Natural arsenic in groundwater and alkaline lakes at the upper Paraguay basin, Pantanal, Brazil

Abstract

A distinctive feature of Nhecolândia, a sub-region of the Pantanal wetland in Brazil, is the presence of both saline (alkaline) and freshwater lakes. Saline lakes were attributed to a past arid phase during the Pleistocene, but recent studies have shown that the geochemistry of the saline lakes arises from the current concentration process of fresh waters that is supplied to the Pantanal every year. The region is mainly used for cattle farming, and more recently eco-tourism is becoming widespread and water conservation agencies have begun to worry about As contents in surface and shallow groundwater. The study was carried out along a 600-m-long transect that linked a freshwater lake to a saline lake. The spatial distribution of As is studied, based on soil morphology, according to pH, redox conditions (Eh) and dissolved organic carbon, and also compared to distribution of major elements. Three main processes, responsible for the chemical variability, were identified: (1) the concentration of the solution under the influence of evaporation and the associated precipitations (Mg-calcite, illite, sodium carbonate); (2) oxidation of sulphides included in clay layers and subsequent development of acid conditions and (3) buffering of acid conditions by clay dissolution. Arsenic contents are not affected by the last two processes (2 and 3), but evolve in proportion to the concentration. Arsenic concentrations in water ranged from 0.11 μg/l to 3.68 mg/l, i.e., 368 times higher than the World Health Organization guidelines for drinking water. The As speciation, as calculated from field conditions, indicated that the As(V) redox state dominated in the groundwater and in the lakes, whereas some proportion of As(III) could occur on the shore and in the sediments of the saline lake. Arsenic(III) from the sediment can be remobilized and dispersed into the water following the disturbance of the sediments by cattle that come to drink water. Arsenic contents increased with most of the other dissolved species; hence, competitive adsorption prevents dissolved As regulation by solid phase reaction. Moreover, the pH values increased above 8 with increasing As, favouring As(V) desorption. As a consequence, the good correlation established between As contents and electrical conductivity (r2 = 0.97) should help to indirectly evaluate the As contents in shallow groundwater and lakes of the region.