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AOC 55 RCRA Facility Investigation Report
Section 1- Introduction
1.1 - Background and Site Description
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.
AOC 55 was used as a landfill for on-post disposal of waste material prior to 1950. The site is approximately 3.6 acres in size and contains approximately 34,700 cubic yards of waste material. The site was initially identified by the presence of waste exposed during a flooding event in July 2002.
Background information regarding the location, size, and known historical use of AOC 55 is also included in the Environmental Encyclopedia (Volume 1-3, AOC 55). Volume 1-3 also includes a Chronology of Actions and a Site-Specific Work Plan for AOC 55.
1.1.2.2 Potential Sources of Contamination
AOC-55 consists of potential contaminated source areas associated with land disposal of non-hazardous waste. The site was believed to have been used for on-post disposal of waste material prior to 1950. These areas have been identified by the presence of trash exposed along Salado Creek during a previous (July 2002) flooding event. Initial site visits in 2002 indicate that the waste consists of metal debris, and ash material. Therefore, the COCs potentially associated with AOC 55 are: VOCs, SVOCs, and metals.
The AOC-55 disposal area is located south of Tenberg Drive, bisected by Salado Creek and approximately 3.6 acres in size. Initial site surveys indicate that the waste consisted of ash material and a minor amount of metal debris that appeared to have been generated from an incinerator operation. The landfill also contained bottles that appeared to be waste from the on-base cafeteria. The time of the landfill operation is unknown, but is thought to be between 1900 till 1950 based on aerial photos and on dates found on bottles and plates from the disposal area.
AOC 55 is centrally located within CSSA, in the northeastern portion of the Inner Cantonment. The site is defined on the northern edge by Tenberg Drive and on the eastern edge, mostly by Salado Creek (Figure AOC 55-1). The AOC is roughly rectangular, approximately 480 feet north-south and 250 feet east west. The site boundaries were expanded during the preliminary excavations presented in this report to include a small area east of Salado creek. This area is approximately 100 by 100 feet and was included due to the discovery of waste material similar to that uncovered within the original AOC 55 boundaries.
RMU 3 and AOC 61 are located approximately 250 feet east-northeast and approximately 680 feet east of AOC 55, respectively. AOC 42 and 58 are located just within a ¼ mile radius of AOC 55. They are both situated north of AOC 55.
1.2 - Site Environmental Setting
1.2.1 Site Soils and Topography
The Trinity and Frio soils are native to AOC 55 (Figure AOC 55-2). Detailed descriptions of the CSSA soil types are given in the CSSA Environmental Encyclopedia (Volume 1-1, Background Information Report). The Trinity and Frio soils frequently subjected to flooding and are the main channel soils for Salado Creek and a large tributary that joins the creek in southwestern CSSA. Some areas are subject to thin sediment depositions, while other areas are scoured. Trinity soils are 3 to 5 feet deep and composed of clayey to gravely loam. Frio soils are a dark grayish-brown clay loam, 3 to 4 feet deep. Vegetation associated with this soil type may consist of elm, hackberry, oak, mesquite, and other thorny shrubs, Texas wintergrass, Johnson grass, buffalo grass, Bermudagrass, and annual weeds.
AOC 55 lies at an elevation of approximately 1,220 feet above sea level, sloping gently to the southeast. Natural surface run-off would be expected to flow eastward toward the nearby Salado Creek (Figure AOC 55-2).
The Upper Glen Rose is the uppermost geologic strata in the area of AOC 55 (Figure AOC55-3). 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 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 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 consist of silty dolomite, marl, calcareous shale, and shaley limestone. The geologic strata dip approximately 10 to 12 degrees to the south-southeast at CSSA.
There are two major fault (shatter) zones at CSSA: the North Fault Zone and the South Fault Zone. AOC 55 lies just south of the Northern Fault Zone (Figure AOC55‑3). Additional information on structural geology at CSSA can be found in the Environmental Encyclopedia (Volume 1-1, Background Information Report).
At CSSA, the uppermost hydrologic layer 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 that static well levels are discordant in adjacent wells completed in different beds. Principal development of solution channels is limited to evaporite layers in the Upper Glen Rose Limestone. In general, ground water at CSSA flows in a north to south direction. However, local flow gradient may vary depending on rainfall, recharge and possibly well pumping.
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 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 (Volume 1-1, Background Information Report).
Zones at CSSA (Figure AOC 55-3). 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 (Volume 1-1, Background Information Report).
