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Three-Tiered Long Term Monitoring Network Optimization Evaluation
Section 2 - Site Description
The location, operational history, geology, and hydrogeology of CSSA are briefly described in the following subsections.
CSSA is an active installation located in Bexar County, approximately 19 miles northwest of downtown San Antonio, Texas. Its higher headquarters is the Red River Army Depot in Texarkana, Texas. The mission of CSSA is the receipt, storage, and issuance of ordnance materiel as well as quality assurance testing and maintenance of military weapons and ammunition. Because of its ordnance mission, CSSA is a restricted-access facility.
CSSA consists of 4,004 acres immediately east of Farm to Market Road (FM) 3351, and approximately half a mile east of Interstate Highway (IH) 10 (Figure 2.1). Camp Bullis borders CSSA on the north, east, and southeast. The land on which CSSA is located was used for ranching and agriculture until the early 1900s. Six tracts of land were purchased by the U.S. Government during 1906 and 1907 and designated the Leon Springs Military Reservation.
Land surrounding CSSA is primarily residential or used for ranching. Nearby communities and subdivisions include Leon Springs, Leon Springs Villa, Hidden Springs Estates, The Dominion, Fair Oaks Ranch, and Jackson Woods. Ranching and agricultural land is intermingled with the developed communities. The IH 10 and Ralph Fair Road intersection includes separate commercial businesses. A strip center at the northwest corner of CSSA also contains businesses that serve the city of Fair Oaks Ranch.
2.1.2 Investigative and Remedial Activities
A total of 84 sites, consisting of 39 solid waste management units (SWMU), 40 areas of concern (AOC), and five rifle management units (RMU) were identified at CSSA in previous investigations. Analytical data suggest that tetrachloroethene (PCE), trichloroethene (TCE), and cis-1,2-dichloroethene (DCE) are the primary contaminants of concern (COC) in groundwater, and that metals are the primary COC in soil. As of October 2004, a total of 26 SWMU and/or AOC sites have been closed. Over 60 sites have been investigated, and remediation is currently being conducted at 34 sites. However, only three sites investigated are considered to be likely sources for the groundwater contamination within the Middle Trinity aquifer. These include two SWMUs (B-3 and O-1) located near well CS-16 and AOC-65 located near the SW corner of the post (Figure 2.2). Additional information on these site investigations is included in the CSSA Environmental Encyclopedia; specifically the Groundwater Investigation and Associated Source Characterizations Report, SWMU B-3 Characterization (Parsons 1996), Interim/Stabilization Measures and Partial Facility Closure Report for SWMU O-1 (Parsons, 2000) and AOC 65 Interim Removal Action Report (Parsons 2003). The CSSA Environmental Encyclopedia is maintained as the Administrative Record for CSSA under provisions of the Administrative Order on Consent issued to CSSA on May 5, 1999, pursuant to �3008(h) of the Safe Drinking Water Act (SDWA). The CSSA Environmental Encyclopedia is available in hard copy format and on the internet.
SWMU B-3 was a landfill area thought to have been used primarily for garbage disposal and trash incineration conducted in a series of trenches. It consists of a series of trenches. In 1991, chlorinated hydrocarbons were detected in groundwater from well CS-16 approximately 500 feet north-northwest of SWMU B-3. The concentrations were above drinking water standards and prompted several investigations aimed at identifying possible source areas that could have contributed to the contamination. Various investigations including geophysical surveying, surface and subsurface soil sampling, and soil gas sampling, indicated PCE and TCE were present at SWMU B-3. The presence of these chlorinated hydrocarbons indicated SWMU B-3 as a likely source area for the contamination detected in well CS-16.
Removal actions were performed at SWMU B-3 for removal of soil volatile organic compound (VOC) contamination from the eastern most trench. Three drums of unknown origin were removed and disposed off-site along with 732 loose cubic yards (LCY) of hazardous media and 1,242 LCY of Class 2 non-hazardous materials. In addition, over 5,500 LCY of cover soil were properly characterized, stockpiled and used as backfill and cover.
