H_2023_OAA_Six_Groundwater_Sites_LTM_Report_FINAL_comp_Part1.pdf

FINAL

2023 ANNUAL LONG-TERM

MONITORING REPORT

Other Aberdeen Areas – Six Groundwater Sites Aberdeen Proving Ground, Maryland

August 2025

Prepared for:

Prepared by:

Contract No.: W56ZTN-17-D-0009 Delivery Order: W56ZTN-21-F-0110

DISTRIBUTION RESTRICTION STATEMENT

APPROVED FOR PUBLIC RELEASE.

DISTRIBUTION IS UNLIMITED. #250214-01

RENOVA-SOVEREIGN JOINT VENTURE

3417 Sunset Avenue Ocean Township, New Jersey 07712

2023 Annual Long-Term Monitoring Report

Other Aberdeen Areas – Six Groundwater Sites Final

Aberdeen Proving Ground, Maryland August 2025

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TABLE OF CONTENTS

1.0 INTRODUCTION

1.1 Sites Overview

1.2 Site Characteristics and Historical Contamination

1.2.1 Building 525

1.2.2 Building 3327

1.2.3 Tower Road

1.2.4 Building 507

1.2.5 Building M600

1.2.6 DRMO

2.0 ANNUAL OPERATIONS AND MAINTENANCE RESULTS FOR 2023

2.1 Site 23 – Building 525

2.1.1 Building 525 West

2.1.2 Building 525 East

2.2 Site 28f – Building 3327 Former UST Site

2.2.1 Building 3327 – System Operations Summary

2.2.2 Building 3327 – Maintenance

2.3 Site 29 – Tower Road

2.3.1 Tower Road – System Operations Summary

2.3.2 Tower Road – Maintenance

2.4 Site 32 – Building 507

2.4.1 Building 507 – System Operations Summary

2.4.2 Building 507 – Maintenance

2.5 Site 33 – Building M600

2.5.1 Building M600 – System Operations Summary

2.5.2 Building M600 – Maintenance

2.6 Site 16 – DRMO

2.6.1 DRMO – Maintenance

3.0 ANNUAL LTM GROUNDWATER RESULTS FOR 2023

3.1 Site 23 – Building 525

3.1.1 Plume Assessment for Site 23 – Building 525

3.2 Site 28f – Building 3327 Former UST

3.2.1 Plume Assessment for Site 28f – Building 3327 UST

3.3 Site 29 – Tower Road

3.3.1 Plume Assessment for Site 29 – Tower Road

3.4 Site 32 – Building 507

3.4.1 Plume Assessment for Site 32 – Building 507

3.5 Site 33 – Building M600

3.5.1 Plume Assessment for Site 33 – Building M600

3.6 Site 16 – DRMO

3.6.1 Plume Assessment for Site 16 – DRMO

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4.0 SAMPLING EVENT QUALITY CONTROL

5.0 SUMMARY AND CONCLUSIONS

6.0 RECOMMENDATIONS

7.0 REFERENCES

LIST OF FIGURES

Figure 1 Location of Other Aberdeen Areas at US Army Garrison Aberdeen Proving Ground

Figure 2 Other Aberdeen Areas Site Locations

Figure 3 Building 525 Well Locations and 2023 TCE Groundwater Plume

Figure 4 Building 3327 Well Locations and 2023 PCE Groundwater Plume

Figure 5 Tower Road Well Locations and 2023 TCE Groundwater Plume

Figure 6 Building 507 Well Locations and 2023 TCE Groundwater Plume

Figure 7 Building M600 Well Locations and 2023 PCE Groundwater Plume

Figure 8 DRMO Site Well Locations and 2023 TCE Groundwater Plume

Figure 9 TCE Plume Over Time Building 525 (Site 23) Other Aberdeen Areas

Figure 10 PCE Plume Over Time Building 3327(Site 28f) Other Aberdeen Areas

Figure 11 TCE Plume Over Time Tower Road (Site 29) Other Aberdeen Areas

Figure 12 TCE Plume Over Time Building 507 (Site 32) Other Aberdeen Areas

Figure 13 PCE Plume Over Time Building M600 (Site 33) Other Aberdeen Areas

Figure 14 TCE Plume Over Time DRMO (Site 16) Other Aberdeen Areas

LIST OF TABLES

Table 1 Sample Results for Site G23 (Building 525)

Table 2 Sample Results for Site G28f (Building 3327)

Table 3 Sample Results for Site G29 (Tower Road)

Table 4 Sample Results for Site G32 (Building 507)

Table 5 Sample Results for Site G33 (Building M600)

Table 6 Sample Results for Site G16 (DRMO)

Table 7 Field Duplicate Sample Results

LIST OF APPENDICES

Appendix A CVOC Groundwater Concentration Trend Plots

Appendix B Field Forms: Groundwater Sampling Field Forms

Appendix C Laboratory Analytical Reports iii

ACRONYMS AND ABBREVIATIONS

% percent µg/L micrograms per liter

APG Aberdeen Proving Ground bgs below ground surface

BBW biobarrier well (well name code)

BRW bioremediation well (well name code)

CERCLA Comprehensive Environmental Response, Compensation, and Liability Act

CF chloroform

COC contaminant of concern

CVOC chlorinated volatile organic compound

DCA dichloroethane

DCE dichloroethene

DOC dissolved organic carbon

DO dissolved oxygen

DoD Department of Defense

DRMO Defense Reutilization Program and Marketing Office

EISB enhanced in situ bioremediation

ELAP Environmental Laboratory Accreditation Program

EX extraction (well name code)

FS Feasibility Study

INJ injection (well name code)

IRG Interim Remedial Goal

IRP Installation Restoration Program

ISCO In-Situ Chemical Oxidation

LTM Long-Term Monitoring

LUC Land use controls

MCL Maximum Contaminant Level

MNA Monitored Natural Attenuation

MS/MSD matrix spike/matrix spike duplicate

MW monitoring well

OAA Other Aberdeen Areas

O&M Operations and maintenance

ORP oxidation reduction potential

OU operable unit

OW oleate well (well name code)

