Characterizing sources and natural attenuation of nitrate contamination in the Baix Ter aquifer system (NE Spain) using a multi-isotope approach.

Nitrate pollution is a widespread issue affecting global water resources with significant economic and health effects. Knowledge of both the corresponding pollution sources and of processes naturally attenuating them is thus of crucial importance in assessing water management policies and the impact of anthropogenic activities. In this study, an approach combining hydrodynamic, hydrochemical and multi-isotope systematics (8 isotopes) is used to characterize the sources of nitrate pollution and potential natural attenuation processes in a polluted basin of NE Spain. δH and δO isotopes were used to further characterize the sources of recharge of the aquifers. Results show that NO is not homogeneously distributed and presents a large range of concentrations, from no NO to up to 480mgL. δN and δO of dissolved NO identified manure as the main source of nitrate, although sewage and mineral fertilizers can also be isotopically detected using boron isotopes (δB) and δS and δO of dissolved sulphate, respectively. The multi-isotope approach proved that natural denitrification is occurring, especially in near-river environments or in areas hydrologically related to fault zones. δS and δO indicated that denitrification is not driven by pyrite oxidation but rather by the oxidation of organic matter. This could not be confirmed by the study of δC that was buffered by the entanglement of other processes and sources.