Upscaling Local Submarine Groundwater Discharge Fluxes to Long Island Sound Using Radionuclides and Thermal Infrared Imagery

Aerial thermal infrared (TIR) overflights were performed along a coastline cut into a semi-consolidated coastal aquifer system in order to upscale measurements of submarine groundwater discharge (SGD) derived from radionuclide surveys and cluster-well sampling. Two contrasting sites were chosen along the shoreline of Long Island Sound, CT-NY, USA. Site 1 was a sandy glacial outwash beach down gradient from medium-density residential housing, and site 2 was a sandy barrier beach, characteristic of many coastlines. Satellite TIR imagery resolution was found to be too coarse to resolve the distribution of SGD along Long Island. Upscaling airborne TIR data suggests that satellite imagery would require at least 30m spatial resolution for identifying SGD along Long Island. High resolution airborne TIR data identified distinct shoreline segments on the scale of 0.1km that were likely influenced by a mixture of diffuse fresh and circulated seawater SGD. These findings are further supported by pore water salinity, ^224,228Ra and ²²²Rn, where fresh and brackish pore water at Site 1 were high in dissolved ²²²Rn (15,000 ± 9,000 dpm 100L^-1), low in ²²⁴Ra (43 ± 32 dpm 100L^-1) and moderately high in ²²⁸Ra (126 ± 54 dpm 100L^-1). SGD composed of circulated seawater only was found to occur where there was a lack of a TIR signature. In these locations, pore water salinity was consistently saline with moderately high ²²²Rn (10,600 ± 3,900 dpm 100L^-1), ²²⁴Ra (580 ± 290 dpm 100L^-1) and ²²⁸Ra (370 ± 240 dpm 100L^-1), reflecting longer pore water residence times. Using this multifaceted approach, fluxes of ²²⁴Ra and ²²⁸Ra (1.4x10⁹ and 4.5x10⁹ dpm m^-1 y^-1, respectively, August 2014) were upscaled to the entire length of the Long Island Sound estuary utilizing the shoreline’s fractal dimension. Resultant SGD fluxes equal 6.9x10¹¹ ± 2.0x10¹¹ and 3.6x10¹² ± 3.1x10¹² m³ y^-1 for ²²⁴Ra and ²²⁸Ra, respectively.