE and TCE concentrations above Maximum Contaminant Levels (MCLs) north of the T-25 Area. Pumping of groundwater from extraction wells MW-39BHP4 and MW-40BR to the GWETS began in late August 2010.

2011. Extraction well EW-2 was taken offline in December 2011 as recommended in the 2010 annual report, to optimize the capture and treatment of the Buildings 63, 2, and 45 Area plume. Groundwater samples from well EW-2 remain below MCLs for both PCE and TCE.

2012. The GWETS was modified to eliminate the use of the air stripper and associated equipment (see ESD; USEPA 2013). A review of historical influent characteristics and the treatment process indicated that effective treatment of the influent groundwater could be achieved by using only the LGAC. The original equipment remains on the Site. The current treatment operations are shown on Figure 3-1.

2013-2020. A PTS was installed for MW-96B-4 to address suspended solids and coliform detected in this well, which was reportedly caused by a sanitary sewer line leak discovered and repaired in the Spring of 2012. The PTS was installed from December 10 through 27, 2013 by National Environmental Systems (NES). An underground leak in the pumping line for MW-96B-4 was discovered during PTS testing in February 2014. After repairs were made to the pumping line, the PTS was put back into operation in August 2014. In August 2016, the PTS was taken off-line after multiple quarters with no detections of coliform in the influent samples. Following a coliform detection in August 2018, the PTS was brought back online in September 2018. Quarterly sampling of the PTS influent and effluent streams for coliform was not performed in 2020, as the piping to MW-96B-4 was damaged during construction activities associated with the NSSC new warehouse Building 26.

2018. Extraction wells EW-5 and MW-40BR were taken offline based on recommendations in the Technical Memorandum: Evaluation of Data from 2016-2017 Optimization Investigation (Plexus, 2017b), as they no longer contributed to the capture of target capture zones.

2020. Extraction well MW-90B-4 was not operational during 2020 due to well fouling that prevented groundwater recharge rates suitable for supporting continuous groundwater extraction. Rehabilitation efforts were completed by Plexus as part of their Final Optimization Plan (Plexus, 2018a) scope of work; however, the rehabilitation failed to improve the groundwater production rate from MW-90B-4 to a level where sustained groundwater extraction was viable. In an effort to restore MW-90B-4 as a viable groundwater extraction well, Mabbett is scheduled to implement an oxalic acid-based amendment rehabilitation program during June 2021.

3.2 2020 GWETS Operations and Maintenance

Watermark Environmental Inc. (Watermark) has operated and maintained the GWETS since 2015, under subcontract with Plexus from 2015 to 2020 and currently under subcontract to Mabbett (since September 2020).

The year 2020, extending into 2021, was unprecedented with issues associated with the COVID-19 pandemic.

These issues affected and continue to affect almost all aspects of work, including travel restrictions and difficulties with procuring equipment and supplies. As a result, the Army has permitted unplanned overlap between Plexus’ contract to optimize the GWETS and Mabbett’s contract to operate and maintain the GWETS to account for delays in the completion of optimization measures caused by the pandemic. Watermark has provided continuity in the GWETS operation, maintenance, and monitoring during the transition from Plexus to Mabbett.

The down-well pumps in the Buildings 22 and 36 Area and in the Buildings 63, 2, and 45 Area tend to become clogged and require cleaning on a regular basis due to the turbidity of extracted groundwater and oxidation of dissolved iron and/or manganese in the extracted groundwater. This requires the removal of the clogged pump for cleaning and replacement of the removed pump with a spare pump while the cleaning is performed. Typically, © 2022, Mabbett & Associates, Inc. 8 March 7, 2022 one or two down-well recovery well pumps require removal and cleaning each month.

Other routine maintenance activities include (but are not limited to) the following:

Filter bag replacement, typically completed on a weekly basis, based on differential pressure reading values at the bag filter housing documented by the GWETS operator.

