Arsenic Distribution and Geochemistry in Island Groundwater of the Okavango Delta in Botswana

Abstract

Groundwater samples were collected along an island transect in the Okavango Delta, Botswana, in 2007. A 1000-fold increase in arsenic concentration was found along the transect over a distance of 250 m. Arsenic was positively correlated to conser- vative elements (Na and Cl), electrical conductivity, alkalinity, DOC, pH, potassium and sulfate and negatively correlated to nitrate. Results from this work suggest that the enrichment of As in the island groundwater of the Okavango Delta is a result of a complex interplay between (1) concentration by evaporation/transpiration; (2) reduc- tive dissolution of Fe oxyhydroxides, masked by reprecipitation; and (3) competitive interaction between HCO3 – and As for the same sorption sites as pH increases. The predominant process controlling the very elevated levels of arsenic in the island center groundwater is probably the effect of the evapo-transpiration. However, it is pro- posed that reductive dissolution of oxide and hydroxide of minerals containing Fe and Mn is the initial step for the release of arsenic from the sediment to the ground- water although there was no correlation of arsenic with iron or manganese due to reprecipitation of oxides of these metals.