Arsenic-rich shallow groundwater in sandy aquifer systems buffered by rising carbonate waters: A geochemical case study from Mannar Island, Sri Lanka

Abstract

Major ion, trace elements, and stable isotope analyses were performed on groundwater samples collected from Mannar Island in the northern Indian Ocean. Arsenic concentrations up to 34 μg/L have been observed in groundwater samples from the island. In addition, 23% of extensively used shallow drinking water wells showed higher arsenic contents than the recommended value by the World Health Organization (10 μg/L). Groundwater in the island showed pH values between 6.9 and 8.9 and was dominated by Na+, K+, Ca2+, Mg2+, HCO3 −, Cl− and SO4 2−. The δ18OH2O and δ2HH2O composition of most groundwater plotted very close to the local meteoric waterline, however, some wells showed enriched isotope compositions that are most likely due to evaporation. Sea water intrusion in this island was likely of minor importance as indicated by the major ion composition. An approximatedmass balance calculation using chloride concentrations indicated that out of the 35 investigatedwells only 6 near-shore wells were influenced by sea water intrusion up to about 15%. Even though this is a sandy aquifer, groundwaters were characterized with higher contents of dissolved inorganic carbon (DIC) (2.11– 10.9 mmol/L). The corresponding δ13CDIC values varied from −19.4‰ to −6.5‰. Except for a few samples with values approaching −20‰, these isotope values are more typical for carbonate dissolution and equilibration of CO2 in the aquifer. This study shows that the underlying carbonate system may buffer the aqueous geochemistry