HYDROGEOCHEMICAL MODELING OF THE FATE AND MOBILIZATION OF GROUNDWATER ARSENIC IN THE AQUIFERS OF ANDEAN FORELAND, ARGENTINA

Hydrogeochemical modeling and morpho-tectonic analyses methods have been employed to understand the specific geological links for the wide spread occurrences of arsenic (As) in the Santiago Del Estero province of the Chaco-Pampean plain of Argentina with the geotectonic framework. The study area is dotted with several hot springs, and is a part of the retro-arc foreland of the active continental arcs of the Andean orogenic belt. Rhyolitic volcanic glass fragments and ash are notable constituents of the aeolian-fluvial aquifer sediments of the study area, and are related to the events of extrusive magmatism in the vicinity. Groundwater chemistry suggests a dominance of oxidizing condition, along with some indicators of locally reduced. Very high Na, Cl^- and SO₄^2- concentrations have been detected in some of the the groundwaters. It is hypothesized that the groundwater chemistry has largely evolved by the dissolution of the volcanic glasses and mixing of saline surface water, located in some adjoining salinas, which are probably partially evaporated remnants of a palaeo inland sea. Flow path modeling, stability diagrams and thermodynamic analyses undertaken in this study indicate that the dominant evolutionary processes include chemical weathering of silicate and evaporites, in monosialitization dominated weathering. Plagioclase feldspar and volcanic glass seems to the major dissolution phases, leading to solute source. As a consequence of this geologic setting, the primary provenance of the As was probably introduced in the system by mixing or dissolution of precipitates of mantle-sourced, As(III)-carrier hydrothermal fluids or As-laden extrusives. Subsequently, As(III) oxidized to As(V), and gets sorbed on metal oxides and hydro-oxides present in the aquifer sediments.. However, co-influxed oxy-anions, with similar ionic radii and structure, compete with As and leads to succeeding As liberation. Further, the transition of the Ca-rich groundwater to Na-rich groundwater, by mixing with water from salinas and/or evaporative concentration, might have led to counter-ion effect, leading to further enrichment of groundwater by As.