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Final AOC 65 Interim Removal Action Report
Section 1 - Introduction
The purpose of this interim removal action (IRA) was to excavate soil material containing volatile organic compound (VOC) levels greater than Texas Commission on Environmental Quality (TCEQ) Risk Reduction Standard No. 1 (RRS1) criteria from AOC-65 at Camp Stanley Storage Activity (CSSA). Demonstration that all surface soils containing VOCs concentrations above detectable levels were removed can qualify the surface-based portion of the site for partial closure under TCEQ�s RRS1. However, further demonstration that VOCs and metals have been removed from beneath Building 90 must be completed. Excavating the contaminated media outside Building 90 (soil overlying bedrock) under this interim measure reduces the amount of contaminants available for migration to the groundwater at AOC-65 and surrounding areas.
Soil samples were collected from the borings advanced around Building 90 prior to execution of the IRA at AOC-65. The purpose of these characterization samples was to delineate the lateral extent of contaminated material to be excavated during the IRA and to profile the waste for disposal. The vertical extent is defined as impenetrable bedrock encountered at 1.5 to 3 feet below grade across the site. Following execution of the IRA, confirmation samples were collected from the excavated area to verify that the contaminated media had been removed.
1.1 � Site Location and History
AOC-65 consists of potential VOC source areas believed to be associated with Building 90. This includes an inactive, subslab, concrete-lined pit on the west side of the interior of Building 90, and an area extending outside Building 90 along abandoned drain lines and related stormwater ditches. These areas have been implicated as possible VOC source areas by the presence of contaminants detected during a previous soil gas survey (Soil Gas Survey Technical Report, Parsons, February 2002) and the Resource Conservation and Recovery Act (RCRA) facilities investigation (RFI) report completed in 2001 (AOC-65 RFI Report).
Tetrachloroethene (PCE) and trichloroethene (TCE) were formerly used at CSSA to degrease ordnance in a metal vat located inside Building 90. Currently a citrus-based cleaner is used to clean ordnance, but prior to 1995 chlorinated solvents such as PCE and TCE were used. Further background information regarding the location, size, and known historical use of the site is included in the CSSA Environmental Encyclopedia (Volume 1-3, AOC-65).
1.2 - Geology and Hydrogeology
The Crawford and Bexar stony soils predominate AOC-65, although there is significant evidence that soils in the area were dramatically reworked during construction of Building 90 and the associated paved roadway. The Crawford and Bexar stony soils are thin (typically less than one foot thick), stony, very dark gray to dark reddish brown, non-calcareous clays that typically occur in broad, nearly level to gently undulating areas. Detailed descriptions of all CSSA soil types are given in the CSSA Environmental Encyclopedia (Volume 1-1, Background Information Report, Soils and Geology).
Bedrock at the AOC-65 area is the Upper Glen Rose limestone. Approximately 30 feet of this formation is present and is underlain by the Lower Glen Rose Limestone. The Lower Glen Rose is estimated to be 300 feet thick beneath CSSA. The local geology and hydrogeology is the subject of the groundwater recharge study that is currently underway at AOC-65 (Volume 1-3, AOC-65 Cleanup Plan, Groundwater Recharge Study).
Surface topography near AOC-65 slopes gently (less than two percent grade) to the southeast. Storm water run-off in the area is generally towards the southeast. Runoff on the paved area around AOC-65 and gutters from Building 90 formerly discharged to a drainage ditch north and west of the building that directs water to the east and southwest.
Heavy rainfalls in July 2002 illustrated the deficiencies in the drainage network around Building 90, prompting CSSA to pursue a complete upgrade of the Building 90 drainage network as part of this project. Modifications of the gutters and unlined drainage ditch were included as part of the site restoration efforts following completion of the excavations. No streams are present near AOC-65 so drainage is primarily through man-made ditches or by overland flow with rapid infiltration into the underlying formation.