7. Draft Final_SAFB_SS005_2024 Annual GW Monitor Rpt with AF RTCs.pdf

DEPARTMENT OF THE AIR FORCE

AIR FORCE CIVIL ENGINEER CENTER

JOINT BASE SAN ANTONIO LACKLAND TEXAS

25 March 2025

MEMORANDUM FOR Illinois Environmental Protection Agency ATTN: Mr. Jordan Simpkins Bureau of Land

Federal Site Remediation Section 2520 W. Iles Ave.

P.O. Box 19726 Springfield, IL 62704

FROM: Joseph Weber

AFCEC/CZOM

701 Hangar Road, Bldg 531

Scott AFB, IL 62225-5035

SUBJECT: Air Force Response to Illinois EPA comment for Draft Final Annual

Groundwater Monitoring Report, 2024, Site SS005, February 2025, Scott Air Force Base (AFB), IL

1. The Air Force submits the following response to the Illinois EPA comment for the Draft Final Annual Groundwater Monitoring Report, 2024, Site SS005, February 2025, Scott AFB, IL for your review.

a. Illinois EPA comment dated 13 March 2025:

PDF Page 51, Bullet Point 1: “The well depths measured during the June 2024 monitoring event did not indicate any occluded wells screens. However, multiple monitoring wells had a measured well depth deeper than their well depth at completion (i.e., installation), which was determined from historical SS005 reports.” Please elaborate on how multiple monitoring wells have a measured well depth greater now than at the time of well’s completion.

b. Air Force response:

Explanation for the deeper well depths was provided in the second paragraph in Section 3.1 on PDF Page 37 which states, “Monitoring wells SS05-IW02, SS05-MW04S through SS05-MW07S, and SS05-MW09S through SS05-MW13S had a measured well depth greater than the well depth at completion by an average of 0.48 feet with the greatest difference 0.94 feet for SS05-MW12S. For these monitoring wells, the difference between the measured and completion wells depths was attributed to well construction diagrams that documented general depths for monitoring well components (i.e., well screen, total well depth, etc.). The actual well completion depth should be close to the measured well depth for each monitoring well. For SS05-IW02, SS05-MW04S through

DEPARTMENT OF THE AIR FORCE

JOINT BASE SAN ANTONIO LACKLAND TEXAS

SS05-MW07S, and SS05-MW09S through SS05-MW13S, the measured well depth between 2016 and 2023 was within +/- 0.1 feet of the respective June 2024 measured well depth for 108 of 116 total measurements.”

The text in Section 4 beginning on PDF Page 51 is intended to be a summary of the information presented in Sections 1, 2, and 3 of the annual report. Based on the information presented in Section 3.1, PDF Page 37, revision to Bullet Point 1, PDF Page 51 is not required. Please note that a well depth evaluation with results similar to that stated in Section 3.1 was provided in the Final Annual Groundwater and Soil Monitoring Report, 2021 (URS Group, Inc., August 2023), Final Annual Groundwater Monitoring Report, 2022 (URS Group, Inc., March 2024), and Final Annual Groundwater Monitoring Report, 2023 (URS Group, Inc., December 2024).

2. Please provide comment or concurrence by April 8, 2025. If you have any questions, please contact me at 618-256-2125 or email joseph.weber.16 @us.af.mil.

Joseph Weber Restoration Project Manager cc: Ben Christensen, URS

Patrick Reilley, Plexus Eliud Burgos, AFCEC/CZR

DRAFT FINAL

ANNUAL GROUNDWATER MONITORING

REPORT, 2024

SITE SS005

Scott Air Force Base

Prepared for:

2261 Hughes Avenue, Suite 155

Lackland AFB, Texas 78236-9853

Contract FA8903-14-C-0009

February 2025

DRAFT FINAL

Annual Groundwater Monitoring Report, 2024 Site SS005

Scott Air Force Base

Prepared for:

2261 Hughes Avenue, Suite 155 Lackland AFB, Texas 78236-9853

Contract FA8903-14-C-0009

February 2025

Prepared by:

URS Group, Inc.

12120 Shamrock Plaza, Suite 300 Omaha, Nebraska 68154

In Association With:

TABLE OF CONTENTS

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Section 1 Introduction ..................................................................................................................... 1-1

1.1 Report Organization ................................................................................. 1-1

1.2 Authority .................................................................................................. 1-2

1.3 Purpose and Scope ................................................................................... 1-2

1.4 Site Description ........................................................................................ 1-3

1.4.1 Installation Location and Description .......................................... 1-3

1.4.2 Site Location and Description ...................................................... 1-4

1.4.3 Site-Specific Geology .................................................................. 1-5

1.4.4 Site-Specific Hydrogeology ......................................................... 1-6

1.4.5 Groundwater Use ......................................................................... 1-7

1.4.6 Surface Water............................................................................... 1-7

1.5 Previous Site Investigations ..................................................................... 1-8

1.6 Remediation Objectives ........................................................................... 1-9

1.6.1 Soil COCs .................................................................................... 1-9

1.6.2 Groundwater COCs ...................................................................... 1-9

1.7 Remedial Actions ................................................................................... 1-10

1.7.1 Soil COCs .................................................................................. 1-10

1.7.2 Groundwater COCs .................................................................... 1-11

1.7.3 OWS-4 Removal ........................................................................ 1-15

Section 2 Summary of Field Activities ........................................................................................... 2-1

2.1 Annual Groundwater Monitoring ............................................................ 2-1

2.1.1 Monitoring Well Inspection and Repair ...................................... 2-1

2.1.2 Groundwater Level and Well Depth Measurements .................... 2-1

2.1.3 Groundwater Sample Collection .................................................. 2-2

2.2 Investigation-Derived Waste ................................................................... 2-4

Section 3 Summary of Results ....................................................................................................... 3-1

