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Work Plan Overview |
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Site-Specific Work Plan |
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Field Sampling Plan |
(Volume 1-4, DO5068 Addendum); and |
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Health and Safety Plan |
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
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 38 consists of an open pasture area south of North Outer Drive and immediately southwest of Well CS-16, near the approximate center of CSSA (Figure AOC38-1, based on the 1998 aerial photograph), based on the 1998 aerial photograph), based on the 1998 aerial photograph). AOC 38 was included in a 1995 soil gas survey conducted to identify potential sources of groundwater contamination found at Well CS-16. At that time, the site was known as the “South Pasture.”
AOC 38 is a 35-acre tract that is located in the northernmost portion of the inner cantonment area (Volume 1-3, Site Location Map). The general shape of the site is 1,400 feet long (east-west) by 1,100 feet wide (north-south). The site is bounded by a fence line and paved road to the north and east, trees to the west, and Salado Creek to the south.
One livestock well (Well CS-4) is located in the southwest portion of AOC 38. The livestock well is inactive, but it is included in the quarterly groundwater monitoring program at CSSA. A livestock corral exists in the northeast corner of AOC 38, and the entire site is used occasionally for cattle grazing. Some agricultural equipment, including metal cones, is stored on the site, just to the west of the corral. A six-inch underground water supply line runs through the northwestern portion of the site. This water line has apparently been capped.
During the installation of a fiber optic communications line in July 2000, a trench containing expended munitions was encountered in the northwest corner of the site. Approximately one acre of AOC 38 was separated from the previously defined area of AOC 38 and designated SWMU B-71. The area of SWMU B-71 is now excluded from investigation for AOC 38. Additional information regarding investigation at SWMU B-71 is located in Volume 1-2, Solid Waste Management Units.
A previous report (Technical Memorandum on Surface Geophysical Surveys) indicated that a bermed burn area containing metal wire, wood, rock, and ashes was situated in the north-central portion of the site. However, this bermed area could not be located during field investigations in 2000. It is possible that this area was part of what is now SWMU B-71.
Additional background information regarding the location, size, and known historical use of AOC 38 is included in the Environmental Encyclopedia (Volume 1-3, AOC 38). Volume 1-3 also includes a Chronology of Actions and a Site-Specific Work Plan for AOC 38.
1.1.2.2 Potential Sources of Contamination
No formal waste management activities were known to occur at AOC 38, with the exception of the area now designated SWMU B-71. Additional investigation and remediation will be conducted separately for SWMU B-71. Due to AOC 38’s proximity to Well CS-16, a soil gas survey was conducted in 1995 that 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 38 are chlorinated VOCs related to Well CS-16.
AOC 38 is located near the approximate center of CSSA, in the northernmost portion of the inner cantonment area. AOC 38 is approximately 2,700 feet from the nearest facility boundary to the west. The site is immediately southwest of Well CS-16 at the intersection of North Outer Drive and Moyer Road. The boundaries of AOC 38 are North Outer Drive to the north, Moyer Road to the east and Salado Creek to the south (Figure AOC38-1).
1.2 - Site Environmental Setting
1.2.1 Site Soils and Topography
The soils at AOC 38 are classified as Trinity and Frio soils (located along two unnamed creeks which merge and flow toward Salado Creek), Krum Complex soils, and Crawford & Bexar stony soils (Figure AOC38-2).
The Trinity and Frio soils are the main channel soils for Salado Creek and its tributaries. The soils occur in narrow, long, and irregularly shaped areas. They are flooded at least once a year after heavy rains. Trinity and Frio soils make up approximately 60 percent of the soils at AOC 38. Trinity soils make up the majority of this soil type and are 3 to 5 feet deep, clayey to gravelly loam. Frio soils are 3 to 4 feet deep, dark grayish-brown clay loam soils. Tarrant soils are generally well drained with slow to rapid runoff depending upon the slope and the permeability is moderately slow.