Soil vapor extraction (SVE) pilot tests and treatability were performed at SWMU B-3 before and after the removal actions. Based on initial SVE pilot tests and the first 12 months of operations and maintenance, removal of approximately 290 pounds of VOCs has occurred. Based on these findings, SVE appears to be an effective method for removing VOCs from the SWMU B-3 trenches. Additionally, SVE has been identified as a possible remedial alternative to reduce levels of residual contaminant in bedrock.
A second site identified as a possible source of contamination was the oxidation pond, also referred to as SWMU O-1. The pond was constructed in 1975 and wastes were trucked to the oxidation pond from a solvent storage tank. The pond liner was apparently damaged during bulldozing. No records are available to indicate whether or not disposal of the sludge or residue contained in the oxidation pond occurred before damage to the liner. Due to its proximity to contaminated well CS-16, investigations were initiated at SWMU O-1 in 1995. Surface geophysical surveys, soil sampling and soil gas surveys were performed. Approximately 80 LCY of soil material were excavated during the liner investigation. A field treatability study was initiated to test the efficacy of electrokinetic treatment on metals contamination. Additional soil was excavated and removed in November 1999 and in 2000. Surface and subsurface soil was transported and disposed of off-site. The excavation area was backfilled and a low-permeability clay liner was constructed over the site. Six inches of topsoil were placed on top of the clay liner, and a vegetative surface was established on the topsoil. Texas Commission on Environmental Quality (TCEQ) approved a partial facility closure of the surface soil zone located within the boundaries of SWMU O-1 in April 2002.
The third site identified as a groundwater contamination source area at CSSA was AOC-65 which is located at the southwest corner of the post. AOC-65 included two sub-slab, concrete-lined vaults, one on the west side, one inside Building 90 as well as associated drain lines and ditches extending outside Building 90. A metal vat was installed in the western vault inside Building 90 prior to 1966 and removed in 1995. The vat was used for cleaning ordnance materials with chlorinated liquid solvents, such as PCE and TCE. In 1995, after removal of the former solvent vat, a metal plate was welded over the concrete vault. Use of PCE and TCE solvents were replaced by citrus-based cleaners. The use of the second vault, located within the middle of the interior of Building 90, is not known. It was backfilled and capped with concrete at an unknown date. Building 90 continues to be used for weapons cleaning and maintenance.
A soil gas survey, performed in January and February 2001, revealed a PCE plume in the soil beneath and to the south and west of Building 90 AOC-65 Soil Gas Survey Results, January - February 2001 (Parsons 2001). Soil borings were advanced and sampled and monitoring wells were installed and sampled. The soil gas survey indicated the presence of a PCE contaminant plume underlying Building 90 and extending primarily to the west and southwest from the building. Based on sampling results, it appears the lateral extent of the PCE plume in the soil gas is generally confined to the immediate vicinity of Building 90. Soil in the area where the drainage line from Building 90 meets the drainage ditch contained the highest COC concentrations. However, in the bedrock samples (21.0 to 21.5 feet below ground surface), concentrations only slightly exceeded background. Groundwater samples collected from both inside and outside the soil-gas survey plume contained PCE.
Geophysical investigations were performed to identify subsurface features such as fractures, faults, and karst dissolution that may be controlling the migration of contaminants. Identification of these features was used to direct installation of piezometers (PZ)s and an SVE system near Building 90. The geophysical methods utilized at AOC-65 include electrical resistivity, microgravity, very low frequency (VLF), EM, shear-wave seismic reflection, induced polarization (IP), and spontaneous potential (SP). These methods were selected based on their ability to detect changes in physical properties associated with fractures, faults, and karst features. The surveys were implemented in a phased approach with the results of one phase providing direction for subsequent phases. Removal of near-surface contamination and the installation of two SVE systems were conducted. Geologic correlations from core and geophysical logs indicate at least three faults cross the AOC-65 area.