PCE tetrachloroethene

QC quality control

RA-O Remedial Action-Operations

RI Remedial Investigation

ROD Record of Decision

RPD relative percent differences

SPC specific conductance iv

SRG Secondary Remedial Goal

TCA trichloroethane

TCE trichloroethene

USACE U.S. Army Corps of Engineers

USEPA United States Environmental Protection Agency

UST underground storage tank

VC vinyl chloride

1.0 INTRODUCTION

This Annual Long-Term Monitoring (LTM) Report has been prepared under the U.S. Army Environmental

Command Contract Number W91ZLK-11-C-0003. The report presents an annual summary for 2023 of the operation and maintenance (O&M) of remediation systems and the groundwater sampling results at the

Other Aberdeen Areas (OAA) Six Groundwater Sites, located at the U.S. Army Garrison Aberdeen Proving

Ground (APG), Maryland. These sites have been designated for cleanup activities under APG’s Installation

Restoration Program (IRP).

The purpose of this Annual LTM Report is to demonstrate that the enhanced in-situ bioremediation (EISB) systems are effectively removing chlorinated volatile organic compounds (CVOCs) from the groundwater at the OAA-Six Groundwater Sites with the objective of cleaning up CVOCs to meet remedial goals and ultimately the U.S. Environmental Protection Agency’s (USEPA) Maximum Contaminant Levels (MCL) criteria for drinking water as set forth in the Record of Decision (ROD). Remedial goals in the ROD include

Interim Remedial Goals (IRGs) and Secondary Remedial Goals (SRGs). These goals were developed for each site to support and map remediation progress. IRGs are a CVOC concentration goal developed for the portions of the site’s groundwater plume where active remediation will occur. Achievement of the IRGs will reduce CVOC contamination to a point that will allow the remaining dissolved-phase CVOC concentrations to continue to decline towards the MCL via monitored natural attenuation (MNA), within the designated 30-year time period. SRGs were also developed and represent a target concentration of 75% of the primary IRG to allow for potential CVOC rebound concentrations which could occur when active remediation is concluded (APG, 2006a).

Groundwater samples are collected annually as a routine part of the O&M at these sites. Sampling includes monitoring of geochemical indicator parameters (pH, specific conductance [SPC], dissolved oxygen [DO], and oxidation-reduction potential [ORP]) and annual sampling for CVOCs in key indicator wells. These key indicator wells are located within historical source areas, downgradient plume centers, and plume edges. These groundwater results serve to:

• establish a record of trend data for the LTM,

• demonstrate a site’s cleanup progress,

• determine the extent of the site’s CVOC plumes, and

• compare the analytical results against the remedial goals.

In accordance with the ROD, if monitoring of a site’s key wells indicate achievement of IRGs site-wide, the LTM sampling protocols will be reevaluated, and sampling modified to assess the site conditions for future remediation technologies and approaches.

Currently, groundwater at six sites is sampled annually for LTM purposes. These six sites, which are discussed in this report, include:

1. Building 525 (Site 23),

2. Building 3327 (Site 28f),

3. Tower Road (Site 29),

4. Building 507 (Site 32),

5. Building M600 (Site 33), and

6. Defense Reutilization and Marketing Office (DRMO) Metal Scrap Yard (Site 16).

This report summarizes the O&M activities and presents groundwater monitoring data at the OAA-Six

Groundwater Sites for 2023. Annual LTM groundwater samples were collected in April and May 2023.

The data are summarized to present the current understanding of site conditions, and the progress toward site cleanup. The O&M for four sites (Building 525, Building 3327, Tower Road, and Building 507) is primarily comprised of activities associated with their respective active groundwater remediation systems.

Two sites (Building M600 and DRMO) have no currently active remediation systems; however, the sites continue to be maintained and monitored.

1.1 Sites Overview

Established in 1917, APG is an U.S. Army facility and is the nation’s oldest active proving ground. APG is located in the northeast portion of Maryland, on the western shore of Chesapeake Bay (Figure 1). APG occupies a land area of approximately 72,500 acres, with low-lying marshes, flat meadows, woodlands, and gently rolling hills. The majority of the installation lies on two peninsulas, separated by the Bush River.

The northeastern peninsula is known as the Aberdeen Area and the southwestern peninsula is referred to as the Edgewood Area (formerly the Edgewood Arsenal). The APG installation also includes the Carroll Island and Graces Quarters areas, which are located east of the Edgewood Area on the Gunpowder River. The waters of the Chesapeake Bay, the Bush River, the Gunpowder River, as well as numerous creeks and ponds, comprise nearly half of the area owned or leased by the installation, which includes approximately

103 miles of shoreline.

Other Aberdeen Areas – Six Groundwater Sites

The Aberdeen Area has 10 operable units (OUs) identified in the IRP. The OAA-Six Groundwater Sites are located within OU6. The six sites of OU6 along with their IRP site designation number (shown on

Figure 2) include:

1. Site 23 – Building 525 Site

2. Site 28f – Building 3327 Underground Storage Tank (UST) Site

3. Site 29 – Tower Road Site

4. Site 32 – Building 507 Site

5. Site 33 – Building M600 Site, and

6. Site 16 – DRMO Metal Scrap Yard.

The OU6 sites have CVOC groundwater contamination originating from historical site activities, such as the use of chlorinated solvents in the degreasing of military equipment. Based on the results and analyses of Remedial Investigations (RIs) and Feasibility Studies (FSs) conducted for OU6, the following CVOC constituents have been identified in the groundwater:

• tetrachloroethene (PCE),

• trichloroethene (TCE),

• 1,1-dichloroethane (DCA),

• cis-1,2-dichloroethene (DCE),

• 1,1-DCE,

• vinyl chloride (VC), and

• 1,1,1-trichloroethane (TCA).

