2. UK510_Scott AFB_DF_Proposed Plan_Mar2025.pdf

DRAFT FINAL

PROPOSED PLAN

SITE UK510

Scott Air Force Base

Prepared for:

AIR FORCE CIVIL ENGINEER CENTER

2261 Hughes Avenue, Suite 155

Lackland AFB, Texas 78236-9853

Contract FA8903-14-C-0009

March 2025

PROPOSED PLAN FOR UK510

Scott Air Force Base, Illinois i

TABLE OF CONTENTS

1 Introduction 2 Site History and Background

Bulk Fuel Facility (SS005) PCB Spill Site (SS-21) 2012 RI for UK510 2017 and 2021 Supplemental RI for UK510 2024 Addendum to Supplemental RI for UK510 6

3 Site Characteristics Topography and Drainage Climate Geology and Soils Hydrogeology Land Use Ecological Resources

4 Scope and Role of the Action 5 Summary of Site Risks

Baseline Human Health Risk Assessment Screening Level Ecological Risk Assessment ... 11

6 Preferred Alternative 7 Community Participation 8 References 9 Glossary of Terms

FIGURES

Figure 1 – Site Location Map Figure 2 – Site Features Map – Western Portion of

UK510

Figure 3 – Site Features Map – Eastern Portion of UK510 Figure 4 – Sediment/Soil Locations 2012 and 2017 –

Western Portion of UK510 Figure 5 – Sediment/Soil Locations 2012 and 2017 –

Eastern Portion of UK510 Figure 6 – Surface Water Locations 2012 and 2021–

Western Portion of UK510 Figure 7 – Surface Water Locations 2012 and 2021 –

Eastern Portion of UK510

ATTACHMENTS

Attachment 1 – Public Comment Form

ACRONYMS AND ABBREVIATIONS

AFB Air Force Base ALM Adult Lead Methodology AST above ground storage tank bgs below ground surface BHRA Baseline Human Health Risk Assessment CERCLA Comprehensive Environmental Response, Compensation, and Liability Act CFR Code of Federal Regulations COPC contaminant of potential concern COPEC contaminant of potential ecological concern CTE central tendency exposure FS Feasibility Study HGL HydroGeoLogic, Inc.

HI hazard index HMW high molecular weight HQ hazard quotient IAC Illinois Administrative Code IEPA Illinois Environmental Protection Agency IEUBK Integrated Exposure Uptake Biokinetic

Model for Lead in Children JP-4 jet propellant grade 4 LOAEL Lowest-Observed-Adverse-Effect Level MSA Metropolitan Statistical Area NFA No Further Action NCP National Oil and Hazardous Substances

Pollution Contingency Plan NOAEL No-Observed-Adverse-Effect Level PAH polycylcic aromatic hydrocarbon PAL project action level PCB polychlorinated biphenyl PEC Probable Effects Concentration RI Remedial Investigation RME reasonable maximum exposure Shaw Shaw Environmental & Infrastructure, Inc.

SLERA Screening Level Ecological Risk Evaluation SRO soil remediation objective SRP Site Remediation Program SVOC semi-volatile organic compound TACO Tiered Approach to Corrective Action

Objectives TEC Threshold Effects Concentration TRV toxicity reference value TTI Tetra Tech, Inc.

UCL upper confidence limit URS URS Group, Inc.

USAF United States Air Force USEPA United States Environmental Protection

Agency VOC volatile organic compound

Scott Air Force Base, Illinois 2

Site UK510 Scott Air Force Base, Illinois Department of the Air Force

1 Introduction This Proposed Plan1 identifies No Further Action (NFA) as the Preferred Alternative for Site UK510, also referred to as South Ditch at Scott Air Force Base (AFB) in St. Clair County, Illinois. NFA means the site is protective of human health and the environment and no further remedial action is required based on knowledge of site conditions.

This document is issued by the United States Air Force (USAF), the lead agency for site activities, and the Illinois Environmental Protection Agency (IEPA) as the support agency. The USAF will select a final Remedy for UK510, in consultation with IEPA, after reviewing and considering all information submitted during a 30-day public comment period for this Proposed Plan. The USAF or IEPA may propose modifications to the Preferred Alternative or propose a different action based on new information or public comments. Therefore, the public is encouraged to review and comment on the Proposed Plan.

The USAF is issuing this Proposed Plan as part of its public participation responsibilities under Section 117(a) of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), Title 42 United States. Code Section 9617(a), and Title 40 Code of Federal Regulations (CFR) 300.430(f)(3) of the National Oil and Hazardous Substances Pollution Contingency Plan, also known as the National Contingency Plan (NCP). The USAF is addressing UK510 under CERCLA in accordance with Executive Order 12580, as amended by Executive Order 13016.

This Proposed Plan summarizes information presented in the Final UK510 Remedial Investigation (RI)/ Feasibility Study (FS) (URS Group, Inc. [URS], 2022), the Final RI Addendum (URS, 2025), and other documents contained in the USAF Administrative Record file for UK510. A link to this Administrative Record website is included at the bottom of the text box shown in the adjacent column.

The USAF and IEPA encourage the public to review these documents to gain a more comprehensive understanding of UK510 and the Proposed Plan for NFA at the site.

PUBLIC COMMENT PERIOD:

(April 28, 2025 – May 28, 2025)

The USAF invited the public to review and comment on this Proposed Plan for UK510 within the Legal Section of the Belleville News Democrat (Belleville, Illinois), the Mascoutah Herald (Mascoutah, Illinois), and the O’Fallon Progress (O’Fallon, Illinois). The USAF will accept written comments on the Proposed Plan during the public comment period. Comment letters must be postmarked by May 28, 2025, and should be submitted to:

375th AMW/Public Affairs Attention: Christine Spargur 901 South Drive, Building 700W Scott AFB, Illinois 62225-5103 Email: christine.spargur@us.af.mil Phone: 618-256-4241

For an extension, send a written request to Ms.

