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Background Information Report

Groundwater Resources

Groundwater Hydrology

Groundwater occurrence and movement at CSSA has been studied since 1992, and in part due to the complex karst environment, it continues to be studied. The results of a preliminary evaluation of groundwater contamination in 1992 were included in a report entitled "Hydrogeologic Report for Evaluation of Groundwater Contamination at Camp Stanley Storage Activity, Texas," (ES, 1993d). This work was updated in the “Groundwater Investigation and Associated Source Characterizations” (Parsons ES, 1996d).

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 principle 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 200 to 300 ft below ground level (bgl).

Lower Trinity aquifer. The lower Trinity aquifer is made up of the Hosston Sand and the Sligo Limestone of the Travis Peak Formation. These strata do not outcrop in the study area. The Hosston and the Sligo thicken in a downdip direc­tion (south and southeast) to as much as 500 combined feet near the Balcones fault zone. At CSSA, they have an average combined thickness of 370 feet. The lower boundary of the lower Trinity aquifer is composed of Paleozoic basement rocks.

The lower Trinity aquifer receives infiltration recharge from the overlying Hensell Sand (Bexar Shale) in the updip direction, where the overlying Hammett Shale and Cow Creek Limestone are thin or absent. Some recharge may come from leakage of overlying strata in areas where the Hammett Shale is disrupted by faulting. The lower Trinity aquifer is artesian, the Hammett Shale acting as a hydraulic confining unit. The average coefficient of trans­missivity in the lower Trinity aquifer is approximately 10,000 gallons per day per foot (gpd/ft) (Ashworth, 1983). The groundwater gradient is generally to the south and southeast except in areas of continuous pumping, where flow is directed towards the active wells. Discharge from the lower Trinity aquifer occurs primarily by pumping from wells.

Middle Trinity aquifer. The middle Trinity aquifer consists of the Cow Creek Limestone, the Bexar Shale (Hensell Sand), and the lower Glen Rose Limestone. The average combined thickness of the aquifer members is approximately 460 feet. The only member found in outcrop is the lower Glen Rose, which has been mapped north of CSSA along Cibolo Creek and within the central and southwest portions of CSSA (Figure 5).

The lower Glen Rose portion of the middle Trinity aquifer derives its recharge from direct precipitation on the outcrop and stream flow infiltration. Stream flow loss has been observed in Cibolo Creek between the towns of Boerne and Bulverde, where stream flow is diverted underground via sinkholes except during flood stages. This is the only area of lower Glen Rose that is considered to be part of the recharge zone for the Edwards aquifer (Edwards, 1987). In fact, the Glen Rose Limestone is sometimes considered to be a confining unit below the Edwards aquifer (USGS, 1985).

In the area of CSSA, the Bexar Shale acts as a hydrologic barrier to vertical leakage except where faulted; therefore, most recharge to the Cow Creek Limestone comes from overlying updip formations. It is inferred that the Cow Creek is in natural hydraulic communication with the lower Glen Rose. The middle Trinity aquifer appears to be unconfined in the CSSA area.

The average coefficient of transmissivity in the middle Trinity aquifer is 1,700 gpd/ft. Groundwater flow is towards 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 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 completed as open holes without well screens. To maximize yield, typical water wells in the area that are completed in the middle Trinity are also open to the upper Trinity aquifer.

There were nineteen water wells at CSSA (Figure 5). These wells have been used for drinking water supply and livestock. Wells 1, 9, 10, 11, G, H, and I are the only active wells remaining. Two Glen Rose monitoring wells were added in April 1996. Wells 2, 3, 4, D, and 16 are currently used for groundwater monitoring. Well 1 is used by CSSA, though located on Camp Bullis property. Five wells were plugged in September 1994 and reported to TNRCC.

Groundwater movement at CSSA in the middle Trinity aquifer was defined using October 1992 through May 1997 water well data. Groundwater elevations were calculated from measured depths to static water levels in the nineteen water wells. Figure 8 is the groundwater potentiometric map from June 1995 data. Groundwater flow direction is generally to the south-southeast with gradients of 0.003 to 0.06 foot per foot.

Prior maps indicated flow gradients southwest and southeast of B-3 and O-1. In this April 1996 map, groundwater now appears to flow from well 16 and D towards the source areas B-3 and O-1. Figure 8 presents two possible explanations of groundwater flow.

Upper Trinity aquifer. The upper Trinity aquifer consists of the upper Glen Rose Limestone. Recharge to the upper Trinity aquifer is from direct precipitation to upper Glen Rose Limestone outcrop and from stream flow infiltration. Additional recharge in the Camp Bullis area occurs from reservoirs built by the NRCS on Salado and Lewis Creeks, approximately 1 mile south of CSSA (USGS, 1973).

No transmissivity values have been determined for the upper Trinity aquifer. Movement of groundwater in the upper Trinity is restricted to lateral flow along bedding planes between marl and limestone, where solution has enhanced permeability. Static water levels in adjacent wells completed in different beds within the upper Glen Rose are not concordant, demonstrating the possibility that beds are not hydraulically connected by avenues of vertical permeability. The only place where extreme development of solution channels has been reported is in evaporite layers in or near the outcrop of the upper Glen Rose Lime­stone. Discharge from the upper Trinity aquifer is predominantly from natural rejection through seeps and springs and from pumping.

The upper Trinity aquifer is, in general, unconfined. Fluctuations in water levels in the upper Trinity are predominantly a result of seasonal rainfalls and pumping from domestic and public wells.

Groundwater Quality

The following information is summarized from TWDB report 273 (Ashworth, 1983), “Hydrogeology of the Lower Glen Rose Aquifer” (Hammond, 1984), and “Hydrogeology of the Camp Bullis Area” (Waterrus, 1992).

Lower Trinity aquifer. In the CSSA area, the lower Trinity aquifer yields fresh to slightly saline water with a TDS content ranging from 900 to 1,500 mg/l. Approximately 40 miles updip of the area, the aquifer yields water with a TDS content of 500 mg/l or less. The lower Trinity aquifer is not used as a primary source of water. The overlying Hammett Shale is reported to be a heaving shale and must be cased off in order to complete a well in the lower Trinity aquifer. The high cost of constructing a well in the lower Trinity makes it economically unfeasible to pump for most purposes.

Middle Trinity aquifer. The middle Trinity aquifer yields fresh to slightly saline water throughout the area. Water in the lower member of the Glen Rose Lime­stone is normally very good quality, although hard. This water increases in dissolved solids content near the Balcones fault zone owing to an increase in sulfate ions. No wells are known to have been completed only in the Cow Creek Limestone; there­fore, the water quality of that limestone member is unknown. The middle Trinity aquifer is the most widely used source of water in the area. Most water wells in the area obtain middle Trinity water for public supply, irrigation, domestic, and livestock purposes.

Upper Trinity aquifer. Upper Trinity water is generally of poor quality and most wells achieve only low production. Evaporite beds in the upper Trinity introduces excessive sulfate in the water. Few wells obtain water solely from the upper Trinity aquifer.

Edwards aquifer recharge and transition zones. 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 near the Edwards aquifer. Two such zones are located north and south of CSSA. 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 mile 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.