3.1 Well Depth Evaluation ............................................................................. 3-1

3.2 Groundwater Flow Direction ................................................................... 3-1

3.3 Groundwater Analytical Results .............................................................. 3-2

3.3.1 Data Quality Evaluation ............................................................... 3-2

3.3.2 PAH Results ................................................................................. 3-3

3.3.3 Metals Results .............................................................................. 3-6

3.3.4 Natural Attenuation Parameters ................................................... 3-9

Section 4 Summary and Conclusions ........................................................................................... 4-1

4.1 Summary .................................................................................................. 4-1

4.2 Conclusions .............................................................................................. 4-4

Section 5 References ...................................................................................................................... 5-1

TABLE OF CONTENTS

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List of Tables Table 1-1 Hydrogeologic Zones at Scott Air Force Base Table 1-2 Monitoring Well Construction Details Table 1-3 Summary of Remediation Objectives Table 2-1 Summary of Groundwater Measurements and Elevations – 2024

Monitoring Event Table 2-2 Summary of Groundwater Quality Parameters – 2024 Monitoring

Event Table 2-3 Summary of Sample Analysis – 2024 Monitoring Event Table 2-4 Summary of Quality Assurance/Quality Control Samples – 2024

Monitoring Event Table 3-1 Summary of Groundwater Analytical Results – 2024 Monitoring Event Table 3-2 Comparison of PAH Groundwater Concentrations – 2023 and 2024

Monitoring Events Table 3-3 Comparison of Metals Groundwater Concentrations – 2023 and 2024

Monitoring Events

List of Figures Figure 1-1 Scott Air Force Base Location Map Figure 1-2 Site Location Map Figure 1-3 Site Features Map Figure 1-4 Site Remediation Areas Figure 1-5 Soil Excavation Limits Figure 3-1 Groundwater Elevation Map, June 2024 Figure 3-2 PAHs in Groundwater, 2024 Figure 3-3 Metals in Groundwater, 2024

List of Appendices Appendix A Field Documentation Appendix B IDW Documentation Appendix C Laboratory Analytical Reports Appendix D Data Verification Documents Appendix E GSI Mann-Kendall Toolkit Outputs

ACRONYMS AND ABBREVIATIONS

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% percent +/- plus or minus AFB Air Force Base AFCEC Air Force Civil Engineer Center AST aboveground storage tank bgs below ground surface COC contaminant of concern CSI Comprehensive Site Investigation CSIR Comprehensive Site Investigation Report DO dissolved oxygen DoD Department of Defense ERM Environmental Resources Management, Inc.

ERP Environmental Restoration Program ft/day feet per day ft/ft feet per foot GRO groundwater remediation objective GSI GSI Environmental, Inc.

Hach Hach Company HGL HydroGeoLogic, Inc.

IAC Illinois Administrative Code IDW investigation-derived waste IEPA Illinois Environmental Protection Agency Illini Illini Environmental, Inc.

iSOC® in-situ Submerged Oxygen Curtain J estimated JP jet propellant LOD limit of detection LUC land use control μg/L micrograms per liter MAROS Monitoring and Remediation Optimization System mg/L milligrams per liter MNA monitored natural attenuation mS/cm milliSiemens per centimeter MTBE methyl tertiary-butyl ether mV millivolts No. Number NTU nephelometric turbidity unit OES Optimized Exit Strategy

ACRONYMS AND ABBREVIATIONS

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ORP oxidation-reduction potential OWS oil water separator PAH polycyclic aromatic hydrocarbon PBR Performance-Based Remediation Plexus Plexus Scientific Corporation QA quality assurance QC quality control QSM Quality Systems Manual RAP Remedial Action Plan ROR Remediation Objectives Report RSL Regional Screening Level SGS SGS North America, Inc.

SIM selected ion monitoring Solinst Solinst Canada, Ltd.

SOP Standard Operating Procedure SRO soil remediation objective SS005 Site SS005 SW United States Environmental Protection Agency Solid Waste Test Method UFP-QAPP Uniform Federal Policy-Quality Assurance Project Plan URS URS Group, Inc.

U.S. United States USEPA United States Environmental Protection Agency UST underground storage tank

SECTIONONE Introduction

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URS Group, Inc. (URS), as a subcontractor to Plexus Scientific Corporation (Plexus), completed an annual groundwater monitoring event at Scott Air Force Base (AFB) Environmental Restoration Program (ERP) Site SS005 (SS005). This report discusses the field activities, field results, and laboratory analytical results associated with the annual groundwater monitoring event completed in June 2024 for SS005.

1.1 REPORT ORGANIZATION

This report includes the following sections:

• Section 1 – Introduction: Describes the organization and purpose of this annual monitoring report and provides a background of SS005 including location, site-specific geology and hydrogeology, and site history which includes discussion on previous site investigation, remedial objectives, and remedial actions.

• Section 2 – Summary of Field Activities: Summarizes the field activities completed for the annual groundwater monitoring event, any deviations for the field activities, and management of investigation-derived waste (IDW).

• Section 3 – Summary of Results: Provides a discussion of the groundwater elevation and analytical results for the annual groundwater monitoring event, compares contaminant of concern (COC) concentrations to the remediation objectives, and evaluates the natural attenuation of COC concentrations at SS005.

• Section 4 – Summary and Conclusions: Provides a summary and the conclusions for this annual monitoring report.

• Section 5 – References: Lists the references cited in the preparation of this annual monitoring report.

• Appendix A – Field Documentation: Contains field documentation associated with the 2024 annual groundwater monitoring event such as Static Water Level and Well Inspection Form, Well Occlusion Evaluation form, Water Sample Collection Field Sheets, and well repair photo documentation.