Spargur by May 28, 2025.

PUBLIC MEETING:

If requested during the public comment period, the USAF will provide a public meeting to explain the Proposed Plan. Oral and written comments would also be accepted at this meeting. If requested, the meeting will be held at the O’Fallon Public Safety Building, 285 North Seven Hills Road, O’Fallon, Illinois, 62269, on a date and time to be determined.

For more information, see the USAF Administrative Record at these locations:

Website: https://ar.cce.af.mil/

375 Civil Engineering Environmental Office 701 Hangar Road, Building 531 Scott AFB, Illinois 62225-5035 Phone: 618-256-2387 Hours: Monday - Friday 8:00 a.m. - 4:00 p.m.

Southwestern Illinois College Library IS Room 1025 2500 Carlyle Avenue Belleville, Illinois 62221 Phone: 618-235-2700 ext. 5204 Hours: Monday - Thursday 8:00 a.m. - 8:00 p.m., Friday 8:00 a.m. - 4:00 p.m.

1 Boldface terms are defined in glossary.

mailto:christine.spargur@us.af.mil

Scott Air Force Base, Illinois 3

2 Site History and Background Scott AFB is in St. Clair County, Illinois, approximately 25 miles southeast of St. Louis, Missouri, in southwestern Illinois (Figure 1). The city of Belleville, Illinois, is located approximately 6 miles southwest of the installation, and the city of O’Fallon, Illinois, is located approximately 5 miles northwest of the installation. Scott AFB property encompasses approximately 3,545 acres of United States Government-owned and easement-held land.

UK510 is identified as the South Ditch and a west to east trending linear drainage area that flows via a generally open channel for approximately 1.75 miles, before discharging to Mosquito Creek northeast of the Scott AFB wastewater treatment plant (Figure 1). The Scott AFB flightline as well as areas north of UK510 discharge surface runoff and stormwater into the channel (Figures 2 and 3).

The Scott AFB Bulk Fuel Facility (SS005) and polychlorinated biphenyl (PCB) Spill Site (SS-21) were investigated as potential contaminant sources for sediments and/or soils deposited within UK510. A detailed summary and evaluation of the previous investigations completed at UK510, and at adjacent sites where contaminant sources were suspected, were documented in the Final UK510 RI/FS (URS, 2022). The most significant findings from these previous investigations are summarized in the remainder of this section.

Bulk Fuel Facility (SS005) Site SS005 is approximately 1,500 feet east (i.e., downstream) from where UK510 begins near the Belleville Gate and 70 feet north of the centerline of UK510 (Figure 2). SS005 includes three above-ground storage tanks (ASTs), four underground storage tanks, a pump house, tanker truck offloading facilities, and associated fuel distribution piping. Each AST is within a concrete-lined and bermed containment area. In 1977, civilian personnel at Scott AFB reported that approximately 13,000 gallons of jet propellant grade 4 (JP-4) fuels were lost in a spill incident at one of the SS005 ASTs. No other contaminant releases were historically reported at SS005 during its operational lifespan. During a conversion in 2000 from earthen berm AST containment structures to the current concrete-bermed structures, approximately 2,000 cubic yards of benzene, toluene, ethylbenzene, xylene, and polycyclic aromatic hydrocarbon (PAH)-contaminated soils were removed from SS005 (HydroGeoLogic, Inc. [HGL] 2008).

Multiple investigations were completed for SS005 to assess impacts to site media and adjacent sites including UK510. These investigations included a Basewide RI/FS from 1988 to 1991 (Environmental Management

Resources, Inc., 1992), investigation of former Building 508 in 2004 (a site north of SS005 and not suspected of impacts to UK510) (Cape Environmental, 2004), and a site-specific RI/FS in 2007 (HGL, 2008). A goal of this latest RI/FS for SS005 was to investigate any impacts to the adjacent and downgradient portions of UK510.

During the 2007 RI/FS at SS005, seven sediment (i.e., 0 to 1 foot below ground surface [bgs]) and six surface water samples were collected south of site within UK510. Preliminary results indicated that thirteen semi-volatile organic compounds (SVOCs) and nine metals slightly exceeded the screening criteria in at least one of the samples. The slightly elevated SVOCs and metals observed in the surface water and sediment samples were considered ubiquitous to the environment and were considered to be the result of typical anthropogenic and naturally occurring processes and not suspected to have been linked to SS005 or UK510-specific contaminant sources (HGL, 2008).

Investigation activities not associated with UK510 were completed at SS005 between 2007 and 2013 and identified petroleum contaminants resulting from known site release (HGL, 2014). Site SS005 remains active with investigation, monitoring, and remedial actions following the procedures under the Illinois Site Remediation Program (SRP) as petroleum releases are excluded from CERCLA. The IEPA concurred with SS005 being excluded from CERCLA and following SRP procedures (IEPA, 2011).

PCB Spill Site (SS-21) Site SS-21 is located in the Civil Engineering storage yard and encompasses Building 540 and portions of the paved areas surrounding the building. Building 540 is a restricted-access building that was formerly used as a temporary storage area for transformers, capacitors, and other equipment regulated for PCBs under the Toxic Substances Control Act (United States Environmental Protection Agency [USEPA], 1976). SS-21 is approximately 1,000 feet east/downstream from where UK510 begins near the Belleville Gate and 95 feet north of the centerline of UK510 (Figure 2).

Multiple investigations have been performed for SS-21 including a Basewide Preliminary Assessment/Site Inspection in 1995 (Parsons Engineering Science, Inc., 1995), a Preliminary Assessment/Site Inspection in 2005 of Scott AFB PCB sites including SS-21 (HGL, 2006), a Basewide Due Diligence Investigation between 2006 and 2008 (Tetra Tech, Inc. [TTI], 2008a), and a RI between 2009 and 2010 (HGL, 2011a). The RI for SS-21 was scoped to address data gaps from the other previous investigations at SS-21 and to assess any impacts to

UK510.

