Summary of the Statement of Basis

Area of Concern 18 - Potential Trench Disposal Area

at the Sunflower Former Army Ammunition Site

JUSTIFICATION:

AOC 18, Potential Trench Disposal Area, is located in the northeast quadrant of SFAAP encompassing approximately 34 acres. Based solely on a review of a 1948 aerial photograph, the U.S. Environmental Protection Agency (EPA) designated AOC 18 as a trench disposal area. Confirmation that the area was used for disposal activities at SFAAP is not available. However, it appears that it was a staging yard used during the initial construction of SFAAP or a western extension of Solid Waste Management Unit (SWMU) 01.

A Relative Risk Site Evaluation (RRSE) was conducted in 2003 by Battelle. Twenty-one surface soil samples were collected and analyzed for target analyte list (TAL) metals, semi-volatile organic compounds (SVOCs), and volatile organic compounds (VOCs). Antimony, cobalt, and lead exceeded the residential target media cleanup levels (TMCLs). However, the location of the TMCL exceedances for antimony and lead could not be confirmed during the RFI/CMS (2008-2010). X-ray fluorescence (XRF) data indicated that lead concentrations were below TMCLs in the surrounding areas. Cobalt was not used in historical processes at SFAAP. Therefore, the exceedances for antimony, cobalt, and lead were not considered for additional investigation during RFI activities nor are they considered to be contaminants ofconcern (COCs). However, benzo(a)pyrene is considered a COC for surface soil.

In 2008, forty subsurface soil samples, one surface water/sediment sample pair (taken at the point where the upslope drainage enters the Livestock Watering Pond-Figure 1), were collected and analyzed for general chemistry parameters (including ammonia, nitrate, and sulfate), RCRA metals, explosives, SVOCs, and VOCs. Benzo(a)pyrene was the only analyte that exceeded the TMCL for soil.

An RFI/CMS was performed in 2016 and identified benzo(a)pyrene as the sole COC in soil at AOC 18. Excavation and disposal were selected as the corrective action alternative and surface soil contaminated with benzo(a)pyrene was to be excavated to a depth of 0.5 feet below ground surface (bgs) (CAPE, 2020).

During 2019-2020, Corrective Measures Implementation (CMI) excavation and field activities were conducted. A total of 95.32 CY of benzo(a)pyrene-contaminated soil was excavated and disposed of off-site. Confirmation samples were collected and analyzed for benzo(a)pyrene. Post-excavation samples were below the Target Media Contaminant Level (TMCL) in three of the four sampling grids. The fourth sampling grid was above the TMCL and required an additional one-foot depth excavation. Aft this excavation, the sample collected was below the TMCL and no additional excavation was necessary (CAPE, 2020). All other media: surface water, sediment, subsurface soil, and site-specific groundwater tested below the TMCLs.

Groundwater will be further evaluated during the Army's assessment for site-wide groundwater contamination in Groundwater operational unit #4 (GWOU-4).

Proposed Remedial Measure

Excavation and removal ofapproximately 95 cy (cubic yards) of benzo(a)pyrene-contaminated soil was disposed of off-site at an appropriate non-hazardous waste landfill. Post-excavation confirmation samples were collected, and restoration of the excavated areas was completed. Therefore, no further action is required for surface soil. All other media tested below the respective TMCLs.

Groundwater at the site will be evaluated for site-specific and site-wide COCs during the Army's assessment of Groundwater Operable Unit #4 (GWOU #4).

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