Présentations Colloques

Session 8.09: Hydrogeoecology: a focus on groundwater ecology
Hernández Del Amo (orateur)
Analysis of nitrate attenuation conditions in groundwater from an ecological approach
Nitrate pollution is one of the main concerns for water management in the Osona region (NE Spain), as high nitrate concentrations are frequently found in groundwater, reaching up to 500 mg NO3- L. The main source of nitrate in the Osona aquifers is slurry and manure applied to crops as fertilizers. **Nitrate can be reduced by dissimilatory nitrate reduction to ammonia (DNRA) or by denitrification, being the later the unique process that could ensure nitrogen removal in groundwater. Although several studies have observed natural groundwater denitrification in Osona area, mainly related to pyrite oxidation, little or no attention has been made on bacterial functional marker genes involved in nitrate reduction. In this work, functional gene abundances together with hydrochemical and isotopic characteristics (by means of δ+15NNO3 and δ+18ONO3 isotopic signatures of NO3-) were used to characterize nitrate reduction conditions from an ecological approach. Samples were taken from eight wells in similar geological formations in the Osona area. Hydrochemical and isotopic data were measured jointly with those of genes implied in the two key steps of denitrification (nirK, nirS, nosZI and nosZII). According to the +18ONO3 enrichment factor, three wells showed a degree of denitrification from 10 to 25%. The relative contribution of nirK and nirS were on the range of 0.76-3.30 and 0.02-0.32 respectively. The abundance of nosZ genes was one order of magnitude lower than for nirS and nirK suggesting a limited complete reduction. Wells with different denitrification percentages show similar abundances of denitrification genes. Such finding suggest that genetic potential for groundwater denitrification exist overall the study area, yet the appropriate conditions depending on the hydrogeochemical environment are key factors to locally trigger denitrification rates at efficient levels.**