No site-specific information regarding groundwater is available. However, the nearest well, CS-MW4-LGR, which is a monitoring well, is located approximately 250 feet southeast of AOC 55, at approximately the same elevation as AOC 55 (Figure AOC 55-2). The most recent (May 2003) groundwater level reading for CS-MW4-LGR was at 67 feet below ground surface, with a groundwater flowing south-southwest in the area around AOC 55. The May 2003 sampling of CS-MW4-LGR revealed only barium, nickel, and zinc in concentrations slightly above MCLs Further information about CS-MW4-LGR may be found in Volume 5 of the Environmental Encyclopedia, which compiles the Quarterly Monitoring Reports for CSSA.
Well CS-MW5-LGR is located just over ¼ mile from AOC 55 but, because it is situated at a significantly higher elevation, it is not expected to provide helpful groundwater information for AOC 55.
The nearest surface water body is the Salado Creek, which is immediately adjacent to AOC 55 on the eastern boundary of the site (Figure AOC55-1). This creek exits CSSA south of AOC 55.
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. Several building, which are contained in the H&I sections of the Inner Cantonment, are located within a ¼ mile radius of AOC 55.
A land use survey discussing local and possible future uses of groundwater and surface water, a water well survey, and sensitive environmental areas survey at CSSA were completed on 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 constiuent pathways is presented in Section 3 of the Technical Approach Document for Risk Evaluation (Volume 1-6).
AOC 55 is located in the center of CSSA, in the northeast corner of the Inner Cantonment. On-post well CS-MW4-LGR is located about 250 feet southeast of AOC 55.
The nearest potential habitats for local endangered species (Volume 1-1, Background Information Report) are approximately 2,100 feet east-southeast (Golden-Cheeked Warbler) and 7,800 feet northeast (Black-Capped Vireo).
A small herd of cattle is maintained on CSSA by the US Department of Agriculture’s (USDA)-Agricultural Research Center (ARC). The cattle roam freely throughout the Inner Cantonment and in selected areas of the North and East Pastures. 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 the Technical Approach Document for Risk Evaluation (Volume 1-6). 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. None are located within ¼ mile of AOC 55.
AOC 55 was selected for soil gas testing to determine whether VOCs may be associated with past landfill use. A detailed description of the methodology is included in the Environmental Encyclopedia behind the Soil Gas Surveys tab in Volume 3-1.1. The work at AOC 55 was performed with soil gas surveys at SWMU B-3, SWMU B-4, AOC 57, AOC 63, AOC 65 and the WWTP from January 2, 2001 through February 23, 2001. Sampling depths varied from 1 foot to 8 feet below grade and were usually determined by refusal. A total of 54 points were sampled on a grid layout that extended beyond the estimated lateral extent of the AOC. Grid spacing was approximately 100 feet on the north side of the road and 50 feet on the south side of the road for the 54 grid points.
Minor amounts of benzene, toluene, and xylene were detected in soil gas samples throughout the soil gas survey, but these detections are disregarded as artifact contamination for closure considerations. The BTEX compounds are attributed to the gas generator used to power the vacuum pump and/or to the combustion engine of the geoprobe truck. The frequency of detectable BTEX compounds decreased substantially after the gas generator was moved away from the sampling area, and after the rig was shut down during sample collection.
PCE was detected in three sample locations (AOC 55-17, AOC55-18, and AOC 55-27) at concentrations ranging from 0.08-0.11μg/L. Upon review of the data, it was discovered that high levels of PCE were detected in SWMU B-3 samples that were analyzed immediately prior to these three samples. Based on the lack of PCE in all other sample locations at AOC 55, cross-contamination during analytical procedures is the likely source of the detectable PCE. Because data review occurred after completing the field work, there was not an opportunity to verify that PCE does not actually exists in the AOC 55 soil gas.
From the soil gas survey results, it appears that no significant VOC contamination has been released to, or is present in, the soil and groundwater underlying AOC 55. No additional investigation activities are proposed for AOC 55 with regard to VOC soil gas contamination.
As described in Section 4.3 of the Risk Assessment Technical Approach Document (Volume 1-6), CSSA has opted to pursue closure of AOC 55 under the Risk Reduction Rule (RRR) (30 TAC §335 subchapter S). If the concentrations of PCOCs at the site do not exceed background concentrations appropriate to the sample media, then the site will be closed using RRS1. If samples collected from the site do exceed background concentrations, 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 then be sought. A notification of intent to close sites identified to date (including AOC 55) in accordance with the former RRR was sent to the TNRCC on July 12, 1999. Acceptance of this notification was received on October 5, 1999.
RRS1 requires the site be closed following removal or decontamination of waste, waste residues, and contaminated operation system components and following the demonstration of attainment of cleanup levels (30 TAC §335.554). If closure requirements under RRS1 are attained and approved of by the TNRCC Executive Director, then the owner is released from the deed recordation requirement.
Since the COCs for AOC 55 include metals, the cleanup levels will be background levels established for CSSA soil and Glenrose Limestone.