The primary contaminants of concern (COCs) at these sites are either TCE or PCE, which were the focus in the development of the various site-specific IRGs. The remaining CVOC constituents are products from the degradation of TCE and PCE via microbial, geochemical, or anthropogenic processes.

In January 2006, a ROD (APG, 2006a) was issued specifying components of a remedy to be applied at each of the sites in OU6 to address the CVOC groundwater contamination. The remedy for each site included the following components:

• The use of in-situ reductive dechlorination (biologically induced breakdown of CVOCs) technology to reduce contaminant concentrations in groundwater.

• Natural attenuation (long-term continuation of contaminant breakdown and dilution) of lower concentrations of CVOCs.

• Tiered monitoring (periodic groundwater monitoring in which the frequency of monitoring events typically is reduced as trends toward site clean-up are established).

• Natural attenuation monitoring and assessment to demonstrate that once the IRGs are reached, contaminant concentrations will continue to decline via natural attenuation to levels below the

USEPA MCL criteria for drinking water.

• Land use controls (LUC) prohibiting the use of groundwater until MCLs are reached.

• Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) 121(c)

Five-Year Reviews.

The remedial design (APG, 2006b) developed each site’s EISB system for implementation of the remedy. These systems included active groundwater recirculation in the CVOC source areas, for all sites except the DRMO.

The DRMO EISB system consisted of periodic amendment injections into the subsurface via a network of injection wells. The injected amendments consisted of a colony of lab-grown dechlorinating microbes paired with a slowly consumed microbial food source, such as edible-oil. The food provides a source of electron donors which are critical in the microbial reductive dechlorination process. The water used for the injections was extracted from the DRMO’s groundwater using submersible pumps in monitoring wells.

Active recirculation was not used at the DRMO.

The active recirculation systems started operating at five sites between October 2006 and March 2007.

Generally, the recirculation systems extract groundwater, and then amend the collected groundwater with pH buffer and a lactate-based microbial food source, which is rapidly consumed in the subsurface by microbes, spurring growth activity. The amended water is then injected back into the source area to contact and treat the CVOC contaminants via EISB. Initially, lab-grown dechlorinating microbes were added to the injected amended water. These active recirculation systems were installed with extraction and injection well networks in the vicinity of the source areas.

The Building 507 and Tower Road sites were designed with the passive biobarrier technology to address the dissolved phase CVOC plume downgradient from the source areas. Passive biobarriers consist of a series of wells where a microbial food source, pH buffer, and microbe strain capable of degrading chlorinated solvents is fed into the aquifer, promoting EISB in an area surrounding the well location. These wells are typically arranged in a line, perpendicular to the groundwater flow, spaced such that the applied amendments form a barrier or zone of EISB in the vicinity of the arranged line of wells. The site’s contaminated groundwater flows through, reducing contaminant concentrations when in contact and treated by the enhanced microbial colonies within the barrier. Biobarrier wells (BBWs) were installed at the

Building 3327 site toward the end of the initial period of performance (2009); amendment application to condition the barriers was completed in July and November of 2019 and bioaugmentation occurred on

September 24th and December 11th of 2019.

Active remediation systems operations ceased in September 2009. In mid-2011, the inactive recirculation components of the EISB systems were restarted to re-initiate enhanced bioremediation at the OAA-Six

Groundwater Sites. The effect of the operational stoppage in 2009 into 2011 typically demonstrated rebounds in CVOC concentrations and indicated that the efficacy of EISB at the sites had diminished.

1.2 Site Characteristics and Historical Contamination

This section summarizes the pertinent environmental conditions and EISB system configurations for the sites, as presented in the 2006 ROD and Final Remedial Design (APG, 2006a and 2006b). The RIs conducted at

OU6 established the conceptual understanding of the subsurface, including the aquifer units, groundwater flow, and the historical maximum CVOC concentrations detected at each site. The ROD designated the primary COCs and associated IRGs for each site, which are presented in Section 3.0 of this report.

Appendix A features plots of the concentration of CVOC constituents over time, which were generated from the available groundwater sampling results at select monitoring locations at each site. The trends illustrated in these plots provide evidence of a remedy’s performance in progressing each site toward the respective cleanup goals. Data collection began in the 2006 to 2007 timeframe, when the site’s current monitoring well networks and EISB systems were installed.

1.2.1 Building 525

Groundwater at the Building 525 site flows south-southwest toward Woodrest Creek. Surface water flows toward the nearest storm inlets and/or surface water drains or discharges directly to the subsurface in unpaved areas. These storm sewers ultimately discharge into Woodrest Creek.

Soils are characterized as silty clays and fine sands that coarsen with depth, becoming silty and gravelly sands. The surficial aquifer is composed of clay, silts, silty clay, silty sand, sand, and fine gravel. The bottom of the surficial aquifer unit is situated at a depth of approximately 25 feet (ft) below ground surface

(bgs). Groundwater in the surficial aquifer is contaminated with CVOCs.

Elevated groundwater CVOC concentrations have been reported on either side of Building 525. Results of sampling events conducted in 2006 indicated the maximum groundwater CVOC concentrations by constituent were:

• 1,1,1- TCA (4,500 micrograms per liter [µg/L]),

• TCE (2,660 µg/L),

• 1,1-DCE (893 µg/L), and

• carbon tetrachloride (510 µg/L)

Although 1,1,1-TCA was the highest CVOC concentration historically detected, TCE comparatively presented a higher potential carcinogenic risk and remains the primary COC for the site. Low levels of CVOCs were detected in surface water samples suggesting that the CVOCs dissolved in groundwater were discharging to Woodrest Creek. However, all surface water CVOCs detected have been below screening criteria.

