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AOC 39 RFI/Closure Report
Section 1 - Introduction
On May 5, 1999 an Administrative Consent Order was issued to CSSA pursuant to §3008(h) of the SDWA, as amended by the RCRA, and further amended by the HSWA of 1984. In accordance with the RFI requirements of the Consent Order, this report has been prepared to document the environmental condition and site closure requirements of AOC 39 and to recommend further investigation, if necessary, or to provide documentation necessary for site closure. The main objectives of the AOC 39 investigation are to determine if the site meets TNRCC requirements for closure, as described in Section 1.4, and to meet requirements of the Consent Order.
This specific RFI was performed by Parsons ES under the U.S. Air Force AETC Contract F41689-96-D-0710, Delivery Order 5068. AFCEE provided technical oversight for the delivery order. Based upon the AOC 39 pre-defined SOW, a set of work plans were established to govern the field work. These include:
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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.
1.1.2.1 Site DescriptionAOC 39 consists of an open, grassy area north of North Outer Drive, approximately 950 feet west of Well CS-16 and approximately 400 feet west of Well CS-D, near the approximate center of CSSA (Figure AOC39-1, based on the 1998 aerial photograph). Research and field investigation do not indicate that AOC 39 was used for waste disposal activities in the past. In 1995, a soil gas survey was conducted to identify potential sources of groundwater contamination found at Well CS-16. AOC 39 was included in this soil gas survey in the area designated as “Well 16 West.”
AOC 39 is a one-half 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 750 feet long (east-west) by 75 feet wide (north-south). The physical boundaries of AOC 39 are identified by North Outer Drive to the south, an intermittently flowing creek (a tributary of Salado Creek) to the east, and the inner cantonment fenceline to the north (Figure AOC39-1). AOC 39 is used occasionally for cattle grazing, but no other activities currently take place at the site. There are no overhead utilities at AOC 39. Two abandoned livestock wells exist within AOC 39, specifically, abandoned livestock supply wells CS-B and CS-C (Figure AOC39-1). A metal pipe which is of unknown origin but may be a remnant of abandoned Well CS-B, protrudes from the ground surface in the northwest corner of the site.
Background information regarding the location, size, and known historical use of AOC 39 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 39.
1.1.2.2 Potential Sources of Contamination
No formal waste management activities were known to occur at AOC 39. Due to AOC 39’s proximity to Wells CS-D and CS-16, a soil gas survey was conducted in 1995 and chlorinated VOCs were detected at AOC 39 (as described in Section 8, Technical Memorandum on Soil Gas Surveys, Parsons ES 1996). The compounds detected in the AOC 39 soil gas samples are the same compounds which resulted in the decommissioning of Well CS-16 as a water supply well. The COCs for AOC 39 are the same chlorinated VOCs that have been detected in wells CS-D and CS-16.
AOC 39 is located near the approximate center of CSSA, in the northernmost portion of the inner cantonment area (Figure AOC39-1). AOC 39 is approximately 2,600 feet from the nearest facility boundary to the west. The site is west of wells CS-D and CS-16 and immediately north of North Outer Drive.
1.2 - Site Environmental Setting
1.2.1 Site Soils and Topography
Three soil types have been identified at AOC 39. The soil types occurring from west to east across AOC 39 are classified as Brackett-Tarrant association, Krum Complex and Trinity and Frio soils, respectively (Figure AOC39-2).
Brackett-Tarrant association soils are formed on hills with 8 to 30 percent slopes and consist primarily of soils that developed over limestone. At CSSA, this soil type is found north of the inner cantonment, in the north pasture. The slopes of ridges are Tarrant soils which are clayey, calcareous, and very dark grayish-brown. The Brackett soils are light grayish-brown and calcareous. Brackett-Tarrant association soils cover approximately 10 percent of AOC 39, at the west area boundary, at an elevation of 1,260 feet. These soils are well drained, with moderate permeability. Runoff is low on slopes of grades less than three percent and high on slopes greater than 20 percent.
Krum Complex soils occur on slopes of two to five percent, occupying ‘foot’ slopes below Brackett and Tarrant soils. At AOC 39, Krum complex soils cover approximately 60 percent of the area, from an elevation of 1,260 feet in the west to approximately 1,235 feet in elevation toward the east. 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 are underlain by limestone and range in depth from 15 to 60 inches. Krum Complex soils are generally well drained with slow to rapid runoff and the permeability is moderately slow.
Trinity and Frio soils are located along the Salado Creek tributary that is the eastern boundary of AOC 39. 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 and are flooded after heavy rains. Trinity and Frio soils make up approximately 30 percent of the soils at AOC 39. Trinity soils make up the majority of this soil type and are a clayey to gravelly loam from 3 to 5 feet deep. Frio soils are 3 to 4 feet of dark grayish-brown clay loam. These soils are moderately well drained, with slow permeability. Runoff is low on slopes of grades less than one percent and medium on slopes ranging from one to three percent. Flooding is common except where soils are protected.
Detailed descriptions of the CSSA soil types are provided in the CSSA Environmental Encyclopedia (Volume 1-1, Background Information Report, Soils and Geology). Although a subsurface investigation has not been performed for AOC 39, 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 39 except at sediment deposition sites along the creek channel.
