Arsenic concentrations in soils and sediments of the southern Pampean Plain, within Clarameco River Basin (Argentina)

NUMA NAHUEL SOSA; SAUGATA DATTA; MARCELO ARISTIDES ZARATE

San Francisco

Congreso; AGU Fall Meeting; 2015

American Geophisics Union

The Pampean plain is an extensive flatland covering ~1000km2 of central and northern Argentina. It consists of a large mantle of Neogene and Quaternary Aeolian (loess) deposits locally reworked by fluvial systems; the sediments are mostly of volcanoclastic in composition derived from the Andes Cordillera. The region shows one of the highest groundwater As concentrations of the world which cause serious problems to human health. Volcanic glass, a common constituent of the Pampean loess is thought to be the main source of As, while others emphasize the importance of Fe oxy-hydroxides as another potential source. In both cases, the oxidising and high pH conditions of the Chaco-pampean groundwater leads to the dissolution of the particles and the release of As to water.A variation of As content related to lithogenic factors is evident from our study in Claromecó River Basin (Southern Pampean plain): the oldest Mio-Pliocene fluvial facies (MPFF) have reduced arsenic content (2.6mg/kg) compared with the most recent Late Pleistocene fluvial facies (11.6mg/kg; LPFF). Furthermore, the calcrete and the paleosols developed in fluvial facies present significantly different As content. While the calcrete of the MPFF unit show a low content of As (3.9 mg/Kg), the paleosols of the LPFF show the higher As content of the fluvial facies (16.5 mg/Kg). On the other hand, present-day soils show the highest As content, especially in the illuvial horizons (23.3 mg/Kg).Preliminary results demonstrate a sedimentological control embarking differences in As concentrations between fluvial facies. These differences are probably attributed to a major hydraulic gradient during the MPFF, which is reflected in grain size and in fluvial structures, which probably was followed by washed out sediments. A geomorphological control was observed through an increase of As concentrations from the interfluves (MPFF) to the valleys (LPFF) as well as from the upper to the lower basin zone within the LPFF. Calcrete and paleosols developed in MPFF and LPFF respectively, reflects the different geomorphological conditions. A good correlation between them and the As content exists and is attributed to the presence of Fe oxy-hidroxides in the paleosols of the LPFF. Finally, present soils show high As concentrations in B and Bt horizons probably due to the grain size and clay mineralogy.