The locations of the EISB system’s oleate wells, recirculation wells (extraction wells and injection wells), and monitoring wells are shown on Figure 3. Oleate wells are a cluster of direct injection wells where potassium oleate was used as the primary remediation amendment. The Building 525 recirculation system currently includes four extraction wells and five injection wells. One extraction well (G23-EX-01) and two injection wells (G23-INJ-01 and G23-INJ-05) are installed on the east side of Building 525. Three extraction wells (G23-EX-02, G23-EX-03, and G23-EX-04) and three injection wells (G23-INJ-02, G23-

INJ-03, and G23-INJ-04) are installed on the west side of Building 525.

1.2.2 Building 3327

Groundwater at the Building 3327 site flows northeast toward Swan Creek. Since most of the area within the site is graded and paved, surface water flows toward the nearest storm inlets and/or surface water drains.

These storm sewers ultimately discharge to Swan Creek.

Lithologic data collected from stratigraphic borings and monitoring wells, advanced during previous investigations, were used to obtain a general understanding of site geology to approximately 74 feet bgs. In general, the site geology is characterized by silts, gravels, and fine sands of varying thicknesses to the maximum drilling depth of 74 feet.

Groundwater at the site is contaminated with CVOCs, mainly PCE from former USTs. CVOC impacts appeared highest at depths approximately 50 to 72 feet bgs, decreasing with depth; indicating that contamination is within the unconfined surficial aquifer. In 2005, the highest detected concentration of

CVOCs at the site was PCE at 450 μg/L. Other CVOC compounds were detected near or below the MCLs.

Locations of recirculation wells (extraction and injection), monitoring wells, and BBWs are shown on

Figure 4. Currently, the Building 3327 recirculation system includes four extraction wells (G28f-EX-01, G28f-EX-02, G28f-EX-03, and G28f-EX-04) and three injection wells (G28f-INJ-01, G28f-INJ-02, and

G28f-INJ-03). In September of 2019, the conversion of G28f-EX-04 to G28f-INJ-04 was tested to observe the ability of this well to function as an injection well closer to the UST source area.

1.2.3 Tower Road

Groundwater at the Tower Road Site flows north-northeast toward Dipper Creek. Since most of the area within the site is graded and paved, surface water flows toward the nearest storm inlets and/or surface water drains. Ultimately, these storm sewers discharge to Dipper Creek.

In general, the site geology is characterized by silty material with inter-bedded layers of coarse sand, underlain with organic silty clays. Borings were advanced to a maximum depth of 140 feet bgs. The surficial aquifer is underlain by a semi-confining organic clay unit approximately 75 to 90 feet bgs.

Groundwater in the surficial aquifer is contaminated with CVOCs including TCE, 1,1,1-TCA, 1,2-DCE, and their degradation products. The highest CVOC groundwater concentration detected was TCE at 15,000 μg/L in the vicinity of the former USTs near Building 361 (monitoring well location 361-MW-02A).

Historically, the CVOC plume at the site is primarily composed of TCE.

Locations of BBWs, recirculation wells (extraction and injection wells, now known as Bio-Remediation

Wells [BRW], labeled BRW-01 through BRW-12), and monitoring wells are shown on Figure 5. The active groundwater recirculation system was originally designed to extract groundwater from six extraction wells, amend the extracted groundwater with sodium carbonate and sodium lactate, and inject into the subsurface via six injection wells. However, due to limited extraction well yields (typically less than 1.0 gallon per minute combined from the six extraction wells), the system was reconfigured to maximize the combined groundwater extraction flow rate by extracting groundwater from two to three of the highest yielding wells.

Currently, the Tower Road recirculation system operates with two extraction wells (G29-BRW-03 and G29-

BRW-05) and three injection wells (G29-BRW-01, G29-BRW-04, and G29-BRW-09).

1.2.4 Building 507

Groundwater at the Building 507 Site flows east-southeast toward the Spesutie Narrows, located approximately 400 feet east of Building 507. Surface water flows toward the nearest storm inlets and/or surface water drains or discharges directly to the subsurface in unpaved areas. These storm sewers ultimately discharge to the Spesutie Narrows.

The site’s geology is comprised of an upper silt layer extending from grade to 10 feet bgs and is underlain by a sand layer containing discontinuous silty lenses from 10 to 23 feet bgs. This comprises the site’s surficial aquifer, which is underlain by a sandy, clayey silt layer that was observed to a boring termination depth of 147 feet bgs. The water table is typically three to four feet bgs. CVOC impacts to the surficial aquifer appeared highest at depths of approximately eight to 18 feet bgs, decreasing with depth.

The CVOCs TCE and VC were detected at elevated levels. The historically highest CVOC concentration detected in groundwater was TCE at 10,500 μg/L in 2005. Based on these results, CVOC impacts appeared to be limited to the surficial aquifer. The dissolved phase TCE plume appeared to extend approximately

1,000 feet south/southeast from Building 507 to the Spesutie Narrows.

Locations of BBWs, recirculation wells (extraction and injection), and monitoring wells are shown on Figure

6. The Building 507 recirculation system includes three extraction wells (G32-EX-01, G32-EX-02, and G32-

EX-03) and five injection wells (G32-INJ-01, G32-INJ-02, G32-INJ-03, G32-INJ-04 and G32-INJ-05).

1.2.5 Building M600

Groundwater at the M600 site flows north-northeast toward the Spesutie Narrows. Surface water flows toward the nearest storm inlets and/or surface water drains or discharges directly to the subsurface unpaved areas. Ultimately, these storm sewers discharge into the Spesutie Narrows.

In general, the site geology consists of interbedded sand, silt, and clay layers from grade to approximately

96 feet bgs. A clay layer was observed from 96 to 102 feet bgs. Based on results from groundwater samples collected during previous investigations, the CVOC impacts appear greatest at approximately 10 to 20 feet bgs and decrease with depth, which suggests that the contaminated groundwater (primarily TCE) is limited to the unconfined surficial aquifer.

