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July 1999

CSSA supplies water to its facility from several on-base wells. Monitoring of these water supply wells for VOCs was initiated in 1991 when regulations first required it. Prior to that date, only monitoring for bacteriological constituents was conducted. Prior to 1991, the CSSA water supply wells included Wells 1, 9, 10, 11, and 16. Wells 2, 3, and 4 were also formerly used for water supply. Wells D, G, H, and I are agricultural water supply wells. TCE, PCE, and DCE were found at concentrations above drinking water standards during routine testing of groundwater at well 16 in August 1991. During additional testing later that year, DCE above the MCL was also detected in well D, which is located approximately 100 meters west of Well 16.

In 1992, an on base water well inventory, geologic survey, and downhole camera survey were conducted. Further groundwater sampling and analyses confirmed contamination was still present above MCLs in wells 16 and D, and that conditions were such that contaminants could be transported through fractures in limestone to the open borehole wells. In 1995, a second phase of the investigation was conducted to identify potential source areas for groundwater contamination. Source characterization actions included surface geophysical surveys at potential waste disposal/site source areas and continued surveys of open pasture areas and/or abandoned wells within a 1-mile radius around well 16. Survey results showed twenty-six anomalies potentially related to waste management activities. Drilling of shallow soil borings and collection of soil samples and grab water samples at potential source areas indicated chlorinated hydrocarbons or metals at SWMUs B-3, O-1, and B-19 above site background levels. Results of subsequent soil gas surveys performed at potential source areas and in open areas around well 16 strongly indicated two sources of chlorinated hydrocarbons - SWMUs B-3 and O-1. The sites are about 400 to 1,200 feet downgradient of well 16. TCE soil gas concentrations in these units were detected up to 10,000 ug/l, whereas other potential sites and open areas (pastures) located up and downgradient of well 16 were typically three to four orders of magnitude less at 0.1 to 10 ug/l. The soil gas survey results also strongly indicated fracturing or faulting within the area around well 16. Results of additional characterization of these sites are described under the SWMU B-3 and SWMU O-1 headings in this document. Pilot-scale treatability studies have been conducted at each of these sites: electrokinetics at O-1 and soil vapor extraction at B-3.

Evaluation of the local hydrogeologic system has been on-going since November 1992. Groundwater monitoring of the middle Trinity aquifer has showed a typical flow direction to the south-southeast with occasional changes to the south-southwest during decreasing water level conditions. Groundwater gradients typically range from 0.003 to 0.1 foot per foot. Cumulative monitoring results show that the aquifer water level responds rapidly to percolation from rain events, and that chlorinated hydrocarbon concentrations in affected wells typically increase after rain events. Since the initial TCE discovery, a total of 10 groundwater sampling events have occurred at CSSA. These events have occurred in November 1992, May 1994, December 1994, February 1995, April 1995, June 1995, December 1995, January 1997, October 1997, and November 1998. Summaries of these sampling events can be found in Volume 5, of the Camp Stanley Environmental Encyclopedia.

Other past hydrogeologic investigation activities included geophysical downhole well logging, installation of surface casing in seven wells, and plugging of abandoning of four wells. After source characterization results indicated fractures or faults in the vicinity of well 16, intensive field mapping found three displacements of key stratigraphic beds. Ground penetrating radar profiling, used to better delineate subsurface conditions, detected multiple offsets or other subsurface disturbances in the north inner cantonment, in the vicinity of well 16. A seismic reflection survey conducted from well 16 towards the southern boundary of CSSA indicated three areas of offset in the Bexar Shale. The Bexar Shale consists of approximately 80 to 150 feet of silty dolomite, marl, calcareous shale, and shaly limestone. These results and other investigative data showed that two northeast-southwest fault zones exist at CSSA, one in the area of well 16, and one along the southern CSSA boundary. Displacement varies from 1 to 30 feet, typically with downdropped blocks to the southeast, and near vertical fault planes.

In 1996, two Lower Glen Rose monitoring wells were installed southwest and southeast of well 16. Use of data from drill cuttings, core samples, and gamma logging of pilot holes allowed more detailed geologic descriptions of the area. The core samples confirmed that Lower Glen Rose limestone is massive to fossiliferous, has fractures and occasional zones of perched moisture (vadose zone) above the saturated portion of the Aquifer. It is likely that these perched zones contain water for some indeterminate period of time after rain events. In addition, logging of the core samples showed that the top 20 to 30 feet of the Bexar Shale is more characteristic of limestone than shale, despite the increase in gamma counts observed in geophysical logs (an increase which is typically associated with shale beds in this stratigraphy). Water level measurements after installation of the new wells showed that even though the overall flow direction was to the southeast, the area between well 16 and wells immediately downgradient had an anomalous easterly flow direction. Sampling and analysis of groundwater samples from the two wells showed levels of PCE only slightly above MCLs and lower than concentrations in wells 16 and D. These findings suggest that the contaminants migrate along complex subsurface fracture pathways associated with the northern fault zone at CSSA. Significantly, PCE concentrations have not been detected above MCLs in wells west and southwest of CSSA wells 16 and D. In 1997, CSSA voluntarily had offsite three private wells sampled and analyzed for volatile organics and metals. The locations of these wells are shown in the Groundwater Investigation and Associated Source Characterizations Report (Parsons ES, June 1996). .It was found that the offsite wells did not contain target analytes above MCLs. A second round of off-site well sampling is planned under the RL74 project, summer/fall timeframe, 1999.

Future plans regarding groundwater at CSSA include continued monitoring, installation of additional monitoring wells, and development of a site-specific groundwater model. All of these activities will be initiated in late 1999.