After near-surface soil was removed along the former drain line and ditch, engineering controls were constructed to minimize the amount of precipitation recharge infiltrating the source zone.
2.2 - Geology and Hydrogeology
The oldest and deepest known rocks in the CSSA area are Paleozoic age (225 to 570 million years ago) schists of the Ouachita structural belt. They underlie the predominant carbonate lithology of the Edwards Plateau. The Cretaceous age sediments were deposited as onlapping sequences on a submerged marine plain and, according to well logs and outcrop observations, these sediments thicken to the southeast. The Cretaceous System stratigraphy includes the Trinity Group Travis Peak Formation shallow marine deposits. The Travis Peak Formation attains a maximum thickness of about 940 ft and is divided into five members, in ascending order: the Hosston Sand, the Sligo Limestone, the Hammett Shale, the Cow Creek (CC) Limestone, and the Hensell Sand (and Bexar Shale (BS) facies). Overlying the Travis Peak Formation, but still a part of the Cretaceous-age Trinity Group, is the Glen Rose Limestone. For this study, the units of interest are the Glen Rose Limestone, BS, and CC Limestone that form the Middle Trinity aquifer.
The Hammett Shale, which overlies the Sligo Limestone, has an average thickness of 60 feet. It is composed of dark blue to gray fossiliferous, calcareous, and dolomitic shale, pinches out to the north of CSSA and attains a maximum thickness of 80 feet to the south. Above the Hammett Shale is the CC Limestone, which is a massive fossiliferous, white to gray, shaley to dolomitic limestone that attains a maximum thickness of 90 feet down dip in the area. The youngest member of the Travis Peak Formation is the Hensell Sand, locally known as the BS. The shale thickness averages 60-80 feet, and is composed of silty dolomite, marl, calcareous shale, and shaley limestone, and thins by interfingering into the Glen Rose Formation.
The upper member of the Trinity Group is the Glen Rose Limestone. The Glen Rose Limestone was deposited over the Travis Peak BS and represents a thick sequence of shallow water marine shelf deposits. This formation is divided into upper and lower members. At CSSA, the Glen Rose is exposed at the surface and in stream valleys.
The Upper Glen Rose (UGR) consists of beds of blue shale, limestone, and marly limestone with occasional gypsum beds (Hammond, 1984). Based on well log information, the thickness of the upper member reaches 500 feet in Bexar County. The thickness of this member at CSSA is estimated from well logs to be between 20 and 150 feet.
The Lower Glen Rose (LGR), underlying the UGR, consists of a massive fossiliferous limestone, grading upward into thin beds of limestone, marl, and shale (Ashworth, 1983). The lower member, according to area well logs, is approximately 300 feet thick at CSSA. Isolated areas of reef rock have also been identified in the LGR. The boundary between the upper and lower members of the Glen Rose Limestone is defined by a widespread fossil stratigraphic marker known as the Corbula bed (Corbula martinae) (Whitney, 1952). The Corbula bed is 0.5-5 feet thick and contains small pelecypod clamshells, which are three to five millimeters in diameter. Presence of Corbula fossils indicates a slightly more saline depositional environment than fossils found above and below the Corbula. A gypsum bed has also been identified near the Corbula bed.
The geologic units present at CSSA were informally divided into hydrostratigraphic units to provide a framework for describing the local hydrogeology. Three aquifers are present in the area of CSSA: the Upper, Middle, and Lower Trinity. The Travis Peak Formation and the Glen Rose Formation are the principal water-bearing units. Only the Middle and Upper Trinity aquifers are addressed for this study.The following hydrostratigraphic descriptions are based on work performed by the USGS, in which the UGR member has been informally divided into five mappable units within Camp Bullis and CSSA. For this report, the UGR Limestone has been subdivided into five mappable intervals (UGR[A-E]). Exposures of units UGR(A, B, and C) are limited to the very highest elevations within the post, with Unit A being present only atop Schasse Hill at the southern edge of CSSA. The lower two units, UGR(D and E), comprise over 83 percent of the outcrop at CSSA.
