3. Draft Final Monitored Natural Attenuation Report, Site OT007, Scott AFB.pdf

DRAFT FINAL

2024 Annual Monitored Natural Attenuation Report Site OT007 – Former Sludge Weathering Lagoon / Bulk

Fuel Facility Scott Air Force Base, Illinois

Prepared for:

U.S. Air Force Civil Engineer Center

AIR FORCE CIVIL ENGINEER CENTER

2261 Hughes Avenue, Suite 155

Lackland AFB, Texas 78236-9853

Contract FA8903-14-C-0009

March 2025

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DRAFT FINAL

2024 Annual Monitored Natural Attenuation Report

Site OT007 – Former Sludge Weathering Lagoon / Bulk Fuel Facility

Scott Air Force Base, Illinois

Prepared for:

U.S. Air Force Civil Engineer Center

AIR FORCE CIVIL ENGINEER CENTER

2261 Hughes Avenue, Suite 155

Lackland AFB, Texas 78236-9853

Contract FA8903-14-C-0009

March 2025

Prepared by:

Plexus Scientific Corporation 5510 Cherokee Ave., Suite 350

Alexandria, VA 22312

Draft Final 2024 Annual MNA Report Site OT007 – Former Sludge Weathering Lagoon/Bulk Fuel Facility

Scott Air Force Base, Illinois i

TABLE OF CONTENTS

1.0 INTRODUCTION

1.1 Report Organization

1.2 Purpose of Report

1.3 Project Location

2.0 BACKGROUND INFORMATION

2.1 Physical Setting

2.1.1 Site-Specific Geology and Hydrogeology

2.1.2 Land Use

2.1.3 Groundwater Use

2.2 Previous Investigations

2.2.1 2013 Comprehensive Site Investigation Report

2.2.1.1 Summary of Analytical Results in Groundwater and Surface Water

2.2.1.2 Selected Remedy

2.2.2 2013 Groundwater Monitoring Event

2.2.3 2015 Groundwater Monitoring Event

2.2.4 2016 Groundwater Monitoring Event

2.2.5 2017 Soil Characterization Investigation

2.2.6 2017 Groundwater Monitoring Event

2.2.7 2018 Groundwater Monitoring Event

2.2.8 2019 Groundwater Monitoring Event

2.2.9 2020 Oxidant Injection Event

2.2.10 2020 Groundwater Monitoring Event

2.2.11 2021 Groundwater Monitoring Event

2.2.12 2022 Groundwater Monitoring Event

2.2.13 2023 Groundwater Monitoring Event

3.0 SUMMARY OF FIELD ACTIVITIES

3.1 Soil Characterization Sampling

3.1.1 Quality Assurance/Quality Control Samples

3.2 Groundwater Monitoring

3.2.1 Water Level Measurements

3.2.2 Groundwater Sampling Activities

3.2.3 Quality Assurance/Quality Control Samples

3.3 Sample Preservation and Shipping

3.4 Investigation-Derived Waste Disposal

3.5 Surface Water Monitoring

3.6 Work Plan Deviations

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4.0 DATA QUALITY EVALUATION

4.1 Quality Assurance/Quality Control Sampling

4.2 Data Validation

4.3 Data Quality Assessment and Usability

4.3.1 Precision

4.3.2 Accuracy

4.3.3 Representativeness

4.3.4 Comparability

4.3.5 Completeness

4.3.6 Sensitivity

4.4 Data Quality Conclusions

5.0 SUMMARY OF ANALYTICAL RESULTS

5.1 Zone 1 Groundwater Flow Direction and Hydraulic Gradient

5.2 Groundwater Analytical Data

5.2.1 Volatile Organic Compounds Results

5.2.2 Monitored Natural Attenuation Parameter Results

5.3 Mann-Kendall Statistical Evaluation

5.4 Soil Analytical Data

6.0 CONCLUSIONS AND RECOMMENDATIONS

6.1 Groundwater MNA Conclusions

6.2 Soil Characterization Sampling Conclusions

6.3 Recommendations:

7.0 REFERENCES

Scott Air Force Base, Illinois iii

FIGURES

Figure 1-1 Scott AFB Location Map Figure 1-2 Site Vicinity Map Figure 2-1 Site Location Map Figure 2-2 March 2011 Zone 1 and December 1988 Zone 2 Potentiometric Map and

Historical Sample Locations Figure 2-3 March 2011 Groundwater and Surface Water Sample Exceedances Figure 3-1 Soil Boring Locations June 2017, June 2020, June 2021, June 2022, June 2023, and June 2024 Figure 5-1 Potentiometric Surface Map June 2024 Figure 5-2 Selected Results and Groundwater Exceedances June 2024 Figure 5-3 OT07-MW06 VOC Concentration and Groundwater Elevation Over Time Figure 5-4a OT07-MW02 TOC and Sulfate Concentrations Over Time Figure 5-4b OT07-MW04 TOC and Sulfate Concentrations Over Time Figure 5-4c OT07-MW05 TOC and Sulfate Concentrations Over Time Figure 5-4d OT07-MW06 TOC and Sulfate Concentrations Over Time Figure 5-5 Soil Boring Results June 2024

TABLES

Table 1-1 Site OT007 Groundwater Remediation Objectives Table 2-1 Site OT007 Historical Synoptic Water Levels Table 3-1 Site OT007 Water Quality Parameters Table 5-1 Site OT007 Monitoring Well Construction Details Table 5-2 Groundwater Elevations at Site OT007 Table 5-3 Site OT007 Summary of June 2024 Groundwater Results Table 5-4 Site OT007 Summary of June 2024 Soil Results Table 6-1 Annual Groundwater Sample and Analysis

APPENDICES

Appendix A Field Forms Appendix B Analytical Reports Appendix C Data Quality Validation Reports Appendix D Waste Disposal Documentation Appendix E Mann-Kendall Analysis Supporting Documentation

Scott Air Force Base, Illinois iv

Scott Air Force Base, Illinois v

ACRONYMS AND ABBREVIATIONS

# Number % Percent µg/kg micrograms per kilogram µg/L micrograms per liter AFB Air Force Base AFCEC Air Force Civil Engineer Center AOC Area of Concern AST Aboveground Storage Tank bgs Below Ground Surface CERCLA Comprehensive Environmental Response, Compensation, and Liability Act cis-1,2-DCE cis-1,2-dichloroethene CoC Chain of Custody COC Contaminant of Concern COPC Contaminant of Potential Concern CSIR Comprehensive Site Investigation Report CVOC Chlorinated Volatile Organic Compound DL Detection Limit DO Dissolved Oxygen DoD Department of Defense ERM Environmental Resources Management, Inc.

