5th UF Water Institute Symposium Abstract

Submitter's Name	Jonathan Martin
Session Name	Springs IV - Temporal Dynamics
Author(s)	Jonathan B. Martin,  Department of Geological Sciences, University of Florida (Presenting Author)
	Marie Kurz,  Hydrogeology Department, Helmholtz Centre for Environmental Research–UFZ
	Mitra B. Khadka, Department of Geological Sciences, University of Florida
	Controls and effects of time-varying apparent ages in karst spring water
	Water quantity and quality in karst aquifers may depend on decadal-scale variations in recharge or withdrawal, which we hypothesize could be assessed through time-series measurements of apparent ages of spring water. We test this hypothesis with analyses of various age tracers (3H/3He, SF6, CFC-11, CFC-12, CFC-113) and selected solute concentrations (dissolved oxygen (DO), NO3, Mg, and SO4) from 6 springs in a single spring group (Ichetucknee springs) in northern Florida over a 16-yr period. These springs fall into two groups that reflect shallow short (Group 1) and deep long (Group 2) flow paths. CFC-12 and CFC-113 concentrations yield the best apparent ages. These tracers show a 10 to 20 year monotonic increase in apparent age from 1997 to 2013 and indicate most water discharged during the study recharged the aquifer within a few years of 1973 for Group 2 springs and 1980 for Group 1 springs. The increase in apparent age included the flood recession following Tropical Storm Debby in mid-2012. Inverse correlations between apparent age and DO and NO3 concentrations reflect reduced redox state in older water. Positive correlations between apparent age and Mg and SO4 concentrations reflect increased water-rock reactions. Concentrated recharge in the decade around 1970 resulted from nearly 2 m of rain in excess of the average that fell between 1960 and 1980, followed by a nearly 4 m deficit to 2014. The excess rain coincided with two major El Niño events during the maximum cool phase in the Atlantic Multidecadal Oscillation. Although regional water withdrawal increased nearly 5 fold between 1980 and 2005, withdrawals represent only 2 to 5% of Ichetucknee River flow and are less important than decadal-long variations in precipitation. These results suggest that aquifer management should consider climate cycles as predictive tools for future water resources.