Shallow High-Resolution Seismic-Reflection Imaging of Karst Structures within the Floridan Aquifer System, Northeastern Florida

Abstract

Water quality monitoring of production, industry and monitor wells tapping the upper Floridan aquifer have shown increases in chloride concentrations and/or temperature in some wells in Duval and St. Johns Counties, Florida. Meanwhile, in other nearby wells, the chloride concentrations and/or temperature are unchanged. It is postulated that the areal and vertical variability of temperature and/or chloride concentrations can be explained by the localized upward migration of poorer quality (more saline) water from zones in or below the Lower Floridan aquifer. The conduits for this upward flow are believed to be along isolated geologic structures (fractures and enlarged joints, solution conduits, and/or paleosinkholes) that have breached the middle semi-confining unit of the Floridan aquifer system. A reconnaissance investigation was conducted to determine if land-based high-resolution seismic reflection techniques could be used to identify the presence of site specific karst geologic structures. The Mini-Sosie method was used here because it works well in the physically restrictive and culturally noisy urban environment and was thought to have potential since the majority of the sites to be investigated are located within urban or residential areas. Data quality is considered to be good to very good and was successful in imaging formational contacts and inter-formational units that define the boundaries of the Floridan aquifer system within the depth interval of 30–375 m. At the Fort George Island site a buried sinkhole, with an area of at least 200 by 475 m of deformation, was imaged on a set of near perpendicular reflection profiles. This karst feature, with at least 20 m of vertical downwarping, overlies an interpreted solution conduit that clearly penetrates to a depth of at least 375 m. The profile at the Bakersville site images near-surface sinkhole deformation that not only appears to be related to a speculated deeper solution conduit source for subsurface Floridan aquifer contamination but also appears to have had a recent influence on the surface hydrology as well.