Application of nitrogen and boron isotopes for tracing sources of anthropogenic contamination in Monforte-Alter do Chão aquifer system, Portugal

The Monforte-Alter do Chão aquifer system represents one of the main water resources in Alentejo region (SW Portugal), and is located in an area of intensive agriculture and cattle breeding. The groundwater shows varied chemical composition, reflecting fast chemical changes associated with the water–rock interaction, with waters of Ca–HCO₃, Na/Ca–HCO₃, and Na–HCO₃ facies. Some of the waters have strong evidence of nitrogen contamination with nitrate concentrations above 50 mg/l. The implementation of the European Nitrates Directive and the Water Framework Directive has led to the designation of large areas as Nitrate Vulnerable Zones, for being at risk from agricultural nitrate pollution: groundwater containing or that could contain more than 50 mg/l of nitrates. Corrective measures taken to reverse the trend can only be established when the contaminant source is known. In this study, the isotopic compositions of NO₃ (δ¹⁵N and δ¹⁸O) and B (δ¹¹B) were used to identify the sources of the nitrogen contamination. Despite the occurrence of some evaporation processes, the δ¹⁵N and δ¹⁸O isotopes allowed to distinguish two groups of waters with different sources of nitrates, fertilizers, and soils or manure and domestic effluents. The analysis of the NO₃ source based on δ¹¹B isotope ratio pointed out contamination by pig farms effluents. However, considering the land occupation and the strong livestock activities in the area, it is possible that the adsorption of ¹¹B isotopes onto clays or the high pH of the groundwater could explain the high δ¹¹B values.