Evidence for Denitrification Processes in Nitrate Polluted Groundwater (Catalonia, NE Spain) using a Multi-Isotopic Approach
Universidad Girona, Spain
An understanding of the origin and fate of nitrate, as well as of the biogeochemical processes controlling nitrate attenuation in aquifers, is crucial for improving water resources management, and preserving the quality of groundwater supplies and groundwater-dependent surface waters. Six vulnerable areas to nitrate pollution from agricultural sources in Catalonia (NE Spain) (Maresme, Osona-Lluçanès, Baix Empordà, Selva and Garrotxa) have been studied by coupling conventional hydrogeochemical data with a multi-isotope approach (D, 18OH2O, 15NNO3, 18ONO3, 34SSO4, 18OSO4, 13CDIC and, for the Baix Empordà samples, 11B) in a research project focused on tracking the sources of nitrate pollution and assessing the chemical reactions affecting the solutes involved in denitrification processes (NO3-, SO42- and HCO3-). According to dissolved nitrate and sulfate isotopic compositions, the main origin of nitrate pollution in the different areas are: synthetic fertilizers (Maresme; Vitòria et al., 2005), animal manure (Osona-Lluçanès, Baix Empordà), and a mixed source in Selva and Garrotxa areas. The use of boron isotopic composition (Widory et al., 2005) in the Baix Empordà demonstrated that sewage is not a potential pollution source in this area. Moreover, the coupled use of nitrogen and oxygen isotopes of dissolved nitrate confirmed active denitrification in the Osona-Lluçanès, Baix Empordà and Selva areas, being not significant in the Maresme and Garrotxa areas. The isotopic composition of the ions involved in denitrification reactions indicated a relationship between pyrite oxidation and nitrate attenuation in the Osona area. On the other hand, in the Baix Empordà, Selva and Lluçanès areas, denitrification is mainly linked to organic matter oxidation.