Sources of Water for San Marcos Springs in San Marcos, Texas

The Edwards Aquifer Authority investigated the hydrogeology of the San Marcos Springs area in San Marcos, Texas. The purpose of the study was to better define the source water for San Marcos Springs, which provides habitat for eight threatened and endangered species including Texas wild rice and the San Marcos salamander. Defining the areas of contribution to San Marcos Springs is important for managing springflow during droughts. San Marcos Springs is the second largest spring complex in Texas, discharging an average of 177 cubic feet per second. Previous studies indicated that San Marcos Springs had both regional and local sources, although the relative contributions were not quantified.

This study concluded that San Marcos Springs receives both local and regional flow with a greater component being derived from local sources during wet years and a greater component of regional flow during dry years. Three major fault blocks, part of the Balcones Fault Zone, influence groundwater flow from Bexar County eastward toward both San Marcos and Comal springs. Groundwater flows through the Artesian fault block and either discharges from Comal Springs or, depending upon stage, bypasses Comal Springs and discharges at San Marcos Springs. During dry periods, water levels in the Comal Springs and Hueco Springs fault blocks decline below Comal Springs orifice elevations, which precludes local recharge, although the orifices continue to discharge water from the Artesian fault block. During wet periods, San Marcos Springs is also recharged by water infiltrating from Cibolo Creek, the Guadalupe River, Dry Comal Creek, Sink Creek, and other streams, especially when a groundwater mound forms under Onion Creek. Despite the large volume of water that these streams carry at times, they contribute relatively small amounts to San Marcos springflow.