Hydrochemistry of Pampean Lakes

Abstract

Numerous shallow lakes can be found in the Argentine Pampean plain. In Southern Pampa most of them are classified as freshwater or subsaline lakes, whereas lakes of Northern and Western Pampas are from subsalines to hypersalines, with higher TDS concentration during the wet season, when water balance is usually negative, than in the dry season. Pampean lakes are mostly alkaline, without significant regional or seasonal variations in pH values. Waters of most Southern Pampean lakes are of the bicarbonate-sodium-type, whereas Northern Pampean lakes show a composition from bicarbonate-sodium-type to chloride-sodium-type. Western Pampean lakes show the widest compositional range. Weathering and evaporation-precipitation processes are the main mechanisms that control the Pampean lakes water chemistry. Most of them are oversaturated in Ca and Mg carbonates, theoretically evolving from calcite composition towards the precipitation of Na-carbonates, where halite is the final mineral that will precipitate. Monovalent ions (Cl-, K+, and Na+) are mainly present as free aqueous ions, whereas HCO3- is the most abundant carbon species. Other elements (S, Ca, and Mg) tend to form complexes. The stable isotopic compositions of Pampean lakes also evidence the evaporation processes, which are more intense during the wet season. Groundwater-surface water interactions in several lakes of Northern and Western Pampas, modeled using environmental tracers (i.e. δ18O and 222Rn), indicate that groundwater discharge occurs at different scales.