The Final RI Report (HGL, 2011a) for SS-21 identified no significant PCB concentrations in surface water

Scott Air Force Base, Illinois 4 samples collected within UK510. Although PCBs and PAHs were detected in SS-21 RI sediment samples (i.e., 0 to 0.5 feet bgs) collected within UK510, the spatial distribution of the results did not indicate SS-21 as a potential source of elevated PCBs or PAHs in sediments at UK510. The highest PCB concentrations were observed in sediment samples collected from locations upstream (east) of SS-21. The report also stated that PAH concentrations were similar upstream (east) and downstream (west) of SS-21 and were consistent with the IEPA Metropolitan Statistical Area (MSA) values. The Final RI Report (HGL, 2011a) indicated elevated PAH concentrations were most likely due to storm water runoff from the adjacent industrial areas including the asphalt parking lot surrounding SS-21. Following the CERCLA process, a Proposed Plan (HGL, 2011b) and Final Record of Decision (HGL, 2012) for NFA were submitted in November 2011 and July 2012 (signed in August 2012), respectively, for SS-21.

2012 RI for UK510 An RI was completed in 2012 by Shaw Environmental & Infrastructure, Inc. (Shaw) to assess impacts specific to UK510 sediment, subsurface soil, and surface water by storm water outfalls and runoff from the industrialized and urban areas of Scott AFB, as these potentially contributed to concentrations of PAH and PCB in sediment and subsurface soil identified during prior investigations. The investigation was completed in two phases to define the limits and potential sources of identified contaminants. Phase 1 included sediment (i.e., 0 to 0.5 feet bgs), subsurface soil (i.e., 2 to 3 feet bgs), and surface water samples to define the limits and types of contaminants along the length of UK510. Phase 2 included sediment sampling within storm water conveyance pipes that discharge to UK510 in the area where contaminants were identified during Phase 1.

Analytical results for Phase 1 samples did not indicate a significant presence of herbicides, pesticides, and volatile organic compounds (VOCs) within UK510, and these parameters were not analyzed for the Phase 2 samples (Shaw, 2013a and 2013b).

The following paragraphs include discussion of the 2012 RI sediment and subsurface soil analytical results compared to IEPA soil remediation objectives (SROs) and surface water analytical results compared to IEPA water quality standards. The SROs and water quality standards were those in use at the time the Draft RI Report (Shaw, 2013b) was being prepared and may not reflect the most current SROs or water quality standards.

The analytical results in the Draft RI Report (Shaw, 2013b) for PAHs in sediment and subsurface soil samples were compared to the IEPA Tiered Approach to Corrective Action Objectives (TACO) Tier 1 SROs for residential properties. One or more PAHs exceeded their SRO for 27 of 34 sample locations. The majority of the sediment sample locations with exceedances were in the western portion of the UK510 channel from UK510- SD-11 to UK510-SD-100 (Figure 4). Other sediment sample exceedances were western locations UK510-SD- 18 and UK510-SD-110 (Figure 4) and eastern location UK510-SD-17 (Figure 5). Only one subsurface soil sample, UK510-SS-17 (co-located with UK510-SD-17 on Figure 5), contained PAH concentrations greater than their SRO.

The analytical results in the Draft RI Report (Shaw, 2013b) for total PCBs in sediment and subsurface soil samples were compared to the IEPA TACO Tier 1 SROs for residential properties. Five of six sample locations with exceedances were sediment samples in the western portion of the UK510 channel from UK510-SD-04 to UK510-SD-102 (Figure 4). The remaining sample location was in the eastern portion of the UK510 channel at subsurface soil sample location UK510-SS-17 (Figure 5).

The analytical results in the Draft RI Report (Shaw, 2013b) for metals in sediment and subsurface soil samples were compared to the IEPA TACO Tier 1 SROs for residential properties. For arsenic and silver, the IEPA TACO Tier 1 SRO was based on the soil component of the groundwater ingestion route and pH of

6.65 to 6.89. The metals that exceeded their respective SRO included barium, lead, manganese, and thallium.

Eleven of 13 sample locations with exceedances were sediment samples in the western portion of the UK510 channel from UK510-SD-01 to UK510-SD-115 (Figure 4). The remaining sample locations were in the eastern portion of the UK510 channel as sediment sample location UK510-SD-17 and subsurface soil sample location UK510-SS-17 (Figure 5).

The analytical results in the Draft RI Report (Shaw, 2013b) for SVOCs in sediment and subsurface soil samples were compared to the IEPA TACO Tier 1 SROs for residential properties. Carbazole was the only SVOC to exceed its SRO and only within sediment. All six sediment sample locations with exceedances were in the western portion of the UK510 channel from UK510-SD- 100 to UK510-SD-115 (Figure 4). The SRO for carbazole was for the soil component of the groundwater ingestion route. Surface water samples collected from UK510-SW-01 (near UK510-SD-100) and UK510-SW- 09 (near UK510-SD-115) did not contain detectable concentrations of carbazole. Surface water sample locations are shown on Figure 6. Concentrations of carbazole in sediment did not exceed the IEPA TACO Tier 1 SRO for ingestion of sediment.

The analytical results in the Draft RI Report (Shaw, 2013b) for herbicides, pesticides, and VOCs in sediment and subsurface soil samples were compared to the IEPA TACO Tier 1 SROs for residential properties. Detected concentrations of herbicides, pesticides, and VOCs did not exceed their respective SRO.

Scott Air Force Base, Illinois 5

The analytical results in the Draft RI Report (Shaw, 2013b) for surface water samples were compared to IEPA water quality standards. The PAHs of benzo(a)pyrene at locations UK510-SW-01 (Figure 6) and UK510-SW-17 (Figure 7) and dibenz(a,h)-anthracene at location UK510-SW-17 were the only detected chemicals to exceed their water quality standard. These surface water locations were in areas where PAHs in sediment exceeded the SROs.