Interval UGR(A) is approximately 120 feet thick composed of alternating and interfingering medium-bedded mudstone to packstone, with evaporates occurring locally. Interval UGR(A) has been referred to as the �cavernous zone� (GVA, 2000) because of an abundance of caves in the interval. Interval UGR(A) crops out only atop Schasse Hill within the confines of CSSA.
Interval UGR(B) is a 120- to 150-feet-thick interval similar to Interval UGR(A) but with appreciably less cave development and thus less permeability than the overlying interval. Overall, intervals A and B are indistinguishable based on lithology. Interval UGR(B) crops out only atop some of the larger hills (Schasse Hill, Wells Hill, and Steele Hill) within the confines of CSSA. Groundwater occurring within Intervals UGR(A and B) is laterally discontinuous and likely free of contamination. Limited recharge to the zone is through direct precipitation on the outcrop and recharge from Interval UGR(A), and much of that water is believed to be lost to seeps along the base of the outcrop. Some groundwater may leak vertically to lower strata where the outcrop is bisected by faults or fractures.
Interval UGR(C) is a solution zone approximately 10 to 20‑feet thick. Like the underlying Interval UGR(E) at the base of the UGR, it was originally an evaporite bed. It is composed of yellow-to-white calcareous mud with some very thin mudstone layers interspersed with a tendency to form broad, valley-like slopes. Interval UGR(C) only crops out along the slopes of the larger hills (Schasse, Wells, and Steele) within the confines of CSSA. Interval UGR(D) is 135 to 180 feet thick and composed of alternating beds of wackestone, packstone, and marl. Because of its high mud content, the 135 to 180‑foot thick Interval UGR(D) (between the two solutioned evaporite beds (Intervals UGR [C] and UGR [E] and known locally as a �fossiliferous zone�) generally has low porosity and permeability, with some local exceptions. In a few locations, some cavern porosity can be seen in outcrops along fractures.
Interval UGR(D) crops out over most of CSSA (77.5 percent coverage). Most of the developed areas at CSSA are on the Interval UGR(D) outcrop. Likewise, most of the waste management activities that have occurred at CSSA are also within this interval. However, most of the more permeable zones near the top of the unit have been eroded from CSSA, and occur only near the top of hills where less development and waste management activities have occurred. Significant recharge to the zone is through direct precipitation on the outcrop and recharge from overlying intervals. This is the first pervasive stratum across the facility that lends itself to lateral groundwater movement without being cropped out by the intersecting land surface. A significant volume of groundwater is assumed to leak vertically to lower strata where the outcrop is bisected by faults or fractures. This unit has been investigated in depth by RFI activities and groundwater investigations, as well as the background soils study prepared in the Second Revision to Evaluation of Background Metals Concentrations in Soils and Bedrock (Parsons 2002). Groundwater contamination is known to exist within this interval near the source areas of Plumes 1 and 2.
Interval UGR(E) is a 7- to 10‑foot thick solution zone that was originally an evaporite bed, but that has subsequently been dissolved, leaving behind a calcareous mud. The Corbula bed lies at the base of this interval and marks the geologic contact between the UGR and LGR Limestone. The Corbula bed is a thin to very-thin-bedded grainstone. As with Interval UGR(C), this solutioned evaporite bed, which includes the Corbula bed at its base, appears to intercept the downward seepage of water. The interval acts as a lateral conduit for flow, as demonstrated by seeps observable at the surface in outcrop. Groundwater contamination is known to exist within this interval near the source areas of Plumes 1 and 2. The vapor extraction wells (VEW) at B-3 and the shallow PZs (-2, -4, and -6) at AOC-65 are mostly completed within this depth interval, and groundwater concentrations from these wells indicate concentrations greater than those in the main plume within the LGR. At B-3 (Plume 1), cis-1,2-DCE has been reported in excess of 27,000 �g/L, and nearly 3,000 �g/L of PCE were reported. At AOC-65 (Plume 2), lesser concentrations of PCE, generally ranging between 30 �g/L and 60 �g/L, are perched about the LGR.
