For this report, Section 1 provides the site-specific background and target closure standard. Section 2 describes field actions and the closure evaluation. Section 3 summarizes the findings, evaluates attainment of data quality objectives, provides recommendations, and certifies the site closure, if applicable. References cited in this report can be found in the Bibliography (Volume 1-1 of the Environmental Encyclopedia).

1.1 - Background and Site Description

1.1.1   CSSA

General information regarding the history and environmental setting of CSSA is provided in the CSSA Environmental Encyclopedia (Volume 1-1, Background Information Report). In that report, data regarding the geology, hydrology, and physiography are also available for reference.

1.1.2   AOC-43

1.1.2.1   Site Description

AOC-43 consists of approximately 0.6 acres of undeveloped open area with few trees and a sparse cover of native grasses and shrubs. The site is surrounded on the north, east, and south sides by a thicket of oak trees. AOC-43 is bordered on the west side by an unnamed, unpaved access road (Figure AOC43-1). The site was located during a field investigation conducted for Solid Waste Management Unit (SWMU) B-7 in February 1996. Three-inch mortar pieces were observed at the site and removed by UXB, Inc. specialists. An old stone cistern is located along the southern boundary of the AOC. Prior uses of AOC-43 are unknown and no activities are currently practiced there. 

Background information regarding the location, size, and known historical use of AOC-43 is also included in the Environmental Encyclopedia (Volume 1-3, AOC-43). Volume 1-3 includes a Chronology of Actions and a Site-Specific Work Plan for AOC-43.

1.1.2.2   Potential Sources of Contamination

Field investigation indicated three-inch mortar pieces were present at AOC-43. Based on past site use and the present of mortar debris, potential sources of contamination are explosives and metals. 

1.1.2.3   Site Location

AOC-43 is located approximately 1,200 feet north of the inner cantonment in the central portion of the North Pasture. SWMU B-7 is located 50 feet northwest of the site and SWMU B-2 is located approximately 500 feet south. 

1.2 - Site Environmental Setting

1.2.1   Site Soils and Topography

AOC-43 rests entirely on gently undulating Tarrant Association soils. Tarrant soils occur along the outer edges of the Salado Creek floodplain. The soils are shallow and form over hard, fractured limestone. The surface layer is usually about 10 inches thick and consists of a dark grayish-brown, calcareous, clay loam with scattered gravel cobblestones, and flagstones. Gently undulating Tarrant Association soils occur as nearly level and gently sloping areas of typical prairie and plateau topography. It occurs primarily in areas not occupied by streams, such as the north central area of the inner cantonment and the hills north of the inner cantonment. This soil type covers approximately 14.3 percent (572.6 acres) of CSSA. The soils are dark colored, very shallow, calcareous, clayey, and are best suited for native grasses and range use. 

AOC-43 exhibits a thin, 1- to 2-inch, soil profile with scattered outcrops of the limestone. While no descriptions of subsurface conditions have been gathered at this site, it is reasonable to assume that limestone bedrock exists just below the thin soil profile. Detailed descriptions of the CSSA soil types are provided in the Environmental Encyclopedia (Volume 1-1, Background Information Report, Soils and Geology and in Volume 2, Background Metals Levels, Section 2 of the Revised Evaluation of Background Metals Concentrations in Soil Types, Description of Soil Types and Bedrock). 

The site surface dips gently to the west and is sparsely vegetated with native grasses and shrubs (Figure AOC43-2). AOC-43 is at an average elevation of approximately 1,285 feet above sea level (asl). The site slopes from an elevation of 1,290 feet asl in the east to approximately 1,280 feet asl on the western boundary. 

1.2.2   Geology

The Upper Glen Rose is the uppermost geologic stratum in the area of AOC-43 (Figure AOC43-3). 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 where soils are poorly developed or eroded. Where present at CSSA, the Upper Glen Rose may be up to 150 feet thick. It is underlain by the Lower Glen Rose, which is estimated to be 300 feet thick beneath CSSA. The Lower Glen Rose is a massive, fossiliferous, vuggy limestone that grades upwards into thin beds of limestone, marl, and shale. The Lower Glen Rose 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.

Based on current published information, there are two known major fault zones at CSSA: the North Fault Zone and the South Fault Zone. AOC-43 is located 1,600 feet north of the North Fault Zone (Figure AOC43-3). Additional information on structural geology at CSSA can be found in the Environmental Encyclopedia (Volume 1-1, Background Information Report, Soils and Geology).