The Draft RI Report (Shaw, 2013b) included risk assessments for human health and ecological receptors and concluded exposure to chemicals within sediment and subsurface soil at UK510 could result in elevated risks to human health and ecological receptors. The report recommended removal actions for the western concrete-lined portion to UK510-SD-115 (Figure 4) and the unlined portion near UK510-SD-17 (Figure 5). The report also recommended additional investigation to define the source of metals, PAHs, and PCBs in sediments from the adjacent industrial areas that discharge into UK510. A supplemental RI to define the sources of contaminants within UK510 was completed by URS in 2017 and is discussed in the next subsection.

2017 and 2021 Supplemental RI for UK510 The 2012 RI sample results for metals, PAHs, and PCBs were screened against project action levels (PALs) developed in the Final UK510 Supplemental RI Work Plan (URS, 2019). PALs were developed from the following sources:

• Metals in Sediment (i.e., 0 to 0.5 feet bgs) – Lowest value between the IEPA TACO Tier 1 SROs for residential properties (35 Illinois Administrative Code [IAC] 742, Appendix B, Table A [IEPA, 2013]) and IEPA TACO Tier 1 SROs for the soil component of the groundwater ingestion route based on pH of 6.65 to 6.89 (35 IAC 742, Appendix B, Table C [IEPA, 2013]).

• Metals in Subsurface Soil (i.e., 0.5 to 3 feet bgs) – Similar criteria as used for metals in sediment.

Additional criteria for comparison included the Scott AFB basewide background value (TTI, 2008b).

• PAHs in Sediment – IEPA TACO MSA value (35 IAC 742, Appendix A, Table H [IEPA, 2013]). For PAHs without a MSA value, a calculated value for the soil component of the groundwater ingestion route was used and based Class I groundwater remediation objective.

• PAHs in Subsurface Soil – Lowest IEPA TACO Tier 1 SRO of the exposure routes for residential properties (35 IAC 742, Appendix B, Table A [IEPA, 2013]). For PAHs without a SRO value, a calculated value for the soil component of the groundwater ingestion route was used and based on

PAH’s respective Class I groundwater remediation objective.

• PCBs in Sediment and Subsurface Soil – Aroclor-specific USEPA Regional Screening Level (USEPA, 2019). For total PCBs, the IEPA TACO Tier 1 SRO for residential properties via the ingestion route of exposure (35 IAC 742, Appendix B, Table A [IEPA, 2013]) was used.

The purpose of the sample locations completed in 2017 was to delineate concentrations exceeding the PALs for the 2012 RI sample locations and determine potential sources of sediment concentrations within the industrial areas that discharge in UK510. In July 2017, 21 sediment and 4 subsurface soil samples were collected from 22 locations (Figures 4 and 5) in accordance with the Final UK510 Supplemental RI Work Plan (URS, 2019) and analyzed for metals, PAHs, and PCBs. The subsurface soil sample collected at 3 feet bgs for UK510-SB-17 was analyzed only for PCBs to confirm the 2012 results for UK510-SS-17 (co-located with UK510-SD-17) (Figure 5). Sediment samples were not collected for 16 locations proposed within the industrial areas due to the lack of sediment accumulation at the time of sampling.

Locations not sampled are not included on Figures 4 and 5.

The analytical results in the Final UK510 RI/FS (URS, 2022) for metals in sediment and subsurface soil samples were compared to the PALs in the Final UK510 Supplemental RI Work Plan (URS, 2019). Barium, cobalt, lead, and manganese exceeded their PAL at one or more sample locations within and outside of UK510.

Sediment sample exceedances within the UK510 channel were western locations UK510-SD-200, UK510-SD- 201, UK510-SD-202, and UK510-SD-205 (Figure 4) and eastern location UK510-SD-210 (Figure 5).

Sediment exceedances within the industrial areas outside of UK510 were locations UK510-SD-214 and UK510- SD-222 (Figure 4). Subsurface soil sample exceedances were within and outside of UK510 and included locations UK510-SD-200 and UK510-SD-214 (Figure 4).

The analytical results in the Final UK510 RI/FS (URS, 2022) for PAHs in sediment and subsurface soil samples were compared to the PALs in the Final UK510 Supplemental RI Work Plan (URS, 2019). One or more PAHs exceeded their PAL for 10 of 21 sediment sample locations and 3 of 4 subsurface soil sample locations. All sediment and subsurface soil sample locations with exceedances for PAHs were in the western portion of UK510 channel from UK510-SD-200 to UK510-SD-217 (Figure 4). Sediment sample exceedances within the industrial areas outside UK510 were locations UK510-

SD-213, UK510-SD-214, UK510-SD-215, UK510-SD-

222, and UK510-SD-231. Subsurface soil sample exceedances were within and outside of UK510 and

Scott Air Force Base, Illinois 6 included locations UK510-SD-200, UK510-SD-202, and

UK510-SD-214.

The analytical results in the Final UK510 RI/FS (URS, 2022) for PCBs in sediment and subsurface soil samples were compared to the PALs in the Final UK510 Supplemental RI Work Plan (URS, 2019). One or more PCBs exceeded their PAL for 4 of 21 sediment sample locations and 2 of 4 subsurface soil sample locations. All sediment and subsurface soil sample locations with exceedances for PCBs were in the western portion of UK510 (Figure 4). Sediment sample exceedances within the UK510 channel were locations UK510-SD-200, UK510-SD-201, and UK510-SD-202. One sediment sample exceedance within the industrial areas outside of UK510 was location UK510-SD-234. Subsurface soil sample exceedances were within the UK510 channel at locations UK510-SD-200 and UK510-SD-202.

