Abstract

BLANCO, María del Carmen; AMIOTTI, Nilda Mabel  and  ESPOSITO, Martín Eduardo. Arsenic in soils and sediments of the southwestern pampa: origin, accumulation in water and risk for human consumption. Cienc. suelo [online]. 2018, vol.36, n.1, pp.182-195. ISSN 1850-2067.

The existence of high As in phreatic waters of the southern pampean region guided this research with the objective of analizing surface and groundwaters hydrochemistry, evaluating the rol of loess sediments and derived soils in As provision to water and the potential risk of its use as human drinking water. The loess and alluvial soils´ parent materials respectively associated to the plain and alluvial landforms had 6. 4-29 ppm of As in the 50-2000 μm fraction, considered normal values. In the interfluves of Bahía Blanca region, As content of the loess sediments (14.1 ppm - 22 ppm) between the top soil and 2 m depth had a marked variability, attributed to pedogenesis. The decrease of As content detected up to 4 m depth (As: 9. 8-10.4 m) appears inherited of the sedimentary pattern. In the saturated zone between 16 and 42 m (phreatic layer: 16, 7 m mean depth), variability corresponds to a longer residence time of water. The moderately fine textured alluvial soils of Sauce Chico lower basin yielded between 7.2 and 14.5 ppm of As, with a uniform distribution in depth. The excessive As (>0,01 mg L^-1) in the aqueous phase in the Sauce Grande, Sauce Chico and Napostá Grande river basins initiates in the middle sections and becomes elevated towards the river mouth and the phreatic aquifer´s discharge (As: 0.05-0.10 mg L^-1). Waters are oxidizing, HCO₃-Na ó SO₄-Na, highly saline and moderate to strongly alkaline. In most cases, the calculated exposure risk exceeded the risk unit (USEPA , 10 ^{- 5}), with the exception of Sauce Chico lower basin (rainy season). Therefore, in these zones, utilization of phreatic waters with high As for human consumption expose population to a high carcinogenic risk.

Keywords : Unsaturated zone; phreatic aquifers; water quality; arsenic toxicity.

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