Date of Graduation

8-2013

Document Type

Thesis

Degree Name

Master of Science in Geology (MS)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

Van Brahana, John

Committee Member

Davis, Ralph K.

Second Committee Member

Zachry, Doy L.

Keywords

Earth sciences; Carbonate aquifer; Geochemistry; Hydrogeology; Hydrostratigraphy; Karst; Knippa gap

Abstract

The Edwards aquifer is one of the major regional karst aquifers in the United States, with an average withdrawal of 950 million liters per day (L/d). This investigation focuses on the connection between the Uvalde pool and the San Antonio pool of the Edwards aquifer, known as the Knippa Gap, west of the San Antonio metropolitan area in Uvalde County. This is a major zone of recharge to the Edwards aquifer and is approximately 6.4 km wide. The Knippa Gap is bounded by northeast trending faults of the Balcones Fault Zone (BFZ) on the north (specifically the Cooks and Trio Faults), and uplift from the Uvalde salient and igneous intrusive plugs to the south. Aspects of the hydrogeology in the Knippa Gap have been a topic of major interest among researchers in this area for numerous years, however, the exact location and nature of boundaries are undefined, and the discharge through this area is not accurately known. The input data from this investigation will allow for assessments of discharge, better water budget approximations for the San Antonio pool, and determination of accurate flow boundaries and budgets for Uvalde County. This investigation was limited to the transmissive (karstified) portion of the Edwards aquifer within the study area, and is based on previous studies, and newly collected data. The newly collected data include: 1) compilation of a complete table of wells within the study area; 2) redefined placement of flow boundaries (faults) most of which appear to be structurally controlled; 3) hydrostratigraphic analysis of the Knippa Gap area based on drilling and wireline logs; 4) characterization of the depth of karstification within the Knippa Gap; and 5) analyses of water quality within and contiguous to the study area. These data constrain a revised conceptual model of the flow and karstification in this critical area of recharge to the Edwards aquifer, and provide specific lateral boundaries and vertical karstification zones which can be tested quantitatively. Although current interpretations are tentative, it appears this conceptual model will be readily convertible into a digital model that can test hypotheses relating to water levels and spring discharges.

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