The elevation of the ground surface at AOC 39 ranges from approximately 1,260 feet to 1,230 feet above sea level, with the surface sloping toward the creek at the eastern edge of AOC 39. The eastern one-third of AOC 39 has been scoured by the drainage system, with areas of exposed bedrock along the Salado Creek tributary. The site vegetation consists of native grasses only and is regularly mowed.
The Upper Glen Rose Formation is the uppermost geologic strata in the area of AOC 39 (Figure AOC39-3). The Upper Glen Rose Formation 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 the eastern edge of the ground surface at AOC 39 consists of a scoured stream bed, outcrops of the Upper Glen Rose occur only along the stream bed.
Based on published information, there are two known major fault (shatter) zones at CSSA: the North Fault Zone and the South Fault Zone. AOC 39 lies on the extreme northern edge of the North Fault Zone (Figure AOC39-3). Numerous faults with small displacements have been located as near as 250 feet south of AOC 39 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. Additional information on structural geology at CSSA can be found in the Environmental Encyclopedia (Soils and Geology, Background Information Report, Volume 1-1).
Because no subsurface samples were collected at AOC 39, there are no boring logs to document the depth of the Upper Glen Rose below AOC 39. However, the presence of outcrops of the Upper Glen Rose along the Salado Creek tributary to the east of AOC 39 verify that AOC 39 overlies the Upper Glen Rose.
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 at approximately the same elevation. 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. Regional groundwater flow appears to be to the southeast.
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 39 is located at the northern edge of the North Fault Zone at CSSA (Figure AOC39-3). Fracture systems associated with the 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 (Background Information Report, Volume 1-1).
No site-specific information regarding groundwater is available. Two abandoned livestock well locations exist within AOC 39, specifically, abandoned livestock supply wells CS-B and CS-C (Figure AOC39-1). Layne Environmental Drilling plugged these wells in 1994. State of Texas Plugging Reports were filed with TNRCC in September 1994 (Groundwater Investigation and Associated Source Characterization Report, June 1996). The nearest well (Well CS-D) is located approximately 475 feet east of the eastern boundary of AOC 39. Well CS-16 (an inactive drinking water well) is located 800 feet east of the eastern boundary of AOC 39. An inactive livestock well (Well CS- 4) is located in AOC 38, 750 feet south of AOC 39. Water levels have ranged from 252.5 to 256.7 feet below top of casing in Well CS-D and from 260.4 to 269.2 below top of casing in Well CS-16. Well CS-4 has reported water levels ranging from 45 feet below top of casing to the well being dry.
The nearest surface water body is an intermittently flowing tributary of Salado Creek traveling along the eastern boundary of AOC 39 toward the south. The creek enters AOC-39 from the north and merges with Salado Creek approximately 1,400 feet south of the site. The tributary flows from AOC 66 on the north boundary of AOC 39 south across AOC 39.
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 39. The nearest potentially significant cultural resource is Building 43, located 1,560 feet southeast of AOC 39. Former cantonments were also located within 2,700 feet of the site to the west and 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).
No off-base land is located within one quarter mile of AOC 39. A small herd of cattle is maintained on CSSA by the USDA-ARC. The cattle roam freely throughout the Inner Cantonment and 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 39 is located 1,000 feet northeast of hunting stand number 1. 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 39 and none receive surface water discharge from AOC 39.
The previous investigations performed at AOC 39 have been described in Volume 3-2, AOC 39, of the Environmental Encyclopedia. In May through July 1995, due to the proximity of the eastern boundary of AOC 39 to well CS-16, further investigation was performed in an effort to identify potential source areas for contamination at Well CS-16. Prior to being designated AOC 39, this area was referred to as “Well 16 West.” The “Well 16 West” survey conducted in spring 1995 at AOC 39 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 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.
Two anomalies were detected during the survey. At the time that the Technical Memorandum on Surface Geophysical Surveys (Section 11.2, Parsons, 1996) was prepared, the cause of these anomalies was unknown. However, as shown in Figure AOC39-4, the locations of these anomalies correspond with locations of abandoned CSSA livestock supply wells CS-B and CS-C. The locations of these abandoned wells was recently learned during research conducted for the Offsite Well Survey Report (Parsons, 2001c).
In July 1995, soil gas samples were collected at a total of eight points (Figure AOC39-4). The soil gas sample locations shown on Figure AOC 39-4 were part of the soil gas survey conducted for the “Well 16 West” area. Each sample was analyzed for BTEX compounds, total hydrocarbons, cis-1,2-DCE, TCE, and PCE. PCE was detected in seven samples, at concentrations ranging from 0.02 mg/L to 0.18 mg/L. The maximum was detected at grid coordinate location 45,5. TCE was detected at 0.02 mg/L in one sample, “45,0”. No other analytes were detected. Results are provided in Table 9.1 of the Technical Memorandum on Soil Gas Surveys (Parsons, 1996) and Figure AOC 39-5. As shown in Figure AOC 39-5, the PCE plume within AOC 39 is a portion of a larger PCE plume that has been identified within other AOCs in the vicinity of AOC 39.
As described in Section 4.3 of the Risk Assessment Technical Approach Document (Volume 1-6), CSSA has opted to pursue closure of AOC 39 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 39) 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 39 are chlorinated hydrocarbons (specifically detections of the compound PCE) the cleanup levels should be the RLs. Chlorinated VOCs are man-made chemicals and rarely occur naturally.