For this report, Section 1 provides the site-specific background and 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 CSSA Environmental Encyclopedia (Volume 1-1, Bibliography). 

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 36

1.1.2.1   Site Description

AOC 36 is a site that formerly contained 155mm shells filled with mustard gas that were detonated, partially decontaminated, and buried in 1942. According to US Army records, the shells were located, decontaminated, removed, and destroyed in 1958 by the US Army Chemical Corps Technical Escort Unit. Decontamination of possible residual mustard gas was conducted using calcium hypochlorite. A certificate stating that all 155mm mustard filled rounds and resultant scrap metal buried in 1942 were recovered, destroyed, and decontaminated was issued by the Technical Escort Unit Commanding Officer on June 4, 1958. In 1995, a soil gas survey and an EM geophysical survey were conducted to identify potential sources of groundwater contamination found at Well CS-16. At that time, the site was known as “Southeast Well 16.”

AOC 36 is a 0.6-acre tract that is located near the approximate center of CSSA, in the northernmost portion of the inner cantonment area (Volume 1-3, Site Location Map). The general shape of the site is 1,000 feet long (north-south) by 300 feet wide (east-west). AOC 36 is approximately 4,200 feet from the nearest facility boundary to the west. The site is immediately south of Well CS-16 at the intersection of North Outer Drive and Moyer Road. The physical boundaries of AOC 36 are identified by North Outer Drive to the north, Moyer Road to the west, SWMU B-28 to the south, and SWMUs B-3 and B-10 to the east (Figure AOC36-1, Site Location Map Using 1998 Aerial Photograph). There is no development upon AOC 36 except for overhead utilities and roadside drainage control. AOC 36 is utilized for cattle grazing. There are currently no other activities practiced at AOC 36.

There is a small drainage ditch oriented northwest-southeast along which exposed metal piping was observed during 1995 investigations at the site. The drainage ditch is located from Moyer Road from the northwest to the boundary with SWMU B-3 to the southeast. Further information regarding the drainage ditch and exposed metal piping is included in Section 10, Technical Memorandum on Surface Geophysical Surveys, June 1995. The underground piping trends northwest-southeast and originates at SWMU B-3 where it is aboveground. It is possible that the piping is an abandoned water line, supplying water to SWMU B-3 in the past, but this is unverified. Also present at the site in 1995 was an area of reinforced concrete rubble and some scrap wire and metal on the ground surface as well as a telephone pole along Moyer Road. (Volume 3-1, Geophysical Surveys, Section 10, Figures 10.2 and 10.3) 

Background information regarding the location, size, and known historical use of AOC 36 is included in the Environmental Encyclopedia (Volume 1-3, Areas of Concern). Volume 1-3 also includes a Chronology of Actions and a Site-Specific Work Plan for AOC 36.

1.1.2.2   Potential Sources of Contamination

AOC 36 was reportedly used as a mustard gas disposal site. Former activities at AOC 36 included the disposal of mustard gas shells in 1942 and their subsequent removal and site clearance by Army EOD in 1958. AOC 36 was closed by TNRCC in 1999 with regard to mustard gas canisters. However, a soil gas survey conducted in 1995 detected chlorinated VOCs (as described in Section 8, Technical Memorandum on Soil Gas Surveys, Parsons ES, 1996). The compounds detected are the same COCs which resulted in the decommissioning of the Well CS-16 as a water supply well. The COCs for AOC 36 are chlorinated VOCs related to Well CS-16.

1.1.2.3   Site Location

AOC 36 is located near the approximate center of CSSA, in the northernmost portion of the inner cantonment area. AOC 36 is approximately 4,200 feet from the nearest facility boundary to the west. The site is immediately south of Well CS-16 at the intersection of North Outer Drive and Moyer Road. The boundaries of AOC 36 are identified by North Outer Drive to the north, Moyer Road to the west, SWMU B-28 to the south, and SWMUs B-3 and B-10 to the east (Figure AOC36-1). 

1.2 - Site Environmental Setting

1.2.1   Site Soils and Topography

The soils at AOC 36 are classified Krum complex and Tarrant association soils (Figure AOC36-2, Soils and Topographic Map). Krum Complex soils cover approximately 20 percent of the land at CSSA and occur on slopes of two to five percent, occupying ‘foot’ slopes below Brackett and Tarrant soils. At AOC 36, Krum complex soils cover approximately 65 percent of the site. Generally, Krum Complex soils occur downslope of prairie plains. The Krum Complex soils receive sediments and runoff from higher elevation soils and are highly prone to water erosion if unprotected. Krum Complex and Tarrant association soils are generally well-drained with moderately slow permeability. Krum Complex soils are typically dark grayish-brown, calcareous clay dominant soils. The soils are typically calcareous clay, ranging in depth from 15 to 60 inches and are underlain by limestone.

Approximately 35 percent of AOC 36 is covered by Tarrant association soils. Tarrant association soils cover approximately 14 percent of the land at CSSA and cover areas not occupied by streams, such as the north-central area of the inner cantonment, as well as the west sides of Steele and Wells Hills and on the hills north of the inner cantonment. The soils are dark colored, very shallow, calcareous, and clayey, and are best suited for native grasses and range use. Both Krum Complex and Tarrant association soils are generally well drained with moderately slow permeability. 

