Tracing Reversing Groundwater Flows in the Coastal Floridan Aquifer

Two groundwater tracing experiments in the Woodville Karst Plain, Florida have revealed that Wakulla Spring (a 1^st magnitude inland spring) and Spring Creek Springs (a 1^st magnitude spring group approximately 16 km down-gradient from Wakulla Spring on the coast of the Gulf of Mexico) are connected via one or more karst conduits. Both springs display a large range in discharge arising from conduit connections to swallets whose inflows are dependent on rainfall. Prior to 2006, baseflow at both springs was thought to be composed of groundwater flow derived from distant diffuse recharge. After 2006, the Spring Creek vents began to reverse for extended periods during the dry season. During these periods, siphoning water can be observed at the surface of the Spring Creek vents and Wakulla’s discharge rises.

Both tracer injections were performed in a large swallet located 8 km southwest of Wakulla Spring and 8.5 km northwest of Spring Creek. Sampling was conducted at three Spring Creek vents, Wakulla Spring, and at intermediate karst windows. In the first test, the tracer was released prior to Spring Creek’s siphoning period. It was detected first at Spring Creek until the spring vents reversed, then approximately 45 days later at an intermediate karst window north of the injection swallet, and then at Wakulla Spring. In the second test, the tracer was released after Spring Creek had begun reversing. The tracer slowly followed the same path toward Wakulla Spring until Spring Creek began flowing again after which time it reversed direction and quickly flowed south to Spring Creek. These results document the extent to which groundwater flow patterns in the Floridan aquifer are impacted by karst conduits and also demonstrate the immediacy of and extent to which the coastal region of the aquifer can be impacted by saltwater intrusion.