F result greater than or equal to the method detection limit but less than the reporting limit ft/day feet per day ft/ft feet per foot ft/min feet per minute GRO Groundwater Remediation Objective HGL HydroGeoLogic, Incorporated IAC Illinois Administrative Code IDW Investigation-Derived Waste IEPA Illinois Environmental Protection Agency ILWATER ISGS Water Well Database ISGS Illinois State Geological Survey J The positive result reported for this analyte is a quantitative estimate due to discrepancies in meeting certain analyte-specific quality control criteria (data designation) or detection between the method detection limit and reporting limit

LCS/LCSD Laboratory Control Sample/Laboratory Control Sample Duplicate LOD Limit of Detection LOQ Limit of Quantitation LUC Land Use Control

Scott Air Force Base, Illinois vi

ACRONYMS AND ABBREVIATIONS (CONTINUED)

M For 2007, 2010, and 2011 datasets: Matrix effect was present, the concentration is estimated. All other datasets: Manually integrated compound (data designation) mg/L milligrams per liter MNA Monitored Natural Attenuation MS/MSD Matrix Spike/Matrix Spike Duplicate mV millivolt MW Monitoring Well ORP Oxidation Reduction Potential OWS Oil-Water Separator Pace Pace Analytical, LLC PAL Project Action Limit PID photo-ionization detector Plexus Plexus Scientific Corporation PPE Personal protective equipment QA Quality Assurance QC Quality Control QSM Quality Systems Manual R Rejected Data (data designation) RAP Remedial Action Plan ROR Remediation Objectives Report RPD Relative Percent Difference RSK Robert S. Kerr Laboratory RSL regional screening level SM Standard Method SOP Standard Operating Procedure SRO Soil Remediation Objective SRP Site Remediation Program SW Solid Waste TACO Tiered Approach to Corrective Action Objectives TAT Turnaround Time TCE Trichloroethylene TOC Total Organic Carbon U.S. United States UFP-QAPP Uniform Federal Policy-Quality Assurance Project Plan USEPA United States Environmental Protection Agency VC Vinyl Chloride VOC Volatile Organic Compound

Scott Air Force Base, Illinois

1.0 INTRODUCTION

This Annual Monitored Natural Attenuation (MNA) Report provides a summary of the 2024 groundwater monitoring activities. Land use control (LUC) inspections, performed quarterly, are documented by the Air Force separately in an annual letter. The work was performed under Contract Number FA8903-14-C-0009. The 2024 field event was conducted in accordance with the Final Amended Remedial Action Plan for Site OT007 (Final Amended RAP; Plexus Scientific Corporation [Plexus], 2016a), the Final Remedial Action Plan Addendum, Uniform Federal Policy-Quality Assurance Project Plan (UFP-QAPP), Site OT007 Former Sludge Weathering Lagoon/Bulk Fuel Facility, (Plexus, 2019b), and the Comprehensive Site Investigation Report (CSIR) / Remediation Objectives Report (ROR) / Remedial Action Plan (RAP) for Site OT007 (CSIR/ROR/RAP; HydroGeoLogic, Incorporated [HGL], 2013).

Site OT007 encompasses the two former 30,000-gallon jet fuel aboveground storage tanks (ASTs) (Tanks 8552 and 8554) previously known as Area of Concern (AOC) 22, and the former sludge weathering lagoon. The ASTs were used to store jet fuels from 1970 until 1994. In April 1996, the two 30,000-gallon ASTs were removed from the site in preparation for the construction of Taxiway G, which opened in 1998. Subsequent to the AST removal action, in July and August 1996, a remedial action was performed to remove contaminated soil from the former AST area (Burns & McDonnell, 1997). The excavation was as deep as 16 feet in some areas. A total of 6,063 tons of soil was removed from the site. Groundwater encountered during excavation activities limited the excavation depth. The sludge weathering lagoon, which measured 20 feet by 40 feet, was used for approximately two years in the mid-1970s to dry out the sludge removed from the bottoms of the adjacent ASTs. The lagoon was closed in 1981. Closure activities included removing soil to two feet below ground surface (bgs) and backfilling the excavation to grade with sand and gravel (ERM, 1992).

The United States (U.S.) Air Force Civil Engineer Center (AFCEC) contracted Plexus in 2014 to conduct additional investigation, potentially remove impacted soil, and continue annual groundwater monitoring and surface water sampling of the former OT007 Sludge Weathering Lagoon/Army Reserve Bulk Fuel Facility (Site OT007 or “the Site”), located at Scott Air Force Base (AFB), Illinois. In 2011, Scott AFB requested that Illinois Environmental Protection Agency (IEPA) allow the environmental investigation and any needed response action for Site OT007 to stop following the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) statute and implementing regulations because of CERCLA's petroleum exclusion. The IEPA agreed on September 28, 2011 to let Site OT007 follow the Site Remediation Program (SRP) evaluation, cleanup, and reporting process (35 Illinois Administrative Code [IAC] Part 740.435).

Pursuant to SRP requirements, usable results from the investigations at Site OT007 were screened against remediation objectives found in Illinois' Tiered Approach to Corrective Action Objectives (TACO; 35 IAC Part 742). As presented in the Final CSIR/ROR/RAP (HGL, 2013), Scott Air Force Base, Illinois the selected remedial action for Site OT007 is MNA with LUCs. This action was selected to address the groundwater contaminants of concern (COCs) of arsenic, iron, manganese, cis-1,2-dichloroethene (cis-1,2-DCE), and vinyl chloride (VC). The groundwater remediation objectives (GROs) selected for those COCs are presented in Table 1-1. For the metal COCs, the highest background concentrations were selected as their GROs. Background concentrations in groundwater for Scott AFB were established in the Final Basewide Groundwater Dataset Evaluation Technical Memorandum (HGL, 2012). The GROs for cis-1,2-DCE and VC are based on the Class I groundwater values in TACO (35 IAC 742) Tier 1, Appendix B, Table E. The Final CSIR/ROR/RAP (HGL, 2013) identified no surface water or soil COCs or surface water or soil remediation objectives (SROs). LUCs are documented in Appendix B of the LUC Memorandum of Agreement (MOA; USAF, 2008).