During completion of the Baseline Human Health Risk Assessment (BHRA) and Screening Level Ecological Risk Assessment (SLERA) discussed in Section 5, it was determined additional surface water data was required to complete the SLERA. In May 2021, four locations were sampled of surface water on consecutive days (total of eight samples) (Figures 6 and 7). Surface water samples were analyzed for total aluminum and dissolved copper/iron, dissolved organic carbon, and hardness. Detected concentrations of aluminum, copper, and iron were compared to USEPA and IEPA chronic water quality standards. The water quality standards for aluminum and copper were calculated using equations provided by the USEPA for total aluminum and IEPA for dissolved copper. The maximum detected concentrations of total aluminum and dissolved copper, and dissolved iron did not exceed their USEPA or IEPA chronic water quality standard.

The sample results collected in 2012, 2017, and 2021 for sediment, subsurface soil, and surface water were combined into a single dataset and used to complete the BHRA and SLERA for UK510. The BHRA and SLERA were completed in accordance with the Final UK510 Supplemental RI Work Plan (URS, 2019) and Final Risk Assessment Approach Technical Memorandum (HGL, 2009) and are summarized in Section 5.

2024 Addendum to Supplemental RI for UK510 The Final UK510 RI/FS (URS, 2022) recommended remedial action for sediment and subsurface soil at UK510. During preparation of subsequent decision documents, it was determined the future construction/ excavation worker exposure pathway required revision.

This was the only exposure pathway to have unacceptable risks determined by the BHRA in the Final UK510 RI/FS (URS, 2022). Discussion between the USAF and IEPA resulted in a revision to the BHRA specifically for the future construction/excavation worker exposure pathway. No sediment, subsurface soil, or surface water samples were collected for the revised BHRA. The revised BHRA for the future construction/ excavation worker exposure pathway is discussed further in Section 5. The IEPA concurred with the revised BHRA (IEPA, 2024). The risk evaluation results for the future construction/excavation worker are doucmented in the Final RI Addendum (URS, 2025).

3 Site Characteristics Comprehensive descriptions of the environmental setting at Scott AFB and UK510 (i.e., topography and drainage, climate, soils, geology, hydrogeology, land use, and ecological resources) are presented in Section 2 of the Final UK510 RI/FS (URS, 2022). The following is a summary of the information.

Topography and Drainage Scott AFB is located on the west end of the Silver Creek Valley basin characterized by generally flat to gently rolling hills. The ground surface across Scott AFB is generally level. The maximum ground surface elevation is 510 feet above mean sea level along a till ridge just north of the Base’s northern boundary. The lowest ground surface elevation is approximately 420 feet above mean sea level along the Base’s eastern boundary.

Surface water drainage at Scott AFB is directed through underground storm water conveyances (Figures 2 and 3) and multiple creeks (i.e., Ash Creek, South Ditch, Mosquito Creek, and Cardinal Creek [Figure 1]).

UK510, also identified as the South Ditch, receives surface runoff and stormwater from areas north including the flightline and discharges into Mosquito Creek northeast of the Base wastewater treatment plant.

Mosquito Creek and Cardinal Creek discharge into Silver Creek near the Base’s eastern boundary. Ash Creek receives surface runoff and stormwater from the western portion of Scott AFB as well as tributaries located west beyond the Base’s boundaries. Ash Creek discharges into Loop Creek, a Silver Creek tributary, approximately 2.5 miles south of the Base. Silver Creek discharges into the Kaskaskia River approximately 12 miles south of Scott

AFB.

Climate Climate at Scott AFB is characterized by hot, humid summer and moderately cold winter. The growing season typically occurs from early April to late October with most precipitation occurring from March through July.

Geology and Soils The upper 16 inches of soil on either side of the UK510 channel, in ascending order of acreage, includes the Bethalto silt loam, Mascoutah silty clay loam, Caseyville silt loam, and Edwardsville silt loam. These soils are derived from the unconsolidated Peoria and Roxana Silt, which are wind-blown deposits of silt, clay, and fine sand

Scott Air Force Base, Illinois 7 that mantle the underlying glacially derived formations and have a combined thickness up to 20 feet. The base of the UK510 channel is within the Peoria and Roxana Silt.

Underlying the Peoria and Roxana Silt is the Pearl Formation comprised of glacial outwash sand and gravel deposits. Underlying the Pearl Formation on the western portion of UK510 is the Glasford Formation comprised of glacial till. Underlying the Pearl Formation on the eastern portion of UK510 is the Petersburg Silt comprised of silt and minor sand.

Within the UK510 channel, sediment comprises the upper 6 inches of material and subsurface soil comprises the material below 6 inches. The thickness of the subsurface soil can be up to 3 feet in the concrete-lined portions of UK510. Within the unlined portions, subsurface soil is all material below 6 inches. The sediments and subsurface soils at UK510 are composed of silts and silty clays rich in organic matter and sand sized particles of concrete and asphalt from paved areas.

Hydrogeology No significant regional aquifers exist in the area of Scott AFB, but groundwater within unconsolidated deposits and bedrock can be used as potable water. Scott AFB and nearby communities purchase municipal water from Illinois American Water Company, which obtains its water from the Mississippi River. There are no water wells on Base or at UK510 that are used for public consumption.

UK510 regularly contains surface water for extended periods of time, especially during periods of little to no precipitation indicating groundwater discharge from the unconsolidated silt and clay soils adjacent to the channel.

The creeks within and adjacent to Scott AFB do not have an IEPA designated use (e.g., public and food processing water supply use) (IEPA, 2022).

Land Use Scott AFB is an active military base. The installation contains living quarters and recreational facilities, but airbase facilities and runways make up the largest portion of the installation.

The entirety of UK510 is located within the Scott AFB boundaries (Figure 1). There are currently no known day-to-day operations conducted within UK510 (URS, 2022). The nearest residential property is located up-gradient approximately 130 feet northeast of the start of UK510 near the Belleville Gate (Figure 2). Other residential properties are located southeast of Building 570 and beyond the Scott AFB boundary (Figure 2). A fence between UK510 and the residential properties restricts access to the site.

