SUBBOTTOM PROFILING (SBP) INVESTIGATION OF SINKHOLE LAKE STRUCTURE IN BAY AND WASHINGTON COUNTIES, FLORIDA

The sandhill lakes of Bay and Washington counties, Florida are deep, pristine environments. The lakes owe their existence to continual sinkhole activity as observed in limited bathymetric maps and in the appearance of small circular depressions around their perimeters (“string of pearls”) seen on aerial photography especially during low water level periods. However, little investigative information exists that shows the internal (deep) structure of these lakes and how that structure might affect groundwater flow and lake levels as associated with existing and proposed water well field pumping near these lakes. High resolution seismic reflection surveying in the form of a marine subbottom profiler (SBP) was applied over reconnaissance grids of profile lines on a series of these sandhill lakes in order to investigate the deep structure of the lakes for purposes of determining the sinkhole development within each lake and its relationship with the Floridan Aquifer System (FAS). The SBP provided mapping of the bathymetry (maximum 15 - 32 m water depth) and subbottom structure up to an additional 18 – 24 m below the bottom, all with a theoretical resolution of approximately 0.15 m bed thickness. The resulting SBP profiles showed that a) the lakes formed through the coalescence of numerous small sinkhole features, b) the depth of the sinkhole structures intersects and surpasses the depth to the FAS, and c) multiple stages of movement are exhibited in the subbottom layering. This structural information provides more realistic insight into the interaction between groundwater production (well fields) and the health of these fragile lake environments.