The historically maximum concentrations of CVOCs detected were:

• TCE at 99.5 μg/L (from a geoprobe groundwater sample),

• PCE at 136 μg/L, and

• 1,1-DCE at 113 μg/L.

The highest TCE and PCE concentrations were observed north of Building M600, with a dissolved-phase plume extending toward the direction of the marsh. The highest concentration of 1,1-DCE was observed in

MW-3, south of Building M600. Locations of oleate wells, recirculation wells (extraction and injection), and monitoring wells are shown on Figure 7.

Groundwater sampling results in 2015 showed compliance with IRGs and SRGs for the second consecutive year. Therefore, in June 2015, active groundwater and EISB amendment recirculation was stopped. In

January 2016 a letter was sent to the USEPA and MDE notifying them of SRG achievement at the site.

Since then, the site PCE concentrations have continued to remain below SRGs.

1.2.6 DRMO

Local groundwater at the DRMO tends to have an extremely shallow gradient, generally flowing toward the northeast from the source area, creating the historically large CVOC plume at DRMO that extended over

2,500 feet in a northeasterly direction. However, localized groundwater flow directions at the source area

(refer to Figure 8) have been found to flow in a southeasterly direction toward a local wetland area. Locally, surface water flows toward the nearest storm inlets and/or surface water drains or discharges directly to the subsurface unpaved areas, generally sloping to the east/southeast toward the lower wetland area.

The geology is characterized by silty material with interbedded layers of sand underlain by sandy soil interbedded with silt layers. This geology extends from grade to the first continuous confining layer encountered at 98 to 110 feet bgs. However, locally in the source area, a clay layer was discovered in boreholes at a depth from 25 to 30 feet bgs. Groundwater is typically encountered at 17 to 18 feet bgs.

CVOC contamination was encountered no deeper than 35 feet bgs site wide. The historically maximum groundwater concentration of TCE was found in MW-2 at 177 µg/L.

The historical EISB amendment injection wells are no longer in use at the DRMO. The last injections occurred before 2009. An October 2013 technical memorandum outlined a secondary treatment approach using chemical oxidant amendment injections to progress groundwater TCE concentrations below the

MCL. This secondary treatment approach was the result of a site evaluation based on monitoring data collected in 2011 and 2012. During this site evaluation period (from 2012 onward), the concentrations of

TCE were found to be consistently below the IRG of 45 µg/L site wide. Favorable biodegradation conditions existed in only some of monitoring locations (typically within the initial EISB grid injection areas) and some locations appeared to be stable below the IRG. Therefore, the use of chemical oxidant was initiated as a method to speed up achievement of the MCL goals at the site, instead of the longer, 30-year predicted timeframe for natural attenuation.

In 2014, in-situ chemical oxidation (ISCO) injections using sodium permanganate were initiated. A total of

15 new injection wells were installed in March 2014. Sodium permanganate was gravity fed into the injection wells from June through November 2014. The ISCO injections were halted prior to complete implementation of the secondary treatment technology due to regulator concerns regarding the need for the secondary treatment technology and potential metals mobilization as a by-product of ISCO injections.

Figure 8 illustrates the ISCO injection points, monitoring wells, and former bio-injection points.

Following the completion of additional metals groundwater sampling and analysis in 2016, a letter was sent to regulators summarizing the results. The groundwater sampling results showed that none of the detected metals concentrations exceeded their associated MCL and metals mobilization from ISCO injections were not observed. This letter also included an update on the site’s CVOC groundwater concentrations. Sampling results showed a second consecutive year of SRG achievement, ceasing the need for further injection activities at the site. Since then, the site TCE concentrations have continued to trend below SRGs.

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2.0 ANNUAL OPERATIONS AND MAINTENANCE RESULTS FOR 2023

This section presents the 2023 O&M of the EISB systems at the OAA-Six Groundwater Sites. O&M covered in this report was performed for the period of January 2023 through December 2023. At a minimum, each site’s system was inspected, and the system operation parameters were recorded weekly.

The O&M performed at each of the sites for the 2023 calendar year is summarized by site in the following subsections. Routine maintenance activities are not included in these summaries. Examples of omitted routine activities are amendment chemical refills, minor leak repairs/fitting replacements, flowmeter cleaning, standard pump maintenance, and site groundskeeping.

In 2023 active recirculation was paused to initiate a CVOC rebound assessment rest period. Site remedy optimizations are being evaluated and a scheduled rest period will help identify areas that may need enhancement. As mentioned previously, only four sites (Building 525, Building 3327, Tower Road, and

Building 507) have COC concentrations above SRGs and utilize groundwater recirculation systems. While

O&M activities are primarily associated with recirculation operations, the sites were monitored, maintained, and improved where needed throughout the year as to be kept in a state of ready.

2.1 Site 23 – Building 525

Site 23 – Building 525 utilizes two recirculation systems and has been separated into two areas to treat the two CVOC groundwater plumes on-site. These separate recirculation areas are referred to as Building 525

West and Building 525 East. These designations coordinate with the location of the recirculation system in relation to their positioning around Building 525, and the two areas that historically contained the highest

CVOC concentrations.

2.1.1 Building 525 West

The EISB system located at Site 23 - Building 525 West was configured as a continuous run, groundwater recirculation system as the site’s remedy to address the CVOC plume. Extraction pumps continuously forced groundwater into the system manifold, where amendments were introduced in-line with the fluid stream and fed under pressure into the site’s injection wells.

2.1.1.1 Building 525 West – System Operations Summary

There is no annual system operations data to report in 2023 as active recirculation has been ceased due to

SRG compliance. As indicated in the 2006 ROD, once SRGs are attained, the active remediation treatments will cease operation and further degradation of the groundwater plume will occur by natural attenuation processes until MCLs are achieved.

2.1.1.2 Building 525 West – Maintenance

LTM groundwater sampling was conducted on May 3, 2023.

