GEOCHEMICAL TRENDS AND STORM RESPONSES AT SAN MARCOS SPRINGS, TEXAS: NEW PERSPECTIVES AND NEW QUESTIONS

Two major research initiatives, the State of Texas Legislature-funded San Marcos Observing System, and the TCEQ and EPA-funded Spring Lake Watershed Characterization projects, have recently focused on characterizing the Upper San Marcos River watershed. Specific goals include quantifying spatial and temporal variations in water chemistry and nutrients in this groundwater-surface water system. As part of this, continuously logging sondes measure pH, DO, SC, T, and turbidity in six of the largest spring openings at San Marcos Springs, which discharge from the complex karstic Edwards Aquifer into Spring Lake and form the headwaters of the San Marcos River. Periodic samples are collected bi-monthly at all sites for analysis of major ions and liquid water stable isotopes.

Continuous data confirm previously reported patterns and have revealed previously undocumented relationships and phenomena. Confirmed patterns include distinct spring temperature groups, where warmer springs are believed to receive a majority of their flow from deeper sources in a different fault block, and an increase in SC as flow decreases. However, these generalizations do not reveal the array of complex and subtle changes and differences now resolvable using high precision continuous data. Surprisingly, springs assumed to receive much of their recharge from local sources were less influenced by a large storm event (Sept., 2010) than some which were assumed to derive more flow from deeper sources. Additionally, very subtle periodic fluctuations in parameters at certain sites and under certain flow conditions may be related to the effects of pumping combined with shifts in source water from one fault block to another (deeper vs. shallower source) as aquifer levels increase or decrease. We also detected spatial and temporal differences in ion concentrations (including nitrate), indicating changing source zones as aquifer levels change, or in response to the storm event.

Using all these data, we are now able to infer with greater certainty the connections between springs and local recharge areas, to understand interactions between local and regional sources of water supplying this complex spring system, and to characterize how relative contributions from different sources may vary with changes in aquifer conditions.