GEOGENIC ARSENIC IN SHALLOW AQUIFERS OF RÍO DULCE

Abstract: Geogenic arsenic (As) is reported in groundwaters from the shallow aquifers in different parts of the Chaco-Pampean Plain, Argentina. Río Dulce alluvial cone in Santiago del Estero Province (NW Argentina) was selected as an area to investigate the complex hydrological and geochemical conditions that control the distribution and mobilization of As in groundwater from these shallow alluvial aquifers. Average total As (Astot) concentration was 743 µg/L, with significant spatial variability. Arsenic domains with concentrations of up to 14,969 µg/L were also identified in some parts of the study area. In most water samples, As(V) was the dominant species. Average concentrations of Al, Mn, and Fe were 360 µg/L, 574 µg/L, and 459 µg/L, respectively. Extraction of sediments and interlayered volcanic ash with 7M HNO3 indicated AsNO3 concentrations ranging between 2.5-7.1 mg/kg and indicated significant positive correlations with MnNO3, AlNO3, and FeNO3. Oxalate extractions revealed significant fractions of As (Asox) in the sediments (0.4-1.4 mg/kg) and a dominance of oxalate extractable Al- and Mn. Speciation calculations indicated that Fe and Al oxide and hydroxides have the potential to precipitate in the groundwater, suggesting that they may be significant arsenic adsorbents. Mobility of As seems to depend on high pH values locally, related to the dissolution of carbonates driven by cation exchange, and dissolution of silicates. Hierarchical cluster analysi