Based on groundwater sampling events conducted at the Site since 2015, the chlorinated volatile organic compound (CVOC) biodegradation process appears to be effectively reducing the CVOC concentrations. As described in the Final Amended RAP (Plexus, 2016b), the current presence and extent of CVOCs and other volatile organic compounds (VOCs) within the soil at Site OT007 in the vicinity of monitoring wells (MWs) OT07-MW04 and OT07-MW06 and upgradient of both MWs, was evaluated in 2017. The soil investigation phase of the field work was postponed until June 2017 due to the runway closure and repaving effort, which had resulted in more traffic on Taxiway G.

Results of the investigation, along with recommendations, were reported in the Final, Revision 1, Soil Characterization Report (Plexus, 2018c). Several analytes were detected above project action limits (PALs) in saturated soil, including 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, benzene, ethylbenzene, m,p-xylene, o-xylene, naphthalene, and VC (Plexus, 2018c). The PALs for the 2017 soil characterization effort were presented in Worksheet #15 of the Final Amended RAP (Plexus, 2016b). The PALs for the 2017 work are based on the lowest available TACO (35 IAC 742) Tier 1 SROs for residential properties (Appendix B, Table A), industrial/commercial properties (Appendix B, Table B), or the Soil Component of the Groundwater Ingestion Route for Class I groundwater values (IEPA, 2013b). If a TACO (35 IAC 742) Tier 1 SRO had not been promulgated for a given compound (e.g., 1,3,5-trimethylbenzene), then the PAL was set at the U.S. Environmental Protection Agency (USEPA) regional screening level (RSL) for residential exposures (USEPA, 2024) or at a Non-TACO calculated value protective of construction workers (Plexus, 2016b).

The 2017 soil investigation determined that residual contamination was present in the saturated capillary fringe above the groundwater table rather than in unsaturated soil. These results are consistent with historical groundwater data, which indicate higher groundwater contaminant concentrations when the water table is elevated. Additionally, benzene was added to the list of groundwater COCs for the Site after being detected above the GRO for the second time during 2017 (Final 2017 Annual MNA Report, Plexus, 2019a). Based on the results of the soil investigation, chemical oxidant injections within the capillary fringe and shallow groundwater

Scott Air Force Base, Illinois were recommended to abiotically degrade CVOCs and petroleum hydrocarbons at the Site, followed by continuation of the MNA remedy (Plexus, 2018c). As a result, chemical oxidant injections into the capillary fringe were performed in May 2020, followed by the collection of post-injection soil characterization samples on an annual basis to assess the effectiveness of the injections on subsurface soils around the capillary fringe at that time. The PALs for the post-injection soil characterization work were presented in Worksheet #15 of the Final Remedial Action Plan Addendum, Uniform Federal Policy Quality Assurance Project Plan (Final RAP Addendum; Plexus, 2019b). They were selected following the same approach that was used for PAL selection in the Final Amended RAP (Plexus, 2016b), as described above. The PALs for the post-injection soil characterization work are presented in Table 5-4 (see Section 5.4). Additional groundwater monitoring and soil characterization events were performed annually between 2021 and 2024.

In June 2024, an annual groundwater and soil characterization event was performed. The groundwater PALs for contaminants of potential concern (COPCs) are presented in Table 5-3 (see Section 5.2). The groundwater PALs for COPCs are based on the Class I groundwater values in TACO (35 IAC 742) Tier 1, Appendix B, Table E. If such values do not exist for a given COPC, then the PALs are based on USEPA tap water RSLs (e.g., methyl tert-butyl ether) or USEPA Maximum Contaminant Levels (e.g., nitrite).

1.1 Report Organization

This Annual MNA Report is organized as follows:

• Section 1.0 Introduction – provides a general background of the Site and describes the organization and intent of this monitoring report.

• Section 2.0 Background Information – summarizes previous investigations, physical setting, and current land use.

• Section 3.0 Summary of Field Activities – summarizes the field activities conducted in 2024.

• Section 4.0 Data Quality Evaluation – describes the data quality evaluation process used to review and evaluate laboratory analytical data collected.

• Section 5.0 Summary of Analytical Results – provides the results of the sampling and evaluation of the data.

• Section 6.0 Conclusions and Recommendations – presents the conclusions and recommendations for Site OT007.

• Section 7.0 References – lists the reference documents used to prepare this report.

1.2 Purpose of Report

The purpose of this report is to discuss the sampling activities and analytical results for the groundwater monitoring and the post-injection soil characterization sampling event that was

Scott Air Force Base, Illinois conducted on June 25-26, 2024. Analytical results from 2024 were also compared to historical data to assess trends and develop a path forward for future sampling events.

1.3 Project Location

Scott AFB is located in St. Clair County, Illinois, approximately 25 miles southeast of St. Louis, Missouri, in southwestern Illinois (Figure 1-1). The city of Belleville, Illinois, is located approximately six miles southwest of Scott AFB, and the city of O’Fallon, Illinois, is located approximately 5 miles northwest of the base. Scott AFB property encompasses approximately 3,545 acres of U.S. Government-owned and easement land. Agricultural land, retail development, a commercial airport, and residential properties border the base.

Scott AFB was established in 1917 as an aircraft pilot training facility known as Scott Field.

Between 1920 and 1937, dirigible airships were assigned to Scott Field. A base wide construction program began in 1938, and between 1940 and 1942, four concrete runways were constructed. During and following World War II, Scott AFB has been the headquarters for various military commands (Environmental Resources Management, Inc. [ERM], 1992). Today, headquarters for U.S. Transportation Command and Air Mobility Command, U.S. Army Military Surface Deployment and Distribution Command, a reserve aero-medical airlift wing, and an Illinois Air National Guard aviation support facility are stationed at the base. The primary mission of Scott AFB is global mobility support (Scott AFB Website, 2015).

