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Final Solid Waste Management Unit B-4 RCRA Facility Investigation Report
Section 1 - Introduction
On May 5, 1999 an Administrative Consent Order was issued to CSSA pursuant to §3008(h) of the SWDA, as amended by 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 SWMU B-4 and to recommend further investigation, if necessary, or to provide documentation necessary for site closure. The main objectives of the SWMU B-4 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 under the U.S. Air Force AMC Contract F11623-94-D0024, Delivery Order RL83. AFCEE provided technical oversight for the delivery order. Based upon the project SOW, a set of work plans to govern the fieldwork was prepared. 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, RL83); 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 Bibliography (Volume 1-1 of the Environmental Encyclopedia).
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.
Located in the Inner Cantonment Area northeast of the main compound near well CS-16, the SWMU B-4 site is a large sparsely vegetated area of approximately 1.7 acres. Figure B4-1 illustrates the boundary of the site, using the 1998 aerial photograph as a background. The site was reportedly used to burn classified documents, trash, and ordnance-related materiel for an unknown period of time. In October 1992, Parsons initiated a records review of CSSA sites, including SWMU B-4, during a preliminary evaluation of potential groundwater contamination sources.
Background information regarding the location, size, and known historical use of SWMU B-4 is also included in the Environmental Encyclopedia (Volume 1-2, SWMU B-4). Volume 1-2 also includes a Chronology of Actions and a Site-Specific Work Plan for SWMU B-4.
1.1.2.2 Potential Sources of Contamination
Historical records indicate that SWMU B-4 was used as a disposal area for miscellaneous solid waste. Exact dates of the use of the site are unknown. Contaminated media could include surface and subsurface soils, and possibly groundwater, if present.
Possible sources of contamination are metal debris from the solid metal waste, explosive residue from UXO, and VOCs and SVOCs contamination from the possibility of solvent disposal and trash burning. Based on prior use and previous analytical results, the chemicals of potential concern include VOCs, SVOCs, explosives, and metals.
SWMU B-4 is located in the northeast portion of the Inner Cantonment Area (Figure B4-1). Nearby AOCs include: AOCs 36 and 38 to the northwest, AOCs 42, 52, 58, and 62 to the southwest, AOCs 55 and 61 to the southeast, and AOC 64 to the northeast. Surrounding SWMUs include Bldg. 43, B-3, B-10, B-11, B-19, B-28, and O-1, all to the northwest.
The site is situated about 1,400 feet from the northern boundary of the Inner Cantonment Area and approximately 300 feet north of well CS-MW2-LGR (Volume 1-2, Site Location Map).
1.2 - Site Environmental Setting
1.2.1 Site Soils and Topography
The gently undulating Tarrant Association soil unit is the only soil type present at SWMU B-4 (Figure B4-2). At CSSA, the Tarrant soils occur along the outer edges of the Salado Creek floodplain. The soils are thin and form over hard, fractured limestone. The surface layer is usually about 10 inches thick and is a dark grayish-brown, calcareous, clay loam with scattered gravel and cobblestones within, and on the surface layer. At SWMU B-4, the soil horizon is 4 feet thick, or greater, as shown in the soil boring logs (Appendix B).
The areas with Tarrant association soils that are considered gently undulating are typical of prairie and plateau topography. They occur primarily in 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. This soil type covers approximately 14.3 percent (572.6 acres) of CSSA. The soils are dark colored, very shallow, calcareous, and clayey, and are best suited for native grasses and range use.
The elevation of SWMU B-4 varies between approximately 1,230 and 1,240 feet above mean sea level (Figure B4-2). Surface runoff is to the southwest toward the Salado Creek.
1.2.2 Geology
The Glen Rose Formation is the uppermost geologic stratum in the area of SWMU B-4 (Figure B4-3). 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 Formation, 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 (shatter) zones at CSSA: the North Fault Zone and the South Fault Zone. SWMU B-4 lies near the intersection of two fault planes in the North Fault Zone (Figure B4-3). Additional information on structural geology at CSSA can be found in the Environmental Encyclopedia (Volume 1-1, Background Information Report, Soils and Geology).
