AOC 65 Treatability Test Plan Section2

AOC 65 Treatability Test Plan

Section 2 - Site Conditions

AOC 65 consists of potential source areas associated with Building 90. This includes an inactive, sub-slab, concrete-lined pit on the west side of the interior of Building 90, and an area extending outside Building 90 along an abandoned drain line and stormwater ditch. These areas have been identified as having a contaminant plume from a previous soil gas survey (Draft Soil Gas Survey Technical Report, Parsons, August 2001) and the RCRA facility investigation (Draft AOC 65 RFI Report, Parsons, February 2002).

A metal vat was formerly located within the inactive, sub-slab, concrete-lined pit on the west side of the interior of Building 90. The former metal vat contained liquid solvents, including PCE and TCE that were used for cleaning ordnance material. The metal vat was installed inside the concrete-lined pit prior to 1966. In 1995, the metal vat was removed and a metal plate was welded over the concrete pit when PCE and TCE use was replaced with an environmentally friendly citrus-based cleaner. Currently, the citrus-based solvent is stored in a self-contained wash rack located on top of the metal plate/pit which is part of a closed-loop cleaning system.

Additional background information regarding the location, size, and known historical use of the site is included in the CSSA Electronic Environmental Encyclopedia Volume 1-3, AOC 65).

2.2 - Potential Sources of Contamination

The most probable source of contamination in the vicinity AOC 65 is solvent contamination originating from the former metal vat that once stored chlorinated solvents, including PCE and TCE. During the cleaning of ordnance, PCE and TCE may have spilled onto the concrete slab and possibly seeped into the fill materials underlying the building. No cracks or migration pathways are visible. A former drain line connected the vat to a drainage ditch located west of Building 90. The drain line and outfall area may also be contributing to VOC contamination in the area.

In addition to the former solvent vat, floor drains within Building 90 may have contributed to the spread of contaminants beneath and outside of Building 90. The floor drains originally collected fluids from within Building 90 and discharged them to ditches west of the building. Past operations included a steam cleaner which discharged the condensate to the floor drain. The floor drains are currently plugged with concrete and the drain lines have been disconnected. Based on past operations, potential COCs at AOC 65 include VOCs, SVOCs, TPH, PCBs, and metals.

Site Environmental Setting

2.3.1 Site Soils and Topography

The Crawford and Bexar stony soils dominate in the area of AOC 65, although there is significant evidence that soils have been dramatically reworked for construction of Building 90 and associated asphalt-covered roadways.

The Crawford and Bexar stony soils are thin (typically less than 1-foot thick), stony, very dark gray to dark reddish brown, non-calcareous clays that typically occur in broad, nearly level to gently undulating areas. Detailed descriptions of all CSSA soil types are given in the CSSA Environmental Encyclopedia (Volume 1-1, Background Information Report, Soils and Geology).

AOC 65 is at an elevation of approximately 1,200 feet above sea level and the land surface slopes gently (less than 2 percent grade) to the southeast. Therefore, regional run-off generally flows toward the southeast. However, local drainage in the former metal vat area flows toward a ditch west of the asphalt roadways. Also, former floor drains from inside Building 90 and current roof drains from the west side of the building discharge to the ditch west of the asphalt roadways.

2.3.2 Geology

AOC 65 is sited over the Lower Glen Rose Limestone. The Lower Glen Rose is a massive, fossiliferous, vuggy limestone that grades upward into thin beds of limestone, marl, and shale. The Lower Glen Rose is estimated to be 300 feet thick beneath CSSA. 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 mapping conducted by Parsons for CSSA, there are two major fault (shatter) zones thought to be present at CSSA: the north fault zone and the south fault zone. The north fault zone is reportedly located approximately 1 mile north of AOC 65 and the south fault zone is reportedly located approximately 2,200 feet south of AOC 65. More recent mapping of the area by the USGS identified a third fault zone running NE/SW in the Building 90 area. Additional information on structural geology at CSSA can be found in the Environmental Encyclopedia (Background Information Report, Volume 1-1).

