Identifying source and formation altitudes of nitrates in drinking water from Réunion Island, France, using a multi-isotopic approach.

Abstract Nitrate concentrations, water isotopes ( δ 2 H and δ 18 O water ) and associated nitrate isotopes ( δ 15 N nitrate and δ 18 O nitrate ) from 10 drinking water wells, 5 fresh water springs and the discharge from 3 wastewater treatment stations in Reunion Island, located in the Indian Ocean, were analysed. We used a multi isotopic approach to investigate the extent of nitrate contamination, nitrate formation altitude and source of nitrates in Reunion Island's principal aquifer. Water from these study sites contained between 0.1 and 85.3 mg/L nitrate. δ 15 N nitrate values between + 6 and + 14‰ suggested the main sources of contamination were animal and/or human waste, rather than inorganic (synthetic) fertilisers, infiltrating through the subsurface into the saturated zone, due to rainfall leaching of the unsaturated zone at various altitudes of precipitation. Based on δ 15 N nitrate values alone, it was not possible to distinguish between animal and human activities responsible for the contamination of each specific catchment. However, using a multi isotope approach ( δ 18 O water and δ 15 N nitrate ), it was possible to relate the average altitude of rainfall infiltration ( δ 18 O water ) associated with the nitrate contamination ( δ 18 O nitrate ). This relationship between land use, rainfall recharge altitude and isotopic composition ( δ 15 N nitrate and δ 18 O water ) discriminated between the influences of human waste at lower (below 600 m elevation) or animal derived contamination (at elevations between 600 and 1300 m). By further comparing the theoretical altitude of nitrate formation calculated by the δ 18 O nitrate , it was possible to determine that only 5 out of 15 fresh water wells and springs followed the conservative nitrate formation mechanism of 2/3 δ 18 O water  + 1/3 δ 18 O air , to give nitrate formation altitudes which corresponded to land use activities.