Krum Complex soils occur on slopes of two to five percent, occupying ‘foot’ slopes below Brackett and Tarrant soils. At AOC 38, Krum complex soils cover approximately 30% of the site, flanking the Trinity and Friday soils occurring alongside the streambed. Generally, Krum Complex soils occur downslope of prairie plains and receive sediments from higher elevations. 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. Krum Complex soils are generally well drained with slow to rapid runoff and the permeability is moderately slow.
Approximately 10 percent of the soils occurring at AOC 38 are Crawford & Bexar stony soils. They occur as broad, nearly level to gently undulating areas with slopes of 0 to 5 percent. The Crawford soils are stony, very dark gray to dark reddish brown, noncalcareous clay, and are about 8 inches thick. Large fragments of chert and limestone occur on the surface and within the soil profile. Bexar soils range from cherty clay loam to gravelly loam. Detailed descriptions of the CSSA soil types are provided in the CSSA Environmental Encyclopedia (Volume 1-1, Background Information Report, Soils and Geology). Though no subsurface investigation has been performed for AOC 38, the existence of exposed Upper Glen Rose limestone at scoured surfaces indicates bedrock would be encountered within three feet of the land surface across AOC 38 except at sediment deposition sites along the creek channel. Crawford and Bexar soils are well drained with slow permeability when soils are saturated and rapid when soil are dry and cracked. Runoff is low on slopes less than one percent, medium on one to three percent slopes, and high on three to five percent slopes.
The elevation of the ground surface at AOC 38 is approximately 1,230 feet above sea level, with the surface sloping toward Salado Creek which drains north to south across AOC 38. Much of AOC 38 has been scoured by the drainage system, with large areas of exposed bedrock exhibiting conjugate joint sets (fracture patterns) and fault scarp evidence. The site vegetation consists of native grasses on areas of floodplain with isolated clusters of live oak stands.
The Upper Glen Rose is the uppermost geologic strata in the area of AOC 38 (Figure AOC38-3). 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. Since approximately 60% of the ground surface at AOC 38 consists of scoured stream beds, outcrops of the Upper Glen Rose are numerous at AOC 38.
Based on current published information, there are two known major fault (shatter) zones at CSSA: the North Fault Zone and the South Fault Zone. AOC 38 lies on the northern edge of the North Fault Zone (Figure AOC38-3). Numerous faults with small displacements have been located near AOC 38 using mapping, seismic, and GPR techniques. Vertical displacement between 5 to 30 feet have been reported (Volume 1-1, Background Information Report), where the downthrown blocks typically occur on the southeast side of the fault. Fault scarp evidence has been observed within AOC 38. Additional information on structural geology at CSSA can be found in the Environmental Encyclopedia (Soils and Geology, Background Information Report, Volume 1-1).
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 evaporate layers in the Upper Glen Rose Limestone. Groundwater discharge occurs predominantly via natural springs, seeps, and pumping. No evidence that groundwater from the Upper Glen Rose is discharging to natural springs or seeps in the vicinity of AOC 38 was reported during field investigations.
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, with some seasonal fluctuations. However, local flow gradient may vary depending on rainfall recharge and possibly well pumping.
AOC 38 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 (Volume 1-1, Background Information Report).
One inactive livestock well (Well CS-4) is located in the southwest portion of AOC 38. The livestock well is inactive, but it is included in the quarterly groundwater monitoring program at CSSA. The depth to water at CS-4 has varied greatly since monitoring began in 1992. Water levels have ranged from 45 feet below top of casing to 251.14 feet below top of casing in Well CS-4. Between December 1999 and September 2000, the well was dry. Low levels of PCE and TCE have been detected at Well CS-4. Well CS-16, which is an inactive drinking water well, is located approximately 100 feet northeast of AOC 38. Well CS-D is located approximately 100 feet to the north of AOC 38.
The nearest surface water bodies are two intermittent streams which enter AOC 38 across the northern boundary and from the northeastern corner along Moyer Road. These intermittently flowing streams flow across AOC 41, AOC 37, AOC 35, AOC 36, and AOC 66 prior to entering the boundary of AOC 38. Both of these drainages are usually dry except for periods following a heavy rain. The streams merge within AOC 38 and continue to flow south to join Salado Creek within the CSSA boundary.
