T175 Geologic Controls on Chemical Migration in Fractured and Carbonate Aquifers (Posters)

KARST AQUIFER RESPONSE TO VARIATIONS IN DISTRIBUTION AND MAGNITUDE OF RECHARGE AND IMPLICATIONS TO LAND USE PLANNING IN THE WOODVILLE KARST PLAIN OF NORTH FLORIDA

Paper number: T175-38
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

Quantitative groundwater tracing experiments were conducted in the Woodville Karst Plain of north Florida in 2005 and 2006 to identify the fate of aquifer recharge from 1) sinking streams that receive ~60% of the City of Tallahassee's stormwater runoff; and 2) an 810 hectare 75,000 m3/day center-pivot irrigation field that receives 100% of the City's secondarily treated wastewater. Both tests revealed that the water from those sources flows to Wakulla Spring (the largest spring in the basin) at an average rate of approximately 0.4 km/day. More importantly however, results from both tests indicated that the travel time from each of the injection points is dependent on the distribution and magnitude of local recharge. Breakthrough curves derived from multiple tracers injected into swallets of varying input capacity at different times revealed a hierarchical network of conduit pathways that deliver flow to common springs where flow rates are governed by size of the respective conduits and the distribution of inflow. Similarly, breakthrough curves derived from multiple well injections within the City's spray field at different times revealed multiple flow paths that converged on the same springs and flow rates dependent on the amount of inflow to nearby swallets.

These results indicate that the proportions of direct (swallet) recharge and more dispersed matrix recharge in the springs must be controlled by competing hydraulic gradients. One gradient is between the sinking streams and the springs along the conduits, which is subject to high amplitude, high frequency fluctuations driven by rainfall. A second gradient exists between the conduits and the source of recharge to the aquifer matrix that fluctuates over a much smaller range. More broadly speaking, the results of these two tests dramatically reveal the consequences of poor aquifer conceptualization in the process of land planning. In this case, both the City's stormwater discharge and wastewater sprayfield were sited in an area where flow velocities are far greater than assumed resulting in unforeseen transport of contaminated water to the region's most precious groundwater resource.