MAJOR ION CHEMISTRY IN A COASTAL KARSTIC GROUNDWATER RESOURCE LOCATED IN WESTERN IRELAND

Abstract:The extent of seawater intrusion in a coastal karstic aquifer system along the southern shore of Galway Bay, western Ireland, was investigated using spatial and temporal variations in major ion chemistry and nutrient levels. The background water was Ca- and bicarbonate-rich with variable concentrations of Mg. In general, higher Mg concentrations were detected in the Gort lowlands region compared with sites in the Burren, likely due to contact with dolomite layers. Nitrate concentrations ranged from 1.3mg/L to 78.5mg/L as NO3, and dissolved organic carbon concentrations ranged from 0.6mg/L to 4.9mg/L, suggesting that anthropogenic contamination due to surface activities occurs at some locations in the study area. Based on salinity values, on Na and Cl concentrations and on Cl/Br mass ratios, five of the 24 operational wells sampled were in a saltwater-influenced zone. Three of these wells contained <0.5% seawater, and the extent of saltwater influence was dependent on seasonal conditions. Water levels were monitored in six unused wells, and at four of these locations groundwater levels responded to the tidal variations in Galway Bay. During the study period, seawater intrusion and/or tidal influence on groundwater levels was found to occur within 5km of the coastline. The inland extent of seawater intrusion on the karst system is dependent on the hydrologic conditions at the time of measurement. The results provide a baseline dataset from which the influence of climatic and environmental changes on the aquifer system can be assessed in the future.