LGAC vessel backwashing approximately once every 3 weeks, based on differential pressure reading values at the carbon treatment vessels observed by the GWETS operator.

Processing of backwash solution as needed.

Regular cleaning of process flow meters.

Weekly cleaning and calibration of the pH sensor, level transducer and flow meter in the 1,4-dioxane treatment unit.

Chemical drum replacement approximately once per month when the 1,4 dioxane PTS is operating continuously.

In addition to the routine maintenance activities, the following non-routine activities were completed by Watermark performing GWETS operations in 2020:

January: Troubleshooting the power supply to the 1,4-dioxane PTS, the outage of which was initially identified on December 23, 2019.

February: Troubleshooting the power supply and fiber optic line to the 1,4-dioxane PTS. Performed pH and level control adjustments for the 1,4-dioxane PTS. Installed saddle clamps and sample ports at the LGAC vessels, installed covers on electrical boxes in extraction well vaults, replaced the mixer motor at the 1,4-dioxane reaction tank. Tested fiber optic lines at EW-3, EW-4, and in the 1,4-dioxane PTS shed.

Restarted 1,4-dioxane PTS and balanced flows; continued the re-piping of several sample ports for the

GWETS.

March: Troubleshooting MW-94B-4 which identified that the wires in the pull box were corroded and broken.

Troubleshooting the EW-7 transducer. Continued installation of saddle clamps and sample ports on extraction well lines and pre-treatment system. Installed covers on electrical boxed in extraction well vaults. Re-piped the 1,4-dioxane PTS effluent line and installed new check and ball valves. Restarted the 1,4-dioxane PTS and balanced flows. Pulled and replaced the pump at EW-8. Pulled and replaced the pumps at EW-6 and EW-3. Restarted EW-6 and balanced flow. Cleaned recovered pumps for future redeployment.

April: Troubleshooting MW-96B and troubleshooting the EW-7 transducer. Replaced the pressure gauge on bag filter housing unit one. Pulled and replaced pump at EW-3. Cleaned recovered pump for future redeployment.

May: Troubleshooting MW-96B and troubleshooting the EW-7 transducer. Pulled and replaced pump at EW-3. Cleaned recovered pump for future redeployment.

June: Flushed the groundwater conveyance line for EW-3. Repaired damaged acid feed pump head tubing and cleaned up secondary containment area in the 1,4-dioxane PTS. Pulled and replaced the pump at EW-7. Pulled and cleaned the pumps at EW-3 and EW-8. Repaired the leak in the roof at the communication antennae base.

July: Performed decontamination of well development equipment. Installed MW-90B pump after re-splicing damaged power lead but was unable to reduce flow from MW-90B to pump without pumping

© 2022, Mabbett & Associates, Inc. 9 March 7, 2022 the well dry. Based on water level drawdown and flow rates, Watermark recommended redevelopment to clean well screen and sand pack of MW-90B. Placed order for a new motor for 1,4-dioxane PTS pH adjustment mixer.

August: Replaced the mixer motor on pH adjustment tank of the 1,4-dioxane PTS. The 1,4-dioxane PTS was back online on August 4, 2020. Replaced the threaded nipple where pump connects to the riser at the MW-94B wellhead, which had corroded away and was not pumping. Reset breakers for EW-3 and EW- 4 after they had tripped. Pulled and replaced pump for EW-4. Cleaned recovered pump for future use.

A high-level equalization tank alarm was received on August 23, 2020. A remote connection was attempted but was unsuccessful. Personnel mobilized to the site on the morning of August 23, 2020 and observed that there were no plant controls via the plant computer and no internet access. It was concluded that the network router that allows the treatment system controls to communicate with the plant computer and access the internet for remote access and alarm notification had failed.

September: The router and cellular modem were replaced, and a controls engineer scheduled to re-configure both devices to restore communications between the plant computer and treatment system controls, as well as the associated remote access capabilities. The GWETS was placed back online on September 22, 2020.

