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Environmental Cleanup Plan for AOC 65
Section 2 - Site Conditions
2.1 - Site Description and History
AOC 65 consists of potential VOC source areas 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 stormwater ditches. These areas have been identified by the presence of contaminant plumes detected during a previous soil gas survey (Draft Soil Gas Survey Technical Report, Parsons, August 2001) and the RCRA facilities investigation (RFI) completed in 2001 (Draft AOC 65 RFI Report).
PCE and TCE were formerly used at CSSA to degrease ordnance, and were stored 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. The chlorinated solvents were stored in a metal vat placed in a subslab pit located in the southern portion of Building 90. This former vat location is a suspected source of contamination in AOC 65.
The former vat area consists of a concrete-lined pit and is located along the west side of Building 90. Prior to 1966 a metal vat was installed in the pit, but the vat was removed in 1995 and the pit was covered with a metal plate when the switch to a citrus-based cleaner was made. Essentially the concrete-lined pit acted as secondary containment for the metal storage vat, and during operations, solvents may have spilled onto the concrete and seeped into the underlying fill materials. Although no cracks or migration pathways are visible, staining is evident in the low areas at the bottom of the pit. A second, inactive pit is located north of the former metal vat area, but is not considered a likely source of contamination based on results of the soil gas survey (Draft Soil Gas Survey Technical Report, Parsons, August 2001).
In addition to the former solvent pit, floor drains within Building 90 may have contributed to the spread of contaminants beneath and outside Building 90. The floor drains originally collected fluids from within Building 90 and discharged them to ditches adjacent to the building. The floor drains are currently plugged with concrete and the drain lines have been disconnected.
Further background information regarding the location, size, and known historical use of the site is included in the Environmental Encyclopedia (Volume 1-3, AOC 65).
2.2 - 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 1 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).
AOC 65 is sited over the Lower Glen Rose (LGR) limestone. The LGR formation is a massive, fossiliferous, vuggy limestone that grades upward into thin beds of limestone, marl, and shale; it is estimated to be 300 feet thick beneath CSSA. The LGR is underlain by the Bexar Shale facies of the Hensell Sand, which is estimated to be from 60 to 150 feet thick under the CSSA area. The Bexar Shale consists of silty dolomite, marl, calcareous shale, and shaley limestone. The geologic strata dip approximately 10 to 12 degrees to the south-southeast at CSSA.
According to previous geophysical studies conducted by Parsons for CSSA, there are two major fault (shatter) zones believed to be present at CSSA: the North Fault Zone and the South Fault Zone. The South Fault Zone lies approximately 2,200 feet south of AOC 65. Additional information on structural geology at CSSA can be found in the Environmental Encyclopedia (Background Information Report, Volume 1-1). More recent mapping conducted by USGS has identified a fault under and/or near the Building 90 area.
Surface topography at and in the vicinity of AOC 65 slopes gently (less than 2 percent grade) to the southeast, so run-off is generally toward the southeast. However, runoff on the paved area comprising AOC 65 and gutters from Building 90 discharge to a drainage ditch that parallels the west side of the building and curves around the north side of the building before crossing beneath Tompkin Road and emptying into open fields east and southeast of Building 90. The ditch also diverts some drainage immediately to the south into open fields on the southwest side of AOC 65. There are no streams in the vicinity of AOC 65 so drainage from the land surrounding AOC 65 is primarily through man-made ditches or by overland flow with rapid infiltration into the underlying formation. It is unlikely that any drainage from Building 90 discharges into a surface water body (stream or lake).
In general, the uppermost hydrologic layer at CSSA is the unconfined upper Trinity aquifer, which consists of the Upper Glen Rose Limestone. Locally at CSSA, low-yielding perched zones of groundwater can exist in the Upper Glen Rose. Transmissivity values are not available for the Upper Glen Rose. Regionally, groundwater flow is likely enhanced along the bedding contacts between marl and limestone; however, hydraulic conductivity between beds is thought to be poor. This interpretation is based on the observation of differing static well levels in adjacent wells completed in different beds. Principal development of solution channels is limited to evaporite layers in the Upper Glen Rose Limestone. Based on historical data, groundwater in the LGR at CSSA generally flows in a southerly direction. However, local flow gradient may vary depending on rainfall, recharge, and possibly well pumping. Groundwater elevation data from four monitoring wells (CS-MW-6LGR, CS-MW-7LGR, CS-MW-8LGR and CS-MW-10LGR) recently installed near AOC 65 indicate a northerly gradient exists in the LGR in this area (Draft September 2001 Quarterly On-post Groundwater Monitoring Report).
