Source apportionment of nitrate pollution in surface water using stable isotopes of N and O in nitrate and B : a case study in Flanders (Belgium)

Excess application to fields of animal manure or mineral fertilizers is generally assumed to be the major source of nitrate pollution in surface and groundwater in agricultural areas in Flanders (Belgium). The Flemish Environment Agency (VMM) suspects, but could not prove, that discharges of untreated domestic sewage or discharges containing mineral fertilizers from hydroculture in greenhouses are also contributing to the nitrate pollution observed in some of the agricultural areas. The objective of this study was to investigate, using stable isotope data of dissolved nitrate (δ15N and δ18O) and boron (δ11B), if these suspected sources significantly contribute to the nitrate pollution observed in selected surface water monitoring points in Flanders. Based on our nitrate and boron isotope results, nitrate input from domestic sewage was estimated to range between 5 and 50% in the investigated sampling points. Our nitrate isotope results confirmed significant inputs from mineral nitrate fertilizers, potentially originating from greenhouse discharges, in areas with intensive greenhouse cropping. Nitrate isotope data also indicated the potential occurrence of natural nitrate attenuation through denitrification, which could partially explain lower nitrate concentrations generally observed in summer period.