ASSESSING SHALLOW GROUNDWATER VULNERABILITY IN THE SOUTH TEXAS SAND SHEET USING A GIS-BASED DRASTIC MODEL

The South Texas Sand Sheet (STSS) consists predominantly of medium to fine grain eolian sand deposits and sand dunes. It covers six counties with an area of approximately 7.8 × 10² km². Previous studies indicate the STSS’ thickness varies between a few centimeters to tens of meters. Groundwater contamination is of particular concern as groundwater resources are the principal source of water for agricultural activities in this area. Geospatial analysis has been conducted using digital elevation model (DEM) data with Geographic Information Systems (GIS) Spatial Analyst-Hydrology tools to produce Basin and Drainage maps. A DRASTIC model developed within a GIS framework was used to evaluate the groundwater vulnerability of the STSS. Preliminary results suggest that areas with high risk to pollution are mainly located in the eastern portions of the STSS where the physical factors, i.e., high hydraulic conductivity, low gradient, and high water table, allow easy access for contaminants to reach the shallow groundwater aquifer. Results of the groundwater vulnerability in STSS can be used to evaluate possible environmental and economic impacts to the adjacent Lower Laguna Madre (LLM), one of the important lagoon ecosystems on the Texas coast.