Hydrochemical and Isotopic Characterization of the Waters of the Manglaralto River Basin (Ecuador) to Contribute to the Management of the Coastal Aquifer

Coastal aquifers are strategic and fundamental in the development of touristic areas. The coastal aquifer within the Manglaralto River Basin in Ecuador is essential, as it is the only source of water supply for a large part of the northern part of the Santa Elena province. It is a semi-arid region where high volumes of water are pumped from the aquifer, causing a significant drawdown of groundwater levels, thus affecting the water quality. This work aims to characterize the characteristics of groundwater in the coastal aquifer using hydrochemistry and stable isotopes to propose a hydrogeological conceptual model. The methodology for determining the chemical and isotopic characteristics of groundwater follows the following scheme: (i) studies of ionic concentrations using the Piper diagram, (ii) assessment of the origin of salinity through the Cl/Br ratio, the presence of seawater intrusion through the Hydrochemical Facies Evolution Diagram HFE-D, (iii) characterization of precipitation events using stable isotopes (¹⁸O and ²H), and, (iv) development of a hydrogeological conceptual model of the study area. The results indicate that in the basin there are mixing processes of the existing water in the aquifer with recharge water, direct cation exchange processes in the freshening process during recharge, and evaporation in the unsaturated zone. A conceptual model of the flow system in the basin is built, based on the mentioned processes. The main conclusions are: seawater intrusion is present in the areas of the wells located closest to the coast, urban activity through septic tanks is affecting the quality of the aquifer, and rainfall is highly relevant in the different hydrochemical and isotopic processes that operate in the basin.