TURNING A LAKE SINKHOLE EVENT INTO A TRACING EXPERIMENT FORUNDERSTANDING HYDRAULIC CONNECTION BETWEEN LAKES AND KARST SPRINGS

In the week of 1/23/2018, two sinkholes were exposed in Lake Miccosukee, which is located about 20 miles east to Tallahassee, FL in the Florida Panhandle region. The surface area of the lake is 6,257 acres, and part of the lake disappeared due to lake water drainage through the sinkholes into the underlying upper Floridan aquifer. Lake disappearance due to sinkhole opening is not uncommon in Florida, and it happens every 5 ~ 10 years at different lakes. During a lake sinkhole event, the continuous drainage of a large amount of lake water and water-contained contaminants into sinkholes can be viewed as a natural tracer experiment. We hypothesize that, if the contaminant concentrations are monitored in downgradient karst springs, spring runs, and estuaries, we can turn the sinkhole geohazard into knowledge to advance our understanding of karst hydrogeology and solute transport. Testing the hypothesis requires conducting a dye tracing experiment to determine possible flow paths from the lake to several karst springs that may be hydraulically connected to the lake through karst conduits. A Dye tracing experiment was conducted on 4/6/208. A total of 50 pounds of fluorescent dye were introduced at one of the lake sinkholes, and dye arrival was tested at three down-gradient areas where lake water may flow through karst conduits. The dye was detected at only at one area, i.e., the Natural Bridge Spring area. The first arrival was detected on a charcoal packet deployed on 4/20/2018 and collected on 4/27/2018, and the monitor lasted about 100 days until no dye was detected. Fitting the breakthrough curve of the dye tracing experiment to the QTRACER software gives a mean travel time of 854m/day, suggesting that the flow was in a karst conduit not in matrix. The breakthrough curve is similar to a regular breakthrough curve observed in groundwater transport, i.e., with only one peak and a long tail. This may suggest that the conduit between the lake and the spring area is a relatively simple one, not a conduit network. Groundwater geochemistry and isotope analysis for lake and groundwater shows that hydrogen and oxygen isotope is the most effective way to determine hydraulic connection between the lake and its down-gradient karst spring.