Northern Segment of the Edwards Aquifer, Texas: Groundwater Availability Modeling

By

JONES, IAN C.

Texas Water Development Board, Austin, TX

The northern Edwards aquifer is an important source of water for municipal purposes. This aquifer is composed of Cretaceous limestone that occurs along and east of the Balcones Fault Zone and extends north from the Colorado River. Regional groundwater flow is influenced by fracture orientation, especially in the south where they are more abundant, and by stratigraphy. Highest well yields are associated with fractures and karstified collapse zones. Recharge primarily occurs where the Edwards and associated limestones crop out. Recharge takes the form of stream losses through fractures, sinkholes and other karst features. Along the eastern margin of the outcrop zone, gaining streams occur due to groundwater discharge from major and minor springs associated with faults. Discharge also occurs through springs along the Colorado River and by leakage across overlying confining units.

The Texas Water Development Board is developing a numerical model of the northern segment of the Edwards aquifer using MODFLOW. The purpose of this model is to provide information on groundwater availability and to create a tool to evaluate water-management strategies, and assess present and future groundwater availability under normal and drought conditions. Construction of this model includes compilation of hydrogeologic data, e.g., recharge; stratigraphy and structure; rivers and springs; water levels; aquifer properties; and pumping. Calibration will ensure that the model can reproduce past hydrologic conditions. Predictions of future groundwater availability will be based on projected groundwater demand.