Site OT007 is the former Sludge Weathering Lagoon/Bulk Fuel Facility. It is located in the east-central portion of Scott AFB within the controlled movement area of the flight line south of Taxiway G, as shown on Figure 1-2. The Site OT007 study area is largely encompassed by grass and Taxiway G.

Scott Air Force Base, Illinois

2.0 BACKGROUND INFORMATION

The investigation area for Site OT007 encompasses the two former 30,000-gallon jet fuel aboveground storage tanks (ASTs; Tanks 8552 and 8554) previously known as Area of Concern (AOC)-22, and the former sludge weathering lagoon. The Oil-Water Separator (OWS)-14 site is located adjacent to the study area. OWS-14 is being investigated separately under Site SS028 (formerly AOC-6). The Site boundaries and historical features are depicted on Figure 2-1.

The ASTs were used to store jet fuels from 1970 until 1994. In April 1996, the two 30,000-gallon ASTs were removed from the Site in preparation for the construction of Taxiway G, which opened in 1998. Subsequent to the AST removal action, in July and August 1996, a remedial action was performed to remove contaminated soil from the former AST area (Burns & McDonnell, 1997). The approximate extent of the soil excavation is shown on Figure 2-1. The excavation was as deep as 16 feet in some areas. A total of 6,063 tons of soil was removed from the Site. Groundwater encountered during excavation activities limited the excavation depth.

A sludge weathering lagoon, which measured 20 feet by 40 feet, was located at Site OT007 (ERM, 1992). The lagoon was used for approximately two years in the mid-1970s to dry out the sludge removed from the bottoms of the adjacent ASTs. The lagoon was closed in 1981.

Closure activities included removing soil to 2 feet below ground surface (bgs) and backfilling the excavation to grade with sand and gravel (ERM, 1992).

Based on the Tier 1 risk analysis completed as part of the Final CSIR/ROR/RAP (HGL, 2013), cis-1-2-DCE, VC, arsenic, iron, and manganese in the Aquifer Zone 1 groundwater (present from approximately 5 feet bgs to approximately 20 feet bgs) were identified as COCs at Site OT007.

No COCs were identified for soil, surface water, or Aquifer Zone 2 groundwater (present from approximately 20 feet bgs to approximately 35 feet bgs) at that time; however, CVOCs were identified as COPCs in the soil within the vicinity of OT07-MW06 (HGL, 2016). A 2017 Phase 1 delineation/characterization soil investigation conducted by Plexus confirmed that CVOCs were present in soil at the Site (Plexus, 2018c). Based on the results of the 2017 soil investigation, oxidant injections within the capillary fringe and shallow groundwater were recommended to abiotically degrade the CVOCs and petroleum hydrocarbons at the Site, followed by continuation of the MNA remedy (Plexus, 2019b). The recommended oxidant injections were conducted in May 2020.

2.1 Physical Setting

2.1.1 Site-Specific Geology and Hydrogeology

The boring logs for Site OT007 indicate that a brownish-gray silty clay/clayey silt is present from the ground surface to at least 13 feet bgs. A thin greenish-gray clay layer is also consistently present over most of the investigation area at depths ranging from 4.6 to 7.7 feet bgs, ranging in thickness from 0.3 foot to 0.5 feet thick. A dark gray to black, thin silt layer with nodules is encountered throughout the study area at approximately 12 feet bgs. At approximately

Scott Air Force Base, Illinois

20 to 28 feet bgs, a sand unit with gravel is present. All of the permanent MW borings were terminated between 14 and 34 feet bgs (HGL, 2013).

No significant regional aquifers exist in the area of western Illinois where Scott AFB is located;

however, groundwater located within unconsolidated units and bedrock is occasionally used locally as a source of potable water. No drinking water wells exist at the base (HGL, 2013); Scott AFB and nearby communities purchase water from a municipal water distribution system that obtains water from the Mississippi River.

During the 2010/2011 site investigation field activities, two hydrogeologic units, Aquifer Zone 1 and Aquifer Zone 2, were identified and investigated at Site OT007. Aquifer Zone 1 consists predominantly of clayey silts and extends from approximately 5 feet bgs to approximately 20 feet bgs. Aquifer Zone 2 underlies Aquifer Zone 1 and consists predominantly of medium fine sand to gravel of the Pearl Formation. The specific depth of Aquifer Zone 2 at Site OT007 is not well defined but based on regional geology observed at other Scott AFB sites, it may extend from approximately 20 feet bgs to 35 feet bgs (HGL, 2013; Plexus, 2016a). There is no aquitard present between Aquifer Zone 1 and Aquifer Zone 2. However, a distinction is made between the two aquifer zones because of the soil type transition and the higher hydraulic conductivity of Aquifer Zone 2.

Eleven permanent MWs, both within the Site limits and outside the Site boundary, have been installed at Site OT007, seven of which currently exist (Figure 2-2). One well (OT07-MW04D, screened from 22 to 32 feet bgs) was installed, and currently exists, to assess the groundwater conditions in Aquifer Zone 2. The 2010/2011 data indicated that groundwater in Aquifer Zone 1 flows toward the drainage ditch from both the north and the south with the ditch acting as a boundary for groundwater discharge. The drainage ditch was backfilled and converted to a stormwater line in 2014. During the 2010/2011 site investigation the groundwater gradient measured from the north side of the ditch is estimated to be 0.04 feet per foot (ft/ft). Based on the 1988 water levels measured from MW7-1 (abandoned), MW7-2 (abandoned), and MW7-3 (abandoned), flow in Aquifer Zone 2 occurred roughly parallel to the drainage ditch and ran from the northwest to southeast. The calculated gradient based on the 1988 data in Zone 2 is 0.006 ft/ft. Synoptic water level measurements for the MWs, as presented in the Final CSIR/ROR/RAP (HGL, 2013), are provided in Table 2-1.

Based on historical data, the groundwater elevation difference between OT07-MW04 and OT07- MW04D suggests a slightly downward vertical gradient between Aquifer Zones 1 and 2. In February 2011, the groundwater elevation at OT07-MW04 was 0.25 feet higher than the groundwater elevation at OT07-MW04D (HGL, 2013). Gauging data from June 2024 confirmed this downward gradient with a difference of 1.00 feet between OT07-MW04 and OT07-

MW04D.