• Appendix B – IDW Documentation: Contains the waste profile, waste tracking form, waste manifest, and disposal certificate associated with the 2024 annual groundwater monitoring event.

• Appendix C – Laboratory Analytical Reports: Contains the laboratory analytical reports associated with the 2024 annual groundwater monitoring event.

• Appendix D – Data Verification Documents: Contains an overall summary report, tables, and data verification reports for each laboratory analytical report with definitive groundwater analytical data requiring data verification.

• Appendix E – GSI Mann-Kendall Toolkit Outputs: Contains trend graphs and results for groundwater COC concentrations using software from GSI Environmental, Inc. (GSI).

1 Introduction

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1.2 AUTHORITY

The United States (U.S.) Air Force Civil Engineer Center (AFCEC) contracted Plexus under Contract Number (No.) FA8903-14-C-0009 to complete Performance-Based Remediation (PBR) at Scott AFB. Plexus subcontracted URS to complete the contract performance objective for SS005. Regulatory oversight is led by the Illinois Environmental Protection Agency (IEPA).

Restoration activities at SS005 follow the Illinois Site Remediation Program (35 Illinois Administrative Code [IAC] 740), which was approved by the IEPA in their letter dated September 28, 2011 (IEPA 2011).

1.3 PURPOSE AND SCOPE

The performance objective for SS005 under this PBR contract is to achieve the objectives of an Optimized Exit Strategy (OES). An OES is developed for a site that has an approved Decision Document prior to or during the contract period of performance but does not achieve Site Closeout.

Site SS005 is not eligible for Site Closeout as COC concentrations in soil and groundwater are present at SS005 above the remediation objectives and require land use controls (LUCs) to prevent exposure to potential receptors. The Final SS005 OES Plan (URS 2020a) was submitted to AFCEC under separate cover and included the following objectives:

• Maintain compliance with the Remedial Action Plan (RAP) addressing soil and groundwater.

• Correct any noted five-year review deficiencies.

• Apply innovative technologies to minimize the cost and period of operating, monitoring, and treatment.

• Improve the understanding of the conceptual site model, existing systems, and contamination sources to support optimization.

• Provide an analysis of alternatives to aggressively accelerate treatment and progress to expedite Site Closeout.

• Develop performance indicators, decision criteria, and objectives consistent with the performance model for the site to optimize remedial efficiency, assess progress, and demonstrate when the objective has been reached.

This annual monitoring report satisfies the reporting requirements outlined in the Final SS005 OES Plan (URS 2020a) and Final Remediation Objectives Report (ROR)/RAP Addendum (URS 2021a). Information and data presented in this annual monitoring report will be utilized in an OES Effectiveness Report, which will be submitted to AFCEC under separate cover and assess the progress towards the OES objectives.

The purpose of this annual monitoring report is to:

• Document and discuss the field activities and analytical results from the annual groundwater monitoring event conducted in June 2024.

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• Assess natural attenuation processes in groundwater relevant to arsenic, manganese, and polycyclic aromatic hydrocarbons (PAHs).

The following documents provided guidance for the field and laboratory activities required to complete the annual groundwater monitoring event in 2024 at SS005:

• Final Uniform Federal Policy-Quality Assurance Project Plan (UFP-QAPP), PBR (Plexus 2018, identified as Final UFP-QAPP for this report)

• An addendum to the Final UFP-QAPP that included updated laboratory documentation (Plexus 2024).

• Final ROR/RAP Addendum (URS 2021a)

• Final Technical Memorandum, Modification to Groundwater Monitoring Program, 2024 (URS 2024a, identified as Final Technical Memorandum for this report)

• Final Annual Groundwater Monitoring Report, 2023, Site SS005 (URS 2024b, identified as Final 2023 Monitoring Report for this report)

While the Final 2023 Monitoring Report (URS 2024b) was being prepared, the Final Technical Memorandum (URS 2024a) was submitted for IEPA review and provide rationale to change from semi-annual to annual groundwater monitoring beginning in 2024. Semi-annual groundwater monitoring was completed at SS005 from 2016 to 2023 (Section 1.7.2). The IEPA July 10, 2024, correspondence concurred with the change to annual groundwater monitoring (IEPA 2024). The Final 2023 Monitoring Report (URS 2024b) concluded annual groundwater monitoring would begin in 2024.

1.4 SITE DESCRIPTION

1.4.1 Installation Location and Description

Scott AFB is in St. Clair County, Illinois, 25 miles southeast of St. Louis, Missouri, in southwestern Illinois. The city of Belleville, Illinois, is located approximately 6 miles southwest of Scott AFB, and the city of O’Fallon, Illinois, is located approximately 5 miles northwest of Scott AFB (Figure 1-1). Scott AFB property comprises 3,545 acres of U.S. government-owned and easement land. Scott AFB land is used exclusively as an active military base by the U.S.

Department of Defense (DoD). Airbase facilities and runways compose the largest portion of the installation, which also includes living quarters and recreational facilities. Agricultural land, retail development, MidAmerica St. Louis Airport, and residential properties border Scott AFB.

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 basewide 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. Today, Scott AFB is the home of the 375th Air Mobility Wing, a unit of the Air Mobility Command headquartered at Scott AFB. Other mission partners include U.S.

Transportation Command, 932nd Airlift Wing, Defense Information Systems Agency, U.S. Army

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Military Surface Deployment and Distribution Command, and the Illinois Air National Guard 126th Air Refueling Wing. The primary mission of Scott AFB is executing rapid global mobility (Scott AFB 2024).

