Evaluation of the Option to Designate a Separate San Marcos Pool for Critical Period Management

Author SB Johnson and GM Schindel
Year 2008
Description Hydrogeologic relationships between nearby streams and rivers to springflow at San Marcos Springs
Report Number 08-01
Publisher Edwards Aquifer Authority
Location San Marcos Springs
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This report presents the findings of the Edwards Aquifer Authority’s investigation into the hydrogeology and hydrodynamics of water discharging from San Marcos Springs in Hays County, Texas.  The purpose of the report is to determine whether there is sufficient technical justification for distinct aquifer management rules for the designation of a San Marcos Pool in the San Marcos area regarding critical period management.  Historical records indicate that Comal Springs and San Marcos Springs respond differently to rainfall, which could indicate that these springs represent separate hydrologic subbasins or pools within the San Antonio Segment of the Balcones Fault Zone Edwards Aquifer.  The report concludes that water for both San Marcos Springs and Comal Springs is derived from the western portion of the aquifer.  Groundwater flows through the Artesian fault block and either discharges from Comal Springs or bypasses Comal Springs and discharges at San Marcos Springs.  At higher discharge rates, 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.  The report concludes that there is not sufficient technical justification at this time to create a separate San Marcos “Pool” and recommends that an index well be established in the San Marcos area for triggering critical period levels.  The report was prepared by Authority staff in February 2008.