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

Abstract

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,
\nthick volcanic ash layers and Fe-Mn (Al) oxy-hydroxides are also contemplated as a reasonable
\nsource of As in the sediments.
\nIn order to understand the main source of As in the Claromecó river basin (southern
\nChacopampean plain) sediments, paleosols, present-day soils and groundwaters were sampled.
\nThree sedimentological units were identified: a deeper early Pliocene fluvial unit (mean sediment
\nAs~ 4.6 mg/kg), a shallower late Pleistocene fluvial unit (mean As ~ 11.6mg/kg) and Holocene
\nLoess (mean As~ 2.5 mg/kg). Two types of paleosols were characterized: a Pliocene pedogenic
\ncalcrete (mean As∿ 3.6 mg/kg) and a Late Pleistocene hydromorphic paleosol (mean As∿ 16.5
\nmg/kg). The present-day soils were taken into account where mean As is ∿ 10.5 mg/kg. Although
\nmean values for each unit are quite different, statistical analyses (ANOVA) reveal no statistically
\nsignificant difference between As concentrations within the various sedimentological units.
\nHowever, the hydromorphic paleosols and present day soils show a statistically significant
\ndifference from the rest of the units. In these pedogenetic units, the highest As concentrations
\nare located in the Fe-Mn(Al) oxy-hydroxide phases (nodules and rhizo-concretions) as shown by
\nsequential extractions and μ-XRF analysis. From this context, Pliocene pedogenic calcrete shows
\nlow content of oxy-hydroxides and therefore low As. Additionally, As concentrations in waters
\n(mean As∿ 78.42 μg/L) shows higher values in shallower groundwaters coinciding with
\nPleistocene paleosols.
\nThis preliminary study highlights that the sediment deposition may not necessarily represent a
\nsystematic control in the As concentrations even though an increment from the Pliocene to the
\nHolocene in sediment’s mean As values is noticed. However, the late Pleistocene hydromorphic
\nsoils, most likely linked to a shift in the climatic conditions, favored the concentration of As within
\nFe-Mn (Al) oxy-hydroxides phases. These pedogenetic features could represent the principal
\nsource of As enrichment in groundwaters.