Scott Air Force Base, Illinois

As part of the Final CSIR/ROR/RAP (HGL, 2013), seepage velocities in Zone 1 were estimated to range from 9.6 x 10-6 feet per minute (ft/min) to 2 x 10-4 ft/min, or 0.01 feet per day (ft/day) to

0.3 ft/day (HGL, 2013).

In 2014 the drainage ditch south of OT007 was backfilled. Surface water flow was altered as a result and is now towards the east southeast, ultimately making its way to Silver Creek via a drainage ditch south of Building 5032.

2.1.2 Land Use

The area encompassing Site OT007 is listed as aircraft operations and maintenance land use;

however, the majority of the study area is currently covered by grass and Taxiway G. Future land use is expected to continue as aircraft operations and maintenance.

2.1.3 Groundwater Use

The base has been supplied by municipal water since 1917 for its drinking water (ERM, 1992).

Scott AFB and the surrounding communities purchase municipal water supplies from the Illinois American Water Company, which obtains its water from the Mississippi River. No drinking water wells are known to be in use at Scott AFB.

Plexus completed a search of the Illinois State Geological Survey (ISGS) water well database (ILWATER, 2016). Six wells (ISGS Wells Number [#] 19, 20, 21, 27, 31, and 37) were noted to be within 2,500 feet of Site OT007. All six wells are listed as “Scott Field Test” water wells and were drilled in 1942. According to the Final CSIR/ROR/RAP (HGL, 2013), all of the “Scott Field Test” water wells have been abandoned and are no longer active. There are no known water wells on base used for public consumption (HGL, 2013). Well details for these water wells were provided in the Final Amended RAP, Appendix B (Plexus, 2016b).

2.2 Previous Investigations

Multiple site investigations have occurred at Site OT007 since 1988. Detailed results and discussion can be found in the Final CSIR/ROR/RAP (HGL, 2013) and the Final Amended RAP (Plexus, 2016b). It should be noted that both USEPA Solid Waste (SW) Test Methods 6010C and 6020A were used for metals analysis prior to 2018; Method 6020B was used between 2018 and 2023, after which time metals analysis was discontinued. Additionally, method 9056A replaced SM 310 and SM 300 for analysis of anions. The following is a summary of these findings.

2.2.1 2013 Comprehensive Site Investigation Report

Based on the Tier 1 risk analysis completed as part of the Final CSIR/ROR/RAP (HGL, 2013), cis-1,2-DCE, VC, arsenic, iron, and manganese in the Aquifer Zone 1 (present 5 to 20 feet bgs) groundwater at Site OT007 were identified as COCs requiring remediation to protect human health. The groundwater data demonstrated that the CVOCs at OT07-MW06 were attenuating naturally under the anaerobic conditions created by biological degradation of petroleum

Scott Air Force Base, Illinois hydrocarbons. The analytical results also indicate that conditions at OT07-MW02 supported removal of arsenic, iron, and manganese via precipitation as sulfides. No COCs were identified for soil, surface water, or Aquifer Zone 2 (present 20 to 35 feet bgs) groundwater.

2.2.1.1 Summary of Analytical Results in Groundwater and Surface Water

In the 2007 groundwater sample collected from OT07-MW04, trichloroethylene (TCE) exhibited a concentration of 6.4 M (matrix effect was present, the concentration is estimated) micrograms per liter (µg/L), greater than the IEPA TACO (35 IAC 742) Tier 1, Class I, GRO of 5 µg/L. TCE was not detected in the 2010 and 2011 samples from OT07-MW04. In addition, TCE was not detected in the other piezometers or MWs installed at Site OT007. The constituents cis-1,2-DCE and VC, the daughter products of TCE degradation, were detected at concentrations greater than the Tier 1, Class I, GROs in piezometer AOC22-B15/PZ06 and OT07-MW06.

In 2011, the VC concentration at OT07-MW06 was 89.9 M µg/L, and the cis-1,2-DCE concentration was 71.1 µg/L. These exceedances are delineated in the downgradient direction by surface water sample OT07-SW01, collected from the drainage ditch in 2011, to which the Aquifer Zone 1 groundwater discharged at the time. Both concentrations detected in OT07- SW01, 1.2 M µg/L for VC and 1.1 µg/L for cis-1,2-DCE, were less than the surface water PALs (2 µg/L and 1,100 µg/L, respectively; IEPA, 2019). The surface water PALs are based on TACO (35 IAC 302.210), Derived Water Quality Criteria List.

In 2011, only samples from OT07-MW02, exceeded the IEPA TACO (35 IAC 742) Tier 1, Class I, GRO of 10 µg/L for arsenic (81.2 µg/L unfiltered), 5,000 µg/L for iron (40,700 µg/L unfiltered), and 150 µg/L for manganese (14,100 µg/L unfiltered). The surface water sample from location OT07-SW02, collected from the drainage ditch, bounded the extent of metals contamination to the south. The unfiltered arsenic result of 9.2 µg/L F (result greater than or equal to the method detection limit but less than the reporting limit), unfiltered iron (11,300 µg/L), and unfiltered manganese (1,470 µg/L) concentrations were all below the IEPA TACO (35 IAC 742) Tier 1, Class I, GRO or, in the case of manganese, background.

Figure 2-2 depicts the locations of the former piezometers, permanent MWs, surface water samples (2011) and groundwater flow direction discussed above. Figure 2-3 presents the 2011 results for OT07-MW02, OT07-MW06, OT07-SW01 and OT07-SW02. The area of CVOC contamination was estimated to be 7,700 square feet (0.18 acre) surrounding OT07-MW06, and the area of metals contamination was estimated to be 15,600 square feet (0.36 acre) surrounding OT07-MW02.

2.2.1.2 Selected Remedy

According to the Final CSIR/ROR/RAP (HGL, 2013), the selected remedy for the groundwater COCs consists of MNA. During attenuation of the groundwater contamination, LUCs were implemented to (1) prevent installing water supply wells at the Site and, (2) require incorporating vapor intrusion controls into the building design in the unlikely situation that the taxiway is

Scott Air Force Base, Illinois relocated and a building is constructed at the Site. The estimated remedial time frame was 27 years for attenuation of CVOCs and seven years for the metals (HGL, 2013).