In the Hydrogeologic Conceptual Site Model (HCSM), the LGR Limestone has been informally divided into six intervals LGR(A-F), as described below from youngest to oldest.
Exposures of unit LGR(A) are limited to the basal portion of Salado Creek and its tributaries in the central portion of the post (covering 10.8 percent of CSSA�s surface). The remaining older units do not crop out within the post. Interval LGR(A) is defined as the uppermost 50-foot sequence of LGR deposits throughout the CSSA area. The unit is characterized by alternating layers of pale yellow mudstone, wackestones, and packstones.
The top of Interval LGR(B) ranges between 30 to 50 feet beneath the UGR/LGR contact, and the interval is between 30 and 50 feet thick. The interval is characterized as a whitish fossiliferous packstone and grainstone that is evident both in lithologic and geophysical logs. During much of the year, the main aquifer level is well below the elevation of this interval. During these times, groundwater will tend to perch within this zone. Large sinkholes and other solution features have formed in this zone.
Over much of CSSA, Interval LGR(C) exists as a 60-70-foot thick sequence of thin and medium-bedded mudstones below the more permeable grain-supported limestones of Interval LGR(B). The mudstones are described as alternating layers of tannish-brown and greenish-gray bioturbated muds with a low percentage allochemical constituents (e.g., fossils). The rock is competent and highly styolitic (susceptible to diagenetic pressure solutioning). Interval LGR(C) also includes some significant reef structures to the north and south.
Interval LGR(D) is a 65-70-foot thick unit of rock that is characterized by a unique resistivity signature with respect to the overlying and underlying rocks. The change generally represents two resistive packstone layers divided by a less resistive mudstone. The upper and lower packstone layers tend to be approximately 25 feet thick, and are described as interbedded fossiliferous wackestones and packstones that are pale yellow to white in color. The middle layer is more characteristic of a bioturbated mudstone that is tan in color. The localized vugs associated with moldic porosity (fabric selective) can store and transmit limited amounts of groundwater. Interval LGR(E) is a 50-60-foot layer of tan and light brown wackestones with intermittent thin fossiliferous layers and grain-supported rock. The unit is fairly unremarkable, except for the presence of a notable vuggy packstone layer located at the base of the interval.
Interval LGR(F) comprises the main groundwater production zone within the LGR throughout CSSA. Interval LGR(F) is composed of a 45 to 55‑foot reef complex, the lateral extent of which appears to be under the entire confines of CSSA. The occurrence of this reef has been well documented within boreholes drilled at CSSA and neighboring areas. The interval is described as a white to tan, very fossiliferous packstone/grainstone with high fabric selective moldic porosity. The interval is characterized by its relatively low gamma response and high resistivity response. The vuggy porosity left as a result of fossil dissolution has resulted in voids that range from several millimeters to 5 centimeters in size. In some locations, the basal 15 feet of the interval has a pronounced increase in mud content, and a color change to pale brown.
The primary permeability of Interval LGR(F) is moldic (fabric selective) porosity. Extensive testing through packer tests and discrete interval groundwater sampling indicate that the interval is capable of yielding groundwater in excess of 75 gallons per minute (gpm). Where not fabric selective porosity exists in the form of developed fractures, karst, or small caverns, groundwater production can easily exceed 150 to 300 gpm. For the monitoring well program, this interval has been the focus of the investigations where typically the basal 25 feet of the aquifer are monitored for the occurrence of contamination.
