Environmental isotopes (N, S, C, O, D) to determine natural attenuation processes in nitrate contaminated waters: Example of Osona (NE Spain)

Nitrate-contaminated groundwater from an aquifer in the Osona region (NE Spain) was chemically and isotopically (δ15NNO3,δ18ONO3,δ34SSO4,δ18OSO4δ15NNO3,δ18ONO3,δ34SSO4,δ18OSO4, δ  D, δ18OH2Oδ18OH2O and δ13CDICδ13CDIC) characterized. Diffuse- NO3- contamination reached values of 366 mg/L. Nearly 75% of the 37 sampled sites had higher concentrations than the 50 mg/L in NO3- limit for drinking water. To identify the source of pollution δ15NNO3δ15NNO3 and δ18ONO3δ18ONO3 were used with results, for most samples, in the range of pig manure NO3-. Nitrification processes were evaluated by means of the δ18O of NO3- and water. Isotopic data suggested that natural attenuation of NO3- was taking place. This process was confirmed using the δ18ONO3δ18ONO3 coupled with the NO3-/Cl- ratio, avoiding the influence of continuous NO3- inputs. A further insight on denitrification processes was obtained by analyzing the ions involved in denitrification reactions. Although the role of organic matter oxidation could neither be confirmed nor discarded, this approach showed a link between denitrification and pyrite oxidation. Therefore, in areas with no adequate infrastructure (e.g. multipiezometers), such as the one studied, this approach could be useful for implementing better water management.