PHYSICAL AND REMOTELY SENSED EVIDENCE OF CONDUIT FLOW FACILITATING WATER EXCHANGE BETWEEN THE KARST UPPER FLORIDAN AQUIFER AND LAKE SEMINOLE, SOUTHWESTERN GEORGIA AND NORTHWESTERN FLORIDA

Lake Seminole is a 37,600-acre impoundment created in the mid-1950s by the U.S. Army Corps of Engineers (Corps) near the confluence of the Chattahoochee, and Flint Rivers, Spring Creek, and Fishpond Drain in southwestern Georgia and northwestern Florida. The lake provides headwater to the Apalachicola River, which flows 106 miles from Jim Woodruff Lock and Dam to Apalachicola Bay and the Gulf of Mexico. Physical and remotely sensed information derived from water-temperature data, dye-tracing, aerial-photograph interpretation, acoustic Doppler current profiling (ADCP), and hydrographic surveys confirm lake leakage to the karst Upper Floridan aquifer near the dam and groundwater inflow to the lake through solution-enhanced conduits in the limestone that line upstream reaches of the lake's four impoundment arms. Written accounts by Corps geologists in the late 1940s and early 1950s and construction-era photographs document caverns, sinkholes, and conduits in the limestone that would become lake bottom and foundation rock to the dam and confirm future lake-aquifer connection. A mosaic of 53 preimpoundment aerial photographs were orthorectfied to digital images of 1:24,000-scale topographic maps to aid in identifying more than 250 karst features that have the potential to facilitate lake-aquifer water exchange. Hydrographic surveys compared with orthorectified aerial photographs indicate ubiquitous occurrences of circular and elongated depressions in the lake bottom, consistent in shape with sinkholes on land, and some coinciding with locations of in-lake springs. Vortex flow from the lake to the aquifer occurs near the dam at some depressions and near sinkholes and reverse-flowing springs. Dye-tracing studies by the Corps indicate that lake water entering sinkholes upstream of the dam flows through the Upper Floridan aquifer at velocities of about 500 feet per hour to locations downstream of the dam. Here, water "boils up" from the aquifer at Polk Lake Spring located on the western floodplain of the Apalachicola River and in a trench-like conduit along the channel bottom of the Apalachicola River (at the "River Boil"). Water flowing from Polk Lake Spring enters a sinkhole and eventually discharges from the River Boil; ADCP indicates that River Boil discharge ranges from about 140 to 220 cubic feet per second.