Principal Investigator: Sydney T. Bacchus (University of Georgia)
Principal Investigator: George A. Brook (University of Georgia)
Sponsor: GWRI
Start Date: 1996-04-01; Completion Date: 1996-04-01;
Keywords: Depressional wetlands, ground penetrating radar, groundwater mining, karst aquifers, southeastern Coastal Plain, subsidence, sustainable aquifer yield
Description:
The sustainable yield of the Floridan aquifer has been exceeded in west-central Florida. The Southwest Florida Water Management District (District), one of five regional water resource agencies in Florida, recently designated Water Use Caution Areas to reduce damage: 1) to private wells and property from lateral intrusion of saltwater, vertical upconing of mineralized water, and sinkhole formation, and 2) to depressional wetlands and other natural resources from hydroperiod perturbation. Aquifer characteristics vary dramatically from the shallow, semiconfined hydrogeologic system in the North Tampa Bay Water Use Caution Area (NTBWUCA) to a system described by the District as a well-confined, multi-layered aquifer system in the Southern Water Use Caution Area (SWUCA). However, similar damage is occurring in both areas. Destruction of depressional wetlands became apparent around North Tampa Bay municipal well fields in the mid 1980s. In response, the District initiated a geophysical evaluation of selected wetlands in two well fields to determine if structural differences could explain why some wetlands appeared to be more severely affected than others. Of the four geophysical approaches used in that study, ground penetrating radar (GPR) produced the best results. In conjunction with fracture trace analysis, GPR documented differences in geophysical characteristics between wetlands (ie. reflection free zones and dipping reflectors) that could increase the hydrologic connection between the depressional wetland and the production aquifer. A similar geophysical evaluation has not been conducted in depressional wetlands in the SWUCA or in Georgia, where extensive groundwater withdrawals occur for agricultural and other uses. This study was initiated to determine if wetlands in Georgia and well-confined areas in Florida exhibit subsurface discontinuities which could increase the hydrologic connection between the depressional wetland and the production aquifer. Results of geophysical characterization using GPR in this study suggest that depressional wetlands in Georgia and Florida exhibit subsurface discontinuities that invalidate the assumptions of homogeneity and isotropy commonly used in hydrologic models for determining responses to groundwater mining. These findings also suggest that standard approaches for groundwater monitoring, with wells installed in uplands and often in grid configurations, will provide limited information regarding how the hydroperiods of depressional wetlands are being influenced by anthropogenic groundwater perturbations or when the sustainable yield of the aquifer has been exceeded. Depressional wetlands appear to provide a more accurate tool for predicting the sustainable yield of a karst aquifer than the current approach, when using GPR analysis in conjunction with fracture trace analysis. The degree to which discontinuities associated with depressional wetlands are magnified by groundwater mining is unclear and should be investigated.