1.2.3   Hydrology

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 different static water levels in adjacent wells completed at approximately the same elevation. 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 along major drainage ways located in 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, 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 gallons per day per feet (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. 

No site-specific information regarding groundwater is available. However, the nearest well is Well CS-G, which is a livestock well, located 2,500 feet northwest of AOC-43. Static water levels measured from October 1992 through March 2000 have ranged from 316.65 feet below top of casing (BTOC) (January 1997) to 246.14 feet BTOC (October 1997) in Well CS-G (Volume 5, Introduction to the Groundwater Monitoring Program, Table 3).

The nearest surface water body is an unnamed, intermittently flowing stream located approximately 1,000 feet west of AOC-43. The intermittent stream joins Salado Creek approximately 3,000 feet south of AOC-43 (Figure AOC43-1). Salado Creek is also intermittent, and continues southeast past the CSSA boundary and eventually joins the San Antonio River.

1.2.4   Cultural 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. The nearest cultural resource site is a prehistoric open campsite exhibiting a small burned rock midden that is located approximately 2,300 feet north of the site.

1.2.5   Potential Receptors

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 on December 15-16, 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 is presented in Section 3 of the Technical Approach Document for Risk Evaluation (Volume 1-6).

The United States Department of Agricultural, Agricultural Research Center (USDA-ARC) maintains a small herd of cattle on CSSA. The cattle roam freely throughout the Inner Cantonment and in selected areas of the Inner Cantonment and North Pasture. CSSA also manages wild game species for the purpose of hunting. White-tailed deer, axis deer, and wild turkey all roam freely throughout CSSA. A map of deer hunting stands which overlook mechanical feeders and planted food plots is located in Figure 5.2 of the Technical Approach Document for Risk Evaluation (Volume 1-6). AOC-43 is located approximately 100 feet west of hunting stand number 35. Four water tanks are maintained at CSSA for the purpose of sport fishing. Two of the tanks are located in the north western and north eastern portions of the North Pasture while the other two tanks are located near the western boundary of the Inner Cantonment. 

AOC-43 is located in potential habitat for one of the local endangered species, the Black-Capped Vireo. Potential habitat for another local endangered species, the Golden-Cheeked Warbler, is located approximately 2,000 feet northeast (Volume 1-1, Background Information Report, Figure 11).

1.3 - Previous Investigations

AOC-43 was identified during a field investigation conducted at nearby SWMU B-7 in February 1996. Initially designated “SWMU B-7A,” AOC-43 was identified as a trench located south of SWMU B-7 and miscellaneous munitions and mortar pieces were found on the ground surface. No other previous investigations were performed at AOC-43. 

1.4 - Closure Standard

As described in Section 4.3 of the Risk Assessment Technical Approach Document (Volume 1-6), CSSA has opted to pursue closure of AOC-43 under the Risk Reduction Rule (RRR) (30 TAC §335). If the site concentrations do not exceed background, then the site will be closed using Risk Reduction Standard 1 (RRS1). If the site exceeds background, then a determination will be made regarding the feasibility of cleaning the site to meet background concentrations. If the decision is made to clean the site to background, closure under RRS1 will be sought. However, if it is determined that the site cannot be closed to meet background concentrations, then the site will be closed under Texas Risk Reduction Program (TRRP). A notification of intent to close sites identified to date (including AOC-43) in accordance with the former RRR was sent to the TCEQ on July 12, 1999. TCEQ acceptance of this notification was received on October 5, 1999.

RRS1 requires that the site is closed following removal or decontamination of waste, waste residues, and contaminated operation system components; and demonstration of attainment of cleanup levels (30 TAC §335.554). If closure requirements under RRS1 are attained and approved by the TCEQ Executive Director, then the owner is released from the deed recordation requirement.

Since the chemical of concerns (COCs) for AOC-43 are metals and explosives, the cleanup levels should be the reporting limits (RLs) for the explosives. For metals, background levels were statistically calculated for each soil type found at CSSA and the Glen Rose Limestone, and are reported in the Second Revision to the Evaluation of Background Metals Concentrations in Soils and Bedrock at CSSA (Parsons, February 2002). TCEQ approved the background metals evaluation on April 23, 2002.

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