1.4.2 Site Location and Description

SS005 is in the southern portion of Scott AFB as shown on Figure 1-2. SS005 is also referred to in historical documents as Site SS-05 or Bulk Fuel Storage Area. The site boundary for SS005 shown on Figure 1-3 was based on the results from previous site investigations identified in Section 1.5. SS005 encompasses much of the Scott AFB bulk fuel storage and distribution area located within the perimeter fence. The Scott AFB bulk fuel storage and distribution area is designated as “mission critical” by the U.S. Air Force (HydroGeoLogic, Inc. [HGL] 2014a). Site features of the bulk fuel storage and distribution area include the following:

• Perimeter chain link fence to control site access

• Three aboveground storage tanks (ASTs) to store Jet A aircraft fuel (a kerosene-based fuel)

• One AST to store deicing fluid

• Active pump house (Building 560), former pump house (Building 545) for equipment storage, and one ancillary building (Building 544)

• Four underground storage tanks (USTs), each of which is designated to a specific motor vehicle fuel (i.e., gasoline, E-85, biodiesel, and diesel)

• Tanker truck off-loading facilities

• Aboveground and underground fuel distribution piping

• Underground utilities including communication, electric, storm sewer, wastewater, and water

All site features, except underground utilities, are shown on Figure 1-3. Active aboveground and underground piping conveys Jet A fuel between the tanker truck off-loading facilities, Building 560, and the three ASTs designated for storage (i.e., ASTs 8732, 8733, and 8734).

AST 8733 was previously identified as AST 8550, and AST 8734 was previously identified as AST 8570. AST 8732 was constructed sometime after April 1992. Each AST is located within a concrete-lined and bermed containment area. Prior to the installation of the concrete-lined containment area, ASTs 8733 and 8734 were surrounded by an earthen berm structure overlain by gravel. In 2000, the original earthen bermed structure was removed and upgraded to the current concrete-lined and bermed containment area (HGL 2014a). According to Scott AFB personnel, jet propellant (JP) JP-4 (a mixed gasoline and kerosene-based fuel) was replaced by JP-8 (a kerosene-based fuel) within the ASTs between 1994 and 1995, and JP-8 was replaced by Jet-A in October 2014. Separate aboveground and underground piping conveys motor vehicle fuel from the USTs located along the eastern boundary of SS005.

Multiple inactive steel underground fuel lines exist northwest of Building 545 at a depth of 4 feet below ground surface (bgs), believed to extend north of monitoring well SS05-MW13S, but their exact extent is unknown. A steel underground fuel line was formerly located within the grass

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landscape north of the tanker truck off-loading area. The former underground fuel line extended to the north, beneath South Drive, and was connected to two 50,000-gallon JP-8 USTs and a pump house (former Building 508). A 550-gallon UST used to collect water separated from the JP-8 was also located at former Building 508. The USTs, building, and approximately 200 feet of the underground fuel line were removed in 1997 (AmTech Engineering, Inc. 1999). Building 503 was constructed near the location of former Building 508 and serves a similar purpose. The USTs associated with former Building 508 were investigated as separate ERP Sites TU020, TU021, and TU027 and have achieved Site Closeout.

Access to SS005 is controlled by three gates within the perimeter chain link fence. The ground surface is covered by grass landscaping approximately 15 to 20 feet inward from the fence line toward the ASTs. Adjacent to the grass landscaping within the fence line is an asphalt/concrete road that loops around the AST areas. The road is paved with concrete at the tanker truck off-loading facilities and UST area. The area in front of Building 560 is paved with asphalt.

Except for the concrete containment areas, the remainder of the ground surface is covered with gravel underlain by a landscape fabric. A surface water feature, the South Ditch (ERP Site UK510), flows west to east and is located approximately 70 feet from the south side perimeter fence. Surface water features are discussed further in Section 1.4.6.

Residential land use is not planned for the area covered by SS005 and the Scott AFB bulk fuel storage and distribution area. This land use is consistent with the Scott AFB general plan, which indicates this area will remain zoned for use as aircraft operation and maintenance (HGL 2016a).

1.4.3 Site-Specific Geology

Soil sample locations and monitoring wells completed at SS005 during prior investigations indicate site lithology generally consists of unconsolidated materials throughout the vertical thickness explored. Non-native fill materials were encountered from the ground surface up to 6 feet bgs (HGL 2014a). Fill materials consisting of various mixtures of gravel, sand, silt, clay, and cinders were encountered in a majority of soil boring and monitoring well locations completed across the site (HGL 2014a). A possible explanation for cinders at SS005 includes use as railroad ballast for railroad spurs formerly located north of the South Ditch. The railroad spurs were present prior to site development and have been removed.

Underlying the fill materials is a fine-grained unit that ranges from silt to clay and terminates between 24 to 32 feet bgs (HGL 2014a). These fine-grained soils are representative of the Peoria loess and Roxana silt, which consist of windblown deposits of silts, clays, and trace amounts of fine sand (HGL 2014a).

The fine-grained silt and clay unit is underlain by a poorly graded to well-graded sand unit that terminates between 29 to 43 feet bgs and is representative of the outwash deposits of the Pearl Formation (HGL 2014a). The sand unit is discontinuous and may be present as interbedded clayey sand seams at SS005, as evidenced around monitoring well SS05-MW08D (Figure 1-3) (HGL 2014a).

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The sand unit is underlain by a unit consisting of sandy glacial till with thin lenticular bodies of silt, sand, and gravel and representative of the Glasford Formation, which is typically less than 40 feet thick (HGL 2014a).

1.4.4 Site-Specific Hydrogeology

The different water-bearing units for Scott AFB sites have been divided into the four zones identified in Table 1-1.

