Geoelectrical and geochemical characterization of groundwater in a shallow coastal aquifer

Playa Las Glorias is the main tourist destination for the inhabitants of the municipality of Guasave, Sinaloa, México, and it overlies a shallow coastal aquifer. In recent years, the momentum of the tourism industry in the region has brought a growing demand in the consumption of drinking water. Because of this, it is relevant to the precise knowledge of the interface of slightly brackish saltwater for an efficient planning of new wells by the municipal authorities. For efficient groundwater exploitation, it was taken into consideration the productivity of the aquifer and the environmental impact that would cause improper management of water resources. The electromagnetic profiling (EMP) method was applied in Playa Las Glorias, Sinaloa, Mexico. A total of 150 measurements of EMP distributed in 11 profiles were performed, of which 10 were perpendicular to the coast and one parallel to it, using an EM34 meter in horizontal polarization and separation between coils of 10 m and 20 m. The apparent conductivity maps show an anomaly of low conductivity, indicating the presence of fresh or slightly saline water. In this anomalous zone we performed an electric resistivity tomography (ERT) profile to determine the behavior of groundwater salinity finding a shallow layer of freshwater, and as it deepens the salinity increases due to the influence of the sea. In addition, electrical resistivity (R_w) values were measured in groundwater samples that, along with the resistivity values (R_o) for sandy formation obtained from ERT data, allowed us to determine a lineal relationship between both resistivities. A hydro geochemical study was conducted in the site through Piper diagrams and Chadha, indicating a predominance in the region of chlorinated water due to the influence of the nearby sea. Finally, we determined the correlations between the water electrical conductivity values with the Cl-anion and the Na₊ and Mg²⁺ cations for the site, besides finding relationships between Na⁺-Cl^-, Na⁺-Mg²⁺, and Cl^--Mg²⁺ with a superior adjustment to 0.98.