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Groundwater Investigation and Associated Source Characterization
Section 9 - Conclusions and Remedial Alternatives
CSSA has groundwater contamination by solvents in the vicinity of water supply well 16. CSSA intends to characterize and remediate the problem, including identification of sources, pathways, and receptors, and protection of human health, safety, and the environment.
A technical review meeting was held at CSSA on October 19, 1995 to discuss the status and findings of the groundwater investigation and source characterization actions with EPA, TNRCC, AL/OEB, and AFCEE representatives. Based on the characterization of potential source areas through geophysical surveys, drilling and sampling, and soil gas surveys, the two most likely source areas for groundwater contamination are SWMUs O-1 and B-3. The consensus of regulatory agencies and CSSA was that the next step in the groundwater investigation project would be source removal actions at these two sites However, the contaminated drinking water aquifer remains to be the central issue to CSSA.
Additional investigative actions continued and source removal actions were initiated for the B-3 and O-1 SWMUs. Source removal action at the B-3 and O-1 sites consisted of the extraction of volatile organic carbons from soil. A summary of the actions for each area is presented below. Included are recommendations on additional activities for each site.
Based on the results of the investigations, the mass of the contaminant of concern was estimated for O-1. The estimated quantities are 97.8 kilograms (215.7 pounds) of PCE, 0.6 kilograms (1.32 pounds) of cadmium, and 143.2 kg (315.1 lbs) of chromium. The deep aquifer has not been impacted by contamination of heavy metals. The limestone derived clay soil evidently has high retardation capacity, as evidenced by the observation that chromium has not been transported to the water table. These estimates are based on the EM survey regarding the lateral extent of O-1 and subsurface sampling data that was collected. Vertical extent was based on the average soil thickness observed during the liner integrity investigation and drilling activities within O-1.
During the liner investigation, contaminated soils were excavated and placed into an open bermed area. A total of 19 cubic yards of "hot" soils, which initially contained 180 pounds of PCE, were subjected to a volatilization study. This volume represents the soils identified to contain the highest PCE levels. In approximately 3 months, PCE was almost completely removed. This removal action accounts for about 80 percent of the total PCE contamination initially estimated. Continuation of a volatilization effort, accomplished by either in situ methods, such as the SVE system in operation at B-3, or ex situ methods such as the volatilization study, could achieve the goal of removing PCE from O-1 soils, thus eliminating a potential source of continuing contaminant migration to groundwater. Confirmatory characterization of the actual extents of contamination would allow for an evaluation of the specific amounts of soils which require remediation. Continuation of volatilization of VOCs will require a standard air emission exemption permit from TNRCC.
For O-1 to meet TNRCC closure standard 1 requirements, a removal efficiency of 90 percent for chromium, 72 percent for cadmium, and greater than 99.99 percent for PCE would be necessary. Preliminary screening of "proven" and demonstrated remedial technologies has been accomplished. The remedial technologies which has passed the preliminary screening efforts are high temperature thermal desorption working in conjunction with soil washing, and a SVE system working in conjunction with electrokinetic remediation. Although electrokinetic remediation is not a "proven" remedial technology, it may have an applicability because of the near homogeneous nature of the O-1 soils. Phyto remediation may also reduce levels of chromium. However, further investigations are required to identify the effectiveness of this type of remediation technique. Additional investigations associated with the preliminary screening of remedial technologies associated with new or emerging technologies is recommended to allow CSSA to possibly provide innovative remedial actions. Treatability studies of remedial technologies, which pass the prescreening and detail evaluations, is also recommended for the O-1 area.
Table 9.1-1 presents a summary of the remedial alternatives, their respective data objectives and recommended actions for SWMU O-1. A combination of remedial technologies is necessary to remediate PCE and chromium, e.g., SVE and electrokinetic (in situ) treatment, or thermal desorption and soil washing (ex situ).
The reported concentrations of VOCs and the presumed volume of affected subsurface soils were used to estimate the mass of contaminants present at B-3. The estimated masses of VOCs are 850 pounds of TCE and 170 pounds of cis-1,2-DCE. These estimates were made using assumptions regarding the lateral extent of contaminated soils. The assumed lateral extent was limited to the "hot spot" soil gas survey limits. Vertical thickness was based on the average thickness observed during drilling. Additional characterization data is recommended to obtain a more accurate estimate of the soil volume. Determination of the actual perimeter of the trench areas at B-3 could be used to establish the maximum extent of affected soils. This determination could be coupled with additional soils data to provide total VOC mass estimate.
Metals contamination does not appear to be a significant problem in the main B-3 landfill trench. However, metals were detected at concentrations significantly greater than the background levels determined for CSSA soils in some of the samples collected outside the main line of the SVE test system. These elevated levels appear to be isolated in small, localized areas. Assessment of metals and possible treatment options need to be evaluated separate from the SVE pilot test results. Metals concentrations are within the CSSA background concentration range in the primary areas targeted by the SVE pilot system for VOC removal.
Initial soil gas chemistry data confirmed that significant VOCs (primarily TCE and cis-1,2-DCE) are present inside the main B-3 trench, with much lower concentrations in a minor trench located northeast of the main SVE test VEWs and VMPs. Low oxygen content of the deeper soils indicates that a significant amount of biological activity is also occurring. The biodegradation component of VOC removal can be determined by performing an assessment of biological activity. Data can be collected for an assessment of biological activity by monitoring the consumption of oxygen over time, following shut down of active air extraction activities. This would provide additional VOC removal data for predicting the treatment time required at B-3. Soil gas could also be tested for breakdown products of TCE, cis-1,2-DCE, and vinyl chloride.
