SEDIMENTOLOGICAL CONTROL ON ARSENIC OCCURRENCE IN CLAROMECÓ RIVER BASIN (SOUTHERN PAMPEAN PLAIN, ARGENTINA)

High arsenic concentrations from geogenic sources in the Pampean plains groundwatersare a known problem since 1950 in Argentina. However, only during the last 20 years several studies systematically targeted As distribution and mobilization in groundwater in the Pampean plain. Nonetheless many areas within this flatland have not been studied yet and the effects on human health have not been documented.

Claromecó river basin (in the southern Pampean plain) represents one of the poorly studied areas where high As concentration in groundwater was observed. Although many towns use reverse osmosis plant for As treatment, a large portion of the population use pump wells for daily activities as drinking water. Besides, the demand of groundwater for irrigation in the area is growing significantly in the recent years. For that reasons, a study of the As occurrence in the area is particularly important.

The aim of this projectis to relate mineralogy and sedimentological environment to groundwater and surface water in order to understand the hydrochemical processes controlling As within the Claromecó basin. To this purpose, soils and sediments from the Claromecó river basin have been studied in order to understand firstly the possible As sources and the sedimentological controls.

Total As concentrations in 32 samples of sediments collected from the basin rangefrom 1.8-24.9 mg/Kg with a mean of 8.8 mg/Kg. Two samples of rhyolitic ash have As content of3.3-3.6 mg/kg. Five samples of volcanoclastic loess have As content of 1.8-3.5 mg/Kg, while the fluvial sediments (loess like material) exhibit higher As content: 2.8-24.9 mg/Kg.

Preliminary results reveal a sedimentological control of As reflecting fluvial environment to be the highest As source in the basin. Variation of As content are observed in fluvial sediments depending on topography. On this aspect, valley sediments show higher As content (mean of 12.2 mg/Kg) instead of interfluve sediments (mean of 4.2 mg/Kg), which may reflect groundwater discharge and enhanced residence times. Selective extraction data (acid-ammonium oxalate) suggest that much of the As is associated with Fe oxy-hydroxides, probably including other secondary oxide minerals. Moreover, micromorphological analysis shows presence of Fe nodules in valley sediments associated with paleosols.