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Crystalline Basement Aquifer, Llano Uplift, Central Texas:
An Overlooked Minor Aquifer of Texas
Brian B. Hunt
Springs/Edwards Aquifer Conservation District, 1124 Regal Row, Austin, Texas 78748
Fractured and weathered Precambrian igneous and metamorphic rocks of the Llano Uplift are an important groundwater resource and constitute a minor aquifer of Texas. The Crystalline Basement Aquifer (CBA) covers about 1290 mi2 (3350 km2) in portions of seven counties sustaining streams and providing rural water supplies estimated at greater than 3600 acre-ft/yr. A conceptual model derived from literature review, well database and geographic information system (GIS) analysis, geologic mapping, and well field studies is presented.
The CBA appears similar to other hard rock aquifers recognized throughout the world. Hydrogeologic features include a vertical layering consisting of an upper weathered zone and middle fractured zone forming a near-surface aquifer generally conformable to the land surface with shallow depth to water (23 ft [7 m] median). Flow in the aquifer is localized, heterogeneous, and anisotropic, reflecting variations in fractures, lithology, weathering, and topography. At deeper levels, the rocks are fractured allowing potential hydrologic connection to a deeper regional flow regime. Borehole depths are shallow (median 120 ft [37 m]) with small yields (median 11 gallons per minute [gpm]) with a high rate of dry boreholes (about 30%). Fracturing and weathering are critical to the porosity and permeability of the aquifer. The Llano Uplift is dominated by decompressive and Paleozoic-age tectonic fractures, which are amenable to lineament analysis for well-siting. Well yields and water quality are influenced by lithology and metamorphic fabrics.
The CBA is more prolific than previously considered, as fracturing is more extensive than shown on published maps. However, low storage and shallow localized flow systems make the CBA susceptible to droughts and contamination. The CBA satisfies the State’s definition of a minor aquifer because it supplies “relatively small quantities of water in large areas of the State.” Recognition of the CBA as a minor aquifer will lead to increased awareness and greater protection and management of this important resource.
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