Occurrence of Arsenic in Soils and Paleosols of the Claromecó River Basin, Southern Pampean Plain (Argentina)

The Chacopampean plain is an extended flatland characterized by high As within sediments anddrinking waters. The volcanic glass shards, normally present as a major constituent of the Chacopampean plain sediments, are classically considered the main source of As. Nevertheless, thick volcanic ash layers and Fe-Mn (Al) oxy-hydroxides are also contemplated as a reasonable source of As in the sediments. In order to understand the main source of As in the Claromecó river basin (southern Chacopampean plain) sediments, paleosols, present-day soils and groundwaters were sampled. Three sedimentological units were identified: a deeper early Pliocene fluvial unit (mean sediment As~ 4.6 mg/kg), a shallower late Pleistocene fluvial unit (mean As ~ 11.6mg/kg) and Holocene Loess (mean As~ 2.5 mg/kg). Two types of paleosols were characterized: a Pliocene pedogenic calcrete (mean As∿ 3.6 mg/kg) and a Late Pleistocene hydromorphic paleosol (mean As∿ 16.5 mg/kg). The present-day soils were taken into account where mean As is ∿ 10.5 mg/kg. Although mean values for each unit are quite different, statistical analyses (ANOVA) reveal no statistically significant difference between As concentrations within the various sedimentological units. However, the hydromorphic paleosols and present day soils show a statistically significant difference from the rest of the units. In these pedogenetic units, the highest As concentrations are located in the Fe-Mn(Al) oxy-hydroxide phases (nodules and rhizo-concretions) as shown by sequential extractions and μ-XRF analysis. From this context, Pliocene pedogenic calcrete shows low content of oxy-hydroxides and therefore low As. Additionally, As concentrations in waters (mean As∿ 78.42 μg/L) shows higher values in shallower groundwaters coinciding with Pleistocene paleosols. This preliminary study highlights that the sediment deposition may not necessarily represent a systematic control in the As concentrations even though an increment from the Pliocene to the Holocene in sediment’s mean As values is noticed. However, the late Pleistocene hydromorphic soils, most likely linked to a shift in the climatic conditions, favored the concentration of As within Fe-Mn (Al) oxy-hydroxides phases. These pedogenetic features could represent the principal source of As enrichment in groundwaters.