Control of the mobilization of arsenic and other natural pollutants in groundwater by calcium carbonate concretions in the Pampean Aquifer, southeast of the Buenos Aires province, Argentina

Abstract

The water supply for human consumption in the Chaco-Pampean region in Argentina is restricted by the low quality of groundwater due to elevated concentrations of arsenic and other trace elements. Previous studies indicated a complex concurrence of factors and processes that are believed responsible to control the distribution of arsenic in groundwater. For a better understanding of the origin of trace elements in the Pampean aquifer, flow-through experiments with loess and calcrete samples representative of the sediments that constitute the aquifer were carried out in continuous flow reactors. The aqueous solutions were collected and the concentrations of SiO 2 (aq), Ca 2+ , SO 4 2− , Na + , Cl − , F − and trace elements (Ba, Sr, V, and As) were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), high performance liquid chromatography (HPLC) and capillary electrophoresis. The experiments showed differences in the release rate of elements to the solution according to the type of sediment. The highest concentrations of V, Ba, and As were measured in experiments conducted with loess, and these elements were released quickly to the solution in the first minute of the test. In the case of loess, V and As are suggested to be adsorbed on the solid particles surface. Conversely, the experiments conducted with calcrete showed a lower but continuous release of those elements. This last result may indicate that the trace elements were coprecipitated in the calcite. In addition, it was demonstrated that F did not come from the dissolution of minerals such as fluorapatite, but both desorption from solid surface and dissolution from calcite minerals account for the release of F. This study support that both dissolution and adsorption-desorption processes can control the mobility of trace elements, with an emphasis on the role of calcrete in the retention and the mobilization of trace elements in the Pampean aquifer.