Assessment of nitrate sources in groundwater of Monterrey Valley using a multi-tracer approach combined with a probability isotope mixing model

Groundwater quality deterioration by nitrate pollution due to the intensive use of fertilizers in agriculture, release of untreated urban sewage and industrial wastewater, and atmospheric deposition is a worldwide concern. The urbanized and industrialized Monterrey valley has a long record of elevated nitrate concentrations in groundwater with multiple potential pollution sources. This study aimed to fingerprint different sources and transformation processes of nitrate pollution in Monterrey using a suite of chemical and isotopic tracers (δ²H-H₂O, δ¹⁸O-H₂O, δ¹⁵N-NO₃, δ¹⁸O-NO₃) combined with a Bayesian isotope mixing model. The results suggest that soil nitrogen and sewage were the most important nitrate sources. However, the concentrations of nitrate were controlled by denitrification processes in the transition and discharge zones. The approach followed in this study is useful for establishing effective pollution management strategies in contaminated aquifers.