Table 1-1 Hydrogeologic Zones at Scott Air Force Base

Hydrogeologic Zone Basewide Estimated Depth Range

(feet bgs)[1]

Zone 1 (shallow clay/silt zone) 3 to 18 Zone 2 (shallow silt/sand zone) 18 to 35 Zone 3 (lower clay/silt zone) 35 to 50 Zone 4 (lower silt/sand zone) 50 to 60

Notes:

[1] = All depths are approximate (HydroGeoLogic, Inc. 2016b).

bgs = below ground surface

Hydrogeologic Zones 1 and 2 have been investigated at SS005 with COC concentrations exceeding remediation objectives within Zone 1. Zone 1 is the uppermost water-bearing unit consisting primarily of fill material, clay, and silt that begins at or near the ground surface and terminates between 24 and 32 feet bgs. The existing Zone 1 monitoring well network for SS005 is shown on Figure 1-3 and in Table 1-2. For this annual monitoring report, monitoring wells associated with SS005 are identified with the prefix Bldg508, SS05, or SS31.

Zone 1 monitoring wells have well screens above the terminus of Zone 1 with bottom well screen depths ranging from 12.50 to 15.18 feet bgs. Well screen placement for Zone 1 monitoring wells was based on petroleum impacted soils identified during investigation activities (Section 1.5).

Groundwater in Zone 1 is unconfined and has ranged from 2.45 to 6.53 feet bgs, with an average depth of 4.40 feet bgs. The primary inferred groundwater flow direction observed during previous monitoring events has been to the southwest towards the South Ditch with an average hydraulic gradient of 0.010 feet per foot (ft/ft). The groundwater elevation is typically highest in the area around SS05-MW09S and extending north to Bldg508-MW03 and results in groundwater flow directions to the north/northeast, southeast, and west/northwest. Groundwater elevations collected in July 2004 and June 2018 from Bldg508-MW03, and monitoring wells formerly located north of this well indicated groundwater flow north across South Drive from Bldg508-MW03 (Cape Environmental 2004 and Zapata and Burns & McDonnell 2020).

Groundwater elevations for SS05-IW02, SS05-MW08S, SS05-MW09S, SS05-MW12S, and SS05-MW13S have been higher than the other SS005 Zone 1 monitoring wells. Groundwater elevations for SS05-IW02, SS05-MW08S, SS05-MW09S, SS05-MW12S, and SS05-MW13S may be influenced by the concrete berm and containment area surrounding AST 8734 along with surrounding subsurface fill material. Based on field observations, the base of the concrete berm

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and containment area is below the ground surface and may extend to 3 feet bgs. The concrete berm may act as a barrier to groundwater flow when the water table is above 3 feet bgs. The boring logs for SS05-IW02, SS05-MW08S, SS05-MW09S, SS05-MW12S, and SS05-MW13S indicate fill material including sand, gravel, and cinders mixed with silt soil to a depth ranging from 2 to 4 feet bgs (HGL 2014a and 2014b). Drilling logs for locations completed north and south of SS05-MW09S indicate similar fill material extending to 5 feet bgs (HGL 2014a). Groundwater recharge will be higher within the fill material as compared to underlying clay soil.

Zone 2 underlies Zone 1 at SS005 and consists primarily of sand and gravel representative of the Pearl Formation and terminates between 29 and 43 feet bgs. The existing Zone 2 monitoring well network at SS005 is shown on Figure 1-3 and in Table 1-2. Zone 2 monitoring wells have well screens encompassing the Pearl Formation with bottom well screen depths ranging from 35.03 to

38.30 feet bgs. Groundwater in Zone 2 is semiconfined and has ranged from 3.94 to 6.53 feet bgs, with an average depth of 5.06 feet bgs. Depth to groundwater was last measured for the Zone 2 monitoring wells in November 2018 to evaluate basewide groundwater flow. The inferred groundwater flow direction observed at the SS005 area in November 2018 was to the southeast towards the South Ditch (Zapata and Burns & McDonnell 2020). Zone 2 monitoring wells are not included in the current SS005 groundwater monitoring program as COC concentrations did not exceed groundwater remediation objectives (GROs) during sampling in 2007 and 2010 (HGL 2014a).

1.4.5 Groundwater Use

No significant regional aquifers exist where Scott AFB is located. Beyond the Scott AFB boundaries, groundwater located within the unconsolidated units and bedrock is occasionally used as a source of potable water. Hydraulic conductivity values of 10-7 to 10-8 centimeters per second (10-3 to 10-5 feet per day [ft/day]) for Zones 1 and 2 were obtained from slug tests during prior investigations at SS005 and do not meet the criteria for Class I potable water resource groundwater under 35 IAC 620, Section 210 (IEPA 2012). However, groundwater at Scott AFB has been classified as Class I potable resource groundwater. Scott AFB and nearby communities purchase potable water from a municipal water distribution system that obtains its water from the Mississippi River. A St. Clair County ordinance requires municipal water to be used if available.

The LUCs for SS005 prevent drinking water wells from being installed in the affected groundwater zones.

1.4.6 Surface Water

As shown on Figure 1-3, the South Ditch (ERP Site UK510) flows west to east and is located approximately 70 feet from the south side perimeter fence. The South Ditch receives storm water from industrial and administrative areas of Scott AFB located beyond the SS005 boundaries.

Underground storm sewer lines that begin far north of SS005 discharge into the South Ditch approximately 40 feet southwest of SS05-MW06S and approximately 45 feet southwest of the SS005 eastern boundary. Precipitation over SS005 infiltrates into the subsurface, discharges into the South Ditch via overland flow, and collects within the bermed containment area around each AST. Surface water within the bermed containment areas is conveyed via underground wastewater lines to the Scott AFB wastewater treatment plant approximately 1.4 miles east of SS005.