2.3.3 Hydrology

In general, the uppermost hydrologic 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 layers is thought to be poor. This interpretation is based on differences in observed static water levels in wells completed in different layers. Principal development of solution channels is limited to evaporite layers in the Upper Glen Rose Limestone. 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. Groundwater elevation data from four monitoring wells recently installed near AOC 65 indicate a hydraulic gradient to the north (Draft September 2001 Quarterly On-post Groundwater Monitoring Report).

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 middle Trinity aquifer is separated from the lower Trinity aquifer by the Hammett Shale, which is an aquitard and not considered to be part of either the middle or lower Trinity aquifers. The Lower Glen Rose Limestone outcrops north of CSSA along Cibolo Creek and within the central and southwest portions of CSSA. As such, principal 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, which 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 has been estimated at 1,700 gpd/ft., although localized differences may occur. Fracture systems associated with fault zones are thought to affect groundwater flow and be the controlling structural feature for migration of contaminants in the vadose zone at CSSA (Background Information Report, Volume 1-1 Environmental Encyclopedia).

2.4 - Summary of Previous Investigations

In April 2000, two surface soil samples, AOC65-Pit1 and AOC65-Pit 2 were collected from soils beneath the former vat inside Building 90. In addition, one soil boring, SB01, was drilled approximately 50 feet west of Building 90 and approximately 20 feet south of the terminus of a drain line that exits the building in the vicinity of the former metal vat. These initial samples were collected to characterize the site and determine the readiness of AOC 65 for closure under TNRCC RRS1 rules. Results from this initial investigation confirmed VOC and metals concentrations exceeded CSSA background levels.

A more comprehensive investigation including a soil gas survey, installation of 14 soil borings (AOC65-SB2 through AOC65-SB10 and AOC65-MW01 through AOC65-MW05), and installation of five shallow monitoring wells (AOC65-MW01, AOC65-MW02 A-B, AOC65-MW03, AOC65-MW04 and AOC65-MW05) was conducted from January 2001 through April 2001 to further assess the site. Results of this investigation delineated a PCE plume in soil below and to the west and south of Building 90. In addition, a smaller TCE plume was delineated near the former metal vat in Building 90 coincident with the highest PCE concentrations recorded at the site.

In April 2001, groundwater samples were collected from three monitoring wells (AOC 65-MW-01, AOC 65-MW-02 A, and AOC 65-MW-04) and one soil boring (AOC 65-SB-6). Monitoring wells AOC 65-MW-02-B, AOC 65-MW-03, and AOC 65-MW-05 were dry at the time of sample collection so groundwater samples could not be obtained.

Groundwater samples collected in April 2001 were analyzed for metals and VOCs. Metals results indicate that nickel, zinc, and lead exceed RLs in groundwater collected from AOC 65-MW-01, and barium exceeds the MCL in groundwater collected from AOC 65-MW-04. VOC results indicate that PCE exceeded the RL in all four samples collected, even in AOC65-SB6, which is outside the confines of the PCE soil plume. PCE concentrations ranged from 7.8 ppb (AOC 65-MW-04) to 950 ppb (AOC 65-MW-2A). In addition, cis-1,2-DCE and TCE exceeded RLs in AOC65-MW01 and AOC65-MW02 A.

Detailed discussions of previous investigations conducted at Building 90 and AOC 65 are included in Volume 1-3, AOC 65 of the CSSA Environmental Encyclopedia, the Draft Soil Gas Survey Technical Report, Parsons, August 2001, and the Draft AOC 65 RFI Report, Parsons, February 2002.

