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Hydrogeologic Report for Evaluation of Groundwater Contamination

Section 6 - Conclusions and Recommendations

Limestone of the upper Glen Rose Formation is exposed in most areas at CSSA. Thin soil cover and stream alluvium covers the remaining area. The upper Glen Rose is estimated to be 10 to 150 feet at CSSA before the lower Glen Rose is encountered. Beneath the lower Glen Rose Formation is the Travis Peak Formation, which is approximately 625 feet thick and divided into five members. The Glen Rose Formation and the Travis Peak Formation overlie metamorphosed Paleozoic rocks.

Two normal faults and numerous regional and minor fractures traverse the CSSA area. A degree of structural control of the streams in the area is observed, and both topographic relief and regional dip is to the southeast.

The regional geology and hydrogeology corresponds with the site geology and hydrogeology identified during the preliminary work. Groundwater under the CSSA area occurs in three separate aquifers; the upper, middle, and lower Trinity aquifer (lower Glen Rose and Cow Creek Limestone). This is the sole source of water at CSSA, which has nineteen water wells in the middle Trinity aquifer. Of these nineteen wells, all of which were surveyed for horizontal location and casing elevations, four are currently used as drinking water supply, three are used as livestock supply, and eleven wells are inactive, unused, or abandoned. Of the unused or abandoned wells, six are not sealed and are susceptible to surface infiltration.

The middle Trinity aquifer is characterized as a water table aquifer in which surface infiltration and stream flow losses are the primary recharge mechanism. Recharge is relatively fast after heavy rainfall. Groundwater travels through fractures and solution enlarged cavities above and below the water table, and these conduits act as flow channels in both the unsaturated and the saturated zone. Regional groundwater flow is to the southeast at a velocity of 13.6 to 15.9 feet per year, and the local hydraulic gradient was estimated at 0.003 foot per foot.

Numerous zones of vuggy porosity and high permeability were observed in wellbores during the downhole camera survey. Depth to water was approximately 150 feet BGL. Water was observed entering the wellbores of wells 2, 3, 4, 16, and D at elevations of 1,150 to 1,170 feet above mean sea level (approximately 50 to 85 feet BGL) in the vadose zone indicating horizontal and vertical pathways. Small vertebrates such as frogs were seen in wells, and objects including metal and wood pieces were also observed in the boreholes.

Dissolved halogenated volatile organics were detected in wells 2, 3, 4, 16, and D in November 1992. No phase-separated liquids were found in any of the wells. Previous investigations by the TWC and TDH in 1991 indicated these compounds in wells 4, 16, and D at slightly higher concentrations. The groundwater contamination appears to be limited to the north-central area of CSSA. Seven previous waste management areas were identified as potential source areas on aerial photos and during site walks of CSSA.

The extent of groundwater contamination and the amount of migration are unknown. According to current data, the contamination is known to have affected one drinking water supply well (CSSA well 16), which was shut down immediately after the detection of the contaminants. This well is completed as open hole construction in the lower Glen Rose Formation, the Bexar Shale, and the Cow Creek Limestone. No other active supply wells at CSSA have been impacted. The closest active supply well that is downgradient of well 16 is well 1, located about 1.75 miles southeast of well 16. The farthest distance from well 16 to a known impacted well is 1,635 feet to the west-southwest to well 2. No information is available on possible groundwater impacts upgradient and east sidegradient of well 16.

The potential relationship between two faults at CSSA and groundwater movement is not known. The fault or fracture that may be within 100 feet and upgradient of well 16 was noted in only one publication. Surface indications of faulting in this area were not observed during the geologic survey at CSSA. As the offset may be slight, the presence of this fault or fracture may not be identified except as unusual data results that indicate boundary conditions during as aquifer pump test. If that fault or fracture is present, the potential exists for increased migration of contaminants in groundwater along the displacement zone. Similarly, if the dissolved contaminant plume migrates or has migrated to the known fault trending northeast-southwest across the southern portion of CSSA, it is probable that the dissolved contaminants will migrate along that fault plane.

The contaminant source is also unknown. It is suspected that the source area(s) is near well 16, but no other supply wells appear to be threatened at this time. The rate of contaminant migration is estimated at 16 feet per year based on regional groundwater velocities. It does not appear that the two Edwards aquifer recharge zones closest to CSSA (1.5 miles north of well 16 and 5.75 miles southeast of well 16) are threatened or impacted at this time.

