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Soil Pile Disposition Assessment
Section 2 - Environmental Setting
This section provides a brief description of CSSA and its environmental setting. Additional information regarding the history and setting of CSSA is provided in the Environmental Encyclopedia (Volume 1-1, Background Information Report). In that report, information regarding the geology, hydrology, and physiography are also available for reference.
CSSA is located in northwestern Bexar County, approximately 19 miles northwest of downtown San Antonio in south-central Texas. The installation consists of 4,004 acres immediately east of State Highway 3351 (Ralph Fair Road), and is approximately 0.5 mile east of Interstate Highway 10 (Figure 1.1).
CSSA is a restricted-access installation with inner cantonment lands to the southwest and outer cantonment areas to the east and north. Operation buildings and storage magazines are located within the inner cantonment. Outer cantonment land is primarily undeveloped open range. The eastern boundary of CSSA and part of its northern and southern boundaries are contiguous with the Camp Bullis Military Training Reservation. The surrounding area to the west is primarily rural and zoned for residential use. Some residential development is also present west, northwest, and southwest of the installation.
CSSA is characterized by a rolling terrain of hills and valleys in which nearly flat-lying limestone formations have been faulted, eroded, and dissected by streams draining to the east and southeast. In general, soils at CSSA are thin, dark-colored, gravely clays and loams. A detailed description of the CSSA soil types are provided in the CSSA Environmental Encyclopedia (Volume 1-1, Background Information Report, Soils and Geology). A topographic map of the installation is provided as Figure 2.4.1 of the Groundwater Investigation and Associated Source Characterization Report (Parsons ES, 1996).
At SWMUs B-8, B-20/21, B-24, B-28 and the DD Area the Upper Glen Rose Formation is the uppermost geologic stratum. The Upper Glen Rose consists of beds of blue shale, limestone, and marly limestone, with occasional gypsum beds. Generally, it outcrops in stream valleys and at the ground surface. The thickness of the Upper Glen Rose is estimated to be up to 150 feet at CSSA. It is underlain by the Lower Glen Rose, which is estimated to be 300 feet thick beneath CSSA. The Lower Glen Rose is underlain by the Bexar Shale (regionally time equivalent to the Hensell sand), which is estimated to be from 60 to 150 feet thick under the CSSA area. The geologic strata dip approximately 10 to 12 degrees to the south-southeast at CSSA.
Based on current published information, there are known to be two major fault (shatter) zones at CSSA: the North Fault Zone and the South Fault Zone. SWMUs B-8, B-20/21, and B-24, are located north of both of these fault zones, SWMU B-28 is located entirely within the North Fault Zone, and SWMU DD Area is located entirely within the South Fault Zone. Additional information on structural geology at CSSA can be found in the Environmental Encyclopedia (Volume 1-1, Background Information Report, Soils and Geology).
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 thought to be enhanced along the bedding contacts between marl and limestone; however, the hydraulic conductivity between beds is thought to be poor. This interpretation is based on the observation of differing static water levels in adjacent wells completed in different beds. Principle development of solution channels is limited to evaporite layers in the Upper Glen Rose Limestone.
The Middle Trinity Aquifer is unconfined and functions as the primary source of groundwater at CSSA. It consists of the Lower Glen Rose Limestone, the Bexar Shale, and the Cow Creek Limestone. The Lower Glen Rose Limestone outcrops north of CSSA along Cibolo Creek and within the central and southwest portions of CSSA. As such, principle 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 Lower Glen Rose. Fractures and faults within the Bexar Shale may allow hydraulic communication between the Lower Glen Rose and Cow Creek Limestones. Groundwater flow within the Middle Trinity Aquifer is toward the south and southeast and the average transmissivity coefficient is 1,700 gpd/ft (Ashworth, 1983). In general, groundwater at CSSA flows in a north to south direction. However, local flow gradient may vary depending on rainfall, recharge and possibly well pumping.
Fracture systems associated with the two fault zones present at CSSA are thought to affect groundwater flow and to be the controlling structural feature for migration of contaminants in the vadose zone at CSSA (Volume 1-1, Background Information Report, Groundwater Resources).
Cultural resources are prehistoric and historic sites, structures, districts, artifacts, or any other physical evidence of human activity considered important to a culture, subculture, or community for scientific, traditional, or religious purposes. According to the Cultural Resources Survey conducted at CSSA in 1998 (Prewitt, 1998), there are a total of 34 archeological sites, consisting of 20 prehistoric components and 18 historic components. These sites are located throughout the installation.
A land use survey discussing local and possible future uses of groundwater and surface water, a water well survey, and sensitive environmental areas at CSSA was completed during December 15 and 16, 1999. The results of this survey, along with results from a more in-depth survey to identify potential receptors, points of human exposure, and possible constituent pathways are presented in Section 3 of the Technical Approach Document for Risk Evaluation (Volume 1-6). Land within 0.25 mile of SWMUs B‑8, B-20/21, B-24, and B-28 is classified as �Evergreen Forest Land� in Figure 1.1 of the Technical Approach Document for Risk Evaluation. The majority of land within 0.25 mile of SWMU DD Area is classified as �Commercial and Services.� To the south, a small portion within the 0.25 mile radius is classified as �Evergreen Forest Land.�
A small herd of cattle maintained on CSSA by the United States Department of Agriculture � Agricultural Research Center (USDA-ARC) roam freely throughout selected areas of the North Pasture. In addition, wild game species, including white-tailed deer, axis deer, and wild turkey, roam freely throughout CSSA. A map of deer hunting stands which overlook mechanical feeders and planted food plots is shown in Figure 5.2 of the Technical Approach Document for Risk Evaluation.