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Well Upgrade Technical Report
Section 2 - Background
CSSA is located in northwestern Bexar County, approximately 19 miles northwest of downtown San Antonio in south-central Texas (Figure 2.1). The installation consists of 4,004 acres immediately east of State Highway 3351 and is approximately 0.5 miles east of Interstate Highway 10.
There are 14 active wells on CSSA identified by alphabetic or numeric designations (Figure 2.2). Three of these wells (CS-G, CS-H, and CS-I) provide water for stock and wildlife, seven wells are dedicated monitoring wells (CS-MW1-LGR, CS-MW2-LGR, CS-2, CS-3, CS-4, CS-D, and CS-16), and four provide CSSA with potable water (CS-1, CS-9, CS-10, and CS-11). CSSA water well upgrades of Wells CS-1, CS-9, and CS-11 are necessary to ensure a steady supply of potable water and fire suppression capability during periods of drought.
CSSA was initially part of the Leon Springs Military Reservation. State records list wells that are potential predecessors to existing CSSA wells. These �predecessors� were drilled in 1940 (Well CS-1, possibly drilled in 1918), and 1918 (Wells CS-9, CS-10, and CS-11) (Texas Board of Water Engineers 1956). Total depths listed vary from over 1,000 feet below ground level (BGL) to 370 BGL. Historical records for the wells, particularly Well CS-1, are incomplete, and identification of the wells is ambiguous.
All depths discussed from this point are referenced by depth below ground level unless otherwise stated.
Well CS-1 is located on Camp Bullis property immediately to the east of the CSSA inner cantonment (Figure 2.2). The well is enclosed in a cinder block well house and water is piped underground approximately 1.5 miles to the CSSA water storage reservoir. The well house is surrounded by a 10-foot chain-link fence with barbed wire strung on top. In December 1996 CSSA took Well CS-1 off line when it repeatedly failed bacterial tests.
Wells CS-9, CS-10, and CS-11 are located below the water reservoir on McFarland Hill, on the western edge of the inner cantonment (Figure 2.2). All three wells are within a 1,000-foot radius of each other and housed in prefabricated metal sheds placed on concrete slabs. All three wells share a similar layout, the well head being roughly 3 feet above ground level and extending from a 5-foot by 5-foot concrete sump. In 1989, CSSA contracted Louis Bergmann & Sons to perform pump tests on Wells CS-9, CS-10, and CS-11 and acidize Well CS-10. These actions were performed on October 11, 1989 through January 16, 1990.
Well CS-11 is located 1,000 and 700 feet northwest of Wells CS-9 and CS-10, respectively. A new pump was installed in CS-11 on October 11, 1989. A pump test was performed and well yield was measured at 120 gallons per minute (gpm). The well was noted to pump dry after 19 minutes (January 16, 1990). Prior to the upgrade, Well CS-11 consistently failed bacterial tests due to high silt content (CSSA personal communication 1997). Pump and piping were removed from Well CS-11 by Parsons subcontractor, Venture II, Inc., on December 14, 1995 in preparation for well upgrades. Water level measurements were taken by the air-line method prior to piping removal in 1995.
Well CS-9 is approximately 700 feet southwest of Well CS-10. Historic operations indicate that the water level in Well CS-9 was drawn down to the pump intake after a few hours of operation (CSSA personal communication 1997). A pump test performed on October 11, 1989 provided a yield of 76.7 gpm. The water level was drawn down to the pump intake after 65 minutes.
Well CS-10 lies approximately 700 feet southeast of Well CS-11. This well was upgraded by acidification rather than carbon dioxide treatment; however, the acidification offers useful comparative data. A specific capacity test of 2.5 gpm per foot of drawdown was measured on October 11, 1989. The well underwent acidification October 13, 1989. Twenty thousand gallons of hydrochloric acid were injected during the process. Post acidification, specific capacity was measured at 4.3 gpm per foot of drawdown. Before treatment, the well would break suction after several hours of pumping. After treatment, the well was able to operate for several weeks without breaking suction (Louis Bergmann & Sons, letter 1990). Based on increased duration of pumping and increased specific capacity, CSSA considered the acidification at Well CS-10 to provide an adequate increase in production.
Three aquifers are present in the area of CSSA: the upper, middle, and lower Trinity aquifers. These divisions are based upon hydraulic continuity. The Travis Peak Formation and the Glen Rose Formation are the principal water bearing units. Beneath these are metamorphosed Paleozoic rocks which act as a lower hydrologic barrier. Generally, the depths-to-water at CSSA range from approximately 150 to 260 feet bgl.
The primary groundwater source at CSSA is the middle Trinity aquifer, the most prolific producer with the best quality of water of the three Trinity aquifers. These wells are typically completed as open holes without well screens. To maximize yield, water wells in the area that are completed in the middle Trinity are also open to the upper Trinity aquifer.
The middle Trinity aquifer appears to be unconfined in the CSSA area ( Parsons 1996). The average coefficient of transmissivity in the middle Trinity aquifer is 1,700 gallons per day per foot (gpd/ft). Groundwater flow is toward the south and southeast. Flow velocities calculated by carbon-14 analysis indicate a regional flow that ranges from 13.6 to 15.9 feet per year (Hammond 1984). Middle Trinity discharge occurs both artificially through well use and naturally by springs and seeps. Groundwater geochemistry is typical for calcium magnesium bicarbonate type, with total dissolved solids (TDS) content usually less than 500 milligrams per liter (mg/l) (Ashworth 1983).
The Edwards aquifer is not found in the CSSA area. However, the Edwards Underground Water District (EUWD) has defined recharge and transition zones of concern for the Edwards aquifer. Two such zones are located north and southeast of CSSA and are shown on Figure 2.3. One recharge area is along Cibolo Creek where the Lower Glen Rose outcrops. This is the only area of the Lower Glen Rose that is defined as a recharge zone to the Edwards aquifer. The closest area of this recharge zone is 0.5 miles north from the northeast corner of CSSA. A second recharge zone located on Edwards limestone is about 4 miles to the south-southeast. A transition zone is 5 miles southeast of CSSA.