Constraining a density-dependent flow model with the transient electromagnetic method in a coastal aquifer in Mexico to assess seawater intrusion

Saltwater intrusion is one of the most pressing environmental concerns in coastal areas with increased water demands. Water extraction decreases the level of fresh groundwater, reducing the water pressure and allowing saltwater to flow further inland. Tools for simulating, monitoring and managing saltwater intrusion can provide valuable support in decision-making for management options. In this work, such a tool was developed for the arid coastal aquifer of La Paz in Baja California, Mexico. A geomodel was generated by using a transient electromagnetic method (aquifer geometry, saltwater intrusion). This geomodel, in turn, was used to construct a three-dimensional density-dependent flow model to simulate groundwater flow and saltwater intrusion during the past decades. The results confirmed that a large cone of depression had formed due to excessive pumping in the agricultural area. This successively led to the inversion of the hydraulic gradient and seawater intrusion advancement on the order of 6-8 km inland in the northern-central portion of the aquifer. Freshwater recharge mostly originates from the mountainous southeastern portion of the basin. It supplies water for the city of La Paz and counteracts the saltwater intrusion advancement. The alternation of advancement and retreat of the freshwater-saltwater interface could be attributed to changing climate effects and/or policy changes.