Geochemical processes that explain arsenic in groundwater in a basin developed in the Pampean Mountains and piedmont, Córdoba, Argentina

Abstract

Las Peñas stream basin, developed in the Pampean Mountains of Córdoba (Argentina), has hydrogeological characteristics of high interest for local human activities. The objective of this study is to interpret the influence of the geomorphological and lithological features of the basin, their control on the main hydrodynamic and hydrochemical processes and the factors that constrain the occurrence of arsenic (As) in groundwater. The unconfined aquifer develops in fractured rocks and sediments of diverse origin and provenance. Hydraulic gradients and groundwater geochemistry are conditioned by lithology and relief. The groundwater is mainly fresh (electrical conductivities between 586 and 2000 µS/cm), although there are brackish-type samples (up to 3900 µS/cm) located in the piedmont. The groundwater showed calcium and sodium bicarbonate geochemical types, with the local occurrence of mixed types (sodium-calcium bicarbonate and sodium bicarbonate-sulfate). The dominant geomorphological and lithological aspects condition the chemical composition of groundwater while controlling the distribution and concentration of As in solution. The highest As values were found in gently undulating reliefs where loess sediments prevail, especially in the main relict paleosurface area located in the upper basin. Taking this into account, it is assumed that the abundant volcanic glass—which is known as an arsenic bearer—in the regional sediments, is the possible source of this chemical element as a result of hydrolysis. Also, desorption processes in Fe and Mn oxy-hydroxides present in the loess sediments are related to the passage of As ions to the solution. As it is known, not only the source but a specific geochemical environment to allow the transference of chemical elements from minerals to groundwater is needed. In this way, the results, including a multivariate statistical analysis, demonstrate that the highest As values in groundwater, well correlated with high fluoride (F) contents, are related to an alkaline environment of sodium bicarbonate groundwater with high pH values.