Abstract

The Edwards aquifer in south-central Texas is a productive and important water resource. Several large springs issuing from the aquifer are major discharge points, popular locations for recreational activities, and habitat for threatened and endangered species. Discharges from Comal and San Marcos Springs, the first and second largest spring complexes in Texas, are used as thresholds in groundwater management strategies for the Edwards aquifer. Comal Springs is generally understood to be supplied by predominantly regional groundwater flow paths; the hydrologic connection of San Marcos Springs with the regional flow system, however, is less understood. During November 2008–December 2010, a hydrologic and geochemical investigation of San Marcos Springs was conducted by the U.S. Geological Survey (USGS) in cooperation with the San Antonio Water System. The primary objective of this study was to define and characterize sources of discharge from San Marcos Springs. During this study, hydrologic conditions transitioned from exceptional drought (the dry period, November 1, 2008 to September 8, 2009) to wetter than normal (the wet period, September 9, 2009 to December 31, 2010), which provided the opportunity to investigate the hydrogeology of San Marcos Springs under a wide range of hydrologic conditions. Water samples were collected from streams, groundwater wells, and springs at and in the vicinity of San Marcos Springs, including periodic (routine) sampling (every 3–7 weeks) and sampling in response to storms. Samples were analyzed for major ions, trace elements, nutrients, and selected stable and radiogenic isotopes (deuterium, oxygen, carbon, strontium). Additionally, selected physicochemical properties were measured continuously at several sites, and hydrologic data were compiled from other USGS efforts (stream and spring discharge). Potential aquifer recharge was evaluated from local streams, and daily recharge or gain/loss estimates were computed for several local streams. Local rainfall and recharge events were compared with physicochemical properties and geochemical variability at San Marcos Springs, with little evidence for dilution by local recharge.

Hydrologic and geochemical variability at San Marcos Springs was compared with that at Comal Springs and Hueco Springs. A small range of variability was observed at Comal Springs, and a large range was observed at Hueco Springs; variability at San Marcos Springs was intermediate. Comal Springs and Hueco Springs are representative of two endmember Edwards aquifer spring types, with Hueco Springs predominantly affected by local flowpaths and locally sourced recharge and Comal Springs predominantly affected by regional flowpaths and regionally sourced recharge. The geochemistry of discharge at San Marcos Springs from three of its orifices—Deep, Diversion, and Weissmuller Springs—differed among the orifices; these differences in geochemistry are indicative of differences in the sources and flow paths of recharge and groundwater supplying the springs. During the dry period, the geochemistry of Deep Spring indicates that it was affected by a small component of saline groundwater. The geochemistry of Deep Spring was not responsive to changes in hydrologic conditions from the dry period to the wet period, indicating that Deep Spring is likely dominated by regional and less-variable flow paths. Diversion Spring was more responsive to changes in hydrologic conditions, indicating that Diversion Spring was affected by some changes in discharge sources. From the dry period to the wet period, the geochemistry of Diversion Spring became more like that at Deep Spring; the nature of the geochemical changes indicates that increased discharge included an increased component of saline groundwater. Weissmuller Spring was sampled only during the wet period; its geochemistry was similar to that of Diversion Spring, indicating that Weissmuller and Diversion Springs are likely supplied by common flow paths.

Geochemical models (using PHREEQC) indicate that a small amount of saline groundwater (generally less than 1 percent), in addition to a dominant component of regional groundwater flow, is needed to account for the composition of water from Deep, Diversion, and Weissmuller Springs. Potential sources of saline groundwater are the downdip Edwards aquifer saline zone and the Trinity aquifer; while both sources are hydrologically and geochemically plausible, model results indicate that mixing with groundwater from the Edwards aquifer saline zone is likely a better geochemical fit. Little local recharge occurred during the dry period. Geochemical model results for the wet period yielded different proportions of local recharge sources (the Blanco River) to discharge from San Marcos Springs ranging from 0 to less than 30 percent; the proportion was smaller for Deep Spring than for Diversion Spring, which is consistent with the nominal response of Deep Spring to changes in hydrologic conditions. In response to storm events, when focused recharge of dilute surface water is likely to occur, the geochemical response at San Marcos Springs was minor; for example, the modeled proportion of local surface-water recharge contributing to Diversion Spring was mostly less than 10 percent directly following storms and for several subsequent months. These results place further constraints on estimates using PHREEQC and suggest that the proportion of local recharge contributing to discharge at San Marcos Springs is likely on the order of no more than 10 percent.

The results of this study indicate that discharge at San Marcos Springs is dominated by regional recharge sources and flow paths, even during wet hydrologic conditions when aquifer recharge is likely occurring from local streams. Local surface-water recharge sources do not strongly influence the geochemistry of San Marcos Springs discharge. Knowledge of recharge sources to San Marcos Springs and how they vary spatially and temporally is useful for water-resource management strategies and for understanding geochemical and hydrologic processes that affect discharge at San Marcos Springs.

First posted August 29, 2012