2.5 - Geophysical Testing

Geophysical testing was conducted at AOC 65 and the surrounding area to assist in selecting optimum locations for the groundwater recharge study monitoring well placement and the AOC 65 SVE system. The geophysical surveys completed at AOC 65 include electrical resistivity imaging (ERI), microgravity, and very low frequency electromagnetics (VLF). These geophysical methods were selected to provide information regarding location, depth, size, and orientation of subsurface features that may be controlling contaminant migration in the area. The methods utilized were selected based on applicability for detection of specific features of interest. The ERI data were used to provide high-resolution 2-D slices of subsurface features, including the ability to potentially identify fractures, faults, and voids. The microgravity equipment was selected to identify subsurface voids. In addition, the VLF system was selected to potentially identify subsurface fractures.

2.5.1 Electrical Resistivity Imaging

The ERI method involves generating a current field in the ground and then measuring voltage potential within that induced field. The method measures the electrical resistance of the subsurface materials within the current field. The resistance of the materials is a function of the material type, porosity, degree of saturation, and the electrical properties of the pore fluids. With this method, two-dimensional images reflecting changes in the material type, pore fluids, or porosity can be generated.

Equipment utilized for the ERI surveys included the Sting R1 resistivity meter and the Swift multi-electrode cabling system manufactured by Advanced Geosciences, Inc. The cabling system used consisted of two 28-electrode cable sets for a total of 56 electrodes. The cable sets were constructed with 6 meters of interconnecting cable between each electrode, limiting the maximum separation of the electrodes for a survey to this distance. Surveys were performed using the dipole-dipole array with 5-foot electrode spacing. The decision to use 5-foot spacing was based on concerns for interference from buried utilities and other cultural features. Increasing the electrode spacing increases the survey depth, but also increases the separation distance that must be maintained from potential sources of interference. In the area immediately west of Building 90, a gas main, water main, metal security fence, buried power line, and overhead power lines are present and aligned north to south through the area. These items were potential sources for interference in the resistivity measurements aligned north to south. The potential for interference from these items was less for profiles oriented more or less perpendicular to them.

The surveys are performed by placing stainless steel stakes in the ground at 5-foot intervals along the desired profile lines. The electrodes on the Swift cable system are then connected to the stainless steel stakes. The Sting meter is then connected to the cabling system and the survey is performed. During the survey, four electrodes on the cable system are automatically chosen, two are current electrodes and the other two are used to measure the voltage potential at specific locations within the current field. After the four electrodes are selected, the meter sends current into the ground through the current electrodes and measures the voltage potential within the current field. The system then selects four more electrodes on the cable system and another reading is taken. The system continues in this fashion, changing the separation between the four electrodes to increase the survey depth, until an entire data set is collected.

Following collection, the resistivity data is processed using the RES2DINV inversion software. The software attempts to model the distribution of true resistivities in the ground that would result in the data set that was measured. The output from the software is a contour map of predicted true resistivities in the subsurface to the maximum survey depth. To enhance the presentation of the measured resistivity, the model results were plotted using the Surfer contouring software.

Between November 19, 2001 and January 18, 2002, nine ERI profiles were completed in the vicinity of AOC 65. The locations of geophysical survey profiles are depicted on Figure 2.1. The profiles completed include the following:

	WEST80FT conducted in north-south orientation approximately 80 feet west of Building 90,
	WEST0FT conducted in a north-south orientation immediately adjacent to the west loading dock at Building 90,
	LINE 360 conducted in a west-east orientation at station 360 on profiles WEST80FT and WEST0FT,
	LINE 410 conducted in a west-east orientation at station 410 on profiles WEST80FT and WEST0FT,
	LINE 460 conducted in a west-east orientation at station 460 on profiles WEST80FT and WEST0FT,
	LINE 510 - conducted in a west-east orientation at station 460 on profiles WEST80FT and WEST0FT,
	NW-SE1 - conducted at a northwest-southeast orientation
	NW-SE2 - conducted at a northwest-southeast orientation
	SW-NE1 - conducted at a southwest-northeast orientation

Profiles WEST0FT, WEST80FT, NW-SE1, NW-SE2 and SW-NE1 were conducted with 56 electrodes at 5-foot electrode separation. The 56-electrode dipole-dipole survey at the 5-foot station yielded results to a depth of approximately 60 feet. Profiles LINE 360, LINE 410, LINE 460, and LINE 510, were conducted with 28 electrodes at 5-foot separation due to space limitations. The depth for the 28-electrode surveys was approximately 30 feet. ERI survey results are presented in Appendix A.