In a letter to the Air Force dated October 3, 2013, the IEPA approved the remedial approach of MNA coupled with LUCs for the groundwater COCs (cis-1,2-DCE, VC, arsenic, iron, and manganese; IEPA, 2013a).

2.2.2 2013 Groundwater Monitoring Event

In December 2013, OT07-MW02, OT07-MW04, OT07-MW05, and OT07-MW06 were gauged and sampled in accordance with the Final CSIR/ROR/RAP (HGL, 2013) for the initial MNA monitoring event at Site OT007. Surface water was not present in the drainage ditch; therefore, surface water samples were not collected. Complete results can be found in the Final Groundwater Monitoring Report (HGL, 2014).

The COCs cis-1,2-DCE and VC were detected only in the groundwater sample collected from OT07-MW06. All other VOCs were detected at concentrations below detection limits or the Class I GROs. Total and dissolved arsenic, iron, and/or manganese exceeded one or more GROs in December 2013 in the wells OT07-MW02 and OT07-MW05.

Based on the data collected during the 2013 sampling event, recommendations from the Final Groundwater Monitoring Report (HGL, 2014) included use of low-flow sampling techniques rather than passive samplers for future monitoring events and elimination of carbon dioxide from the analyte list. These sampling recommendations were implemented during the 2015 groundwater monitoring events.

2.2.3 2015 Groundwater Monitoring Event

In March/April and September/October 2015, MWs OT07-MW02, OT07-MW04, OT07-MW05, and OT07-MW06 were gauged and sampled in accordance with the Final CSIR/ROR/RAP (HGL, 2013). Surface water was not present in the drainage ditch; therefore, surface water samples were not collected (HGL, 2016). Complete results can be found in the Final Annual Groundwater Monitoring Report OT007 Former Sludge Weathering Lagoon/Bulk Fuel Facility (Final Groundwater Monitoring Report, HGL, 2016).

Results from the March/April 2015 groundwater monitoring event indicated that both cis-1,2- DCE and VC were detected above the Class I GROs in OT07-MW06. The concentration of CVOCs in the remaining Site OT007 MWs were below detection limits (HGL, 2016). The concentration of metals detected in all samples were below their respective GROs.

Results from the September/October 2015 groundwater monitoring event indicated that the concentration of both cis-1,2-DCE and VC in OT07-MW06 were below detection limits (0.5 µg/L). Total and dissolved (field filtered) concentrations of arsenic and iron exceeded their respective GROs at OT07-MW02; however, concentrations of metals detected in the remaining three MWs were below the GROs. Manganese also exceeded the GRO in OT07-MW02, only in the dissolved (field filtered) sample.

Scott Air Force Base, Illinois

To assess long-term concentration trends, HGL plotted the concentration of CVOCs and groundwater elevation over time. Although the September/October 2015 cis-1,2-DCE and VC concentrations were below the GROs, the fluctuations in cis-1,2-DCE and VC concentration over time may be a function of changes in the groundwater elevation near OT07-MW06. HGL concluded in the Final Groundwater Monitoring Report (HGL, 2016) that a direct relationship between CVOC concentrations and groundwater elevation is present, resulting in elevated cis- 1,2-DCE and VC concentrations when relatively high groundwater elevations are observed

(HGL, 2016).

HGL concluded in the Final Groundwater Monitoring Report (HGL, 2016) that the increase in metals concentrations in well OT07-MW02 was likely due to the dissolution of metals resulting from the anaerobic/reducing conditions present in this portion of the Site. The absence of ethene, ethane, and methane, the elevated oxidation reduction potential (ORP) measurements, the reduced total organic carbon (TOC) results, and the non-detect COC results indicated that chlorinated solvents near OT07-MW06 may have completely biodegraded, and groundwater was returning to natural, aerobic conditions. However, the decrease in cis-1,2-DCE and VC concentrations may also be correlated with the lower groundwater table observed in October 2015. Modifications to the sampling network, analytical suite, or sampling frequency were not recommended in the 2015 report. The sampling network and analytical suite remained unchanged for the 2016 annual MNA event. In accordance with the Final CSIR/ROR/RAP (HGL, 2013), MWs OT07-MW02, OT07-MW04, OT07-MW05, and OT07-MW06 were to be sampled semiannually for the first 2 years followed by annual sampling (HGL, 2013).

2.2.4 2016 Groundwater Monitoring Event

In December 2016, MWs OT07-MW02, OT07-MW04, OT07-MW05, and OT07-MW06 were gauged and sampled in accordance with the Final Amended RAP (Plexus, 2016b). Although only cis-1,2-DCE and VC have been identified as COCs for the Site, IEPA requested that the full VOC suite of compounds be analyzed (IEPA, 2016). Complete results can be found in the Final 2016 Annual Monitored Natural Attenuation Report OT007 Former Sludge Weathering Lagoon/Bulk Fuel Facility (Final 2016 Annual MNA Report, Plexus, 2018b).

Arsenic, iron, and manganese exceedances of the GRO were observed in December 2016 in OT07-MW02. Total and dissolved arsenic was detected above the GRO in OT07-MW02 and OT07-MW06; only the total arsenic concentration was detected above the GRO in OT07-MW04.

Total and dissolved iron concentrations in OT07-MW02, and total iron concentrations in OT07- MW06 were also detected above the GRO. With the exception of OT07-MW04, the total and dissolved concentrations were similar, indicating that these metals are primarily present in the dissolved state and impacts due to sample turbidity were low. The TOC concentration of 9.8 milligrams per liter (mg/L) in well OT07-MW02 coupled with the observation of anaerobic/reducing geochemistry suggested that conditions remained favorable for microbial activity to continue and for metals to remain in solution.

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Exceedances of cis-1,2-DCE, VC, and benzene were observed in OT07-MW06. Vinyl chloride was also detected above the GRO at OT07-MW04. The presence of dissolved gases, methane, reduced TOC, anaerobic conditions, and an increase in CVOCs at OT07-MW06 indicated that reductive dechlorination was still occurring at this portion of the Site. A significant increase in the concentration of CVOCs detected at OT07-MW06 was observed in 2016; these concentrations were over 300-fold times the concentrations observed in 2013. However, the increase in cis-1,2-DCE and VC may also be correlated with the higher groundwater table observed in December 2016. Benzene was also detected above the GRO for the first time in OT07-MW06 and was considered a potential COC. Vinyl chloride was also detected above the GRO at OT07-MW04, which historically has not had detections of chlorinated VOCs.