October: Removed EW-3 pump and motor for cleaning to enable groundwater recovery at a higher flow rate.

EW-3 pump and motor were reinstalled after cleaning. Mabbett was on-site with Watermark to identify on-site well locations.

November: MW-39B went down on November 7, 2020 possibly due to a failed motor. The pump and motor for EW-3 were removed for cleaning and subsequently reinstalled. Based on the conditions at the recovery well, maintenance at EW-3 is necessary approximately every 3 to 4 weeks.

December: The pump and motor at EW-3 were removed for cleaning and subsequently reinstalled. The EW-8 pump and motor were temporarily removed for cleaning and to increase groundwater recovery rates from that well. MW-39B remained out of service to identify, obtain, and install the necessary components.

3.3 2020 Pumping Rates

During 2020, a total volume of approximately 19 million gallons (19,298,878 gallons) of groundwater was pumped through the T-25 Area GWETS. Since the beginning of operations in November 1997, a total of approximately 690 million gallons of groundwater has been treated. During 2020, the GWETS was operating approximately 91 percent of the time. The GWETS was shut down a total of 1,032 hours in 2020.

Extraction wells MW-90B-4, MW-96B, EW-5, and MW-40BR were not operational during the 2020 calendar year.

Extraction wells MW-94B-4, MW-95B-4, and MW-39B-HP4 were pumped at a combined average pumping rate of

40.0 gpm in 2020. The extraction wells EW-3 through EW-8 were pumped at a combined average rate of 3.5 gpm in 2020. The total average inflow to the GWETS in 2020 was therefore approximately 43.5 gpm. Well fouling and an associated reduction in pumping rates were addressed as recommended in the Optimization Plan (Plexus, 2018a). The objectives of future sampling events will include measurement and evaluation of the effectiveness of optimization measures and whether additional modifications are warranted.

Each extraction well is pumped at a constant flow rate controlled by a gate valve, with the flow rate set by use of a rotameter or paddle wheel flow meter. Flow rates are recorded manually during weekly O&M inspections. The average monthly flow rate is calculated using the hourly recorded rates documented by the supervisory control

© 2022, Mabbett & Associates, Inc. 10 March 7, 2022 and data acquisition (SCADA) system. Using this approach, downtime is factored into the average. Therefore, average flow rates are not necessarily representative of flow rates during optimal operating conditions, but may be significantly lower, especially for wells exhibiting maintenance issues during the reporting period. Average flow rates for the 2020 annual reporting period are summarized in Table 3-1. The 2020 monthly extraction well flow volumes are summarized in Table 3-2.

The total volume extracted in 2020 was approximately equivalent to that in 2019 (20 million gallons). The Optimization Plan (Plexus, 2018a) outlines the optimization steps that were recommended to increase the effectiveness of the remedy and increase groundwater extraction rates from select recovery wells where fouling has restricted flow to the GWETS.

3.4 2020 GWETS Operation and Maintenance Sampling Program

As stipulated in the O&M Manual, monthly and quarterly water samples were collected from the GWETS in 2020 to monitor remedial performance and to document compliance with MCLs and Massachusetts Contingency Plan (MCP) Method 1 GW-1 standards (1,4-dioxane) based effluent discharge requirements. The GWETS performance sampling program is summarized in Table 3-3.

In accordance with the sampling and analysis schedule in Table 3-3, aqueous samples are collected at the following locations along the GWETS, as illustrated on Figure 3-1:

Influent to the 1,4-dioxane PTS Effluent of the 1,4-dioxane PTS MW-90B-4 influent (not an active recovery well during 2020) MW-94B-4 influent MW-95B-4 influent MW-96B-4 influent (not an active recovery well during 2020) MW-39B-HP4 influent EW-3 through EW-8 influent (EW-5 not an active recovery well during 2020) Location A: after the equalization tank Location D: LEAD LGAC vessel influent Location F: LAG LGAC vessel influent; and Location G: effluent from the GWETS, after the LAG LGAC vessel.

