Reversals of karst springs: Implications for water budgets, water quality, and speleogenesis

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ABSTRACTThis award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Intellectual Merit. Reversal of springs during river flooding (e.g., formation of estavelles) are likely to carry organic carbon to oligotrophic environments where it will be remineralized, changing concentrations of nutrients and terminal electron acceptors such as dissolved oxygen (DO), NO3, and Fe and Mn oxides, and increase pCO2. Elevated pCO2 should drive dissolution at the discharge point rather than at sinking streams. Reversals will be controlled by head gradients between the spring, conduits, groundwater, and rivers, but this simplistic view is complicated by antecedent conditions, the rates of diffuse versus allogenic recharge, hydraulic properties of the aquifer, and aquifer confinement. These potential effects of spring reversals lead to questions addressed here including: (1) What are the relationships between hydraulic head in the aquifer, discharge, diffuse recharge, and aquifer confinement? (2) At what rate is backflooding water returned to the surface? (3) What are the primary solid-fluid reactions controlling backflooded water compositions? These questions will be addressed at four springs (Madison Blue, Troy, Peacock, and Fanning) along the Suwannee River in north-central Florida, all of which experience frequent reversals. Madison Blue, Troy and Fanning springs are sites of on-going monitoring programs that have generated much legacy data about flow and chemistry, which will supplement new data collected as part of this project. Samples will be collected at low frequency during base flow to observe background chemical compositions and at high resolution during a reversal event to construct flood chemographs. Specific conductivity, temperature, DO, and pH will be monitored with reliable off the shelf logging instruments. Data collection and analyses will include estimates of reversal magnitudes using available and new discharge data. Sr isotope ratios and age of water (3H/3He ratios and CFC concentrations) will be used to separate fractions of flood water and groundwater. Major element concentrations will be used to assess saturation states of the water. Measurements of DO, NO3, NH4, Fe2+, Fe3+, Mn, sulfate, sulfide, dissolved inorganic and organic carbon concentrations, and ?Ô13CDICvalues will be used to estimate the magnitude of organic matter remineralization. Data modeling will include MODFLOW to simulate flow and PHREEQC to estimate redox and saturation states.Broader impacts. Similar to many karst areas, the Suwannee River Basin is experiencing rapid land use changes and elevated groundwater withdrawals. The basin is also home to a variety of endangered stygobitic fauna. These issues require careful management of water quality and quantity, both of which will be affected by spring reversals. The Suwannee River has also been proposed as a site for the WATERS network, and its largest tributary, the Santa Fe River, is the site of a funded ¡§test-bed¡¨ project. The test bed project will be linked to this project through combined data handling and storage using Hydrologic Information System (HIS).Study sites are located in state parks, allowing interaction with the general population. Cave diving is common at the parks, and divers will volunteer for sample collection. The project will provide training for undergraduates, who will work with legacy data, graduate students, who will work on the project for their thesis or dissertations. Results will be used as examples in formal classroom training in several courses currently taught at the University of Florida.