A concrete-lined portion of the UK510 channel approximately 2,700 feet in length is between the Belleville Gate and Building 570 (Figure 2). Another concrete-lined portion approximately 600 feet in length is located northwest of the Scott AFB wastewater treatment plant (Figure 3). Approximately 0.5 miles of UK510 is routed through metal culverts covered with grass landscape or under roadways. The remaining portions of the channel are unlined.

The width at the channel base varies from 2 feet within the concrete-lined area near the Belleville Gate to 20 feet within the unlined area near Mosquito Creek. The channel sides include concrete and vegetation cover. In general, the concrete covered sides have a very steep slope and the vegetation covered sides have a moderate slope.

Ecological Resources Natural vegetative communities found at Scott AFB include bottomland forest communities within the floodplain of Silver Creek and upland forest communities located north of Scott Lake and along the western edges of the bottomland forest. The remaining 90 percent of the Base, including UK510, includes improved and semi-improved areas consisting of turf and landscape vegetation that is regularly maintained.

Various wildlife studies and observations indicate that Scott AFB supports a diversity of wildlife species. These studies have identified 210 species of birds, 24 mammal species, 25 species of amphibians and reptiles, 45 fish species, and a large number of aquatic macro-invertebrates.

Even though dense vegetation of grasses and cattail occupies portions of UK510, due to its concrete lining and industrial use as a critical drainage feature for the Base, UK510 does not include appropriate habitat for potentially sensitive species. Species of special interest in the vicinity of Scott AFB are the Indiana bat (Myotis sodalis) and northern long-eared bat (Myotis septentrionalis) (URS, 2022).

4 Scope and Role of the Action This Proposed Plan documents the Preferred Alternative of NFA for UK510. The NFA decision was based on information presented in documents referenced in Section 2. Additionally, the BHRA and SLERA summarized in Section 5, concluded no unacceptable risks to human and ecological receptors from exposure to sediment, subsurface soil, and/or surface water at

UK510.

Remedial action is not recommended for UK510;

however, discharge from upgradient industrial areas that feed into UK510 have the potential to impact sediment, subsurface soil, and surface water quality at UK510.

These potential future risks would be mitigated by future compliance monitoring under the purview of the Scott AFB Environmental Compliance Office.

Scott Air Force Base, Illinois 8

The FS portion of the Final UK510 RI/FS (URS, 2022) was not required. Elements of typical CERCLA Proposed Plans (i.e., which are developed based on a preceding FS) have been omitted from this Proposed Plan for UK510 and include Remedial Action Objectives, Summary of Proposed Remedial Alternatives, and Evaluation of Remedial Alternatives.

Site SS005 remains active with completion of activities and documents that follow procedures set forth in the Illinois SRP. Site SS-21 is not an active site as documented in the Final Record of Decision (HGL, 2012).

5 Summary of Site Risks The USAF has identified NFA as the Preferred Alternative for UK510 based on knowledge of site conditions and conclusions of the BHRA and SLERA documented in the RI portion of the Final UK510 RI/FS (URS, 2022) and the revised BHRA documented in the Final RI Addendum (URS, 2025). The BHRA and SLERA concluded no unacceptable risks to human or ecological receptors from exposure to sediment, subsurface soil, and/or surface water at UK510.

The process followed by the BHRA and SLERA were generally similar and included the following objectives:

• Identify contaminants of potential concern (COPCs) for human receptors and contaminants of potential ecological concern (COPECs) for ecological receptors through screening of sampling results collected in 2012, 2017, and/or 2021 against criteria established by USEPA, IEPA, or other sources.

• Identify the potential receptors and pathways of exposure to the COPCs/COPECs.

• Estimate the risk to the receptors from exposure to the COPCs/COPECs.

The results and conclusions of the BHRA and SLERA are summarized in the following subsections.

Baseline Human Health Risk Assessment The BHRA was completed in accordance with the Final Risk Assessment Approach Technical Memorandum (HGL, 2009) and to evaluate the potential risks to human receptors from exposure to the following COPCs.

Sediment and Subsurface Soil Surface Water

Metals Arsenic Barium Chromium Cobalt Lead Manganese

Metals Arsenic Barium Chromium Iron Manganese Thallium

Sediment and Subsurface Soil Surface Water

Metals (continued) Vanadium

PAHs 1-Methylnaphthalene 2-Methylnaphthalene Benzo(a)anthracene Benzo(a)pyrene Benzo(b)fluoranthene Benzo(k)fluoranthene Chrysene Dibenz(a,h)anthracene Indeno(1,2,3-cd)pyrene

PCBs Aroclor 1248 Aroclor 1254 Aroclor 1260

PAHs Benzo(a)anthracene Benzo(a)pyrene Benzo(b)fluoranthene Benzo(k)fluoranthene Chrysene Dibenz(a,h)anthracene Indeno(1,2,3-cd)pyrene

VOCs Bromodichloromethane Chloroform

The BHRA evaluated the following current and future receptors and exposure pathways:

• Current/Future Adolescent Recreator (person who accesses the site without permission) – Ingestion and dermal contact with sediment and surface water.

• Current/Future Adult Recreator (person who accesses the site without permission) – Ingestion and dermal contact with sediment and surface water.

• Future Outdoor Site Worker – Ingestion and dermal contact with sediment, subsurface soil, and surface water.

• Future Construction/Excavation Worker – Ingestion and dermal contact with sediment and subsurface soil. Standard Occupational Safety and Health Administration requirements prevent construction workers from working inside an excavation with water (HGL, 2009). This receptor was the only receptor addressed in the Final RI Addendum (URS, 2025).

• Future Resident (i.e., child and adult) – Ingestion and dermal contact with sediment, subsurface soil, and surface water. The evaluation is hypothetical as future land use for UK510 is anticipated to remain industrial/commercial.

• Exposure pathways for the current resident were not evaluated. The area surrounding UK510 is industrial/commercial. The nearest residential property is located up-gradient approximately 130 feet northeast of the start of UK510 near the Belleville Gate (Figure 2). Other residential properties are located southeast of Building 570 and beyond the Scott AFB boundary (Figure 2). A fence between UK510 and the residential properties restricts access to UK510.

