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

- δ15N, δ18ONO3 and δ11B confirm pig manure as the main vector of NO3− pollution.
- SO42- and B isotopes indicate also contributions from sewage and mineral fertilizers.
- NO3− isotopes show that NO3− undergoes natural attenuation.
- SO42- isotopes confirm that denitrification is not controlled by pyrite oxidation.
- A unique and comprehensive multi-isotope approach allows characterizing sources of NO3− contamination and processes involved.

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. δ2H and δ18O isotopes were used to further characterize the sources of recharge of the aquifers. Results show that NO3− is not homogeneously distributed and presents a large range of concentrations, from no NO3− to up to 480 mg L−1. δ15N and δ18O of dissolved NO3− identified manure as the main source of nitrate, although sewage and mineral fertilizers can also be isotopically detected using boron isotopes (δ11B) and δ34S and δ18O 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. δ34S and δ18O 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 δ13CHCO3 that was buffered by the entanglement of other processes and sources.

123