The middle Trinity aquifer is unconfined and functions as the primary source of groundwater at CSSA. It consists of LGR Limestone, Bexar Shale, and Cow Creek Limestone. The middle Trinity aquifer is separated from the lower Trinity aquifer by the Hammett Shale, which is an aquitard and not considered to be part of either the middle or lower Trinity aquifers. The LGR Limestone outcrops north of CSSA along Cibolo Creek and within the central and southwest portions of CSSA. As such, principal recharge into the middle Trinity aquifer is via precipitation infiltration at outcrops. At CSSA, the Bexar Shale is interpreted as a confining layer, except where it is fractured and faulted, therefore allowing vertical flow from the up-dip Cow Creek Limestone into the overlying, but down-dip LGR. Fractures and faults within the Bexar Shale allow hydraulic communication between the LGR and Cow Creek Limestone. Groundwater flow within the middle Trinity aquifer is toward the south and southeast, and the average transmissivity coefficient is 1,700 gpd/ft, although localized differences may occur. Fracture systems associated with the fault zones are thought to affect groundwater flow and to be the controlling structural feature for migration of contaminants in the vadose zone at CSSA (Background Information Report, Volume 1-1 Environmental Encyclopedia).
2.3 - Summary of Previous Investigations
In April 2000 two surface soil samples, AOC65-Pit 1 and AOC65-Pit 2, were collected from soils beneath the southern pit. In addition, one soil boring, SB01, was drilled approximately 50 feet west of AOC65 at the terminus of a drain line that exits the building in the vicinity of the former southern vat. These initial samples were collected to characterize the site and determine the readiness of AOC 65 for closure under TNRCC RRS1 rules. Results from this initial investigation confirmed that VOC and metals concentrations exceeded CSSA background levels.
A more comprehensive investigation, including a soil gas survey, installation of 14 soil borings (AOC65-SB2 through AOC65-SB10 and AOC65-MW01 through AOC65-MW05), and installation of five shallow monitoring wells (AOC65-MW01, AOC65-MW02 A-B, AOC65-MW03, AOC65-MW04 and AOC65-MW05) was conducted from January through April 2001 to further assess the site. Results of this investigation determined that soil below and to the west and south of Building 90 was impacted by PCE. In addition, a smaller area of TCE contamination was delineated near the southern pit in Building 90 coincident with the highest PCE concentrations recorded at the site.
In April 2001, groundwater samples were collected from three monitoring wells (AOC65-MW01, AOC65-MW02 A, and AOC65-MW04) and one soil boring (AOC65-SB6). Monitoring wells AOC65-MW02 B, AOC65-MW03, and AOC65-MW05 were dry at the time of sample collection and therefore, groundwater samples were not obtained.
Groundwater samples collected in April 2001 were analyzed for metals and VOCs. Metals results indicate that nickel, zinc, and lead exceed RLs in groundwater collected from AOC65-MW01, and barium exceeds the MCL in groundwater collected from AOC65-MW04. VOC results indicate that PCE exceeds the RL in all four samples collected, even in AOC65-SB6, which is outside the confines of the PCE soil plume. PCE concentrations ranged from 7.8 ppb (AOC 65-MW-04) to 950 ppb (AOC 65-MW2A). In addition, TCE and cis-1,2-DCE exceeded RLs in AOC65-MW01 and AOC65-MW02 A. Subsequent sampling events conducted at these shallow monitoring wells (August and November 2001) confirmed the previous findings, with PCE levels ranging from 9.2 to 3,400 ppb.
Detailed discussions of previous investigations conducted at Building 90 and AOC 65 are included in Volume 1-3, AOC 65 of the Environmental Encyclopedia, the Draft Soil Gas Survey Technical Report, Parsons, August 2001, and the Draft AOC65 RFI Report, Parsons, February 2002.