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 38. The nearest potentially significant cultural resource is Building 43, located 525 feet east of AOC 38. Former cantonments are also located within 1,700 feet of the site to the west and 1,400 feet to the south.
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).
Within one-quarter mile of AOC 38, land use is classified as “Evergreen Forest Land” and “Commercial and Services” in Figure 1.1 of the Technical Approach Document for Risk Evaluation. Three buildings used primarily for storage are located within the 0.25-mile radius of the site, and weapons storage igloos are located within 0.25 mile to the south. The nearest residential area is approximately 0.3-mile to the west, but the nearest school is over a mile to the northwest. Several inactive wells are located within 0.25-mile of the site, including wells CS-d, CS-16, CS-2, CS-3, CS-MW1, and CS-MW2.
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. A map of deer hunting stands that overlook mechanical feeders and planted food plots is located in the Technical Approach Document for Risk Evaluation (Volume 1-6). AOC 38 is located 500 feet east of hunting stand number 1 and 500 feet west of hunting stand number 2. 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. None of the water reservoirs are within ¼ mile of AOC 38 and none receive surface water discharge from AOC 38.
In 1995, due to the proximity of the northern boundary of AOC 38 to Well CS-16, investigations were performed in an effort to identify potential source areas for contamination at Well CS-16. Prior to being designated AOC 38, this area was referred to as the “South Pasture.” The “South Pasture” survey in spring 1995 included soil gas sampling and an EM geophysical survey.
The EM-31 geophysical survey was conducted at the site in March 1995. Grid points were staked at 100-foot intervals with data transects conducted at 50-foot intervals in both north-south and east-west directions. Both apparent conductivity and in-phase EM readings were collected at 2-foot intervals along each transect. A data logger was used to collect readings which were subsequently downloaded and contoured using Surfer contouring software.
Five anomalies were detected during the survey; however, all but one were attributed to underground/overhead utilities or surface metal debris. The one unexplained anomaly is shown as “A” on Figure AOC38-4. A 1.5-inch diameter metal pipe was located on the southwestern end of anomaly A which may have affected the EM readings in this area.
In an effort to further identify the source of this anomaly, field personnel walked the area in August 2001. At the north end of the anomalous area, a portion of a barbed wire fence, presumably washed down during a precipitation event, was found. If this fence part was on the ground during the 1995 geophysical survey, it may have been the cause of anomaly A, together with the 1.5-inch diameter pipe which remains on site today.
In July 1995, soil gas samples were collected at a total of 26 points. Soil gas survey sample locations are shown in Figure AOC38-4. 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 μg/L to 3.1 μg/L. The maximum concentration was detected at the location identified as “South Pasture 110,40,” which was situated in the northeast corner of AOC 38. No other analytes were detected. Results of the July 1995 survey are provided in Table 9.1 of the Technical Memorandum on Soil Gas Surveys (Parsons ES, 1996).
In November 1995, 95 additional soil gas samples were collected in the South Pasture area and analyzed for cis-1,2-DCE, TCE, PCE, and carbon tetrachloride. These additional samples were collected to provide additional information within the area surveyed in June 1995, as well as to extend the survey area to the south and west. During the second survey, TCE was detected at 10 locations at concentrations ranging from 0.02 μg/L to 0.21 μg/L. PCE was detected at 81 locations at concentrations ranging from 0.01 μg/L to 2.6 μg/L; the maximum PCE and TCE concentrations were detected at “South Pasture 80,-40,” in the southeast corner of the site. Results of the November 1995 survey are provided in Table 9.2 of the Technical Memorandum on Soil Gas Surveys, and a PCE soil gas plume map is provided as Figure AOC38-5.
As described in Section 4.3 of the Risk Assessment Technical Approach Document (Volume 1-6), CSSA has opted to pursue closure of AOC 38 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 38) 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 38 are chlorinated hydrocarbons (specifically detections of the compounds PCE and TCE) the cleanup levels should be the RLs. Chlorinated VOCs are man-made chemicals and rarely occur naturally.