Evaluation and source attribution of freshwater contributions to Kinvarra Bay, Ireland, using 222Rn, EC and stable isotopes as natural indicators
Freshwater discharge into the coastal sea is of general interest for two reasons: (i) It acts as vehicle for the transport of contaminants or nutrients into the ocean, and (ii) it indicates the loss of significant volumes of freshwater that might be needed for irrigation or drinking water supply. Due to the large-scale and long-term nature of the related hydrological processes, locating and quantitatively assessing freshwater discharge into the sea require naturally occurring tracers that allow fast, inexpensive and straightforward detection. In several studies, the standard water parameters electrical conductivity (EC) and pH have proven their suitability in this regard. However, while distribution patterns of EC and pH in the coastal sea indicate freshwater discharge in general, a separation between discharging surface water and submarine groundwater discharge (SGD) is not possible with these alone. The naturally occurring radionuclide radon-222 has been shown to be useful in the quantification of SGD and its distinction from surface runoff. This study aimed to evaluate and compare the informative value of the three parameters—EC, pH and radon concentration—in detecting and quantifying SGD by carrying out a case study in a bay located in western Ireland. The results reveal that radon activity is the most sensitive parameter for detecting SGD. However, only the combined evaluation of radon, EC and pH allows a quantitative allocation of groundwater and surface water contributions to the overall freshwater discharge into the sea. This conclusion is independently supported by stable isotope data measured on selected samples.
KeywordsCoastal freshwater discharge Submarine groundwater discharge Radon Stable isotopes of water Environmental tracers
The authors want to thank Reiner Stollberg (Martin Luther University of Halle, Germany) for digital elevation model analyses, Isaac Santos (Southern Cross University, Lismore, Australia) for discussion of oceanographic issues, Jean Wilson (BRG, Trinity College Dublin) for calculation of submerged areas at different tidal stages by remote sensing, and Rachel Cave, Tiernan Henry, and Shane Rooney (National University of Ireland, Galway) for assistance and technical support. Funding for this project was provided by the NUI Galway Griffith Geoscience award that was based on research grant aided by the Department of Communications, Energy and Natural Resources under the National Geoscience Programme 2007–2013.
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