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From the SS005 eastern boundary, the South Ditch flows east 1.4 miles and discharges into Mosquito Creek. From that confluence, Mosquito Creek flows east 0.5 miles and discharges into Silver Creek. Multiple tributaries discharge into Mosquito Creek prior to reaching Silver Creek.

Silver Creek flows south 12 miles and discharges into the Kaskaskia River which flows southwest 22 miles into the Mississippi River. The confluence of the Kaskaskia and Mississippi Rivers is approximately 33 miles south of SS005. The length of Silver Creek adjacent to Scott AFB, as shown on Figure 1-1, and south to the Kaskaskia River does not have an IEPA designated use (i.e., aquatic life, fish consumption, etc.) (IEPA 2018 and 2022). Downstream of where Silver Creek meets the Kaskaskia River, segments of the river have designated uses of aquatic life and/or public and food processing water supply (IEPA 2018 and 2022).

1.5 PREVIOUS SITE INVESTIGATIONS

In 1977, civilian personnel at Scott AFB reported a spill of JP-4 occurred at SS005 associated with AST 8733 (formerly AST 8550). Much of the fuel released was recovered from within the bermed area around the AST. Approximately 13,000 gallons of JP-4 could not be recovered during the spill recovery process. An undetermined amount of fuel was reported to have discharged to the South Ditch. Fuel recovery wells were reported to have been installed in response to the incident;

however, no fuel could be recovered from these wells. No other fuel releases have been reported for SS005 (Environmental Resources Management [ERM] 1992).

Multiple site investigations have been completed at SS005. The data from these investigations was compiled and evaluated in the Final Comprehensive Site Investigation Report (CSIR)/ROR/RAP (HGL 2014a). These investigations included the following:

• Remedial Investigation/Feasibility Study (ERM 1992) conducted in 1988

• Investigations associated with former Building 508 conducted between 1997 and 2004 that were later used to achieve Site Closeout for ERP Sites TU020, TU021, and TU027

• Soil removal conducted in 2000 during the removal of the original earthen berm structure

• Comprehensive Site Investigation (CSI) activities conducted between 2007 and 2013

The Final CSIR/ROR/RAP (HGL 2014a) concluded that all site contaminants in soil and groundwater were delineated, and data indicated limited potential for contaminants to migrate away from SS005. Soil analytical results indicated COCs exceeded soil remediation objectives (SROs) to a depth of 8 feet bgs (HGL 2014a). SROs are discussed further in Section 1.6.1. The completed screening level risk assessment identified benzo[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene, dibenz[a,h]anthracene, and indeno[1,2,3-cd]pyrene as COCs in soil requiring remediation to protect human health under a hypothetical future residential land use scenario (HGL 2014a). The remedial actions identified for soil COCs are discussed in Section 1.7.1.

During CSI activities, groundwater samples were collected from Zones 1 and 2. COCs that exceeded their GRO were identified only in Zone 1 (HGL 2014a). GROs are discussed further in Section 1.6.2. The completed screening level risk assessment identified arsenic, benzene, benzo[a]anthracene, benzo[b]fluoranthene, manganese, methylene chloride, and methyl tertiary-

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butyl ether (MTBE) as COCs in groundwater requiring remedial action to protect human health if groundwater at SS005 were used as a potable water supply (HGL 2014a). Benzo[a]pyrene, dibenz[a,h]anthracene, and indeno[1,2,3-cd]pyrene were later included as groundwater COCs in the Final ROR/RAP Addendum (URS 2021a). The remedial actions identified for groundwater COCs are discussed in Section 1.7.2.

Samples collected from oil water separator (OWS) OWS-4 indicated the presence of PAH contamination in this inactive structure; however, concentrations were not identified in the surrounding soil. Because of the potential for OWS-4 to contribute to PAH concentrations in soil in the future, remediation of this OWS was recommended and is discussed in Section 1.7.3. The location of OWS-4 is shown on Figure 1-4.

An ecological assessment was conducted as part of the risk analysis and concluded that site contaminants do not pose a threat to the plant, invertebrate, and mammalian communities, and pose minimal threat to insectivorous birds exposed via the food web (HGL 2014a).

1.6 REMEDIATION OBJECTIVES

1.6.1 Soil COCs

Soil COCs identified for SS005 in Section 1.5 included benzo[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene, dibenz[a,h]anthracene, and indeno[1,2,3-cd]pyrene, and SROs for these soil COCs were first developed in the Final CSIR/ROR/RAP (HGL 2014a). SROs for surface soil (ground surface to 0.5 feet bgs) were equivalent to the background concentrations within metropolitan areas found in 35 IAC 742, Appendix A, Table H (IEPA 2013). Scott AFB meets the definition of a “metropolitan area,” which is a populated area in a county within a Metropolitan Statistical Area. St Clair County, the county in which Scott AFB is located, is identified as being within a Metropolitan Statistical Area in 35 IAC 742, Appendix A, Table G (IEPA 2013). SROs for subsurface soil (0.5 to 10 feet bgs) were determined from 35 IAC 742, Appendix B, Table A for residential properties (IEPA 2013).

AFCEC proposed the adoption of the United States Environmental Protection Agency (USEPA) Regional Screening Levels (RSLs) for nine PAHs for all active Scott AFB ERP sites (AFCEC 2019). The PAHs selected included benzo[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene, dibenz[a,h]anthracene, and indeno[1,2,3-cd]pyrene. The Final ROR/RAP Addendum (URS 2021a) revised the SROs for SS005 to the USEPA RSLs for industrial soil to more closely align with the current and future industrial use at SS005. The IEPA concurred with the revised SROs in their correspondence dated March 9, 2021 (IEPA 2021a). Table 1-3 summarizes the current SROs for SS005 soil COCs.

