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

Abstract

► Environmental isotopes enable the identification of seawater/groundwater interactions. ► Water isotope data show that groundwater is mostly replenished by winter precipitation. ► We identify salt water intrusion by using δ 18 O of dissolved groundwater sulfate. ► We characterize the sources and the chemical history of groundwater nitrate using δ 18 O and δ 15 N of nitrate. ► Using submarine groundwater discharge data the effect of Ireland’s water quality on the Atlantic coastal zone is discussed. 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 SO 4 2 - in the karstic groundwater that was collected during baseflow conditions with δ 34 S values between 4.6‰ and 18‰ may be composed of S stemming from three principal sources: SO 4 2 - derived from precipitation which is composed of both sea-spray S (δ 34 S: 20‰) and an isotopically light anthropogenic source (δ 34 S: 1–5‰), SO 4 2 - stemming from animal slurries (δ 34 S: ∼5‰), and intruding sea-water SO 4 2 - (δ 34 S: 20.2‰). The isotopic composition of δ 18 O in dissolved groundwater SO 4 2 - collected during baseflow conditions is interpreted as reflecting sea-water intrusion to the karstic coastal groundwater system. The highest δ 18 O values in dissolved groundwater SO 4 2 - were in samples collected near the coast (4.8 ± 0.4‰) and the lowest (2 ± 0.5‰) were collected further inland. The δ 15 N and δ 18 O values of groundwater NO 3 - 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 NO 3 - through denitrification. As a result N loading to Kinvara Bay that is controlled by submarine groundwater discharge (SGD) was calculated as 5 tons/day on average compared to an estimated N-input that derives from precipitation of approximately 2.5 tons/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.