2.2.5 2017 Soil Characterization Investigation

On June 13, 2017, twelve soil borings (OT07-BO-26 through OT07-BO37) were installed to characterize the extent of CVOC contamination in accordance with the Final Amended RAP (Plexus. 2016b). The Final Amended RAP (Plexus, 2016b) indicated that soil borings would be terminated above the water table; however, based on the photo-ionization detector (PID) readings, a decision was made to terminate the borings in the water table at a depth of 10 feet bgs. At least one sample was collected from each boring location from the interval with the highest PID reading. Based on the initial sampling results, an additional ten step-out soil borings were installed on June 20, 2017. Step-out borings were installed to the north, west, and east of OT07-BO-34 and OT07-BO-36 to delineate the extent of VOC impacted soil. Boring locations to the north of OT07-MW04 were limited by the 250-foot taxiway buffer zone. Four (OT07-BO-40, OT07-BO-41, OT07-BO-42, and OT07-BO-46) out of the ten step-out borings sampled had detections of VOCs at concentrations greater than the PALs presented in Worksheet #15 of the Final Amended RAP (Plexus, 2016b). Chemicals detected at elevated levels were primarily petroleum constituents and VC. All four step-out samples that exceeded the PALs were collected in the capillary fringe zone, at depths of 6 to 8 feet bgs (Plexus, 2018c).

The 2017 soil investigation determined that residual contamination was present in the capillary fringe above the groundwater table rather than in unsaturated soil. The results were consistent with historical groundwater data, which indicated higher groundwater contaminant concentrations when the water table was elevated (Plexus, 2018c). Based on the results of the soil investigation, oxidant injections within the capillary fringe and shallow groundwater were recommended to abiotically degrade CVOCs and petroleum hydrocarbons at Site OT007, followed by MNA (Plexus, 2019b).

2.2.6 2017 Groundwater Monitoring Event

In November 2017, MWs OT07-MW02, OT07-MW04, OT07-MW05, and OT07-MW06 were gauged and sampled in accordance with the Final Amended RAP (Plexus, 2016b). Surface water was not present in the drainage ditch; therefore, surface water samples were not collected.

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Complete results can be found in the Final 2017 Annual Monitored Natural Attenuation Report OT007 Former Sludge Weathering Lagoon/Bulk Fuel Facility (Plexus, 2019a).

Arsenic, iron, and manganese exceedances were observed in November 2017 in OT07-MW02.

Total and dissolved arsenic were detected above the GRO in OT07-MW02 and OT07-MW06;

only total arsenic was detected above the GRO in the duplicate sample for OT07-MW06. Total and dissolved iron and manganese concentrations also exceeded the GROs in OT07-MW02. The total and dissolved concentrations were similar, indicating that these metals are primarily present in the dissolved state and impacts due to sample turbidity were low. The TOC concentration of

12.0 mg/L in well OT07-MW02 coupled with the observation of anaerobic/reducing geochemistry suggested that conditions remained favorable for microbial activity to continue and for metals to remain in solution.

Exceedances of cis-1,2-DCE, VC, and benzene were observed in OT07-MW06. The presence of dissolved gases, methane, reduced TOC, anaerobic conditions, and increase in CVOCs at OT07- MW06 indicated that reductive dechlorination was still occurring at this portion of the Site. An increase in the concentration of CVOCs detected at OT07-MW06 was observed again in 2017;

these concentrations were over 400-fold times the concentrations observed in 2013. However, the increase in cis-1,2-DCE and VC may also have been correlated with the higher groundwater table observed in November 2017. Benzene was also detected above the GRO for the second time in OT07-MW06 and was added to the list of COCs.

A limited soil impact was identified during the June 2017 soil sampling event. VOCs were detected in the capillary fringe zone and were likely contributing to the concentrations observed in the groundwater at OT07-MW06. Results of the soil investigation are discussed in a separate technical memorandum (Plexus, 2018c).

2.2.7 2018 Groundwater Monitoring Event

In October 2018, MWs OT07-MW02, OT07-MW04, OT07-MW05, and OT07-MW06 were gauged and sampled in accordance with the Final Amended RAP (Plexus, 2016b). Surface water was not present in the drainage ditch; therefore, surface water samples were not collected.

Complete results can be found in the Final 2018 Annual Monitored Natural Attenuation Report OT007 Former Sludge Weathering Lagoon/Bulk Fuel Facility (Final 2018 Annual MNA Report, Plexus, 2020a).

During the October 2018 event, arsenic, iron, and manganese were detected in each of the four wells. At OT07-MW02, total and dissolved manganese were detected at concentrations that exceeded the GRO; however, total and dissolved arsenic and iron were detected at concentrations below the GROs. Total and dissolved metals were detected at concentrations below the GROs for the other three sampled wells (OT07-MW04, OT07-MW05, and OT07-MW06). Elevated manganese concentrations in OT07-MW02 were 6,750 µg/L (total) and 6,350 µg/L (dissolved), slightly exceeding the GRO (6,120 µg/L). OT07-MW02 experienced a decrease in concentration when compared to total and dissolved manganese concentrations detected in 2017. The TOC

Scott Air Force Base, Illinois concentration of 12.2 mg/L in well OT07-MW02 coupled with the observation of anaerobic/reducing geochemistry suggested that conditions remained favorable for microbial activity to continue and for metals to remain in solution.

Vinyl chloride was detected at 45 µg/L in OT07-MW06, exceeding the GRO of 2 µg/L;

however, there was a significant decrease from concentrations detected in 2016 (860 µg/L) and 2017 (1,000 J [the positive result reported for this analyte is a quantitative estimate due to discrepancies in meeting certain analyte-specific quality control criteria] µg/L). The absence of dissolved gases, methane, reduced TOC, anaerobic conditions, and decrease in CVOCs at OT07- MW06 indicated that reductive dechlorination is no longer occurring at this portion of the Site.

