DISTRIBUTION AND MOBILITY OF GEOGENIC ARSENIC IN THE SHALLOW AQUIFERS OF NORTHEASTERN LA PAMPA PROVINCE, ARGENTINA

High levels of geogenic arsenic (As) in groundwater have been reported in different parts of the Chaco-Pampean Plain where more than 2 million people are exposed to high levels of As and F in drinking water which exceed the WHO drinking waterguideline (10 µg/L) and the national Argentine standard (50 µg/L). Long-term ingestion of contaminated water over a century has caused endemic hydroarsenicism (locally called as HACRE) manifested through differnt forms of skin lesions and internal cancers among the affected population. Arsenic is associated to the weathering of volcanic ash from the Andean volcanic eruptions (90% rhyolitic glass) intercalated in the loess sediments. In the present study, 44 groundwater samples were collected from the shallow aquifers of NE of La Pampa province, covering an area of 100 km². Groundwater was neutral to alkaline (pH 7.43-9.18), predominantly oxidizing (Eh ~0.24 V) with variable EC (456-11,400 µS/cm). Major ion concentrations showed HCO₃^-/Cl^- and Na⁺/K⁺ as dominant anions and cations, respectively. Water type was mostly Na-HCO₃^-. The water samples indicated high degree of mineralization as evidenced by high EC. In discharge areas, high evaporation rates result in high salinity of shallow waters and visible salts incrustations on the dry lakes. Elevated concentrations of NO₃^- and PO₄^3-observed in some wells indicated possible anthropogenic contamination. Groundwater As levels varied from 5.5 to 535 µg/L and 94% of the samples exceeded the WHO drinking water standard limit and 56% exceeded the Argentine standard limit. Fluoride showed wide range of concentrations (0.5-14.2 mg/L) with 78% of samples exceeding the WHO standard limit. Total As concentration is positively correlated with F^-, HCO₃^-, B and V and negatively correlated with Fe, Al and Mn, as these are precipitated as hydroxides. The results suggest that the mobility of As in the shallow Pampean aquifers are predominantly controlled by adsorption/desorption processes from Fe, Mn, Al oxides at high pH. The generation of high pH in groundwater results from carbonate reactions. In addition, Long-term consumption of this groundwater could be a considerable health risk among the population.