The general geology at the site consists of between 5 and 12 feet of fill material overlying highly weathered marly limestone interbedded with massive hard limestone. The fill material is comprised of silty clay with caliche and limestone gravel mixed with waste material. Cretaceous-age foraminifer and pelecypods of the Glen Rose Formation were the predominant fossils identified in the borings. Several fossiliferous zones were observed in the borings: between 21 and 30 ft bgs in B4-SB04, between 33 and 34 ft bgs in B4-SB03, and between 22 and 30 ft bgs in B4-SB05.
In general, the uppermost hydrogeologic 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 that static water 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.
The Middle Trinity aquifer is unconfined and functions as the primary source of potable water at CSSA. It consists of the Lower Glen Rose Limestone, the Bexar Shale, the Cow Creek Limestone, and the Hammett Shale. The Lower Glen Rose Limestone outcrops north of CSSA along Cibolo Creek and within the central and southwest portions of CSSA. Principle recharge into the middle Trinity aquifer occurs 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.
No site-specific information regarding groundwater is available. However, the nearest well (Well CS-MW2-LGR), which is a monitoring well, is located approximately 300 feet south of SWMU B-4. Between April 1996 and March 2000, static water levels have measured between 70.6 feet BTOC (November 1998) to 268.6 feet BTOC (April 1996) in this well (Volume 5, Introduction to the Monitoring Program, Table 3).
The nearest surface water body is the Salado Creek bed that lies approximately 600 feet to the west of the site (Figure B4-1). Salado Creek only contains surface water for brief periods immediately after precipitation events. This small stream exits the CSSA boundary approximately 7,000 feet southeast of SWMU B-4.
1.2.4 Cultural ResourcesCultural 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 historical structures to SWMU B-4 are aboveground magazines and earthen-covered igloos located approximately 800 feet to the west. A prehistoric stone midden is located approximately 4,700 feet to the north in the Outer Cantonment Area (Scott et al., 1998). Cultural resources consisting of World War I era training trenches are located approximately 1,200 feet to the southeast of SWMU B-4.
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 during 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 constituent pathways is presented in Section 3 of the Technical Approach Document for Risk Evaluation (Volume 1-6).
Land use within ¼-mile of SWMU B-4 is classified as “Evergreen Forest Land” and “Commercial and Services” in Figure 1.1 of the Technical Approach Document for Risk Evaluation. Several buildings are present to the south of B-4. These buildings are used by CSSA for storage. Wells located within ¼-mile of the site include monitoring wells CS‑MW1-LGR and CS-MW2-LGR. There are no public water supply wells within a 0.5 mile radius of SWMU B-4. The nearest residential area is approximately 4,500 feet west of the unit. The closest elementary school, Fair Oaks Elementary, is over a mile from the site.
The USDA-ARC maintains a small herd of cattle on CSSA. The cattle roam freely throughout the Inner Cantonment and in 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 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). SWMU B-4 is located approximately 1,200 feet northeast of hunting stand number 3.
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 are located near the western boundary of the Inner Cantonment.
The nearest potential habitats for local endangered species (Volume 1-1, Background Information Report, Figure 11) are 500 feet to the south (Black-Capped Vireo) and about 4,000 feet to the northeast (Golden-Cheeked Warbler).
Due to its proximity to Well CS-16, several previous investigations have been conducted at SWMU B-4. These investigations, which included a geophysical survey, a soil gas survey, and soil boring sampling, were conducted as part of the effort to identify sources of groundwater contamination detected at Well CS-16.
1.3.1 Surface Geophysical Survey
Geophysical surveys were performed at SWMU B-4 in February 1995 as part of the Well CS-16 source characterization study. Results of this survey are described in detail in Section 5 of the Technical Memorandum on Surface Geophysical Surveys (Parsons ES, 1995a).
The EM geophysical survey produced apparent ground conductivity and in-phase EM data. The GPR survey produced profiles of the subsurface. The results of the EM survey were used to choose the location of the GPR transects.
EM readings were obtained every two feet along east-west transects separated by 25 feet. Three anomalous areas were identified on the B-4 EM contour maps; these areas are shown as “A,” “B,” and “C” on Figure B4-4. The anomalies are long, linear features oriented in the north-south direction. The large variation in the EM values of these anomalies infers ground disturbance related to past waste disposal activities. An additional open area was surveyed southwest of SWMU B-4. At the time of the geophysical survey, this area was named B-4A, but is now known as AOC 52. The results showed anomalies (“D,” “E,” and “F”) at AOC 52 that indicated the potential presence of additional trenches in the area.