Benzene, added to the list of COCs in 2017, was not detected in 2018 (Plexus, 2020a).

2.2.8 2019 Groundwater Monitoring Event

In June 2019, MWs OT07-MW02, OT07-MW04, OT07-MW05, and OT07-MW06 were gauged and sampled in accordance with the Final Amended RAP (Plexus, 2016b). Surface water was not present in the drainage ditch; therefore, surface water samples were not collected. Complete and historical results can be found in the Final 2019 Annual Monitored Natural Attenuation Report OT007 Former Sludge Weathering Lagoon/Bulk Fuel Facility (Final 2019 Annual MNA Report, Plexus, 2020b).

During the June 2019 event, arsenic, iron, and manganese were detected in each of the four wells. OT07-MW02 experienced a decrease in concentration when compared to total and dissolved manganese concentrations detected in 2018 at concentrations below the GRO. Total and dissolved arsenic and iron continued to be detected at concentrations below the GROs at OT07-MW02, as well. The TOC concentration of 10.1 mg/L in well OT07-MW02 coupled with the observation of anaerobic/reducing geochemistry suggested that conditions remained favorable for microbial activity to continue and for metals to remain in solution.

Vinyl chloride was detected at 5.41 µg/L in OT07-MW06, exceeding the GRO of 2 µg/L;

however, this was a significant decrease from concentration detected in 2017 (1000 J µg/L) and continued a decreasing trend from 2018 (45 µg/L). The absence of dissolved gases, methane, reduced TOC, anaerobic conditions, and decrease in CVOCs at OT07-MW06 indicate that reductive dechlorination was no longer occurring at this portion of the Site. Benzene, added to the list of COCs in 2017, was not detected in 2019 (Plexus, 2020b).

2.2.9 2020 Oxidant Injection Event

Based on the 2017 soil characterization investigation and groundwater data collected since that investigation, oxidant (Persulfox®) injections were conducted at the Site in May 2020. Injection locations were chosen to address the residual petroleum hydrocarbons and VC in the saturated capillary fringe in the vicinity of OT07-BO-34, OT07-BO-36, OT07-BO-40, OT07-BO-41, OT07-BO-42, and OT07-BO-46. The oxidant injection was conducted in accordance with the Final RAP Addendum (Plexus, 2019b). In June 2020, six soil borings were advanced at the same

Scott Air Force Base, Illinois location and depth interval where exceedances of petroleum hydrocarbons and CVOC concentrations were first observed in 2017 to evaluate the effectiveness of the injection event.

Following the oxidant injection event in 2020, no exceedances of the PALs were observed at boring locations OT07-BO-42 and OT07-BO-46; however, while generally lower than pre-injection concentrations, soil sampling results obtained during the 2020 site investigation indicated that VOC contamination remains present in the saturated soil of the capillary fringe.

The VOC contamination continued to exceed the PALs in the capillary fringe at OT07-BO-34, OT07-BO-36, OT07-BO-40, and OT07-BO-41. The PALs for the post-injection soil characterization work were presented in Worksheet #15 of the Final RAP Addendum (Plexus, 2019b). They were selected following the same approach that was used for PAL selection in the Final Amended RAP (Plexus, 2016b).

2.2.10 2020 Groundwater Monitoring Event

In June 2020, MWs OT07-MW02, OT07-MW04, OT07-MW05, and OT07-MW06 were gauged and sampled in accordance with the Final Amended RAP (Plexus, 2016b). Surface water was not present in the drainage ditch; therefore, surface water samples were not collected. Complete and historical results can be found in the Final 2020 Annual Monitored Natural Attenuation Report OT007 Former Sludge Weathering Lagoon/Bulk Fuel Facility (Final 2020 Annual MNA Report, Plexus, 2020c).

During the June 2020 event, arsenic, iron, and manganese were detected in each of the four wells, however, none of the concentrations exceeded the GROs. The concentrations of arsenic, iron, and manganese showed dissolved arsenic with a decreasing trend and dissolved iron and dissolved manganese with a “probably decreasing” trend at OT07-MW02. The drainage ditch did not contain surface water during the 2020 sampling event, thus the presence of metals in groundwater in the downgradient direction could not be assessed in 2020.

While no metals were detected above the GROs in any of the four wells, the anaerobic conditions will persist if there is sufficient TOC to fuel microbial activity. In June 2020, the TOC concentrations in each well decreased compared to 2019 observation data.

Benzene and cis-1,2-DCE were only detected at OT07-MW06 in June 2020 at concentrations lower than their PAL and GRO respectively. In OT07-MW06, VC exceeded the GRO of 2 µg/L at a concentration of 6.91 µg/L, which is similar to the concentration of VC in the well in 2019.

Low levels of total cis-1,2-DCE and VC were detected in OT07-MW04 for the first time in 2016; these CVOCs have not been detected above the reporting limits since 2017 in this well.

2.2.11 2021 Groundwater Monitoring Event

In June 2021, MWs OT07-MW02, OT07-MW04, OT07-MW05, and OT07-MW06 were gauged and sampled in accordance with the Final Amended RAP (Plexus, 2016b). Surface water was not present in the drainage ditch; therefore, surface water samples were not collected. Complete and historical results can be found in the Final 2021 Annual Monitored Natural Attenuation Report

Scott Air Force Base, Illinois

OT007 Former Sludge Weathering Lagoon/Bulk Fuel Facility (Final 2021 Annual MNA Report, Plexus, 2022).

During the June 2021 event, arsenic, iron, and manganese were detected in each of the four wells, however, none of the concentrations exceeded the GROs.

While no metals were detected above the GROs in any of the four wells, the anaerobic conditions will persist as long as there is sufficient TOC to fuel microbial activity. In June 2021, the TOC concentrations either decreased or remained stable compared to 2020 observation data.

Benzene and cis-1,2-DCE were only detected at OT07-MW06 in June 2021 at concentrations lower than their PAL and GRO respectively. In OT07-MW06, VC exceeded the GRO of 2 µg/L at a concentration of 18.1 µg/L. Low levels of total cis-1,2-DCE and VC were detected in OT07- MW04 for the first time in 2016; these CVOCs have not been detected above the reporting limits since 2017 and remained undetected in 2021 in this well.

2.2.12…