Prior to 2012 and as documented in the 2013 ESD, the GWETS was operated with the air stripper and associated vapor-phase treatment train in service. When the air stripper was operational, air samples were collected at the following locations and were analyzed for TCE and PCE:

Location B: vapor phase effluent from the air stripper, Location C1: vapor phase effluent from the primary VGAC canister, and Location C2: vapor phase effluent from the secondary VGAC canister.

In accordance with the ESD (USEPA, 2013) the air treatment portion of the GWETS has been bypassed since 2012 (Figure 3-1) and air samples are no longer collected.

© 2022, Mabbett & Associates, Inc. 11 March 7, 2022

3.5 Laboratory Analyses

GWETS performance samples were analyzed by ALS. In accordance with the analytical schedule specified in Table 3-3, GWETS performance sampling samples were submitted for laboratory analyses of:

VOCs by USEPA SW 846 Method 8260C 1 ,4-dioxane by USEPA SW 846 Method 8270 in Selected lon Monitoring (SIM) mode Copies of the GWETS laboratory analytical results from September through December are provided in

Appendix C.

3.6 Data Validation

In accordance with the QAPP, monthly, quarterly and semi-annual GWETS performance sampling data are not routinely validated. Monthly, quarterly, and semi-annual results are evaluated when received and, provided the results are consistent with previous results and trends, additional validation to demonstrate the quality of the data are not required.

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4.0 HYDROGEOLOGICAL ASSESSMENT

Groundwater contour maps were developed based on Event 81 groundwater elevation measurements collected on November 2, 2020 while the treatment system was down. The interpreted groundwater contours are provided on Figure 2-1 (Interpreted Water Table Piezometric Surface) and Figure 2-2 (Interpreted Deep Overburden Piezometric Surface). Because the fall 2020 synoptic groundwater level measurement round was completed while the groundwater extraction system was down, the groundwater contours do not represent groundwater flow patterns under pumping conditions and therefore do not depict capture of the contaminant plume. Piezometric head interpretations from previous monitoring reports (Plexus, 2020) indicate that, under pumping conditions, the contaminant plume is contained. The effectiveness of containment and plume capture will be re-evaluated in subsequent monitoring events following the completion of optimization measures.

4.1 Site Hydrogeology

Groundwater flow at NSSC occurs mainly in the glacially derived overburden soils and fractured portions of the bedrock. The overburden across the installation is comprised mainly of outwash sediments and includes sand and gravel, fine to coarse sand, peat, silty sand, and clayey silt. The stratigraphic units vary in order and appear to be laterally discontinuous. Soils generally decrease in grain size with depth, and a laterally discontinuous clayey silt layer is present and apparently forms a semi-confined lower aquifer. The observed depth to bedrock in the vicinity of NSSC ranges from 0 (i.e., at bedrock outcrops) to 199 feet (ft) below ground surface (bgs). Bedrock at the site is primarily granodiorite that can be observed in outcrops along the eastern shore of the South Pond of Lake Cochituate and along Washington Avenue near Pegan Brook.

NSSC is located within the Lake Cochituate drainage basin, which encompasses approximately 17.7 square miles.

The main hydrogeologic features at NSSC and the surrounding area include the South Pond of Lake Cochituate and Fisk Pond. The South Pond of Lake Cochituate, which occupies approximately 0.9 square miles, flows northward to the Middle Pond and then to the North Pond of Lake Cochituate. The North Pond drains via the Cochituate Brook into the Sudbury River. Previous work by ICF indicates leakage from South Pond may act as a source of water to the deeper aquifer.

Groundwater pumping tests performed on one of the WSW Site wells (PW-1 located on the southern portion of the installation) showed that stresses to the deeper aquifer were not readily observed in the shallow water table aquifer (Engineering Technology Associates, Inc. [ETA], 1996).

