SWMU 53 – Burn and Debris Area

Proposed Remedial Measure

The proposed remedy for SWMU 53 was excavation and off-site disposal for surface and subsurface soils and debris materials. Excavation and removal of soil off-site to an appropriate landfill or treatment facility was completed as an Interim Corrective Measure (ICM). All contaminants of concern (COCs) that may impact human health, or the environment were removed as indicated by post-confirmation sampling.

While there are no monitoring wells installed at SWMU 53, groundwater flow direction is inferred based on topography and nearby flow patterns to be easterly toward Kill Creek (Shaw, 2006). Depth to groundwater is estimated at around 3 ft below the gully within bedrock (USACHPPM, 1998).

Groundwater at the site will be evaluated for sitewide COCs during the Army’s site-wide assessment of Groundwater Operable Unit No. 2 (GWOU-#2). A GWOU is a discrete area consisting of a single to many SWMUs and/or AOCs with actual or potential for releases to groundwater, grouped together for purposes of assessment and cleanup. The primary criteria for placement of sites with actual or potential groundwater releases into an operable unit include geographic proximity, similarity of waste characteristics and site type, and the possibility for economies of scale.

Justification

All samples collected during investigations at the site were analyzed for RCRA metals (arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver), plus beryllium, semi-volatile organic compounds (SVOCs), polychlorinated biphenyls (PCBs), pesticides, and explosives.

Five surface soil samples were collected (0 to 0.5 ft depth) along the bank of the drainage channel during the Relative Risk Site Evaluation (RRSE, Battelle, 2003). Metals, SVOCs, and PCB Aroclors were detected, with lead detected in exceedance of the residential TMCLs at one location [1,100 milligrams per kilogram (mg/kg)]. All other detected concentrations were below their respective residential TMCLs.

During the SWMU 53 RCRA Facility Investigation (RFI, Shaw 2006) 59 surface soil samples were collected from a depth of 0.0 to 0.5 ft below ground surface (bgs) along both sides of the drainage channel. A total of 23 metals were detected, including 19 metals at concentrations above accepted background concentrations. Identified contaminants included antimony, arsenic, barium, beryllium, cadmium, calcium, chromium, cobalt, copper, iron, lead, magnesium, mercury, nickel, potassium, selenium, sodium, silver, and zinc. Fifteen subsurface samples were collected between 1.0 to 2.0 ft bgs. Sixteen metals were detected above accepted background concentrations, including antimony, arsenic, barium, beryllium, cadmium, chromium, copper, iron, lead, magnesium, mercury, nickel, selenium, sodium, silver, and zinc (Shaw, 2006). Along an intermittent stream, another 15 sediment samples were collected, where 9 metals (beryllium,

calcium, copper, lead, magnesium, potassium, sodium, silver, and zinc) were detected above background levels. No pesticides, PCBs, SVOCs, or VOCs in soil or sediment samples were detected at concentrations above TMCLs (Shaw, 2006).

The Corrective Measures Study/Corrective Measures Study Work Plan for SWMU 53 Soils Burn and Debris Area North of the Sewage Treatment Plant indicated that groundwater may flow easterly toward Kill Creek (Tetra Tech 2011). COCs in groundwater and proposed monitoring well locations will be addressed during the GWOU-#2 evaluation.

In August 2019, SRS-CAPE conducted Pre-Design Investigation (PDI) sampling at SWMU 53.

Twenty composite soil samples were collected in a 30-ft × 30-ft grid at a depth of 0–1 ft bgs (Cape, 2019). These samples were analyzed for site COCs antimony, arsenic, copper, iron, lead, mercury, hexavalent chromium, and TEQ as 2,3,7,8-TCDD (Cape, 2019). In addition, one soil boring was collected from a depth of 0–1-foot bgs, and sediment samples were collected from both the intermittent stream and the intermittent drainage channel (CAPE, 2019). The results indicated that most sample locations contained at least one COC concentration above the TMCL. Subsurface soil samples were analyzed for chromium speciation to determine whether chromium should be retained as a COC. While hexavalent chromium was not identified as a COC in the CMS, it was still analyzed in accordance with the recommendations of the CMS. Results of the PDI sampling indicated that total chromium and hexavalent chromium exceeded the TMCL of 33.6 mg/kg in several locations; however, the TMCL for total chromium is only applicable in the absence of chromium speciation data (Cape, 2019). Hexavalent chromium was retained as a COC and analyzed in confirmation samples.

