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Hydrogeologic Report for Evaluation of Groundwater Contamination
Section 4 - Site Geology and Hydrogeology
The rock formation exposed at the surface at CSSA is the upper member of the Glen Rose Formation. Using information from published reports on this area and a site walk through the installation as discussed in the geologic survey in Section 5, the thickness of the upper Glen Rose outcrop is estimated to range from less than 10 feet to about 150 feet. The outcrop is at its thinnest in the Salado Creek channel which traverses the site from north to south (Figure 2.2). The lower member of the Glen Rose is not exposed at the installation, but is exposed in the Cibolo Creek channel about 2 miles north of the site. The lower member, according to area well logs, is approximately 300 feet thick in the CSSA area.
Beneath the Glen Rose Formation is the Travis Peak Formation (Table 3.1). The Travis Peak Formation is divided into five members and is approximately 625 feet thick. The Glen Rose and Travis Peak Formations overlie folded, metamorphosed Paleozoic-age rocks.
Figure 4.1 is a geologic map of CSSA showing the line of cross-section and pertinent geologic features. Figure 4.2 is a detailed cross-section of the CSSA facility and adjacent property. These figures were constructed from a site geologic survey visit (discussed in Section 5) and TWC and CSSA records of wells. The geologic map was developed using a previous geologic map (Watterreus 1992), the site visit, and the Geologic Atlas of Texas San Antonio sheet.
The major structural features in the CSSA area are the Balcones system of faults, the regional fracture pattern, and the regional dip. These structures all play a role in groundwater occurrence and movement. Faults and fractures may act as vertical conduits, while the regional dip usually influences the groundwater flow direction. Faults may also act as a hydrologic barrier to groundwater flow.
In the CSSA area, one normal fault has been identified and there is a possible fault or fracture. A known fault traverses the southern portion of the facility (Figure 4.1 and Figure 4.2). An inferred fault (Watterreus 1992) located north of the CSSA facility near Cibolo Creek is shown on Figure 4.1 and Figure 4.2. The fault trends northeast across the well 16 area, but was not identified in the field. The faults are dashed where the locations are estimated.
The dense regional fracture pattern is two series of vertical fractures, one trending northeast-southwest and one trending northwest-southeast. Additionally numerous minor fractures exist in the CSSA area. Most of them are not laterally extensive, but do reach to the groundwater zones. Most streams and rivers follow the northwest-southeast-oriented fractures because of the topographic relief and the northwest-southeast-trending fractures. The local stratigraphic dip is to the south-southeast in the CSSA area.
The CSSA facility is underlain by rocks that make up the Trinity aquifers. These aquifers are three separate hydraulic units: the upper member of the Glen Rose Limestone (upper Trinity aquifer), the lower Glen Rose and Cow Creek Limestones (middle Trinity aquifer), and the Sligo Limestone and Hosston Sand (lower Trinity aquifer) (TWDB 1983).
The lower Trinity aquifer is not used in the CSSA area because of its the low production and the high cost of well completion. The coefficient of transmissivity was determined to be 900 gpd/ft for well 68-19-501, which is located 3 miles west of CSSA and completed in the Hosston Sand (TWDB 1983). The upper Trinity aquifer is also not used widely as a source because of its low production and poor water quality (TWDB 1983). No transmissivity values have been determined for the upper Trinity aquifer.
The primary source of water in the wells at CSSA is the middle Trinity aquifer, the most prolific water producer with the best quality in the area (TWDB 1983). All CSSA wells are completed as open holes without well screens. To maximize yield, wells completed in the middle Trinity are also open to the upper Trinity aquifer. This aquifer is under water table conditions at CSSA because of outcropping near the site.
The CSSA wells are located on a site map (Figure 2.2). The unused livestock supply wells (wells 2, 3, and 4) have depths and water levels that indicate a middle Trinity water source. The livestock wells that are currently in use in the north pasture (wells G and H) were not measured by ES because of piping in the well, but are most likely completed in a manner similar to the unused wells.
Surface infiltration is the primary recharge mechanism, in which water enters the bedrock through the soil and travels through minor and regional fractures and solution channels to the water table. Many solution channels and minor fractures act as groundwater conduits in the middle Trinity. Although the Bexar Shale may act as a vertical migration barrier, the Cow Creek Limestone and the lower Glen Rose Limestone are in hydraulic communication through the open-hole well completions. Therefore, the Hammett Shale is the lower boundary of the middle Trinity aquifer.
Under nonpumping conditions, the general direction of groundwater flow in the middle Trinity aquifer is to the southeast at an average rate of 0.003 foot per foot as measured in October 1992. Figure 4.3 depicts the general potentiometric surface of the middle Trinity aquifer. Under pumping conditions the flow is locally diverted to each pumping well. Flow velocities calculated by carbon 14 analysis indicate a regional flow that ranges from 13.6 to 15.9 feet per year (Hammond 1984).
This production capacity of each well is dependent on the well's location with respect to water-producing fractures and solution cavities. Well number 11 is reported to break suction within 30 minutes if run at 80 gallons per minute (gpm), while wells 9 and 10 are reported to maintain this rate over much longer periods. No information is available on the production capacities of the other wells.
Groundwater in the middle Trinity is primarily of the calcium-magnesium-bicarbonate type, and total dissolved solids content is usually 500 mg/L (TWDB 1983). Figure 4.4 is a Piper diagram showing the relationship of water samples obtained from the distribution systems at CSSA and the Fair Oaks Water Company located 1.5 miles to the north. Fair Oaks obtains its municipal supply from the middle Trinity aquifer. The Piper diagram illustrates that the two waters are similar in natural water quality. The water quality data from selected water supply wells used in Figure 4.4 are presented in Appendix A.
No groundwater samples from individual CSSA wells have been analyzed for natural water quality parameters.
Distance to Edwards Aquifer Recharge Zones
According to the recharge and transition zone maps accessible through the Edwards Underground Water District, the closest recharge zone to CSSA is about 0.5 mile northeast of CSSA's northeast corner (EUWD 1987). This recharge is found along Cibolo Creek and is the only recharge to the Edwards aquifer of the lower Glen Rose Formation. Only a small portion of CSSA, 200 to 400 feet northeast of wells E, F, and G, drains to Cibolo Creek (Army 1991a). This recharge zone is approximately 1.5 miles from well 16.
A second recharge zone to the Edwards aquifer is located about 4 miles southeast of CSSA, and a transition zone is about 1 mile south of that recharge zone. The majority of CSSA land drains southeast into the Salado Creek drainage area, with the exception of the southwest CSSA area that drains to Leon Creek (Army 1982).