Groundwater in the unconsolidated soils at the Site is divided into two aquifers: the unconfined water table aquifer, or “A” interval, and the locally semi-confined, deeper overburden aquifer, or “B” interval. Two wells were advanced to greater than 100 feet deep in the overburden and are referred to as “C” wells. The water table ranges in depth from 2 to 58 feet below ground surface (ft bgs). The most recent status (since spring 2019) of the wells on the Site and a summary of the wells that were accessed for sampling during 2019 and 2020 is provided in Table 4 1.

As described in Section 2.0, groundwater elevation data were collected on November 2 and 3, 2020, from 110 monitoring wells and three lake level (LL) monitoring locations (Table 2-1). No water level reading was taken for Little Roundy Pond as the survey reference point was inaccessible at the time. The Event 81 water level data were used to generate piezometric contours and to determine flow directions for both the water table and deep overburden aquifers. The extraction system was not operational on November 2, 2020. Therefore, the data collected on November 2, 2020 (and presented in Table 2-1 and Figures 2-1 and 2-2) show groundwater conditions following approximately 68 hours of ambient conditions without pumping. The system was brought back online on November 3, 2021.

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4.2 Groundwater Flow Direction

Water level measurements were collected from both on-post and off-post locations during Event 81. Figure 2-1 shows the interpreted groundwater contours for the water table (shallow) aquifer (“A” interval wells) across NSSC based on the groundwater elevation data collected in November 2020. In general, groundwater in the shallow interval flows radially, with an apparent divide trending northeast/southwest across the center of the installation, discharging to the adjacent surface waters at the perimeter of the installation. In the northern portion of the installation, shallow groundwater flow is generally to the north-northwest toward the residential neighborhood.

Figure 2-2 shows the Interpreted groundwater contours for the deeper aquifer “B” interval wells based on the groundwater elevation data collected in November 2020. Deeper overburden groundwater appears to move from east to west beneath NSSC when the GWETS is not operational, discharging to the South Pond of Lake Cochituate.

Local variations where high groundwater elevations were observed to the north of the eastern bay, at the southwestern portion of the Buildings 22 and 36 Area, and the southern peninsula. These three localized higher elevation areas of groundwater shape the deep overburden contours demonstrating flow to the South Pond of Lake Cochituate proximate to the Boiler Plant Area. During previous analyses of deep overburden groundwater elevation data while the GWETS was fully operational, the influence of groundwater pumping was apparent with the distortion of piezometric contours in the vicinity of extraction wells in the deep overburden aquifer; however, as the GWETS was not operational during most of the data collection for Event 81, there was no influence from the GWETS extraction points observed in the Event 81 groundwater elevation data.

Data obtained from the monitoring wells indicate little to no distinction between the upper and lower aquifers to the east, as bedrock rises in that direction eventually outcropping near Washington Avenue.

4.3 Evaluation of Vertical Hydraulic Gradients

Vertical hydraulic gradients were calculated from 11 monitoring well pairs at or near NSSC, as shown on Table 4-

2. Vertical gradients were not evaluated at four well pairs (MW-3/MW-3B, MW-10A/B, MW-113A/B-2 and MW- 122A/B-3) as one of the monitoring wells in each pair was either dry, could not be accessed, or could not be located. Downward vertical gradients, characteristic of a groundwater recharge area, were present across the central (Buildings 63, 2, and 45 Area) and northern (T-25 Area) portions of the installation during Event 81. The observed downward vertical gradients on the central portion of the peninsula support that groundwater recharge of the deep overburden (“B” interval) is occurring from the shallow overburden (“A” interval). Magnitudes ranging from negligible to -0.046 feet per foot (ft/ft) were observed.

At groundwater elevation monitoring locations located within the radius of influence of the active, deep overburden (“B” interval) recovery wells, downward vertical gradients would be expected and attributable to the cones of depression created during active pumping of the recovery well.