Detailed descriptions of the CSSA soil types are given elsewhere in the CSSA Environmental Encyclopedia (Volume 1-1, Background Information Report, Soils and Geology). The elevation of the ground surface at AOC 36 is between 1,220 and 1,240 feet above sea level, with the surface sloping toward the south. The site vegetation consists primarily of oaks and native grasses.

1.2.2   Geology

The Upper Glen Rose is the uppermost geologic stratum in the area of AOC 36 (Figure AOC36-3). 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 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.

Based on published information, there are two known major fault (shatter) zones at CSSA: the North Fault Zone and the South Fault Zone. AOC 36 lies on the northern edge of the North Fault Zone (Figure AOC36-3). Numerous faults with small displacements have been located near AOC 36 using mapping, seismic, and GPR techniques. Vertical displacement between 5 to 30 feet have been reported (Soils and Geology, Background Information Report, Volume 1-1), where the downthrown blocks typically occur on the southeast side of the fault. At least two of these faults have been mapped within AOC 36. Additional information on structural geology at CSSA can be found in the Environmental Encyclopedia (Soils and Geology, Background Information Report, Volume 1-1).

No subsurface soil samples were collected at AOC 36; therefore, there are no soil boring logs to document the depth and/or thickness of the Upper Glen Rose as it occurs below AOC 36. However, the presence of outcrops of the Upper Glen Rose at higher elevations near AOC 36 and at other nearby waste management units including SWMUs O-1, B-3, B-4, B-10, B-19, and Building 43 verify that AOC 36 overlies the Upper Glen Rose. 

1.2.3   Hydrology

At CSSA, the uppermost hydrogeologic 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. Principle development of solution channels is limited to evaporite layers in the Upper Glen Rose Limestone. Groundwater discharge occurs predominantly via natural springs, seeps, and pumping; however, no such discharges have been observed at AOC 36. 

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. 

AOC 36 is located within the North Fault Zone at CSSA. 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).

No site-specific information regarding groundwater is available. However, the nearest well (CS-16), which is an inactive drinking water well, is located approximately 75 feet north of the northern boundary of AOC 36. Well CS-D is located approximately 400 feet to the west-northwest of the northwest corner of AOC 36. Monitoring well CS-MW1 is located approximately 1,000 feet to the southwest of the southern boundary of AOC 36. Monitoring well CS-MW2 is located approximately 1,000 feet to the southeast of the southern boundary of AOC 36. Water levels in CS-MW1 and CS-MW2 have ranged from 240.9 to 247.9 feet BTOC and 253.9 to 264.6 feet BTOC, respectively. 

The nearest surface water body is an ephemeral stream that traverses AOC 36 across the northwestern site corner and flows generally southwest across AOC 36. The ephemeral stream flows across AOC 41, AOC 37 and AOC 35 and then enters AOC 36 across the northern boundary. When flowing, the stream flows south and then southwest, exits AOC 36 and enters AOC 38. The stream is a tributary to Salado Creek, which it joins on AOC 38. No discharge of groundwater to surface seeps or springs at AOC 36 was observed during field investigations. 

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. There are no structures or buildings within the boundaries of AOC 36. The nearest potentially significant cultural resource is Building 43, located 175 feet east of AOC 36. Former cantonments were also located within 3,000 feet of the site to the west and south. 

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 a sensitive environmental area survey were completed for the CSSA facility on December 15 and 16, 1999. The results of this survey, along with results from a more in-depth survey designed 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 use within ¼-mile of AOC 36 is classified as “Evergreen Forest Land” in Figure 1.1 of the Technical Approach Document for Risk Assessment. There are no buildings within ¼ mile of the site, but there are two inactive water supply wells within this radius. Well CS-D is a former agricultural supply well, and CS-16 is a former drinking water well. Both of these wells are currently included in CSSA’s groundwater monitoring program. The closest residential area is approximately ¾ mile to the west, and the nearest school is over one mile to the northwest. Monitor well CS-MW1 is located approximately 1,000 feet to the southwest of the southern boundary of AOC 36. Monitor well CS-MW2 is located approximately 1,000 feet to the southeast of the southern boundary of AOC 36. Water levels in CS-MW1 and CS-MW2 range from 240.9 to 247.9 and 253.9 to 264.6 feet below top of casing, respectively. A small herd of cattle is maintained on CSSA by the USDA-ARC. The cattle roam freely throughout the Inner Cantonment and in selected areas of the 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. AOC 36 is located 750 feet west of hunting stand number 2. Hunting stand number 3 is located 2,000 feet south of the southern boundary of AOC 36. A map of deer hunting stands that overlook mechanical feeders and planted food plots is located in Figure 5.2 of the Technical Approach Document for Risk Evaluation (Volume 1-6). Four water reservoirs are maintained at CSSA for the purpose of sport fishing. Two of the reservoirs are located in the northwestern and northeastern portions of the North Pasture while the other two reservoirs are located near the western boundary of the Inner Cantonment. No water reservoirs are within ¼ mile of AOC 36 and none receive surface water discharge from AOC 36. 

