SIGNIFICANCE OF SOUTH TEXAS SAND SHEET IN WATER RESOURCE MANAGEMENT FOR LOWER RIO GRANDE VALLEY

Conjunctive management of groundwater and surface water becomes extremely important in arid and semi-arid areas such as Lower Rio Grande Valley (LRGV) in South Texas where freshwater sources are scarce and vital. Texas Water Development Board (TWDB) develops Groundwater Availability Models (GAM) for agricultural and urban development for many regions including LRGV. The recharge rate in GAMs for the northern and southern Gulf Coast aquifers (LRGV) varies from 0.09 to 0.43 inch/year while literatures indicated between 0.06 to 6 inch/year (0.3-50% of annual precipitation). The low groundwater recharge is likely influenced by the common caliche soils in Texas as the cementation in such layer greatly reduces water infiltration.

The South Texas Sand Sheet (STSS) occupies more than 780,000 ha (7.8 × 10² km²) and consists predominantly of loose sandy soils and relict sand dunes. It covers six South Texas counties including most of Kenedy and Brooks counties, and parts of Hidalgo, Willacy, Jim Hogg, and Starr counties. Its thickness varies between a few centimeters to 12 meters. The volume of STSS is estimated to be more than 12.6 million acre-feet (15.6 × 10⁹ m³) with assumption of average thickness of 2 meters. Assuming 15% porosity, the water storage capacity of STSS can be around 1.9 million acre-feet (2.3 × 10⁹ m³). The well-sorted medium-sand body in San Isidro area (thickness up to 3 meters) is highly permeable with a potential hydraulic conductivity of greater than 1250 inch/year (1× 10^-4cm/s). The recharge rate in such area with nearly no runoff present should be 80-95% of annual precipitation (10 times of the value used in GAM).

In addition, the eastern portion of STSS is adjacent to Laguna Madre, one of the important lagoon ecosystem on the Texas shoreline. The interactions of fresh/brackish water in STSS with coastal marine ecosystems in Laguna Madre require further studies.

Conceptual model implies that STSS is a significant hydrologic component in South Texas and is important to water resource management for LRGV and nearby coastal regions. The freshwater held in STSS should be taken into account when managing regional water sources, e.g., groundwater recharge and freshwater storage, flood control, and inflows to coastal areas. GIS analysis and modeling are in progress while additional field data are needed.