In total, downward vertical gradients were observed in 7 of the 11 well pairs across NSSC for which gradients were calculated. The largest downward vertical gradients have historically been, and recently confirmed during Event 81, at the MW-120A/B-1 well pair (-0.046 ft/ft), observed to occur in the well couplets located near the elevated central area of the installation. This observation suggests that this area acts as a major recharge zone for the deeper aquifer.

Positive vertical gradients, indicate flow from the deep overburden (“B” interval) into the shallow overburden (“A” interval) were observed at significant magnitudes at the FPG Site, as well as at the Water Supply Well Site. The

0.065 ft/ft upward gradient observed at MW-101A/B-1 is presumed to be attributable to the loss of surface water from Little Roundy Pond, which is located northwest of the intersection of Kansas Street and Heritage Lane

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(Figure 1-2), to the deep overburden (“B” interval) aquifer. As the MW-102A/B-1 well pair, located closest to the shoreline of South Pond demonstrated negative vertical gradient of -0.035 ft/ft, the hydrogeologic conditions at the FPG Site may support the discharge of groundwater from the B interval into South Pond at lower (profundal zone) depth intervals. A positive vertical gradient was also calculated at the MW-14A/B well pair located on the Water Supply Well Site, at a magnitude of 0.030 ft/ft, indicating that deep overburden groundwater discharge to South Pond is not occurring on the southernmost portion of the peninsula.

4.4 Capture Zone Assessment

The capture zone of the GWETS is intended to be evaluated semi-annually by measuring groundwater elevations under pumping conditions. As a synoptic round of groundwater elevational measurements was not performed by Plexus as part of Event 80, the groundwater elevation data collected during the spring of 2020 was insufficient to evaluate the capture zone of the GWETS. In addition, an alarm condition resulting in the shutdown the GWETS in November 2020 did not prompt the automated alarm delivery to the operators, resulting in the collection of groundwater elevation data while the GWETS was both down (November 2, 2020) and operational (November 3, 2020) during Event 81. Given the variability introduced by collecting the synoptic water level round while the system was both down and operational, the Event 81 groundwater elevation data was not suitable for evaluating the capture zone of the GWETS.

As presented in the Technical Memorandum: Evaluation of Data from 2016-2017 Optimization Investigation (Plexus, 2017), groundwater modeling of path line patterns has also been used to evaluate capture zones. This capture zone assessment evaluated the effectiveness of the GWETS in capturing the interpreted plume extents above cleanup goals based on extraction well locations and operating pumping rates. At the time that the evaluation was performed, the models suggested that contaminant plumes exceeding MCLs in groundwater were completely captured within the T-25 Area and the Buildings 22 and 36 Area, and were largely captured at the Buildings 63, 2, and 45 Area.

In an effort to remediate areas outside of the radii of influence of GWETS wells, localized slug volume removal activities were periodically conducted at wells MW-114B-2, MW-165B-2, MW-181B-2, and MW-182B-4 through 2019. Based on the 2017 Groundwater Remediation Optimization Work Plan, the Third 5-Year Review (USACE, 2017) and the USEPA’s concurrence with that review (USEPA, 2017c) the slug removal program has been replaced with pumping from the area via extraction wells. This will be achieved via the two new extraction wells, one near MW125B-2 (identified as EW-11) and one between MW-182B-4 and MW165B-2 (identified as EW-12), as well as converting existing wells MW-182B-4 and MW165B-2 to extraction wells (Plexus, 2018a). Slug removal from MW- 181B-2 was discontinued as the slug removal is no longer necessary due to the installation of the extraction wells.

Slug removal was discontinued at MW-114B-2 since the initiation of the optimization program as TCE and PCE concentrations have remained well below 5 µg/L. Mabbett recommends suspending slug removal at MW-114B-2 until such time that groundwater monitoring data warrant resuming slug removal at this location.

Substantial diminutions to the lateral extents of the cleanup goal exceedance contaminant plumes, resulting from the ongoing operation of the GWETS, have been observed since the 2016-2017 Optimization Investigation.