Kinetics of dissolution processes in loess-like sediments and carbonate concretions in the southeast of the province of Buenos Aires, Argentina

The Pampeano aquifer formed by loess-like sediments provides water to cities and for agricultural uses in the Pampean plain, which is the most productive area in Argentina. Studies on the mechanisms through which this groundwater acquires its chemical composition are scarce and generally make assumption about equilibrium conditions. Few works on total sediments kinetics of mineral dissolution have been made. The main objective is to characterize ions incorporation to the groundwater of the Pampeano aquifer and to estimate the rate of the dissolution of the solid phase of the Pampeano sediments. This work also aims to provide evidence on the effect of particles size on water chemistry, and the changes in mineral structure during dissolution. The methodology included batch experiments on loess and calcrete during 10 h. The kinetics of ions incorporation into water presented variations depending on the types of sediments dissolving and the sizes of particles. Steady values were reached in the first minutes of reaction. Although the principal components of the Pampeano aquifer like calcite, quartz and aluminosilicates are known to have low dissolution coefficient, ions were incorporated fastly into water and saturation of solution appeared in the first minutes of the experiments. Saturation index (SI) calculated by PHREEQC also showed sensitivity to particles size. Observations with loupe and microscope showed modifications on the sediments appearance after batch reactions. For instance, porosity in calcrete increased. Increments in BET (Brunauer-Emmett-Teller) surface area and micropore surface area were measured. Significant changes in sediment chemistry measured by SEM/EDS were observed as well during dissolution processes.