The BS has been subdivided into two intervals BS(A‑B), as described below from youngest to oldest. As expected, these subunits can be quite variable over the extent of CSSA. The BS forms a relatively impermeable aquitard for the overlying LGR water bearing zones. Significant vertical water movement in the BS is anticipated to be through fractures and faults only. CSSA currently has four monitoring wells completed in the BS. For the purposes of this model, Interval BS(A) is defined as the uppermost 25-30-foot sequence of BS deposits throughout the HCSM area. The unit is characterized by alternating layers of pale yellow mudstone, wackestones, and packstones. The BS(A) interval appears to have low porosity and permeability with only not fabric selective fracture porosity evident and no known cavern development. Beneath much of CSSA, the top of interval BS(B) is denoted by a large increase in gamma counts, which peaks and quickly declines. An approximately 10 to 15-feet-thick oyster bioherm also appears to be predominant at the top of BS(B). The basal 20 feet of the BS consists of a platy, fissile mudstone with an olive gray appearance. At this depth the unit is more characteristic of a shale bed with few allochems, and a very low porosity. The BS(B) interval appears to have low porosity and permeability with only not fabric selective fracture porosity evident and no known cavern development.
The CC has been subdivided into two intervals, CC(A-B), as described below from youngest to oldest. Interval CC(A) is defined as the uppermost 50-55-foot sequence of CC deposits throughout the area. The unit is characterized by alternating layers of white and light gray packstones and grainstones. Portions of this interval can be quite permeable from either moldic (fabric selective) porosity or not fabric porosity in the form of dissolutioned vugs, voids, or fractures. Moderate to large amounts of groundwater can be expected to be produced from this interval. This zone has been identified as an interval of interest with respect to groundwater monitoring at CSSA.
The basal 20 feet of the CC Limestone represents a conformable transition with the underlying Hammett Shale. The grainstones and packstones of unit CC(A) grade into a soft olive gray silty mudstone designated unit CC(B). The contact is transitional, with numerous interbeddings between soft shaley members and more competent limestone rock. Bedding units range from a few inches to several feet in thickness. The contact with the Hammet Shale below CC(B) has been defined typically as the greatest gamma peak below the base of the BS.
Historical water level data at CSSA show that the typical groundwater flow gradient is toward the south, with directional variations ranging from the southwest to the southeast, depending on the level of recharge. During extended periods of drought, the flow direction reflects a greater westerly component of flow.
Potentiometric surface maps from previous monitoring events indicate highly varying flow directions in the LGR. From December 2002 through June 2004, the overall direction of groundwater flow is predominately to the south-southeast. Groundwater flow in this unit is apparently influenced by groundwater mounding in the vicinity of well CS-MW4-LGR. Groundwater appears to move in several directions from this groundwater mound, which may be the result of well CS-MW4-LGR intersecting a significant recharge feature. The proximity of CS-MW4-LGR to Salado Creek is possibly the cause of a consistently higher potentiometric surface near this well. Until further control points are established, this mounding effect remains one of the most notable features of the groundwater surface. Figure 2.2 shows the general groundwater flow in the LGR zone at CSSA.
Hydraulic conductivity and transmissivity data were gathered from pumping tests conducted at drinking water wells present at CSSA. Additional hydraulic conductivity and transmissivity data were presented in prior publications. Published hydraulic conductivity values range from 1.4 x 10-3 to 3.5 x 10-3 cm/sec locally and range from 3.4 x 10-5 to 1.0 x 10-3 cm/sec regionally (Hammond, 1984). Site-specific hydraulic conductivity values ranged from 4.2 x 10-4 to 5.7 x 10-4 cm/sec (CSSA, 2002). Published transmissivity values ranged from 5,740 to 16,110 gpd/feet locally and range from 240 to 3,220 gpd/feet regionally (Hammond, 1984). Site-specific transmissivity values range from 1,600 to 2,400 gpd/feet (CSSA, 2001).
2.3 - Nature and Extent of Groundwater Contamination
As a result of previous operations at SWMUs B-3, O-1 and AOC-65, releases of chlorinated VOCs to the environment have occurred from multiple source areas within CSSA. These releases resulted in contamination of the LGR Limestone member of the middle Trinity Aquifer. Detections of solvent contamination (PCE, TCE and cis-1,2-DCE) were first reported in 1991. Starting in 1996, the first of 45 monitoring wells were installed. Well installation continued through September 2003. Off-post contamination was first reported by CSSA in 1999 at private well LS-7. Since that time, solvent contamination has been detected in 26 off-post private and public water supply wells. The U.S. Army installed GAC treatment systems at eight off-post well locations where concentrations exceed 80 percent of the federal maximum contaminant level (MCL) (5 �g/L) for PCE and/or TCE.