Air permeability testing and observed soil gas changes during the pilot test activities suggest the subsurface environment in and around the landfill trench is very complex and difficult to predict. For instance, some points located directly adjacent to an active extraction well exhibited no response, whereas other points located up to 30 or 60 feet from the extraction well were significantly affected. Because preferential pathways have been determined to exist, it may be possible for the installed treatment system to miss an entire area of affected soils because no direct or indirect influence of the subsurface soils is created by any of the VEWs. Additional study of the pilot test system should be performed to obtain greater understanding of the interconnections and preferential air pathways of in the subsurface environment at B-3. This should be accomplished first by performing a series of tests on the existing pilot study system. Additional test wells may be needed to investigate certain portions of the test area, or to confirm the hypotheses inferred from the additional testing of the test system.
According to the extraction rate observed after 6 days of venting, continued extraction from VEW-1 would be able to remove well over the 850 pounds of TCE and 170 pounds of cis-1,2-DCE for one more year of operation. This suggests that either the estimated total quantity of affected soils may be grossly underestimated, or the rates of removal will eventually drop off once the initial pore volumes of soil gas and the easily volatilized VOCs are removed. Testing of the VOC removal rates should be extended to beyond 6 days to assess the long-term effect that continuous SVE exerts on the removal of VOCs at B-3. If significant reduction in VOC removal efficiency is observed, then the test system should be optimized by alternating extraction time with down-time of the system (pulsing). Additional study would be required to create a pulsing schedule that optimizes VOC removal from the site.
Table 9.1-2 presents a summary of remedial alternatives and recommended actions to meet objectives for SWMU B-3.
9.2 - Groundwater Investigation
The hydrogeologic system at CSSA is in an early karst stage and is complex insofar as contaminant migration is difficult to predict. Data from publications about the middle Trinity aquifer, groundwater monitoring data, pumping rates in specific CSSA wells, and general aqueous geochemistry parameters make up the majority of information available about aquifer parameters. The two monitoring wells installed by this project provided crucial but limited data about the magnitude and extent of the plume.
The primary contaminants of concern are PCE, TCE, and cis-1,2-DCE. The lateral and vertical extent of contamination has not been defined, but after 2 years of monitoring, concentration trends seem to be fairly stable, suggesting an upgradient extent of around 1,200 feet and a sidegradient extent of 1,000 to 2,000 feet to the southwest. This assumes that the upgradient concentrations can be attributed in part to a plume in groundwater rather than attributed solely to periodic flushing of contaminants by rainfall into rapidly moving groundwater. The downgradient extent is not known.
The initial remedial alternative proposed is source removal actions at SWMUS O-1 and B-3 combined with groundwater monitoring. Because of the source removal actions initiated and planned, it is critical that groundwater monitoring be continued to determine what changes are affected through these actions. CSSA plans to establish a project to monitor the aquifer during source removal actions. Collection of data on at least a quarterly basis is recommended to determine detailed trends in contaminant concentrations, aquifer water levels, rainfall, and other parameters. Daily data from the well 16 transducer and CSSA weather station is expected to provide additional detail for future monitoring events. The B-3 and O-1 sites can also be periodically monitored for VOC concentrations in soil gas and/or soil for comparison to VOC concentrations in groundwater.
The downgradient groundwater flow direction from the area of sources B-3 and O-1 appears to be easterly. However, the potentiometric water levels should be carefully evaluated through several additional monitoring events involving the new lower Glen Rose monitoring wells. Should the easterly flow direction remain fairly constant through dry and rainy seasons, installation of at least one downgradient well into the middle Trinity aquifer is recommended. If the flow direction varies over time, the water level trends should be assessed for detailed affects on the system from rainfall events to attempt prediction of flow direction. Downgradient wells could then be installed in appropriate locations to monitor the area under seasonal conditions.
Deep well installation in the middle Trinity aquifer is expensive, and may be best deferred until seasonal conditions are determined. However, shallow monitoring wells in the vicinity of B-3 and O-1 may be helpful in delineating effects of source removal actions on the hydrogeologic system. The soil gas survey data (Figure 5.3-2) and estimated fault locations (Figure 8.3-4) should provide a good estimate of well placement in the area around B-3 and O-1. If such wells are undertaken, small pilot holes drilled to determine depth(s) to perched zones after a rainfall event may provide cost-effective data on optimum well locations and depths. Proper well placement is critical because of the anisotropic nature of flow pathways through the fractured and faulted limestone strata.
If monitoring of the sites and groundwater through the source removal actions at B-3 and O-1 does not indicate any changes in the pre-source removal groundwater contaminant concentrations, remedial alternatives in addition to the source removal may be necessary. These alternatives include determination of natural attenuation that may be occurring in the aquifer, an SVE system emplaced into the fault zone to attempt extraction of volatile contaminants in the unsaturated zone above the water table, and a pumping system to contain the plume onsite if the plume could be defined. If the plume could not be defined due to the karst environment, a wellhead treatment unit could be installed at well 16. As the well was installed for water production, it could cost effectively be pumped to re-establish a cone of depression and help mitigate contaminant migration until a more optimum pumping well location is determined. Effective pumping scenarios should first be established to optimize removal of contaminated water. The water could be treated at the surface with an air stripping unit and used for irrigation under the current state approved wastewater permit. CSSA could thus effect water reuse and good land management. Alternatively, the treated water could be mixed with other water for public use. This alternative has been practiced at many Superfund sites (e.g., the South Valley site at Albuquerque, New Mexico).
These alternatives are shown in Table 9.2-1. Because of the degree of uncertainty and high costs associated with these alternatives, it is recommended that they be evaluated for feasibility after completion of the source removal actions and monitoring.