Corrective Measures Investigation (CMI) field activities were conducted from September 30, 2020 through August 2, 2021 in accordance with the SWMU 53 CMI Work Plan and documented in the SWMU 53 Corrective Measures Implementation Completion Report (CMI) (CAPE, 2021). The proposed grid system was expanded, and a total of 117 surface soil delineation samples were collected. Excavation activities initiated in the central historical burn area and expanded based on the results of the soil delineation samples and the observation of visually impacted soils, where soil intermingled with demolition debris and ash containing suspect ACM. Grids with impacted soil were excavated to weathered bedrock or to soil free of suspect ACM. Total depths of the excavated grids ranged from one to two feet along the excavated perimeter to 10 feet bgs along the steep slope on the south side of the channel. Post-excavation confirmatory soil samples were collected from grids that were dug to ACM-free soil. No confirmatory samples were collected from grids where weathered bedrock was encountered.

In accordance with the SWMU 53 CMI Work Plan, post-excavation confirmation samples were collected from the floor of the excavation area at a rate of one 5-point composite sample per 900 square feet (ft2). Samples were analyzed for antimony, arsenic, copper, iron, lead, mercury, hexavalent chromium, and Dioxin TEQ as 2,3,7,8-TCDD. Confirmation sampling was conducted to measure COC concentrations in the soil remaining in place after excavation and confirmed that remaining soil was below residential TMCLs. No confirmation samples were collected from the excavation sidewalls. Due to the presence of bedrock in the excavation area, a clay cap was placed and contoured toward the ditch prior to backfilling with topsoil (CAPE, 2021).

The Army conducted a streamlined health risk evaluation by comparing RFI data to the approved TMCLs for the site. The RFI identified antimony, arsenic, copper, iron, lead, mercury, hexavalent chromium, and TEQ as 2,3,7,8-TCDD as COCs at SWMU 53 based on exceedances of the residential soil TMCLs; therefore, the exposure pathway for soil was considered during the assessment. Evaluated exposure pathways include incidental ingestion, inhalation of airborne particulates (dusts), inhalation of chemicals volatilizing from soil (volatile compounds only), and dermal contact (organic compounds only). The following corrective action objectives (CAOs) were established for surface soil SWMU 53:

  • Mitigate risk to human health via direct contact with impacted surface soil with COC concentrations above TMCLs.
  • Mitigate the potential future migration of contaminants from impacted surface soil to surface soil in drainage pathways leading to Kill Creek.

KDHE believes that proper employment of the KDHE (2021) RSK Manual values result in risk-based remediation that is consistent with federally promulgated standards, including the Safe Drinking Water Act, 42 U.S.C. §300f – 300j-26, and is protective of human health as required by Resource Conservation and Recovery Act, 42 U.S.C. §6901 et seq., including the Hazardous and Solid Waste Amendments (HSWA) and 40 CFR Part 264.101. KDHE Tier 2 risk-based cleanup goals represent concentrations at which the contaminants pose an acceptable human health risk to receptors, including sensitive groups (e.g., children or the elderly), over a lifetime.

Cleanup goals were developed for residents and non-residents, according to the appropriate land-use designation, exposure frequency, and exposure duration. According to the Conceptual Land Use Plan, SWMU 53 falls within the area designated as parks, trails, or open space (Johnson County, 1998). Based on this land use, the potential current and future receptors include:

  • Current Construction/Excavation Workers, current,
  • Residents, future. The assessment determined that site risks were within acceptable limits.

The removal of debris and the excavation of the surface/subsurface soil and sediments removed the COCs for future use at the site.

Additional evaluation of groundwater in the area will occur when work on GWOU-#2 is done.


KDHE Site Documents