Assessment of nitrate and sulfate contamination in groundwater using isotopic and hydrogeochemical tools in three aquifer systems of Northern Mexico

Nitrate and sulfate comprise a significant portion of ionic charge in most natural waters. Groundwater pollution from nitrate is one of the most prevalent environmental problems. Over the past decades, groundwater quality has deteriorated worldwide due to the intensive use of fertilizers in agriculture, the release of untreated urban sewage and industrial wastewater (e.g., mining, smelting, steel manufacturing, kraft pulp, paper mills, and flue gas desulphurization circuits), and natural sources (natural fertilization, bacterial production, atmospheric deposition). These pollution sources contributed to adverse human and biota effects. Furthermore, arid or semi-arid areas are mainly dependent on groundwater resources, which, together with accelerated population growth, generates water stress and often leads to groundwater quality deterioration. To assess these issues, the origin and biogeochemical transformations of nitrate and sulfate in groundwater have been widely studied since the 1970s. A successful tool for tracing pollution sources is the use of the dual-stable isotopic compositions of nitrate (δ15NNO3 and δ18ONO3) and sulfate (δ34SSO4 and δ18OSO4). Unfortunately, despite the ability of the dual-isotope plot to trace the origin of NO3- and SO42- contamination, uncertainties remain because two or more sources may sometimes overlap, hindering the correct differentiation of the origin. For this reason, this Ph.D. research aims to identify and quantify nitrate and sulfate sources in groundwater within three semi-arid areas of Northern Mexico with multiple potential sources using a multi-tracer approach combined with a Bayesian isotope mixing model. The study cases were a highly urbanized industrial area (Monterrey Metropolitan Area), an intensive livestock-agricultural area (Comarca Lagunera Region), and a coastal agricultural aquifer (La Paz aquifer). The approach followed in this research is a useful tool for estimating the contribution of different nitrate and sulfate sources, allowing establishing effective pollution management strategies in contaminated aquifers.

https://orcid.org/0000-0002-5708-0043