2.1.2 Building 525 East

The EISB system located at Site 23 - Building 525 East is configured as a timed, batch run, groundwater recirculation system as the site’s remedy to address the CVOC plume. An extraction pump fills a holding tank with groundwater. The groundwater extraction pump is controlled by a level switch in the holding tank. The recirculation pump connecting the holding tank to the recirculation manifold is controlled by a programmable digital timer. This allows control of injection intervals and frequencies to prevent short-circuiting of injected groundwater to the surface. When a scheduled batch is run, the timer activates the recirculation pump, groundwater is forced into the recirculation manifold, where amendments are introduced in-line to the fluid stream and fed under pressure into the site’s injection wells. The primary bioremediation amendments being used consists of a sodium lactate or emulsified vegetable oil electron donor substrate and a sodium carbonate pH buffer.

2.1.2.1 Building 525 East – System Operations Summary

There is no annual system operations data to report in 2023. Active recirculation is paused for rebound assessment and system maintenance.

2.1.2.2 Building 525 East Maintenance

This section details system maintenance for the Building 525 East EISB system in 2023:

• LTM groundwater sampling was conducted on May 3, 2023.

2.2 Site 28f – Building 3327 Former UST Site

The EISB system located at Site 28f - Building 3327 is configured as a continuous run, groundwater recirculation system as the site’s remedy to address the CVOC plume. Extraction pumps continuously force groundwater into the system manifold, where amendments are introduced in-line with the fluid stream and fed under pressure into the site’s injection wells. The primary bioremediation amendments being used consists of a sodium lactate or emulsified vegetable oil electron donor substrate and a sodium carbonate pH buffer.

2.2.1 Building 3327 – System Operations Summary

There is no annual system operations data to report in 2023. Active recirculation is paused for rebound assessment and system maintenance.

2.2.2 Building 3327 – Maintenance

This section details system maintenance for the Building 3327 EISB system in 2023:

• LTM groundwater sampling was conducted on April 18-20, 2023.

2.3 Site 29 – Tower Road

The EISB system located at Site 29 - Tower Road is configured as a batch run, groundwater recirculation system as the site’s remedy to address the CVOC plume. Extraction pumps fill a holding tank with groundwater. The groundwater extraction pumps are controlled by level switches in the holding tank. When the holding tank reaches a volume threshold, a float switch activates the recirculation pump and groundwater is forced into the recirculation manifold, where amendments are introduced in-line to the fluid stream and fed under pressure into the site’s injection wells. The primary bioremediation amendments being used consists of a sodium lactate or emulsified vegetable oil electron donor substrate and a sodium carbonate pH buffer.

2.3.1 Tower Road – System Operations Summary

There is no annual system operations data to report in 2023. Active recirculation is paused for rebound assessment and system maintenance.

2.3.2 Tower Road – Maintenance

This section details system maintenance for the Tower Road EISB system in 2023:

• LTM groundwater sampling was conducted on May 1-2, 2023.

2.4 Site 32 – Building 507

The EISB system located at Site 32 - Building 507 is configured with a timed, batch run, gravity fed, groundwater recirculation system as the site’s remedy to address the CVOC plume. An extraction pump fills a batch tank with groundwater. The groundwater extraction pump is controlled by level switches in the batch tank. When a scheduled batch is run, a timer activates the recirculation pump and groundwater is forced into the recirculation manifold, where amendments are introduced in-line to the fluid stream and fed into holding tanks adjacent to the injection wells and injected via gravity feeding. The primary bioremediation amendments being used consists of a sodium lactate or emulsified vegetable oil electron donor substrate and a sodium carbonate pH buffer.

2.4.1 Building 507 – System Operations Summary

There is no annual system operations data to report in 2023. Active recirculation is paused for rebound assessment and system maintenance.

2.4.2 Building 507 – Maintenance

This section details system maintenance for the Building 507 EISB system in 2023:

• LTM groundwater sampling was conducted on April 25-26, 2023.

2.5 Site 33 – Building M600

The EISB system located at Site 33 - Building M600 was configured as a timed groundwater recirculation system as the site’s remedy to address the CVOC plume. In June 2015, following the review of LTM groundwater sampling results, active groundwater and EISB amendment recirculation was stopped.

Groundwater sampling results in 2019 showed compliance with IRGs and SRGs for the fourth consecutive year. Groundwater COC concentrations have remained below the IRGs and SRGs since June 2015 and the recirculation system has remained off. Annual LTM groundwater sampling has and will be continued to monitor the natural attenuation of the remaining low-level COC levels and degradation byproducts.

2.5.1 Building M600 – System Operations Summary

There is no annual system operations data to report in 2023 as active recirculation has been ceased due to

SRG compliance. As indicated in the 2006 ROD, once SRGs are attained, the active remediation treatments will cease operation and further degradation of the groundwater plume will occur by natural attenuation processes until MCLs are achieved.

2.5.2 Building M600 – Maintenance

LTM groundwater sampling was conducted on April 26, 2023.

2.6 Site 16 – DRMO

As previously discussed, ISCO injections using sodium permanganate were halted in November 2014 due to State regulator concerns of metals mobilization. Metals sampling completed following injections of sodium permanganate did not indicate metals mobilization within the site groundwater. A letter was submitted to the USEPA and state regulators on 31 August 2016 stating that the site’s TCE concentrations were below the SRG concentration. As indicated in the 2006 ROD, since concentrations below the SRGs were established, the active remediation treatments ceased operation and further degradation of the groundwater plume is expected to occur by natural attenuation processes until MCLs are achieved. As such, site activities transitioned to a monitoring phase involving the collection of groundwater samples for VOCs as well as natural attenuation-based parameters to assess the aquifer chemistry and the further degradation of the remaining COCs present in the aquifer.

2.6.1 DRMO – Maintenance

LTM groundwater sampling was conducted on May 16, 2023.

