Groundwater vulnerability assessment of shallow aquifer 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. STSS covers over six counties with an area of approximately 9.15 x 10(3) km(2) with thickness ranging between less than a meter and over 10 m. Groundwater contamination is of particular concern as groundwater resources are the principal water source in this area. The preliminary conceptual hydro-geologic model implies that the STSS is a significant hydrologic component for shallow groundwater flow and can be potential storage and/or significant environmental flow input into nearby hypersaline Lower Laguna Madre (LLM). Water (fresh/brackish) held in the STSS should be taken into consideration when accounting for regional water balance/budget, e.g., groundwater recharge and storage, flood control, and discharge to coastal areas. The geospatial analysis was conducted using geographic information systems (GIS) spatial analysis tools with well logs from Texas Water Development Board (TWDB) groundwater database and preliminary drilling in the field. A DRASTIC model developed within a GIS framework was applied to evaluate the groundwater vulnerability of the STSS. DRASTIC modeling suggested that STSS with high risk to pollution is mainly located in the eastern portions of the STSS where factors, i.e., high hydraulic conductivity and high-water table, allow easy access for contaminants to reach the shallow groundwater aquifer. The assessment found that around 94% of the unconfined aquifer in STSS has moderate to high vulnerability to contamination. The sensitivity analysis of DRASTIC model showed that the removal of hydraulic conductivity, topography, and soil media parameters caused an increase in groundwater vulnerability to pollution. Moreover, single-parameter sensitivity analysis results suggested that the net recharge rate, hydraulic conductivity, and soil media parameters were more effective in determining the groundwater vulnerability than the DRASTIC model assumed. Results of this study suggest that the groundwater vulnerability in STSS is high and assessment maps can be used to evaluate possible environmental and economic impacts to ecosystems on the adjacent Lower Laguna Madre.