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

Abstract

Nitrate concentrations, water isotopes (δ2H and δ18Owater) and associated nitrate isotopes (δ15Nnitrate and δ18Onitrate) from 10 drinking water wells, 5 fresh water springs and the discharge from 3 wastewater treatment stations in Réunion 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 Réunion Island's principal aquifer. Water from these study sites contained between 0.1 and 85.3mg/L nitrate. δ15Nnitrate 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 δ15Nnitrate 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 (δ18Owater and δ15Nnitrate), it was possible to relate the average altitude of rainfall infiltration (δ18Owater) associated with the nitrate contamination (δ18Onitrate). This relationship between land use, rainfall recharge altitude and isotopic composition (δ15Nnitrate and δ18Owater) discriminated between the influences of human waste at lower (below 600m elevation) or animal derived contamination (at elevations between 600 and 1300m). By further comparing the theoretical altitude of nitrate formation calculated by the δ18Onitrate, it was possible to determine that only 5 out of 15 fresh water wells and springs followed the conservative nitrate formation mechanism of 2/3δ18Owater+1/3δ18Oair, to give nitrate formation altitudes which corresponded to land use activities.