1.6.2 Groundwater COCs

Groundwater COCs identified for SS005 in Section 1.5 included arsenic, benzene, benzo[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene, dibenz[a,h]anthracene, indeno[1,2,3-cd]pyrene, manganese, methylene chloride, and MTBE (HGL 2014a and URS 2021a). GROs developed in the Final CSIR/ROR/RAP (HGL 2014a) for benzene, benzo[a]anthracene, Draft Final Annual Groundwater Monitoring Report, 2024, Site SS005 1-10 Scott AFB PBR

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benzo[b]fluoranthene, methylene chloride, and MTBE were determined from 35 IAC 742, Appendix B, Table E for Class I potable resource groundwater (IEPA 2013). The GROs for arsenic and manganese were the maximum detected concentrations within the Scott AFB background groundwater dataset (HGL 2012). The background dataset represents the range of naturally occurring metals concentrations at Scott AFB.

The Final ROR/RAP Addendum (URS 2021a) did not modify the GROs for arsenic, benzene, benzo[a]anthracene, benzo[b]fluoranthene, manganese, methylene chloride, and MTBE. The Final ROR/RAP Addendum (URS 2021a) did include the soil COCs of benzo[a]pyrene, dibenz[a,h]anthracene, and indeno[1,2,3-cd]pyrene as groundwater COCs, and their GRO determined from 35 IAC 742, Appendix B, Table E for Class I potable resource groundwater (IEPA 2013). Groundwater monitoring activities discussed in Section 1.7.2.2 for benzo[a]anthracene and in Section 1.7.2.3 for benzo[b]fluoranthene included benzo[a]pyrene, dibenz[a,h]anthracene, and indeno[1,2,3-cd]pyrene to monitor concentration trends and natural attenuation. Concentrations of benzo[a]pyrene, dibenz[a,h]anthracene, and indeno[1,2,3-cd]pyrene exceeded their GRO during CSI activities and periodic groundwater monitoring events.

The IEPA March 9, 2021, correspondence concurred with the selected groundwater COCs and GROs for SS005 (IEPA 2021a). Table 1-3 summarizes the current GROs for SS005 groundwater COCs.

1.7 REMEDIAL ACTIONS

1.7.1 Soil COCs

For the SS005 soil COCs indicated in Table 1-3, the remedial action presented in the Final CSIR/ROR/RAP (HGL 2014a) was monitored natural attenuation (MNA). During CSI activities, soil samples were collected in 2010 and 2013 within the vadose and saturated zones and co-located with historical samples collected in 2007. The sample locations were within the soil remediation area shown on Figure 1-4. Soil analytical results for the co-located sampling indicated degradation of the soil COCs in the vadose and saturated zones (HGL 2014a). After implementation of the soil remedy, LUCs were implemented at SS005 to prevent exposure until the SROs for residential land use are attained within the soil remediation area.

In accordance with the Final CSIR/ROR/RAP (HGL 2014a), soil samples were collected annually in 2016 through 2019 from the vadose and saturated zones and co-located with historical samples collected in 2007, 2010, and 2013. Additional soil samples were collected in 2018 and 2019 to delineate soil COC concentrations that exceeded residential SROs. The sample locations were within the soil remediation area shown on Figure 1-4. The Final Annual Groundwater and Soil Monitoring Report, 2019, Site SS005 (URS 2021b) concluded that degradation of the soil COCs was not observed within the soil remediation area. The Final ROR/RAP Addendum (URS 2021a) revised the SROs to the USEPA RSLs (Table 1-3) for industrial soil and the remedial action to remove vadose zone soils exceeding the revised SROs. The IEPA concurred with the revised SROs and remedial action in their correspondence dated March 9, 2021 (IEPA 2021a). SROs are discussed in Section 1.6.1.

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On November 1 through 5, 2021, approximately 85 cubic yards (127.87 tons) of soils exceeding the SROs (Table 1-3) were removed using a tracked excavator. The vertical boundary of the excavation was the extent of the vadose zone near Building 545 and determined to be 4 feet bgs.

The horizontal boundaries of the excavation were determined from direct push soil sample locations with PAH soil concentrations below the SROs and site structures that included Building 545, the active above ground fuel lines, and the concrete berm for AST 8734 (Figure 1-5). Three inactive underground fuel lines were encountered within the excavation area and were not removed (Figure 1-5). Soils exceeding SROs were removed from around the inactive fuel lines. Additional information regarding the soil removal activities was presented in the Final Annual Groundwater and Soil Monitoring Report, 2021, Site SS005 (URS 2023). Upon completion of the soil removal activities, LUCs remain in place to prevent exposure to site soils until SS005 has achieved Site Closeout.

1.7.2 Groundwater COCs

The remedial actions for SS005 groundwater COCs presented in the Final CSIR/ROR/RAP (HGL 2014a) included enhanced aerobic biodegradation, monitoring to confirm attainment of GROs, and MNA. After implementation of the groundwater remedies, LUCs were implemented at SS005 to prevent installation of water supply wells and prohibit a residential exposure scenario. The LUCs for groundwater will remain in place until SS005 has achieved Site Closeout.

In accordance with the Final Record of Decision, Site SS031 (URS 2022), SS31-MW03 was added to the groundwater monitoring program for SS005 beginning in 2023 to monitor concentrations of manganese that exceeded the GRO during remedial investigation activities at SS031 in 2019 and 2020. Concentrations of volatile and semi-volatile organic compounds (including PAHs) in groundwater at Site SS031 did not exceed their GROs for samples collected during the preliminary assessment/site inspection in 2018 and remedial investigation in 2019 through 2020 (URS 2019 and 2021c).

