Natural arsenic in the groundwater of the alluvial aquifers of Santiago del Estero Province, Argentina

Natural occurrences of arsenic has been documented in groundwater of the shallow aquifers of the Chaco-Pampean Plain, Argentina. The distribution of arsenic and mechanisms of its mobilization in the shallow alluvial aquifers was investigated around the city of Santiago del Estero in Northwestern Argentina in order to provide an insight into the complex hydrological and geochemical conditions that yields high As concentrations in groundwater. Significant spatial variations of total arsenic (As-tot) concentrations were observed with an average value of 743 mu g/L. Arsenate was a dominant species in most samples. Average concentrations of Al, Mn, and Fe were 360 mu g/L, 574 mu g/L, and 459 mu g/L, respectively. The 7M HNO3 extraction of sediments and volcanic ash-layer indicated AS(N03) concentrations ranging between 2.5-7.1 mg/kg. AS(N03) indicated a significant positive correlation with Mn-N03, Al-N03, and Fe-N03. Oxalate extractions revealed significant fractions of As (Aso(ox)) in the sediments (0.4-1.4mg/kg) and a dominance of oxalate extractable Al- and Mn. Speciation calculations indicate that Al oxide and hydroxides have the potential to precipitate in the groundwater, suggesting that As adsorption processes may be to some extent controlled by Al oxides and hydroxides. Mobility of As at local scale seems to depend on high pH values, related to the dissolution of carbonates driven by cation exchange, and dissolution of silicates. There is a clear relationship of As with F, V, B and Si, suggesting their common origin in volcanic ash layer. Preliminary conceptual model of arsenic input includes release of As and Al from dissolution of volcanic ash layer, precipitation of Al oxides and hydroxides followed by adsorption of As on Al and Fe phases in sediments, and release of As under high pH conditions.