Reassessment of the Georgetown limestone as a hydrogeologic unit of the Edwards Aquifer, Georgetown area, Texas

Water-Resources Investigations Report 88-4190




The Edwards aquifer consists of geologic units known as the Comanche Peak (oldest) and Edwards Limestones, Kiamichi Formation, and Georgetown Limestone. The Edwards Limestone is the main water-bearing zone. The shallow geologic units dip to the east-southeast at a slope of 50 to 100 feet per mile in the Georgetown area. The Edwards aquifer extends from the western limits of the outcrop to the transition zone from freshwater to saline water to the east. The downdip continuity of the geologic units is interrupted by several faults in the Balcones fault zone. The aquifer is recharged by the infiltration of precipitation and streamflow in the outcrop and is discharged by springs in the outcrop and by wells, evapotranspiration, and leakage through the overlying confining bed in the confined area. Streams in the area regionally flow from the west to the east.

A reassessment of the uppermost geologic unit of the Edwards aquifer, the Georgetown Limestone, was conducted in the Georgetown area, Texas, using: (1) data from six surveys of streamflow gains and losses and ground-water levels, (2) aquifer tests at three clusters of test wells, (3) variation in water-quality characteristics to indicate ground-water circulation, and (4) previous studies. Data from the six surveys did not show a pattern of corresponding streamflow gains and losses with positive (upward) and negative (downward) head differentials, respectively, between the main water-bearing zone of the Edwards aquifer and the streams. A consistent and corresponding pattern was shown only for the subreach containing Berry Springs.

The aquifer tests consisted of "slug" test analyses to determine the transmissive characteristics of the Georgetown Limestone and produced 8 9 hydraulic conductivity values ranging from 1.4 X 10-8 to 2.8 X 10-9 centimeters per second at four of the six test wells. The other two test wells did not produce data suitable for conventional aquifer-test analysis.

An analysis of the water-quality characteristics suggests that the Edwards Limestone and the streams have a significant hydraulic connection but the ground-water circulation between the Edwards Limestone and the Georgetown Limestone is very limited. The only area where a high degree of hydraulic connection between the main water-bearing zone of the Edwards aquifer (Edwards Limestone) and the streams was found is near the updip limits of the Georgetown Limestone, where a nearby major fault occurs and where major springs have developed. These findings suggest that the Georgetown Limestone does not function as a unit of the Edwards aquifer but as a regional confining bed with localized avenues that allow flow to and from the underlying Edwards aquifer.

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USGS Numbered Series
Reassessment of the Georgetown limestone as a hydrogeologic unit of the Edwards Aquifer, Georgetown area, Texas
Series title:
Water-Resources Investigations Report
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Year Published:
U.S. Geological Survey
Publisher location:
Austin, TX
Contributing office(s):
Texas Water Science Center
v, 49 p.
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