GPR surveys were performed at SWMU B-4 using the GSSI SIR-3 system and the Pulse EKKO IV system. The GSSI SIR-3 data, which was collected along east-west transects separated by 50 feet, displayed no evidence of buried trenches. A total of four Pulse EKKO transects were conducted at the site, and one of these resulted in a clear image of two of the EM anomalies identified at SWMU B-4. The location of three separate trenches can be inferred from the GPR profile and the trench bases can be clearly identified. The base of the trenches are estimated to be 10 to 12 feet bgs and the width of each trench, according to the GPR results, is approximately 20 feet.
1.3.2 Soil Boring Sampling
In March 1995, seventeen subsurface soil and rock samples were collected from six borings (boring numbers B4-SB01, B4-SB01A, B4-SB02, B4-SB03, B4-SB04, and B4-SB05) for chemical analysis. The locations of the borings are shown in Figure B4-4. The samples were submitted to Chemron Laboratory of San Antonio, Texas for analysis of metals, VOCs, and SVOCs. SVOCs and VOCs were not detected in any of the samples. Two samples contained metals that exceeded the comparison criteria used at that time. However, based on current background values, cadmium, chromium, copper, lead, and nickel each exceed background in one or more samples. The majority of exceedances were detected in B4-SB01 and B4-SB01A (B4-SB01 had to be resampled due to missed laboratory holding times). In addition, lead exceeded background at B4-SB02 (2-3 ft) and at B4-SB04 (5-6 ft). At B4-SB05 (19.5-21 ft), nickel slightly exceeded background. The highest concentrations were detected at B4-SB01 (6-9 ft). Results of the March 1995 sampling event are summarized in Table B4-1.
Additional information regarding results is provided in Section 6 of the Technical Memorandum on the Soil Boring Investigation (Parsons, 1995).
One grab groundwater sample was also collected from the open borehole B4-SB05, which was drilled to a depth of 27 feet. The sample was submitted to Chemron Laboratory for metals, VOCs, and SVOCs analysis. No VOCs or SVOCs were detected. All metals concentrations were below MCLs, as summarized in Table B4-2.
1.3.3 Soil Gas SurveySoil gas surveys were performed at CSSA during 1995 in an effort to determine possible source areas for the chlorinated hydrocarbon contamination present in groundwater at Well CS-16 and other CSSA monitoring wells. The soil gas surveys were performed in two phases. Initially, a reconnaissance soil gas survey was conducted from June through July 1995, at seven potential source sites. In November and December 1995, a detailed survey was performed to define the extent of contamination identified from the previous survey and provide more in-depth information on the source area. Results are summarized below, but a complete description of results is included in Section 4 of Appendix F, Groundwater Investigation and Associated Source Characterization Report (Parsons ES, 1996a).
A total of 24 reconnaissance samples and 5 follow-up samples were collected at SWMU B-4 and B-4A (now AOC 52) during the soil gas surveys. Soil gas survey sample locations are shown in Figure B4-4 and results are summarized in Table 4.1 and Table 4.2 of the Technical Memorandum on Soil Gas Surveys (Parsons ES, 1996). PCE was detected in several of the soil gas samples from SWMU B-4. The highest concentration detected was 3.8 μg/L and was detected at location C, 2.5 located along the edge of the grid that borders the oxidation pond. All but five soil gas samples were collected from this site during the initial phase of the soil gas survey. The additional samples were collected from the southeast corner of the landfill to define the limits of the PCE contamination in that area. The results of soil gas surveys indicated that the distribution of PCE shows a trend of decreasing concentrations away from the oxidation pond located at the northwest corner of the site.
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 SWMU B-4 under the 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 SWMU B-4) 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 any deed recordation requirements.
As described in Section 1.1.2.2, the COCs for SWMU B-4 are VOCs, SVOCs, explosives, and metals. Under RRS1, the cleanup levels are RLs for VOCs, SVOCs, and explosives, and background levels for metals. Background metals levels were statistically calculated for CSSA soils and the Glen Rose Limestone, and are reported in the Second Revision to the Evaluation of Background Metals Concentrations in Soils and Bedrock (Parsons, February 2002).