Estimation of submarine groundwater discharge from bulk ground electrical conductivity measurements

The utility of bulk ground conductivity (BGC) measurements in the estimation of submarine groundwater discharge (SGD) was investigated at four sites covering a range of hydrogeological settings, namely Cockburn Sound (Australia); Shelter Island (USA); Ubatuba Bay (Brazil) and Flic-en-Flac Bay (Mauritius). At each of the sites, BGC was surveyed in the intertidal zone, and seepage meters were used for direct measurements of SGD flow rates. In the presence of detectable salinity gradients in the sediment, a negative correlation between SGD and BGC was recorded. The correlation is site-specific and is dependent on both the type of sediment and the mixing processes. For example, at Shelter Island the maximum mean flow rates were 65 cm. d^-1 at a BGC of ∼0 ms cm^-1 while at Mauritius maximum mean flow rates were 364 cm d^-1 at a BGC of ∼0 mS cm^-1. BGC measurements are used to estimate SGD over a large scale, and to separate its fresh and saline components. Extrapolating BGC measurements throughout the study sites yields a total discharge of 2.91, 1.59, 7.16, and 25.4 10³ m³ d^-1 km^-1 of shoreline with a freshwater fraction of 41, 24, 29, and 63% at Cockburn Sound, Shelter Island, Ubatuba Bay, and Flic-en-Flac Bay respectively. The results demonstrate that ground conductivity is a useful tracer to survey and separate freshwater and recirculated seawater component of SGD. The presented investigation is a subset within a series of experiments designed to compare different methods to investigate SGD co-organized and carried out by SCOR, LOICZ, IOC and IAEA.