Sea-water/groundwater interactions along a small catchment of the European Atlantic coast

The geochemistry and isotopic composition of a karstic coastal aquifer in western Ireland has shed light on the effect of sea-water/groundwater interactions on the water quality of Ireland’s Atlantic coastal zone. The use of stable isotope data from the IAEA precipitation station in Valentia, located in SW Ireland has facilitated the characterization of groundwater recharge conditions in the western part of Ireland and suggests that groundwater is mostly replenished by the isotopically light winter precipitation. The dissolved SO42- in the karstic groundwater that was collected during baseflow conditions with δ34S values between 4.6‰ and 18‰ may be composed of S stemming from three principal sources: SO42- derived from precipitation which is composed of both sea-spray S (δ34S: 20‰) and an isotopically light anthropogenic source (δ34S: 1–5‰), SO42-stemming from animal slurries (δ34S: ∼5‰), and intruding sea-water SO42- (δ34S: 20.2‰). The isotopic composition of δ18O in dissolved groundwater SO42- collected during baseflow conditions is interpreted as reflecting sea-water intrusion to the karstic coastal groundwater system. The highest δ18O values in dissolved groundwater SO42- were in samples collected near the coast (4.8±0.4‰) and the lowest (2±0.5‰) were collected further inland. The δ15N and δ18O values of groundwater NO3- were between 3.4‰ and 11.4‰ and approximately 7.7‰, respectively, and reflect geochemical conditions in the aquifer that do not promote attenuation of NO3- through denitrification. As a result N loading to Kinvara Bay that is controlled by submarine groundwater discharge (SGD) was calculated as 5tons/day on average compared to an estimated N-input that derives from precipitation of approximately 2.5tons/a. SGD into the bay may result in near coastal sea-water quality changes. These results represent one of the first studies addressing the effect of groundwater quality in Ireland on the European Atlantic coastal zone.