Technical Reports Document Library

Notes:
There is a problem reading pp 3-5 of the pdf file.
Data from Attachment A are available from the Authority upon request.

The purpose of this study was to (1) develop an updated conceptual model of the groundwater systems in Uvalde County that incorporates existing and recently collected data on hydrology, geochemistry, and structural geology, and (2) clearly define the hydrogeologic relationship between the Uvalde pool and the San Antonio pool of the Edwards Aquifer.  The area of investigation consisted of Kinney and Uvalde counties and included the groundwater divide in Kinney County to the west and the Knippa Gap to the east.  It includes all the of the principal aquifers in Uvalde and Kinney counties.  The study was completed in June 2006 by Southwest Research Institute, San Antonio, Texas.  Note: Attachment A data are available from the Authority upon request

[From the Executive Summary]
…The objective of this investigation was to evaluate the groundwater systems in Uvalde County and develop an updated conceptual model that (i) incorporates existing and newly collected data on hydrology, geochemistry, and structural geology, and (ii) clearly defines the hydraulic and hydrogeologic relationship between the Uvalde pool and the San Antonio pool of the Edwards Aquifer. It is imperative to have an accurate, comprehensive understanding of this relationship to be able to effectively manage the water resources of the Edwards Aquifer in Uvalde County and, more generally, all water sources associated with the Edwards Aquifer in and beyond Uvalde County. The boundaries of the study area coincide with those of Kinney and Uvalde counties.

Refined or new conceptual models of the groundwater systems were developed for Kinney and Uvalde counties. Assessment of the stratigraphic and facies geology, structural geology, water chemistry, and hydrogeology provided the basis for the designation of a separate pool in the Edwards Aquifer in Kinney County which extends from a groundwater divide between Mud and Pinto creeks on the west to a zone of low permeability near the Kinney County/Uvalde County line on the east. All evidence suggests that recharge from the West Nueces River basin recharges the Kinney County pool, not the Uvalde pool of the Edwards Aquifer. This new conceptual model does not assert that there is no groundwater flow from the Kinney County pool to the Uvalde pool, only that there is limited or minimal flow.

Evaluations indicate that an average of 70,000 acre-ft annually recharges the Kinney County pool. Of this, approximately 6,000 acre-ft are annually removed by pumping and 20,000 acre-ft are discharged from Las Moras, Pinto, and Mud springs. This hypothesized conceptual model for the Kinney County pool contends that there is sufficient opportunity for discharge of the remaining 44,000 acre-ft, primarily through floodplains, nearby hydrostratigraphic units, and channels associated with rivers and creeks to the south of the Edwards Aquifer recharge zone.

The structure of the Knippa Gap was evaluated to help understand groundwater flow in Uvalde County. Geologic structural maps of the Edwards Aquifer provided an improved understanding of the principal components of groundwater flow through the Knippa Gap. Of greatest interest in this interpretation is the structure of the base of the Edwards Aquifer to the east of the City of Uvalde. There is a prominent ridge in the base of the Edwards Aquifer called the Uvalde Salient that reduces the saturated thickness of the Edwards Aquifer from 600 ft, to the east and west of the Uvalde Salient, to as little as 400 ft at the Uvalde Salient. The resulting topography of the base of the Edwards Aquifer is a series of prominent highpoints in the ridge that constrict groundwater flow through topographic saddles between the high points. This constriction results in a 4-mile wide high-capacity flow channel through central Uvalde County located immediately north of the City of Knippa. Cooks Fault forms its northern boundary and the structure of the Knippa Gap effectively forms its southern boundary. The chemistry of water in the channel is different from water south of the channel. Groundwater elevations suggest a groundwater trough coincident with the channel, which is an indication of high-transmissivity flow. Groundwater flow in the channel is essentially doubled by the addition of recharge from the Frio River-Dry Frio River basin at the Knippa Gap. The combined effect of the structural constriction and focused recharge at the Knippa Gap cause a damming of groundwater upgradient and to the west of the Knippa Gap.

Recharge and discharge volumes of most of the Edwards Aquifer in Uvalde County are highly uncertain, although progress was made during this investigation to limit this uncertainty. Recharge from the West Nueces River is believed to recharge the Kinney County pool, not the Uvalde pool. Discharge from the Edwards Aquifer through floodplains, nearby hydrostratigraphic units, and channels associated with the Nueces, Leona, and Frio rivers is hypothesized by this investigation to significantly exceed previous estimates. These conceptual changes call for rethinking of the existing water balance calculation for Uvalde County which is based on the long-term average of 226,300 acre-ft/yr of recharge from the Nueces River-West Nueces River and the Frio River-Dry Frio River basins. Using reasonable assumptions and characterizations, it is calculated that approximately 270,000 acre-ft/yr flows through the high-capacity flow channel from the Uvalde pool to the San Antonio pool of the Edwards Aquifer.

Although the assumptions inherent in this calculation make this estimate uncertain, it appears likely that total recharge in Uvalde County at least meets, if not exceeds, previous estimates. If the hypotheses provided in this report are valid, then recharge from the Nueces River and the Frio River-Dry Frio River basins, distributed recharge, and inflow from the Trinity and Austin Chalk aquifers, needs to be evaluated to improve estimates of the water balance of the Uvalde pool.