2.5.2 Microgravity

Microgravity surveys were performed to assess the potential presence of voids or dissolution features in the bedrock unit. This method involves measuring the vertical component of gravity at a point using a highly sensitive gravity meter. Microgravity has proven to be a very useful tool for detecting the presence of karst-related features for environmental and engineering projects.

A LaCoste and Romberg Model G gravity meter was selected for the survey. This meter was selected because of its capability to measure relative gravity variations to 0.001 mGals (1 Gal = 1 cm/s²). Modeling of potential gravity anomalies associated with dissolution features indicated that an air-filled or water-filled cavity with a 2-foot radius could potentially be detected with this meter to a depth of at least 50 feet.

Four microgravity profiles were performed near AOC 65. The microgravity profiles coincided with the ERI profiles WEST80FT, WEST0FT, LINE 460, and LINE 510 (see Figure 2.1). The microgravity readings were collected at 10-foot intervals along the survey lines. Prior to conducting the surveys, the land surface elevation at each measurement point was surveyed to an accuracy of 0.01 feet. t each station, a leveling plate was placed onto the ground and the meter placed on the plate and leveled. The internal horizontal beam was then released and the gravity reading was recorded once the meter stabilized. The internal horizontal beam was then re-clamped to minimize fluctuations between readings and the system was moved to the next location. In addition, a local base station was established and gravity measurements were collected approximately every 30 minutes to monitor the drift of the meter during the day.

Microgravity data were entered into a spreadsheet for final processing. To arrive at the final residual gravity anomaly, the measured gravity reading at each point was corrected for effects due to elevation and changes in latitude. In addition, a trend correction was applied to the two north-south profiles, WEST0FT and WEST80FT, to remove regional gravity trends. The measured data along with the gravity anomaly plots are included in Appendix A.

2.5.3 Very Low Frequency Electromagnetics

VLF surveys were attempted at the AOC 65 site. The method utilizes the very low frequency transmitters located throughout the world to identify conductive linear subsurface features such as fractures. The very low frequency transmitters are normally used by the Department of Defense for communication purposes. At distances greater than 500 km from a transmitter the electromagnetic wavefront will essentially be a planar feature and can be used for subsurface characterization. If a fracture is present in the bedrock and the orientation of the fracture is aligned toward a VLF transmitter, the primary electromagnetic field will generate a secondary electromagnetic field in the fracture. The system measures the electromagnetic fields and if a secondary field is present, the location can be determined. The meter also compares the phase shift between the primary field and the secondary field inducted in the ground. This amount of the phase shift is a function of the size and conductivity of the feature.

To begin the VLF survey, a test survey was conducted over a known fracture at the site. The fracture was visible at land surface with an opening of approximately 4 inches. Data was collected at 10-foot stations along a line oriented perpendicular to the strike of the fracture. Initially, the survey was performed using the VLF transmitter at Cutler, Maine, which was the strongest signal available at the site. After this initial survey proved unsuccessful for detecting the fracture, additional test surveys were performed using different transmitters and measurements spacing. Each of the test surveys proved unsuccessful for identifying the fracture.

After repeated unsuccessful attempts to identify the fracture in the test area, surveys were attempted along lines in the AOC 65 area. VLF data were collected along a resistivity profile line south of Building 89. This survey was preformed at 10-foot stations. As with the test area, the data recorded did not identify fractures in the bedrock. Low readings were recorded when measurements were taken beneath over-head power lines, suggesting that the instrumentation was functioning properly. Additional surveys were attempted in this area using the optimum orientation for the various VLF signals available. All surveys attempted failed to yield results indicating the presence of fractures.