1.3 - Previous Investigations

In 1995, due to the proximity of the northern boundary of the area now designated AOC 36 to Well CS-16, an investigation was performed in an effort to identify potential source areas for contamination at Well CS-16. The “Southeast Well 16” survey in spring 1995 at AOC 36 included soil gas sampling and an EM geophysical survey. 

1.3.1   Geophysical Survey

An EM-31 geophysical survey was conducted at the site in March 1995. Grid points were staked at 50-foot intervals with data transects conducted at 25-foot intervals in both north-south and east-west directions. Both apparent conductivity and in-phase EM readings were collected at two foot intervals along each transect. A datalogger was used to collect readings which were subsequently downloaded and contoured using Surfer contouring software. Dense vegetation in the area prevented full coverage of the EM survey at the area called “Southeast Well 16.”

Three anomalies were identified at the site, but all were associated with surface features. One anomaly was caused by buried metal piping which crosses the north portion of the site, another was caused by the telephone pole, and the third by steel-reinforced concrete rubble on the west side of the site.

A GPR survey was also performed using the GSSI SIR-3 system. The GPR survey was conducted at 50-foot intervals in the north-south orientation. No anomalies were detected except for a small hyperbolic signal in response to the underground metal pipe mentioned above. Additional information regarding the geophysical survey can be found in Section 10 of the Technical Memorandum on Surface Geophysical Surveys (Parsons ES, 1995).

1.3.2   Soil Gas Surveys

In July 1995, soil gas samples were collected at a total of 29 points. Soil gas survey sample locations are shown in Figure AOC36-4. Each sample was analyzed for BTEX compounds, total hydrocarbons, cis-1,2-DCE, TCE, and PCE. PCE was detected in every sample, at concentrations ranging from 0.02 mg/L to 28 mg/L. The maximum concentration was detected at the location identified as “Southeast Well 16-M,40,” which is situated east of AOC 36. TCE was detected in four samples at concentrations between 0.04 mg/L and 0.98 mg/L. The maximum TCE concentration was detected during the survey conducted when the site was defined as “Southeast Well 16”, at location “AA,5.” Soil gas survey point AA,5 is in the central portion of AOC 36. No other analytes were detected. Results of the July 1995 survey are listed in Table 8.1 of the Technical Memorandum on Soil Gas Surveys (Parsons ES, 1996).

In November 1995, 10 additional soil gas samples were collected in the Southeast Well 16 area and analyzed for cis-1,2-DCE, TCE, PCE, and carbon tetrachloride. These additional samples were collected to extend the survey area to the east in the area of the maximum PCE concentrations and to the south in the area of the maximum TCE concentrations. During the second survey, PCE was detected in every sample, at concentrations ranging from 0.56 mg/L to 18 mg/L. The maximum was detected at location “M,45” east of AOC 36. TCE (0.15 mg/L) and carbon tetrachloride (0.2 mg/L) were detected in only one sample, “AA,100.” Results of the November 1995 survey are provided in Table 8.2 of the Technical Memorandum on Soil Gas Surveys and a PCE soil gas plume map is provided in Figure AOC36-5.

1.3.3   Mustard Gas Investigation

In 1997, a surveying subcontractor attempted to locate the coordinates of the former mustard gas shell excavation as given by the Technical Escort Unit in 1958. The survey identified the location in the proximity of SWMU B-28, however, there were concerns regarding the accuracy of the original 1958 coordinate system. Two former employees were interviewed during a site visit that placed the excavation site 150 feet north of surveyed area, which located the disposal site in the southern portion of AOC 36. As stated in Section 1.1.2.1, the mustard gas shells were disposed at AOC 36 in 1942 and subsequently excavated and destroyed in 1958. At that time, the Army Chemical Corps Technical Escort Unit certified that the site was clear and decontaminated of mustard gas contamination. In February 1997, a magnetometer sweep was completed of the surveyed areas. Areas with noted anomalies were excavated and sifted to determine if the anomaly was due to buried UXO or mustard gas shells. Metal items determined to be free of explosives were placed in roll-off containers for disposal. No UXO was discovered. AOC 36 was closed by TNRCC in 1999 with regard to mustard gas contamination. Further information regarding mustard gas shells at this site may be found in the Mustard Gas Investigation Report, located in the Environmental Encyclopedia (Volume 3-2, AOC 36). 

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 36 under the TNRCC Risk Reduction Rule (30 TAC §335). If the site concentrations do not exceed background, then the site will be closed using 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 TRRP. A notification of intent to close sites identified to date (including AOC 36) in accordance with the former RRR was sent to the TNRCC on July 12, 1999. TNRCC acceptance of this notification was received on October 5, 1999.

RRS1 requires that the site be 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 TNRCC Executive Director, then the owner is released from the deed recordation requirement. 

Since the COCs for AOC 36 are chlorinated hydrocarbons (specifically detections of the compounds PCE, TCE, and cis-1,2-DCE) the cleanup levels should be the RLs. Chlorinated VOCs are man-made chemicals and rarely occur naturally. 

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