The highest concentrations of the COCs PCE, TCE and/or cis-1,2-DCE occurred at on-post monitoring wells CS-D, CS-16-LGR, CS-MW16-CC, CS-MW1-LGR, CS-MW2-LGR, in various zones of the four WB wells and in wells near Building 90 (AOC-65-MW2A, AOC-65-PZ01-LGR, AOC-65-MW1-LGR, AOC-65-PZ05-LGR, and AOC-65-MW1-LGR). A detection of cis-1,2-DCE occurred in CS-4 but no PCE or TCE was detected. Detections occurred at concentrations below the MCL on-post in wells CS‑MW9-LGR to the north, CS-MW5-LGR and CS-MW17-LGR to the east, CS-1 to the south, CS-MW10-LGR to the southwest, and CS-9, CS-10, and CS-MW18-LGR to the west. Well CS-1 is beyond the southern boundary of CSSA and is located on Camp Bullis.
The highest concentrations of the COCs PCE, TCE, and/or cis-1,2-DCE detected off-post occur at wells OFR-3, RFR-10, RFR-11, LS-2, LS-6, and LS-7. These wells are located approximately 1,000 to 2,000 feet from the CSSA southwestern boundary at Leon Springs Villa. Detections at concentrations below the MCL have been reported in off-post wells JW-29 located approximately 4,000 feet to the west, I10-2 located approximately 4,200 feet to the southwest, LS-4 located approximately 4,200 feet to the south, and HS-2 located approximately 1,200 feet to the south.
The groundwater plume associated with SWMUs O-1 and B-3 exists in the north-central area of the post (Plume 1) and has migrated off-post to the south and west. The groundwater plume associated with AOC-65 at the southwestern boundary of the post (Plume 2) has migrated off-post and has impacted off-post drinking water sources. These plumes are the focus of this Monitoring Network Optimization (MNO) evaluation. The COCs for both plumes include PCE, TCE, and cis-1,2-DCE. Groundwater contamination is most widespread within the LGR water-bearing unit. Previous investigations demonstrated that most of the contamination resides within the LGR.
Within Plume 1, concentrations above the MCL for PCE and/or TCE are detected in wells CS-D, CS-MW1-LGR, CS-MW2-LGR, and the CS-MW16 cluster. Concentrations in excess of 200 �g/L for PCE and/or TCE have been reported at CS-D, CS-16-LGR, and CS-MW16-CC. This plume has advectively migrated southward to CS-1 (on Camp Bullis), and west-southwest toward the CSSA drinking water wells CS-9, CS-10, and CS‑11, and to several off-post public and private wells. Over most of the plume area, contaminant concentrations are below 1 �g/L. In contrast, little to no contamination is detected in the BS and CC within Plume 1.
Contamination at Plume 2 originated at or near AOC-65 and Building 90, and has spread southward and westward from CSSA. The highest concentrations of COCs were reported adjacent to the source area (13,400 �g/L) in CS-WB03-UGR-01, December 29, 2004. Within the CSSA boundary, concentrations in excess of 100 �g/L have been reported in perched groundwater intervals above the main aquifer body. However, once the main aquifer body is penetrated, trace levels are reported. Off-post, concentrations in excess of the MCLs have been detected in private and public wells with open borehole completions. Concentrations exceeding 30 �g/L have been reported 1,200 feet west-southwest of CSSA at RFR-10. Vertical profiling within that well shows that discrete intervals within uncased upper strata contribute PCE concentrations at over 90 �g/L. Only sporadic, trace concentrations of solvents have been detected in BS and CC wells within Plume 2. The general extent of plumes 1 and 2 are shown on Figure 2.1. The groundwater monitoring program at CSSA is fully described in Section 3.