15N and 18O isotopes as tracers of agricultural groundwater contamination in Osona (NE, Spain)

An area of 31 km2 is used in order to study the complexity of its groundwater hydrodynamics, to characterise the nitrate contamination, to evaluate the contribution of each pollutant source (i.e. fertilisers and pig manure) and to identify the fractionation processes of ammonium volatilisation, nitrification and denitrification. To do this, 38 groundwater samples were collected during October 2001 and chemical and isotope data (15N NO3 and 18O NO3) were determined. The δ 15N NO3 have permitted to determine that the groundwater contamination in the area is mainly due to the high rates of pig manure used as organic fertilisers in agriculture. The chloride concentrations in waters are also due to this agricultural practise and can be used as a tracer of the pig manure contamination. The δ 15N NO3 together with the δ 18O NO3, permit to identify the fractionation processes that affect groundwaters (volatilisation, denitrification). The use of other isotopic values (δ 34S SO4, δ 18O SO4, δ 18O H2O or δ 13C HCO3) could permit a better evaluation of the denitrification process