Characterizing near-surface karst system under three stormwater retention basins in Silver Springs, Florida

Water resources in karst systems are particularly vulnerable to contamination by polluted surface water, such as stormwater runoff, due to their specific geologic and hydrogeological characteristics. Better understanding of near-surface karst systems may inform contaminant transport and protective management of karst aquifers and springs. Ground penetrating radar (GPR) and electromagnetic (EM) geophysical data were collected in three stormwater retention basins within the Silver Springs springshed in Florida to investigate configuration of the near-surface karst system. The reported sinkhole data indicated 14 sinkhole occurrences with diameter ranging from 0.6 to 6 m in Basin 1, 11 sinkholes with diameter ranging from 0.6 to 3 m in Basin 2, and no sinkholes in Basin 3. Several sinkholes were identified by visual field survey, including a 2-m diameter sinkhole in Basin 2 that connected stormwater runoff directly with the surficial aquifer. GPR (270 MHz with 110 to 150 nanosecond time ranges) was accomplished in 3 m perpendicular transects, with penetration depth of 6 to 6.7 m. EM34-3 was applied with 10-m intercoil spacing in vertical and horizontal coplanar modes. 1.5% of survey area in Basin 1 is covered by surface sinkholes, while 2.5% of the area is covered by detected subsurface karst anomalies. An overall correlation can be observed between surface and subsurface features in Basin 1. Low conductivity EM zones in the vertical (rather than horizontal) coplanar are correlated fairly well with the detected GPR anomalies. Surface sinkholes consist approximately 0.4% of survey area in Basin 2 while no considerable subsurface anomaly was detected by GPR, suggesting no correlation between surface and shallow karst system. The spatial variation of EM response in the vertical coplanar is relatively correlated with the spatial distribution of sinkholes in Basin 2. Though surface sinkhole was not recorded or observed in Basin 3, approximately 11% of the survey area is covered by subsurface anomalies. The near-surface karst system in the study area is highly heterogeneous, indicating that transport rates and contaminant transformations in the subsurface are likely to be similarly heterogeneous