REGIONAL CHARACTERIZATION OF SPATIAL VARIABILITY IN SUBMARINE GROUNDWATER DISCHARGE: UNDERSTANDING FLUX SIGNATURES IN THERMAL INFRARED DATA AND GEOLOGIC CONTROLS

Airborne thermal infrared (TIR) imagery and concentrations of geochemical tracers were applied to regionally assess submarine groundwater discharge (SGD) in Smithtown Bay, NY, on the south shore of Long Island Sound, where SGD primarily occurs as recirculated, diffuse seepage. TIR imagery is commonly used to detect temperature differences between buoyant, cooler groundwater and warmer surface waters for locating SGD, an important geochemical conduit of non-point source pollution to coastal waters. A continuous 222-Rn survey was performed at low tide along the coastline to ground-truth the TIR imagery. Radon and radium are naturally elevated in groundwater relative to surface waters, making them useful tracers of SGD. Based on the thermal signature of the seepage face, locations of greater thermal signature (spatial distribution and delta-T) are utilized to select areas for piezometer transects and time-series discharge measurements (via 222-Rn). Along the 12 mile shoreline, 222-Rn concentrations range from 51 to 321 dpm 100L^-1. Previous investigations found elevated concentrations of 223-Ra and 224-Ra near the shoreline to be 37 dpm 100L^-1 and 56 dpm 100L^-1, respectively, suggesting an SGD equivalent of several thousand cubic meters per meter of shoreline per day. Elevated nutrient levels and 222-Rn concentrations are associated with cool, surface water anomalies at various sites throughout Smithtown Bay. ArcGIS is used to combine USGS DEM and geologic map datasets with the TIR imagery and geochemical data to display the spatial variability of SGD and its inherent relationship with geologic controls (i.e. onshore topography). Excess nutrient loading via SGD may contribute to the persistently observed summer-time hypoxia that recurs in Smithtown Bay.