HYDROGEOLOGICAL AND WELL-SITING INVESTIGATION IN A SHALLOW, GENERALLY LOW-PERMEABILITY ALLUVIAL SYSTEM - TECOLOTE FARM, MANOR, TEXAS

Tecolote Farm east of Austin, Texas, is a family-run organic farm that irrigated with groundwater produced first from a pre-1940 hand-dug well did not go dry during the 1950-1956 drought of record and then from a small capacity well drilled in 2005. Since then, both wells have gone dry because of a combination of continued drought and installation of large capacity wells in the aquifer. The University of Texas’ 2009 groundwater field methods class used the farm as a project to investigate potential well sites. The site is near the confluence of Decker and Gilleland Creeks in the floodplain of the Colorado River. These sediments form a complex aquifer of sand/gravel lenses in a generally fine-grained alluvial system. Electromagnetic (EM) and electrical resistivity (ER) surveys were run on seven selected transects, Decker Creek was gauged, and a water-table map of the area was prepared. EM and especially ER showed anomalies of lower conductivity/higher resistivity bodies that may represent sand or gravel deposits surrounded by higher conductivity/lower resistivity clays. Five sites were identified as potential sites for future drilling of wells. Three of the anomalies were sampled with either a hand auger or cored with a Geoprobe. These confirmed the presence of sand and gravel underneath clayey alluvial top stratum, two of these three definitely flowed water. The potential well sites are identified based on these conclusions. A secondary objective of the study was to characterize stream and groundwater flow in the study site area through the measurement of groundwater levels in wells, analysis of well logs, stream gauging, and water chemistry analysis. The alluvial aquifers likely have limited interaction with Decker Creek or with nearby high-capacity wells located north of the farm because of the low-permeability top stratum and high lateral variability of the channel deposits.