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

The Osona District (NE Spain) is classified as vulnerable zone of nitrate contamination from agricultural sources by the Directive 91/767/CEE. In this region (1,263.8 km2) more than 1000 pig farm exploitations exist, most of them situated in the central part of the area. In 1999, livestock in the area consisted of 927,250 pigs, 90,024 cows and 57,391 sheep. This intensive activity produces large quantities of manure, mainly coming from the pig farms; a small part of them is processed in treatment plants and the rest is spread onto the fields as organic fertilisers. This practise produces a diffused contamination in the aquifers of the area with high concentrations of nitrates, up to 365 ppm, which is more than 7 times the permitted concentration in drinking waters. Additionally, synthetic fertilisers are also used in the area, contributing to the groundwater contamination. 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 (15NNO3 and 18ONO3) were determined. A correlation between chloride and nitrate concentrations exists, indicating that they have the same origin. Their spatial isoconcentrations are distributed comparably to the piezometry showing an accumulation of nitrates and chlorides downflow. Therefore, as chloride is a conservative ion, it can be used as tracer to know where the maximum inputs of nitrogen have been applied and/or to study the hydrodynamics of the pollutant constituents. Isotopic values range from +2.2 per mille to +21.0 per mille for the δ15NNO3 and from +5.1 to +11.8 per mille for the δ18ONO3. Most of the groundwater samples have δ15NNO3 values heavier than +8 per mille, corresponding to nitrates coming from the pig manure (δ15NNH4 between +8 and +15 per mille). Only three samples have lower values corresponding to synthetic fertilisers contaminated waters (fertilisers have a δ15NNtotal near the 0 per mille) or by a mix of the two inputs. As the pig manure or the synthetic ammonium fertilisers are spread onto the fields, two reactions take place: the urea hydrolysis and the ammonium volatilisation increasing the residual ammonium isotopic composition. In the unsaturated zone ammonium is nitrified into nitrates. Samples with higher isotopic values (δ15NNO3 from +15 to +21 per mille and δ18ONO3 from +8 to +12 per mille) are affected by the denitrification process, which produces an enrichment of the isotopic compositions δ15NNO3 and δ18ONO3. Denitrification in the area is believed to be caused principally by the oxidation of pyrites present in the aquifer materials, mainly marls and limestones. Denitrification caused by the organic matter oxidation is difficult to be observed as the high natural bicarbonate concentrations buffers any relation between this concentration and the nitrates or carbonates isotopic compositions. The δ15NNO3 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 δ15NNO3 together with the δ18ONO3, permit to identify the fractionation processes that affect groundwaters (volatilisation, denitrification). The use of other isotopic values (δ34SSO4, δ18OSO4, δ18OH2O or δ13CHCO3) could permit a better evaluation of the denitrification process