Integration of Multiple Geophysical Techniques to Image a Submarine Groundwater Discharge Zone

Discharge of fresh groundwater has been identified as a major contributor of N loads to many estuaries, especially in the mid-Atlantic region.  Delineation and characterization of the groundwater discharge zones can be difficult, especially in areas where geologic factors force discharge past the shoreline to submarine locations.  Lack of understanding of the characteristics of the discharge zone can lead to great uncertainty in the forms and mass of N reaching estuaries and identification of appropriate practices to manage or mitigate N loads.  Geophysical measurements allow collection of detailed spatial data that can be used to identify potential submarine groundwater discharge (SGD) zones and to focus monitoring and sampling efforts in those areas.

 In Indian River Bay, Delaware marine seismic and continuous resistivity profiling survey methods were used to delineate the geometry and infer lithology of sub-bottom geologic units, infer 3-D distribution of pore water salinity, and identify potential SGD zones.  These data focused field efforts to characterize the SGD area that included shallow pore water testing, seepage meter measurements, drilling and testing of monitoring wells, and downhole geophysical logging.  These data indicate that discontinuous confining units consisting of fine-grained Holocene-age estuarine deposits and Plio-Pleistocene tidal flat deposits allow for the formation of an offshore fresh plume of groundwater and are controlling the location and rates of SGD and the 3-D geometry of the freshwater plume.  Geophysical data greatly improved the chances for successfully characterizing the SGD zone and measuring important physical and geochemical processes that occur during SGD.