Characterization of aquifers and their associated hydrogeochemical processes: Case of the Tongoy aquifer, Chile

Abstract

A methodology is developed, with an integral approach (geological, statistical, hydrological, hydrochemical, soil-water interaction), which, based on the hydrochemical characterization of any aquifer, allows to identify and consistently establish its relationship, with the types of soil and the hydrogeochemical processes associated with the soil-water interactions of the place. This methodology is applied for the first time in the coastal aquifer between Río Elqui and Río Limarí (Coquimbo Region, Chile), since there are no previous studies of this type in that area, being a water resource of great importance for human consumption and productive activities. A dynamic sampling of the water from wells distributed in the study area is carried out measuring in situ electrical conductivity, pH and temperature, and later, in the laboratory, the concentrations of major ions are determined using ICP-MS (EPA 200.8), Ionic Chromatography (EPA 300.1) y Volumetric technics (APHA/AWWA/WEF, 2012). Statistical analysis in conglomerates and principal components shows the relationship between the chemical composition of these waters and four soil types (wind deposits, alluvial deposits, transgressive marine sequences, intrusive rocks) where the wells are located. Most of the waters sampled in the study sector are classified as mixed towards the interior of the continent (chlorinated-bicarbonated-sulfated, sodium-magnesium-calcium). Dominant ions (chloride, sodium) are observed only at the coastline. It is observed that the presence of the mayority ions is determined in the first place by cationic processes in the marine environment and carbonate leaching, which are associated with weathering, seawater intrusion, leaching and reverse cation exchange. Secondly, the major ions originated from carbonate leaching processes in non-marine environments, associated with weathering and leaching, mainly in sectors far away from the salt wedge and/or marine transgressions. The supersaturation of salts (aragonite, calcite, dolomite) and the behavior of electrical conductivity are also correlated with the soil types based on the main components (CP1, CP2). The most important environmental process identified, which accounts for the behavior of electrical conductivity in the aquifer, corresponds to the cationic processes in the marine environment (CP1). The proximity to the coastline and the spatial distribution of marine transgressions are the factors that mostly affect a higher concentration of major ions and soil-water interaction processes. The comprehensive analysis allows to describe and explain the spatial hydrochemical characteristics of the aquifer, and how its major ions and soil-water interaction processes identified are consistent with the types of soil where the wells are located.