2.6 - Summary of Geophysical Results

Geophysical surveys were completed in the vicinity of AOC 65/Building 90 between November 19, 2001 and January 18, 2002 employing three geophysical methods. The methods utilized included ERI, microgravity, and VLF. Nine ERI profiles, four microgravity profiles were completed. Results of the VLF surveys did not provide useful data in the AOC 65 area and, as a result, additional surveys were not attempted. A summary of the results for each method is included in the following sections. Processed survey results and data are included in Appendix A.

2.7 - ERI

Subsurface anomalies were identified on all ERI profiles. On many of the profiles, both strong and weak low conductivity features were present. Typically, the strong anomalies are 60 ohm-meters or less, with many below 10 ohm-meters. The resistivity of the weak anomalies is above 60 ohm-meters but less than the bedrock unit. Both the strong and weak anomalies are interpreted as fractures/faults with water and/or clay in the aperture. Many of the strong (conductive) anomalies begin at a depth 15 bgl surface and extend downward to the bottom of the profiles. Groundwater was measured at depths of 15 to 20 feet in monitoring wells AOC65-MW01 and AOC65-MW02A during the surveys, suggesting that the conductive anomalies may reflect the presence of water.

The width of the anomalies observed in the profiles is exaggerated by the modeling program due to the relatively large resistivity contrast between the resistive bedrock and conductive fracture. On many of these linear features, the trend of the anomaly to the land surface is not well defined. The lack of definition of the upper portions of the fractures/faults can be attributed to infilling of the fracture coupled with a lower moisture content resulting in a lower resistivity contrast between the fracture aperture and the bedrock material. Due to the exaggeration of the anomaly widths and the lack of definition to land surface, the exact locations and orientations for the suspect fractures are difficult to determine.

To establish trends in the resistivity data, locations of strong and weak resistivity anomalies from each profile were plotted on a map. Linear trends in the alignment of the anomalies were viewed as indications of potential fracture/fault locations. Anomalies that did not exhibit a linear trend are suspect locations of karst features or localized changes in stratigraphy. The interpreted locations of anomalies from the ERI data are included on Figure 2.2.

Two specific anomalies of note do appear in the ERI data. On profiles WEST0FT, WEST80FT, and NW-SE1, a rather broad conductive anomaly occurs at locations 294, 300, and 87, respectively.As shown on Figure 2.2, the alignment of this feature trends in an east to west direction across the site. The feature is a suspect fault zone with the increased width the result of breccia zones having higher porosity and water content.

The other feature of note on the east-west ERI profiles is a conductive feature that appears on all profiles crossing a line approximately 60 feet east of the fence along Ralph Fair Road. The north-south alignment raised suspicion since fractures are not known to be aligned in such an orientation at CSSA. Upon further inspection, an abandoned heavy-gauge electrical power line was found buried in this location. The power line was removed because of concerns that interference from this line was masking other features. Additional ERI surveys were then performed at location LINE 460 and LINE 510 to assess the change in survey results following removal of the power line. Results of the follow-up surveys revealed that the conductive feature was still present. The conductive feature may be the result of bedrock being severely broken during construction of the bedrock trench for the power line and different backfill material from native rock and soil. The fracturing coupled with moisture from recent rain events may explain the strong anomaly.

2.7.1 Microgravity

Several microgravity anomalies were detected in the two north-south profiles. The microgravity responses for most of these anomalies were generally less than 0.005 mGals. This small microgravity response could result from a wide range of potential target situations, from a small feature at a relatively shallow depth to a large target at a considerable depth. Exact estimates of the depth and size of the anomaly is not possible from the microgravity results.