1.7.2.1 Enhanced Aerobic Biodegradation

During CSI activities at SS005, MTBE concentrations in groundwater exceeded the GRO of 70 micrograms per liter (µg/L) with the highest concentrations observed at SS05-MW08S and SS05- MW09S (HGL 2014a). The selected remedial action for MTBE was active treatment using in-situ Submerged Oxygen Curtain (iSOC®) technology to enhance aerobic biodegradation. Installation of the iSOC® remediation system was conducted between February and March 2014 and described in detail in the Final Interim Remedial Action Completion Report (HGL 2014b). Figure 1-4 shows the locations of SS05-MW08S and SS05-MW09S and former iSOC® injection wells SS05-IW01, SS05-IW02, and SS05-IW03 within the remediation area for organics. The former injection wells were completed with well screens in hydrogeologic Zone 1. Former injection well SS05-IW02 remains at the site and is utilized as a monitoring well for groundwater MNA. The iSOC® remediation system began operation on March 18, 2014, and operated continuously until February 9, 2018.

After completion of the March 2018 groundwater monitoring event, MTBE concentrations in groundwater did not exceed the GRO for all monitoring wells sampled (URS 2020b). The IEPA

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concurred on June 5, 2018, with shutdown of the iSOC® remediation system and completion of semi-annual groundwater monitoring for rebound of MTBE concentrations (IEPA 2019).

Groundwater monitoring activities were completed in September 2018, June 2019, and December 2019, and MTBE concentrations did not exceed the GRO during each monitoring event (URS 2020b and 2021b). The Final Annual Groundwater and Soil Monitoring Report, 2020, Site SS005 (URS 2021c) indicated the iSOC® remediation system would be decommissioned in 2021. The IEPA June 25, 2021, correspondence concurred with decommissioning of the iSOC® remediation system (IEPA 2021b). Except for SS05-IW02, all components of the iSOC® remediation system were removed from SS005 as of November 5, 2021 (URS 2023).

1.7.2.2 Monitoring to Confirm Attainment of GROs

During CSI activities, groundwater analytical results for samples collected in June 2013 indicated concentrations of benzene, benzo[a]anthracene, and methylene chloride did not exceed their GRO (HGL 2014a). The selected remedial action for benzene, benzo[a]anthracene, and methylene chloride was continued groundwater monitoring to confirm concentrations do not rebound above their GRO.

After June 2013, groundwater samples for analysis of benzene and methylene chloride were collected at the following monitoring wells (Figure 1-4) and frequency:

• 2014 – March 2014 from SS05-MW11S, SS05-MW12S, and SS05-MW13S and May 2014 from all monitoring wells excluding SS05-IW02 and SS31-MW03

• 2015 – Quarterly from all monitoring wells excluding SS05-IW02 and SS31-MW03

• 2016 and 2017 – Semi-annually from all monitoring wells excluding SS05-IW02 and

SS31-MW03

• 2018 and 2019 – Semi-annually from all monitoring wells excluding SS31-MW03

• 2020 – December 2020 from SS05-MW13S

Groundwater concentrations of benzene and methylene chloride did not rebound above their GRO.

The Final Annual Groundwater and Soil Monitoring Report, 2020, Site SS005 (URS 2021d) concluded that monitoring for these volatile organic compounds would be discontinued during future groundwater monitoring events. The IEPA June 25, 2021, correspondence concurred with the removal of benzene and methylene chloride from future groundwater monitoring events (IEPA 2021b).

After June 2013, groundwater samples for analysis of benzo[a]anthracene were collected at the following monitoring wells (Figure 1-4) and frequency:

• 2014 – March 2014 from SS05-MW12S and SS05-MW13S and May 2014 from SS05-MW08S, SS05-MW09S, SS05-MW12S, and SS05-MW13S

• 2015 – Quarterly from SS05-MW08S, SS05-MW09S, SS05-MW12S, and SS05-MW13S

• 2016 and 2017 – Semi-annually from SS05-MW08S, SS05-MW09S, SS05-MW12S, and

SS05-MW13S

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• 2018 and 2019 – Semi-annually from SS05-IW02, SS05-MW05S, SS05-MW08S, SS05-MW09S, SS05-MW12S, and SS05-MW13S

• 2020 through 2023 – Semi-annually from SS05-IW02 and SS05-MW09S

Groundwater concentrations of benzo[a]anthracene exceeded the GRO of 0.13 µg/L in June 2015 for SS05-MW08S and multiple times for SS05-IW02 and SS05-MW09S including monitoring events completed in 2023 (URS 2021b and 2024b). The most recent Mann-Kendall trend analysis for each monitoring well indicated a decreasing trend for SS05-MW08S, no trend for SS05-IW02, and a stable trend for SS05-MW09S (URS 2021b and 2024b). The Final 2023 Monitoring Report (URS 2024b) concluded attenuation of benzo[a]anthracene concentrations in groundwater was not complete at SS005. The Final ROR/RAP Addendum (URS 2021a) included a plan for annual groundwater monitoring in which groundwater samples collected from SS05-IW02 and SS05-MW09S would be analyzed for benzo[a]anthracene. The IEPA March 9, 2021, correspondence concurred with the monitoring plan for SS005 (IEPA 2021a). The Final 2023 Monitoring Report (URS 2024b) and Final Technical Memorandum (URS 2024a) indicated annual groundwater monitoring would begin in 2024. Groundwater monitoring activities completed in 2024 are discussed in Section 2.1.

1.7.2.3 Monitored Natural Attenuation

During CSI activities, groundwater analytical results for samples collected in June 2013 indicated concentrations of arsenic, benzo[b]fluoranthene, and manganese exceeded their GRO. The selected remedial action for arsenic, benzo[b]fluoranthene, and manganese was MNA.

Summary of MNA for Metals

After June 2013, groundwater samples for analysis of arsenic and manganese were…