3.0 ANNUAL LTM GROUNDWATER RESULTS FOR 2023

According to the 2006 ROD (APG, 2006a), the Selected Remedy for the Six Groundwater Sites is intended to prevent human exposures to COCs, and to prevent migration of COCs from the Six Groundwater Sites.

Areas of high groundwater CVOC concentrations are being remediated by anaerobic reductive dechlorination through EISB. The selected remedy is designed to reduce the higher concentrations of

CVOCs in groundwater to the IRGs via enhanced bioremediation. Once IRG concentrations are attained at a site, the active groundwater recirculation technology of the EISB system may be terminated, and the

CVOC plume treated via monitored natural attenuation (MNA) (or another technology if MNA is not appropriate at the site) until MCLs are achieved. IRGs have been attained at the Building M600 site, DRMO site, and western portion of the Building 525 site.

The ultimate remedial goal for CVOCs at each of the OAA-Six Groundwater Sites is to achieve contaminant concentrations in groundwater below the MCLs. The MCLs are enforceable groundwater contaminant concentration criteria, intended to be protective of human drinking water, the most restrictive groundwater use. IRGs and SRGs were developed for each site to facilitate each site’s specific CVOC remediation strategy. Each site’s IRG is based on a primary COC, typically the CVOC detected at the highest concentration for each site during its RI. The IRG concentrations were developed under the assumption that once IRGs are achieved, remaining CVOC concentrations will reduce via natural attenuation to MCLs within a 30-year timeframe.

Per the ROD, SRGs are more conservative concentrations than the IRGs. When the SRG concentration is reached, CVOC concentrations are anticipated to remain below IRGs, even in cases where contaminant concentrations rebound. This would allow for the natural attenuation of the CVOCs to proceed and reduce concentrations to MCLs within the 30-year timeframe as predicted in the groundwater modeling. Whereas in the case of achieving the IRG, a case of rebound could put concentrations over the IRG, and the timeframe for natural attenuation would be set longer than modeled. Despite their development, the SRGs are not to be considered as performance standards for the remedies.

The objective of the LTM program is to provide periodic and continuing assessment of the nature and extent of the CVOC groundwater contamination beneath the Six Groundwater Sites. The current LTM effort includes annual sampling and analysis for CVOC concentrations dissolved in the groundwater at each of the sites.

Figures 3 through 8 show the locations and the primary COC results of selected monitoring wells at each of the six sites sampled in 2023. The wells selected are considered representative of the CVOC plumes and provide the data required to assess CVOC plume status and EISB system performance. Generally, wells selected were from: within the historical contaminant source areas, within the historical areas of the CVOC plume, and outside of the plume extents. Monitoring wells upgradient and downgradient of the groundwater contamination were also selected, if available, to provide a baseline with which to compare other wells within the plumes, if necessary. Additionally, Figures 3 through 8 also show the areal extent of the groundwater plume of each site’s primary COC above its site-specific IRG, based on the 2023 sampling results.

The purpose of this section is to present the annual groundwater sampling analytical results and data interpretation. Analytical results for samples collected in 2023 are presented in Tables 1 through 7.

Appendix A contains temporal trend plots of the concentration of CVOC constituents generated from the available record of groundwater sampling results at select monitoring locations from each site, typically beginning in year 2006 or 2007 when the EISB systems were installed, and remedial action operations (RA-

O) began. The plots illustrate the COC concentration trends at these monitoring locations and provide additional evidence of progress toward cleanup goals.

All LTM sampling, field analysis, and water level measurements were performed following standard field procedures contained in Appendix J of the Standard Operating Procedures Work Plan for CERCLA RI/FS

(Generic Work Plan) (U.S. Army Corps of Engineers [USACE], 1995). Appendix B contains the field logs generated during the 2023 LTM groundwater sampling events. Logs include results from geochemical parameter collection via water quality instruments to assess stabilization during low-flow groundwater sampling.

Samples were collected via a low-flow sampling system. Groundwater was extracted via submersible pump, or a dedicated bailer. Groundwater was extracted at a rate no higher than 500 milliliters per minute. Water quality parameters (DO, pH, SPC, ORP) were recorded periodically (typically every five to ten minutes).

Once water quality parameters had stabilized to within approximately 10% for the three consecutive recorded measurements, groundwater samples were collected in appropriate containers for analyses. The samples were then sent to a Department of Defense (DoD) Environmental Laboratory Accreditation

Program (ELAP) certified laboratory. All samples were analyzed for the following CVOCs using USEPA

Method 8260C: PCE, TCE, cis-1,2-DCE, 1,1-DCE, 1,1-DCA, VC, and 1,1,1-TCA. Selected monitoring wells were also analyzed for dissolved organic carbon (DOC), and dissolved gases (ethene, ethane and methane). The detailed laboratory reports containing all analytical results for the 2023 calendar year are contained in Appendix C.

3.1 Site 23 – Building 525

Based on historical uses of the Building 525 Site and results of the RI, the COCs identified in groundwater at the site are PCE, TCE, 1,1-DCE and VC. Of these COCs, the most prevalent or primary COC for the site is TCE. Remediation goals for Site 23 - Building 525 were developed for TCE and are summarized in the table below:

Remediation Goals for Building 525

COC

MCL

(µg/L)

IRG

(µg/L)

SRG

(µg/L)

TCE 5 50 37.5

The results for Building 525 were divided into the West and East areas based on the historical locations of the CVOC source areas, and the extent of their plumes.

Building 525 West

Groundwater samples were collected from two extraction wells (G23-EX-03 and G23-EX-04) and four monitoring wells (G23-MW-18s, G23-MW-23s, G23-MW-24s, and G23-MW-27s) in 2023. CVOC, DOC, and dissolved gases analytical results from the 2023 sampling event are summarized in Table 1. The TCE results are shown on Figure 3 and summarized below.

• TCE concentrations at G23-EX-03 (6.1 µg/L), G23-MW-23s (12 µg/L), and G23-MW-24s (18 µg/L) were below the IRG of 50 µg/L and SRG of 37.5 µg/L but above the MCL of 5 µg/L.