Gravity anomalies were detected at stations 70 and 60 on profiles LINE 460 and LINE 510, respectively. These anomalies are believed to be related to the bedrock trench associated with the former buried power line identified during the ERI surveys. No other anomalies were detected on the LINE 460 and LINE 510 surveys. Locations of the microgravity anomalies are included on Figure 2.2.

2.7.2 VLF

As previously mentioned, the VLF method did prove useful for identifying the fractures and faults in the AOC 65 area. There are numerous reasons why the VLF survey was unsuccessful. First, for the method to work the orientation of the fracture must be within approximately 20 degrees of the direction toward the selected transmitter. Second, the selected signal must be sufficiently strong to generate a secondary field that can be measured by the instrument. The depth of penetration of the electromagnetic field will be influenced by the electrical conductivity of the bedrock with conductive materials significantly reducing the depths to which the fractures can be detected. In addition, the electrical resistivity of the fracture should significantly contrast adjacent bedrock material to cause the secondary field to be generated.

With regard to applications at CSSA, the inability of the equipment to detect the fractures is likely a sum of these potential complications. As previously mentioned, the strongest signal at the site was associated with the transmitter at Cutler, Maine and strength of the signal was at the lower range of the manufacturer�s recommendation. Signals from the next two strongest transmitters, Annapolis, Maryland and Seattle, Washington were below the manufacturer�s recommended minimum levels. In addition, as observed in the resistivity data, the conductivity of the bedrock unit is somewhat lower than anticipated for limestone terrain because of the presence of clay and shale layers within the limestone. The low bulk conductivity of the bedrock combined with the small electrical resistivity contrast of the fractures also complicates the ability to locate these features with this method.

2.8 - Additional Data Needs

The geophysical data collected through January 18, 2002, identified several subsurface anomalies. Of the methods employed, the ERI surveys appear to be providing the most useful data. Several anomalies have been identified that are suspect faults, fractures, and voids. Additional ERI surveys should be performed to identify faults, fractures, and voids to the west, south and east of Building 90. This data will help identify subsurface features that are controlling the migration of the PCE, TCE and their degradation products. Identification of these features is critical to understanding the migration of the PCE/TCE material from the AOC 65 unit into the subsurface and the subsequent distribution of these compounds in the absorbed, vapor, dissolved, and free phase.

Selection of the three geophysical methods was based on the methods complementing one another to help identify the features of interest. As previously mentioned the ERI method appears to be proving useful results. Results of the microgravity data identified anomalies that are potential voids; however, the exact nature, depth, and size of these features are impossible to accurately determine with current processing techniques. Comparison of the microgravity and ERI data shows limited correlation suggesting that either the gravity anomalies are deeper than the investigation depth of the ERI survey or smaller than the resolution capabilities of ERI. In addition, the VLF method was intended to assist with the identification of fractures and faults; however, this method is not providing useful data at the site. Therefore, additional data are needed to supplement the ERI data to help define the exact locations of the fractures and faults and to allow a more effective design for the AOC 65 SVE system.

To complement the ERI data, shallow seismic reflection surveys should be performed in the AOC 65/Building 90 area. Shallow 2D and 3D seismic reflection surveys are a proven method for identifying faults, fractures, and cavities to depths of 600 feet. Additionally, seismic shear wave surveys may also prove effective in identifying the fractures and faults. Both 2D reflection and shear wave seismic surveys can be performed immediately west of Building 90 and in locations where other surveys are not possible (between buildings, etc.). The 3D seismic surveys are conducted over larger areas and as such would be limited to areas such as those immediately south of Building 90.

Additional geophysical methods that may prove useful include spontaneous potential and borehole televiewer. The spontaneous potential method, also known as natural potential, has proven to be effective at detecting the minute currents generated in the ground resulting in the flow of fluids through the material. According to recent information, this method has successfully been utilized at neighboring Camp Bullis, to identify Karst features. The borehole flow meter method produces a picture of the borehole and may prove useful in assessing the size and orientation of fractures and voids in piezometers and VEW/VMP borings prior to well completion.

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