Scott Air Force Base, Illinois 9

• There are no maintenance or construction/ excavation activities regularly performed in Scott AFB stormwater ditches including UK510, so the current receptors evaluated by the BHRA were limited to adolescent and adult recreators.

• All inhalation pathways for sediment, subsurface soil, and surface water were considered incomplete.

Since sediment and subsurface soil are continually saturated with water within UK510, particulates within air would not be generated. COPCs that could volatilize tend to be dispersed and diluted in outdoor air. IEPA TACO Tier 1 values for the inhalation exposure route are not established or magnitude greater than values for the ingestion exposure route for the COPCs.

The risk to human receptors from exposure to COPCs was evaluated quantitatively by estimating a cancer risk level for each carcinogenic COPC and a hazard quotient (HQ) for each non-carcinogenic COPC. Cancer risk levels are expressed as a probability; for example, 1 in 10,000 (1 x 10-4), which means that for every 10,000 people that are exposed to a carcinogenic COPC, one additional person may develop cancer over their lifetime than would normally be expected. Per the NCP (40 CFR 300.430[e][2][i][A][2]), the acceptable cancer risk range is 1 in 1,000,000 (1 x 10-6) to 1 in 10,000 (1 x 10-4). IEPA TACO Tier 1 SROs for COPCs are calculated to pose a cancer risk no greater than 1 in 1,000,000 (1 x 10-6) (IEPA, 2013). For each receptor exposure scenario, cancer risks were estimated for each carcinogenic COPC.

The resulting cancer risks were also summed to yield an upper-bound estimate of cancer risk due to multiple carcinogenic COPC exposures.

Non-cancer effects for individual COPCs are expressed as a HQ that provides a rough estimate of potential toxicity but does not represent a statistical probability of an adverse effect occurring over a lifetime of exposure.

For each receptor exposure scenario, HQs were calculated for the COPCs and summed to yield a total hazard index (HI). An HI less than or equal to 1 indicates that no adverse non-cancer health effects are expected to occur, even to sensitive individuals over a lifetime of exposure. An HI greater than 1 indicates potential concern for non-cancer health effects and the need for further evaluation about exposure and toxicity. An HI greater than 1 for a single COPC does not require further evaluation. An HI greater than 1 based on multiple COPCs requires identification of the major contributors and their effects on target organs and/or organ systems.

Reasonable maximum exposure (RME) and central tendency exposure (CTE) human health risks were calculated for both cancer and non-cancer risks based on incidental ingestion of and dermal contact with sediment and subsurface soil. The RME is an estimate of the maximum exposure that can reasonably be expected to occur. The CTE represents a more typical exposure for the average individual.

Risks from exposure to lead were evaluated separately from the other COPCs using the USEPA Integrated Exposure Uptake Biokinetic (IEUBK) Model for Lead in Children and Adult Lead Methodology (ALM) in accordance with the Final Risk Assessment Approach Technical Memorandum (HGL, 2009). The IEUBK model predicts lead concentrations in air, sediment/soil, and water that are acceptable for areas where children are likely to be present. The ALM predicts the lead concentrations in sediment/subsurface soil that would be appropriate for non-residential areas (e.g., industrial/ commercial). Lead was not selected as a COPC for surface water. The IEUBK model and ALM utilize the site mean lead concentration to determine the acceptable concentrations used for the risk evaluation. For UK510, the mean lead concentrations for current and future sediment exposure were less than the IEPA TACO Tier 1 SROs for residential land use, industrial/commercial land use, and the construction worker. Therefore, a quantitative evaluation of lead using the IEUBK model and ALM was not completed because the potential risks associated with current and future exposure to lead were regarded as minimal.

The results and conclusions from the risk evaluation for each exposure pathway are summarized as follows:

• Current Adolescent Recreator Scenario – The combined sediment and surface water cancer risks are 5 x 10-6 (RME) and 3 x 10-6 (CTE). The cancer risks are at the low end of the USEPA 1 x 10-6 to 1 x 10-4 risk management range. No individual COPC in sediment or surface water exceeded a cancer risk level of 1 x 10-6. The combined non-cancer HIs for sediment and surface water are 0.17 (RME) and 0.08 (CTE). Both HIs are less than the USEPA threshold value of 1. Exposure via ingestion or dermal contact to COPCs within sediment and surface water is unlikely to pose unacceptable cancer risks or adverse non-cancer health effects to current adolescent recreators.

• Current Adult Recreator Scenario – The combined sediment and surface water cancer risks are 5 x 10-6 (RME) and 6 x 10-7 (CTE). The cancer risks are less than or at the low end of the USEPA 1 x 10-6 to 1 x 10-4 risk management range. No individual COPC in sediment or surface water exceeded a cancer risk level of 1 x 10-6. The combined non-cancer HIs for sediment and surface water are 0.07 (RME) and 0.02 (CTE). Both HIs are less than the USEPA threshold value of 1. Exposure via ingestion or dermal contact to COPCs within sediment and surface water is unlikely to pose unacceptable cancer risks or adverse non-cancer health effects to current adult recreators.

• Future Adolescent Recreator Scenario – The combined sediment and surface water cancer risks https://www.lawinsider.com/dictionary/reasonable-maximum-exposure-rme

Scott Air Force Base, Illinois 10 are 3 x 10-6 (RME) and 2 x 10-6 (CTE). The cancer risks are at the low end of the USEPA 1 x 10-6 to 1 x 10-4 risk management range. No individual COPC in sediment or surface water exceeded a cancer risk level of 1 x 10-6. The combined non-cancer HIs for sediment and surface water are 0.12 (RME) and 0.06 (CTE). Both HIs are less than the USEPA threshold value of 1. Exposure via ingestion or dermal contact to COPCs within sediment and surface water is unlikely to pose unacceptable cancer risks or adverse non-cancer health effects to future adolescent recreators.