• TCE concentrations at G23-EX-04 (1.7 µg/L), G23-MW-18s (11.4 µg/L), and G23-MW-27s

(0.41 J µg/L) were below the MCL.

Building 525 East

Groundwater samples were collected from one extraction well (G23-EX-01) and nine monitoring wells (G23-

OW-04, G23-MW-03s, G23-MW-05s, G23-MW-11s, G23-MW-13s, G23-MW-14s, G23-MW-15s, G23-

MW-17s, and G23-MW-25s) in 2023. CVOC, DOC, and dissolved gases analytical results from the 2023 sampling event are summarized in Table 1. The TCE results are shown on Figure 3 and summarized below.

• TCE concentrations at G23-OW-04 (62 µg/L) and G23-MW-11s (230 µg/L) were above the IRG.

• TCE concentrations at G23-MW-03s (12 µg/L), G23-MW-17s (7.5 µg/L), and G23-MW-25s (26 µg/L) were below the SRG but above the MCL.

• TCE concentrations at G23-EX-01 (0.92 J µg/L), G23-MW-05s (2.5 µg/L, G23-MW-13s (4.8 µg/L), G23-MW-14s (<0.5 µg/L), and G23-MW-15s (<0.5 µg/L) were below the MCL.

3.1.1 Plume Assessment for Site 23 – Building 525

The Building 525 plumes are effectively being treated with the EISB system as overall TCE concentrations have decreased when compared to results from previous sampling events. Figure 9 illustrates how the TCE plume has reduced in area since baseline sampling in 2006. Appendix A provides well trend graphs for

TCE. Specific observations from Appendix A on TCE trends are summarized below:

Building 525 West

• G23-EX-03: Except for an anomalous spike in 2019, the TCE concentrations have been below the

SRG of 37.5 µg/L since 2017. Overall TCE concentrations have reduced by more than an order-of-magnitude since baseline sampling. The presence of cis-1,2-DCE (8.2 µg/L), 1,1-DCE (0.66 J µg/L) and 1,1-DCA (0.88 J µg/L) in 2023 indicates reductive dechlorination is occurring.

• G23-EX-04: TCE concentrations have been below the SRG of 37.5 µg/L since 2014, including a relatively minor spike in 2019. Since 2021, TCE concentrations have been below the MCL. In

2023, with TCE concentrations continuing to be below the MCL and degradation compounds present, groundwater conditions are supporting reductive dechlorination.

• G23-MW-18s: TCE concentrations have been below the MCL of 5 µg/L since 2014. In 2023 TCE concentrations were (1.4 µg/L), 1,1-DCA (1.3 µg/L) and VC (0.58 J µg/L) were the only other

CVOCs detected. This combined with TCE concentrations below MCL for a long period of time, and relatively low methane concentration in 2023 (650 µg/L) suggests reducing conditions are present and stable but not as active as in the past due to the overall lower availability of CVOCs in the area of this monitoring location.

• G23-MW-23s: Following a significant reduction during the first few years of RA-O and compliance with the SRG since 2013, TCE concentrations have been gradually decreasing. In 2023 TCE concentrations were 12 µg/L. The presence of methane (4.7 µg/L) and cis-1,2-DCE (19 µg/L) indicate reductive dechlorination is occurring.

• G23-MW-24s: Following a reduction during the first few years of RA-O, TCE concentrations have been below the SRG since 2015. TCE concentrations in 2023 were at 18 µg/L The presence of cis-

1,2-DCE (5.1 µg/L) and VC (0.31 J µg/L) in 2023 was slightly lower than last year, but still indicates conditions are favorable for reductive processes.

• G23-MW-27s: TCE concentrations have been below the MCL of 5 µg/L since 2014.

Building 525 East

• G23-EX-01: TCE concentrations have been below the MCL of 5 µg/L since 2016.

• G23-OW-04: TCE concentrations have fluctuated above and below the IRG of 50 µg/L since the start of RA-O. Current concentrations are just above the IRG at 62 µg/L, which is slightly lower than 2021 and 2022 exceedances. TCE concentration fluctuations suggest residual source material may remain in the area represented by this well located under the building.

• G23-MW-03s: Following a significant reduction from 2014 to 2016, TCE concentrations have remained stable between the SRG of 37.5 µg/L and MCL of 5 µg/L through 2023. The residual

TCE underneath the building is likely serving as a secondary source for this monitoring well, which is located just outside the building footprint.

• G23-MW-05s: TCE concentrations have been below the MCL of 5 µg/L since 2014.

• G23-MW-11s: TCE concentrations significantly decreased during the first few years of RA-O and remained relatively stable below the SRG from 2012 to 2017. However, TCE concentrations have been gradually increasing above the IRG since 2020. Although again in 2023, the continued presence of all degradation compounds indicates reductive dechlorination is occurring. TCE concentration increases suggests residual source material may remain in the area represented by this monitoring well (and G23-OW-04) as it was installed in the vicinity of two former site USTs containing solvents and fuel oil.

• G23-MW-13s: TCE concentrations significantly decreased during the first few years of RA-O, spiked in 2014, and then significantly decreased again. TCE concentrations had been at or below the MCL of 5 µg/L since 2017. Following a slight exceedance of the MCL in 2022, concentrations are back below the MCL in 2023 at 4.8 µg/L. Like G23-MW-03s and G23-MW-11s, it is possible that groundwater VOC concentrations in this area is influenced by the residual contamination present under the building footprint. Methane concentrations in 2023 (2.4 µg/L) were significantly lower than in and 2022 which is attributed to the recirculation rest period and the well’s close, downgradient position from injection well G23-INJ-05.

• G23-MW-14s: Except for low level detections in 2018 and 2020, TCE concentrations have been below the MCL of 5 µg/L since 2009.

• G23-MW-15s: TCE concentrations have been below the…