DOLINE AND AQUIFER CHARACTERISTICS WITHIN HERNANDO, PASCO, & NORTHERN HILLSBOROUGH COUNTIES, FLORIDA

An investigation to examine the relationship between the spatial distribution of doline features and the hydraulic conductivity (K_x) of the underlying Upper Floridan aquifer was conducted in a 4,400 km² area of west-central Florida. Hydraulic properties of an aquifer are traditionally estimated from field tests and the values obtained are often more site-specific in nature than a regional characterization of the aquifer. Establishing a relationship between karst geomorphology and the characteristics of the underlying aquifer could provide a useful technique for estimating regional-scale values of hydraulic conductivity or other hydraulic properties which could then be applied to a groundwater model.

The Upper Floridan aquifer is a limestone and dolomite aquifer approximately 300 meters to 450 meters thick, containing numerous sinkhole and karst features. Water supplies in the area are provided almost exclusively by the Upper Floridan aquifer. Hydraulic conductivities in the aquifer may vary by three orders of magnitude in the study area. The aquifer has been extensively subjected to karst activity, and the intensity of this activity is controlled largely by the distance of the aquifer from land surface, and the thickness and continuity of the clay confining unit overlying the Upper Floridan.

Using depression data derived from U.S. Geological Survey (USGS) maps, doline densities (D_D) were calculated over the study area, and doline-area ratios (R_D) were generated and compiled by county, physiographic province, and model-derived K_x zones. Results from these analyses indicate that areas of high D_D coincide with northern portions of the study area where aquifer confinement is poor. Values of R_D calculated by county also support the observation of increased karst activity in the north. Of particular significance, however, are the results indicating a positive linear correlation between R_D and model-derived values of K_x (r²=0.97). These results may provide a useful technique for ground-water modelers to estimate regional-scale values of hydraulic conductivity or other hydraulic properties.