Arsenic in groundwater and sediments from La Pampa Province, Argentina

Arsenic in pumped groundwaters from the Quaternary loess aquifer of northern La Pampa, Argentina, has concentrations in the range <4–5300 μg/L, most being present as the oxidised arsenate form. Other anions and oxyanions (B, F, Mo, V, U) also often have high con-centrations. These trace elements show positive correlations with both pH and alkalinity. Arse-nic concentrations are particularly high in pumped groundwaters and porewaters beneath small topographic depressions which act as zones of seasonal discharge and restricted groundwater flow. Evaporation of water in these small internal drainage systems can be significant but is not responsible for the observed high As concentrations. Accumulation of As (dissolved and sorbed) through flow towards the depression and lack of flushing are likely controls. The As may be derived from a number of minerals but sorption/desorption reactions involving Fe ox-ides and possibly Mn oxides are considered important controls on the mobility of As, sorption being weakest at high pH. Modelling suggests that competition from other anions, especially vanadate, for binding sites on Fe oxides can further enhance the concentrations of As in the groundwater.