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. delta H-2 and 6180 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. 618N and 6180 of dissolved NO3- identified manure as the main source of nitrate, although sewage and mineral fertilizers can also be isotopically detected using boron isotopes (delta B-11) and delta S-34 and delta O-18 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. delta S-34 and delta(18)o 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 delta C-13(CHO3) that was buffered by the entanglement of other processes and sources. (C) 2016 Elsevier B.V. All rights reserved.