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Groundwater Pumping Tests
CS-10 and CS-16
Section 2 - Literature Review
The purpose for conducting the literature review was to identify published aquifer testing results applicable to the Middle Trinity aquifer as a comparison to results of the pumping tests conducted for this project and to summarize and document broader knowledge of aquifer parameters than identified in this limited study. Information obtained during the literature review also aided in designing the parameters composing the aquifer analysis. The published literature sources included in the review included:
| | Ashworth, John B., 1983. Ground-Water Availability of the Lower Cretaceous Formations in the Hill Country of South-Central Texas, Texas Department of Water Resources (TDWR), Report 273. |
| | Hammond, Weldon, W., Jr., 1984. Hydrogeology of the Lower Glen Rose Aquifer, South-Central Texas, Ph.D. dissertation, The University of Texas at Austin, 43p. |
| | Mace, Robert E., Ph.D., Ali H. Chowdhury, Roberto Anaya, Shao-Chih (Ted) Way, September 2000, Groundwater Availability of the Trinity Aquifer, Hill Country Area, Texas: Numerical Simulations Through 2050, Texas Water Development Board � Report No. 353. |
| | Parsons, 1993. Hydrogeologic Report for Evaluation of Groundwater Contamination at Camp Stanley Storage Activity, Texas, March 1993 |
| | Arnow, Ted, 1959. Texas Board of Water Engineers Bulletin 5911, Ground-Water Geology of Bexar County, Texas, October 1959. |
The following paragraphs discuss the stratigraphy and aquifer properties reported for the Middle Trinity aquifer as identified within the published literature.
The Middle Trinity aquifer consists of materials representing the Lower Glen Rose Limestone Formation, the Hensell Sand Formation, and the Cow Creek Limestone Formation (Mace et al., 2000). Mace et al. further documents that the Middle Trinity aquifer is overlain by the Upper Glen Rose Limestone Formation and underlain by the Hammett Shale Formation. The Bexar Shale Formation is a local facies change of the Hensell Sand within the site vicinity. The Bexar Shale is not considered to be a principal water-bearing unit.
The Lower Glen Rose Limestone is approximately 300 feet thick and consists primarily of massive bedded limestone with a few layers of marl and marly limestone. Regionally, the Bexar Shale thickness averages from 70 to 150 feet. However, in the CSSA vicinity the Bexar Shale is typically 70 to 80 feet in thickness. Parsons (1993) documents that the Bexar Shale has been identified to function as an aquitard for groundwater communication between the Lower Glen Rose and the Cow Creek Limestone. It is composed of silty dolomite, marl, calcareous shale, and shaley limestone, and thins by interfingering into the Glen Rose Formation. The Cow Creek Limestone is a massive fossiliferous, white to gray, shaley to dolomitic limestone that attains a maximum thickness of 90 feet in the CSSA area. However, because most water production wells in northwest Bexar and Kendall Counties are open borehole completions, the Lower Glen Rose and the Cow Creek Limestones are considered hydraulically connected.
The CSSA Environmental Encyclopedia (Volume 1-1, Background Information Report, Soils and Geology) documents elevations of the various geologic formations comprising the Middle Trinity aquifer in the vicinity of both groundwater pumping tests conducted for this project. The Mean Sea Level (MSL) elevations of these formations are summarized in the Table 1.
Table 1
Upper and Lower Middle Trinity Aquifer Formation Contact Elevations (feet above MSL)
| | Lower Glen Rose Formation | Bexar Shale Formation | Cow Creek Formation |
| Site Area | Groundwater Elevation* | Lower Contact** | Upper Contact** | Lower Contact** | Upper Contact** | Lower Contact (Estimated) |
| CS-10 | 1,036 | 910 | 910 | 827 | 823 | 759 |
| CS-16 | 1,037 | 925 | 925 | 855 | 855 | 790 |
| *Groundwater Elevation measured at the start of each respective pumping test. |
| ** Based on formational contacts picked from cross section A-A� and B-B (Figures 6 and 7 of Volume 1-1, Background Information Report) �. Contacts at CS-10 have been inferred from borehole geophysics performed at well CS-11. |
Parsons (1993) characterizes the Middle Trinity aquifer as a water table (unconfined) aquifer. In his dissertation, Hammond (1984) described regionalized and localized aquifer parameters that can be encountered in the Lower Glen Rose aquifer. According to Hammond, both regional and local groundwater systems occur within the Lower Glen Rose portion of the Middle Trinity aquifer. The regional system is dominated by syndepositional porosity where groundwater moves within poorly interconnected avenues of low permeability. Localized systems produced by post-depositional solution activity are characterized by megaporic voids, solutional channels, and caves. The regional trend of fractures controls the occurrence of localized systems, orientation of caves, and locations of many of the modern streams.
Hammond (1984) identified the transmissivity of the Lower Glen Rose as typically ranging from 240 gpd/ft to 3,220 gpd/ft for the regional system, and 5,740 to 16,110 gpd/ft for higher porosity localized systems. Furthermore, Hammond (1984) identified the typical range in hydraulic conductivity for the Lower Glen Rose regional system as varying from 0.73 gpd/ft2 to 22 gpd/ft2 (3.4 x 10-5 centimeters per second [cm/sec] to 1.0 x 10-3 cm/sec). Hydraulic conductivity values ranging from 29 gpd/ft2 to 74 gpd/ft2 (1.4 x 10-3 cm/sec to 3.5 x 10-3 cm/sec) were reported for localized permeable systems. A pumping test conducted at Camp Bullis, adjacent and east of CSSA, in association with the Hammond (1984) study, identified the transmissivity and hydraulic conductivity of the Lower Glen Rose as 3,220 gpd/ft and 15 gpd/ft2 (7.1 x 10-4 cm/sec), respectively. The high transmissivity and hydraulic conductivity values identified for local areas are due to discriminating (i.e., heterogeneous) secondary porosity induced by dissolution of limestone.
The findings of Hammond (1984) are corroborated by Ashworth (1983) who reported an average transmissivity value of 1,700 gpd/ft for the Middle Trinity aquifer. Hammond (1984) cited that pumping tests conducted for the Middle Trinity aquifer in Kerr and Kendall Counties identified a storativity value of 0.00003.
As reported in the Texas Board of Water Engineers Bulletin 5911 (Arnow, 1959), an analysis of wells in the CSSA and Camp Bullis vicinity shows that well yields decrease from east to west. It is possible this condition is a function of distance from the Edwards Limestone Formation outcrop. The Edwards Limestone outcrop denotes the location of the Balcones Fault Zone (BFZ). Well yields at Camp Bullis to the east were reported to be about four times greater than at CSSA (Arnow, 1959). Because groundwater exits the Glen Rose into the BFZ of the Edwards Limestone, the groundwater flow velocity increases toward Camp Bullis. Proportionally, this causes more dissolution of the Glen Rose limestone along joints, bedding planes, and faults, resulting in higher well yields at Camp Bullis as compared to CSSA.
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