Recommendations

Based on the hydrogeologic information summarized above, ES recommends that further work be performed to accomplish the following goals:

An SI of groundwater contamination will be performed to accomplish the above goals. This will include drilling and sampling of soil borings, monitoring well construction, downhole geophysical logging, and groundwater sampling. Soil and groundwater samples will be analyzed for selected chemical constituents to determine the nature and level of contamination. Soil samples will also be tested for geotechnical parameters to determine soil physical characteristics. Aquifer tests will be performed to determine geohydraulic parameters such as transmissivity, storativity, well yield, and boundary conditions potentially related to faulting. Groundwater modeling will also be performed if necessary to evaluate remedial options and groundwater recovery system with respect to specific contaminants. The specific actions recommended to accomplish this SI are further discussed in the work plan for groundwater evaluation (ES, March 1993).

In addition, ES recommends that some existing water wells may need to be plugged or upgraded to conform to current well construction specifications outlined in the Texas Water Well Drill Board (TWWDB) "Permanent Rule Changes" in 31 Texas Administrative Code (TAC) Chapter 287 and the Texas rules and regulations for public water systems, 31 TAC 290.38-290.41.

The two recommendations for upgrading CSSA wells to meet current regulations are: (1) plugging the entire depth of the well with cement of bentonite mud, or (2) construction of surface completions and capping of existing unused wells. Some unused wells may require additional casing to seal off shallow water zones to prevent migration of undesirable waters into the wells.

Water supply wells 1, 9, 10, 11, G, H, and I may not conform to all specifications in the newly promulgated 31 TAC 290.41. These wells should be more closely evaluated with respect to the following items:

These wells appear to conform to other specifications stated in 31 TAC 290.41. The wells and the entire water supply system at CSSA, however, will be evaluated more fully in the ES environmental assessment report (due to CSSA as a draft in the spring of 1993).

Water supply wells 1, 9, 10, 11, G, H, and I do not appear to be threatened by groundwater contamination at this time. Wells G, H, and I are located upgradient of well 16 and water sampling has indicated no impact to the groundwater in these areas. Wells 9, 10, and 11 are located up and side gradient of well 16 and water sampling has indicated no impact to groundwater in this area. Well 1 is located 1.75 miles downgradient of well 16 and again water sampling has indicated no impact to groundwater in this area.

Currently wells E and F which are upgradient of potential source areas and well 16 are plugged. Well E is plugged at the surface, adn well F is plugged at 20 feet BGL. No information on te plugging method or depth is available. These wells do not appear to be at risk from potential source areas at this time; therefore, no action is necessary. Based on the results of the SI, the status of these wells will be reevaluated for potential risk for migrating contamination.

Well 5 and 6 are unused at his time. If CSSA plans to use these wells in the future, then proper surface completions should be constructed. Additional casing may not be needed if the potential for shallow water contamination is deemed minimal. If CSSA does not plan to use these wells, then they should be properly plugged according to TWWDB specifications.

Wells A, B, and C are currently abandoned and or plugged. Wells A and C have concrete at the surface and no information is available on plugging. Well B is plugged at 8 feet BGL and no information is available on the plugging depth and method. This well should be plugged to ground surface with grout to prevent surface water infiltration. Based on the results of the SI, an evaluation will be made as to whether these wells A and C should be further investigated.

Wells 2, 3, 4, and D will be upgraded or plugged. To reduce SI costs, it is recommended that these wells be upgraded for use as monitoring wells during the SI rather than plugging them and installing new monitoring wells.

Well 16 is currently inactive. This well should be upgraded for use as a groundwater monitoring and pumping test well during the SI.

All plugging and upgrading should be performed by a State of Texas licensed water well driller with proper OSHA hazardous waste training. Appropriate well plugging forms will be sent to the TWWDB within 30 days of plugging.

In addition to evaluating future well upgrades or plugging, ES recommends that CSSA address the caves found during the hydrogeologic evaluation. The caves may have archaeological significance or contain endangered species. It is beyond the scope of this preliminary evaluation to perform such an assessment.