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Excessive arsenic concentrations above the Argentinean and WHO guidelines for drinking water (10 lg L -1 ) affects shallow aquifers of the southern Pampean Plain (Argentina) hosted in the Pampean and the Post Pampean formations (loess and reworked loess; Plio-Pleistocene-Holocene). Health problems related to high As concentrations in drinking waters are known as Endemic Regional Chronic Hydroarsenicism. Hydrochemistry of shallow groundwaters and soil geochemistry were investigated aiming to (1) understand the partition of As in the solid phase and its relationship with unacceptable As concentrations in waters, (2) identify the provision source of As to groundwaters. Only 5% of the samples had As concentrations \10 lg L -1 ; in 27% As concentrations ranged from 10 to 50 lg L -1 and in 58% it reached 60-500 lg L -1 . The coarse fraction (50-2,000 lm) hosts about 27% of the total As in the solid phase, being positively correlated to Ba (p \ 0.01; r 2 = 0.93). About 70% is included in the \2 lm fraction and had positive correlations of As-Fe (p \ 0.05; r 2 = 0.85) and As-Cr (p \ 0.05; r 2 = 0.68). Soils and sediment sand fractions of vadose zones are the primary sources of As in shallow groundwater while adsorptiondesorption processes, codisolution-coprecipitation, and evaporation during the dry seasons raise As concentrations in waters exceeding the guideline value for drinking water.

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