HYDROGEOLOGICAL INVESTIGATION OF THE DECKER AND GILLELAND CREEK WATERSHEDS, TRAVIS COUNTY, TEXAS—IMPLICATIONS FOR ALLUVIAL GROUNDWATER DEVELOPMENT

The Decker and Gilleland Creek watersheds of central Texas and their related shallow subsurface aquifers support a variety of municipal, domestic, and agricultural consumer needs. These watersheds consist of Quaternary alluvium and terrace deposits composed of interbedded clay-rich and gravelly-sand layers overlying Cretaceous clays (Del Rio and Navarro Clays). This hydrogeologic survey builds on a previous field methods class study to characterize the three-dimensional structure of the shallow aquifers in the Decker and Gilleland Creek watersheds. Urban development combined with a severe drought has caused an increasing number of wells to become dry as shown in our well survey. Gilleland Creek is currently a losing stream. Decker Creek is perched which was also concluded in a 2009 study, based upon stream gauging and water chemistry, but which contradicts other studies that inferred that both creeks are gaining streams. 2-D and pseudo 3-D electrical resistivity (ER) profiles reveal significant heterogeneity in the alluvium: low resistivity clay at 0-6 m depth, fine sand to silt at 6-8 m depth, lenses of high resistivity gravelly sand embedded in low resistivity clay at 8-18 m depth, inverse grading with increasing moisture content at 18-39 m depth, and saturated soil (water table) at 39 m and below. These ER results were compared with a continuous Geoprobe core from 0-12.19 m depth. The Geoprobe soil cores support variations in alluvial material inferred from ER profiles. Analysis of the core revealed that the volumetric moisture content in the deepest layer sampled (12.19m) was low, ~3%. We hypothesize that this and deeper gravelly sand lenses are buried channels that are connected to nearby, high yield wells. Our ER and Geoprobe analyses indicate that remnant shallow sand and gravel channels supply high yield wells throughout the study area. We infer that three factors are responsible for the recent increased drawdown in these sand and gravel channels: (1) the pumping rates of nearby wells, (2) preexisting drought conditions, and (3) the extent of remnant channel connectivity between the wells.