Sinkholes are a common geologic feature and one of the prevailing landforms in karst terranes, where they pose significant hazards to property and the environment. Although many new sinkholes develop naturally, their increasing frequency corresponds to the accelerated development of groundwater and land resources. This paper deals with the engineering problems associated with siting a pipeline on a mantled karst terrane in Central Florida by presenting the results of a detailed geophysical investigation of the size and risk-level of buried sinkholes. Areas that were at significant risk for sinkhole collapse or subsidence, based on the hydrogeologic and geomorphic analysis of the route, were initially investigated with a centerline ground penetrating radar (GPR) survey. Based on the interpretation of irregularities in the ubiquitous sand/clay boundary and the shallow groundwater level in the GPR images, incongruities on the radar record were placed in three categories:

1. hydraulically active sinkholes, through which shallow groundwater is leaking downward into the deeper karstified limestone, thus providing a potential mechanism for erosion and collapse;

2. hydraulically inactive sinkholes; and

3. those in which the signatures that may be related to sinkhole development were inconclusive.

This three-level classification of the buried sinkholes was then refined by natural potential (NP) measurements to minimize the number of sinkholes in the third category and place them in either the first or second category. Because the first category of sinkholes has the highest risk of collapse or subsidence a high density grid of GPR measurements was used to define their extent and to detect groundwater leakage where it was significant enough to cause a visible decline in the water table. The NP method was used to augment the GPR and was highly effective in delineating areas of groundwater recharge in the mantled karst terrane of Central Florida.