• Future Adult Recreator Scenario – The combined sediment and surface water cancer risks are 3 x 10-6 (RME) and 5 x 10-7 (CTE). The cancer risks are less than or at the low end of the USEPA 1 x 10-6 to 1 x 10-4 risk management range. No individual COPC in sediment or surface water exceeded a cancer risk level of 1 x 10-6. The combined non-cancer HIs for sediment and surface water are 0.05 (RME) and 0.02 (CTE). Both HIs are less than the USEPA threshold value of 1. Exposure via ingestion or dermal contact to COPCs within sediment and surface water is unlikely to pose unacceptable cancer risks or adverse non-cancer health effects to future adult recreators.

• Future Outdoor Site Worker Scenario – The combined sediment (including subsurface soil) and surface water cancer risks are 3 x 10-6 (RME) and 4 x 10-7 (CTE). The cancer risks are less than or at the low end of the USEPA 1 x 10-6 to 1 x 10-4 risk management range. No individual COPC in sediment or surface water exceeded a cancer risk level of 1 x 10-6. The combined non-cancer HIs for sediment and surface water are 0.05 (RME) and

0.013 (CTE). Both HIs are less than the USEPA threshold value of 1. Exposure via ingestion or dermal contact to COPCs within sediment and surface water is unlikely to pose unacceptable cancer risks or adverse non-cancer health effects to future outdoor site workers.

• Future Construction/Excavation Worker Scenario – Risks were evaluated based on maximum detected concentrations and the 95% upper confidence limit (UCL) using USEPA ProUCL Statistical Software (USEPA, 2016). The 95% UCL represents a more likely exposure concentration because it is based on the site mean concentration and accounts for exposure to concentrations across the entire site.

The risk evaluation based on maximum detected concentrations was revised as documented in the Final RI Addendum (URS, 2025). The USEPA default exposure frequency of 250 days per year and exposure duration of 1 year used in the Final UK510 RI/FS (URS, 2022) overestimated the risk to the future construction/excavation worker, as a worker was unlikely to spend 250 days per year at individual sample locations where maximum concentrations were identified. The revised risk evaluation pathway used an exposure frequency of 30 days per year and exposure duration of 6 weeks (42 days or 0.115 year), as recommended by the IEPA for a short-term construction worker scenario. The IEPA concurred with the revised risk evaluation (IEPA, 2024). The risk evaluation results for the future construction/ excavation worker in the Final RI Addendum (URS, 2025) supersede the results in the Final UK510 RI/FS (URS, 2022) and are presented in this Proposed Plan.

The sediment (including subsurface soil) cancer risk based upon exposure to maximum concentrations was 9 x 10-8 (RME). The cancer risk was less than the USEPA 1 x 10-6 to 1 x 10-4 risk management range. No individual COPC in sediment exceeded a cancer risk level of 1 x 10-6. The non-cancer HI for sediment was 0.3 (RME) and less than the USEPA threshold value of 1. The cancer risk and non-cancer HI for the CTE scenario were not calculated as maximum detected concentrations were used in the revised evaluation. Exposure via ingestion or dermal contact to COPCs within sediment is unlikely to pose unacceptable cancer risks or adverse non-cancer health effects for future construction/ excavation workers within UK510.

The sediment (including subsurface soil) cancer risks based upon 95% UCL concentrations are 9 x 10-7 (RME) and 2 x 10-7 (CTE). The cancer risks are less than the USEPA 1 x 10-6 to 1 x 10-4 risk management range. No individual COPC in sediment exceeded a cancer risk level of 1 x 10-6. The non-cancer HIs for sediment are 0.3 (RME) and 0.5 (CTE). Both HIs are less than the USEPA threshold value of 1. Based on 95% UCL concentrations, exposure via ingestion or dermal contact to COPCs within sediment and subsurface soil is unlikely to pose unacceptable cancer risks or adverse non-cancer health effects for future construction/ excavation workers within UK510.

• Future Resident Scenario – The combined sediment (including subsurface soil) and surface water cancer risks are 6 x 10-5 (RME) and 2 x 10-5 (CTE). The cancer risks are within the USEPA 1 x 10-6 to 1 x 10-4 risk management range. Individual COPCs of arsenic (via ingestion of surface water), chromium (via ingestion of sediment and surface water), and benzo(a)pyrene (via ingestion of sediment) had cancer risks greater than 1 x 10-6. The combined non-cancer HIs for sediment and surface water are 1.0 (RME) and 0.5 (CTE). Both HIs did not exceed the USEPA threshold value of 1. The non-cancer HIs are based on a child scenario because the child represents the more sensitive population and is

Scott Air Force Base, Illinois 11 protective of the adult population. The cancer and non-cancer risks for a future resident are hypothetical since future land use for UK510 is anticipated to remain industrial/commercial.

Exposure to COPCs in sediment, subsurface soil, and surface water is unlikely to pose unacceptable cancer risks or adverse non-cancer health effects.

The results BHRA in the Final UK510 RI/FS (URS, 2022) and the revised BHRA in the Final RI Addendum (URS, 2025) support the conclusion of no unacceptable risks to human receptors from exposure to COPCS in sediment, subsurface soil, and/or surface water at

UK510.

Screening Level Ecological Risk Assessment The RI portion of the Final UK510 RI/FS (URS, 2022) included a SLERA completed in accordance with the Final Risk Assessment Approach Technical Memorandum (HGL, 2009) to evaluate the potential risks from exposure to COPECs to the following ecological receptors:

• Benthic Invertebrate Community – Benthic invertebrates recycle nutrients and condition the sediment and subsurface soil. They can be an important prey species for other ecological receptors. Benthic invertebrates are evaluated as a community receptor exposed to sediment and subsurface soil via direct contact.

• Freshwater Aquatic Community – A healthy aquatic community is critical to maintenance of stream function. Members of this community serve as prey species for other ecological receptors.