Interaction between lakes' surface water and groundwater in the Pantanal wetland, Brazil

The Nhecolândia subregion (area ~ 26,921 km²), in central-southern Pantanal, is marked by the presence of hundreds of alkaline–saline lakes, located on the top of sand hills, and thousands of non-alkaline lakes adjacent to the sand hills. This study aimed to provide a better understanding of the hydrological dynamics in the Nhecolândia, which is scarce and controversial, based on piezometric monitoring, isotopic data and chemical analyses of the surface water and groundwater of an alkaline–saline lake, a non-alkaline lake and the sand hill between them. The potentiometric data indicated that both alkaline–saline and non-alkaline lakes act as recharge zones, the former in all seasons and the later only in the wet season. These results are corroborated by downward flow near the zones of recharge, indicated by hydraulic head measurements in multilevel wells around the non-alkaline lake and in mini-piezometers installed in the studied alkaline–saline lake and two more lakes in the region. Also, δ¹⁸O and δ²H values of the alkaline–saline lake surface water became more depleted than the surrounding groundwater in the wet season, confirming that the inflow to the alkaline–saline lake in this season was by the more depleted rainwater and not by the isotopically enriched groundwater. The water chemistry data is also in agreement with recharge from the alkaline–saline lake, even though this recharge is limited by a low-K layer. Because of this layer, the non-alkaline lake does not dry off during the dry season. This set of evidence demonstrated that the groundwater recharge in the study area occurs in the topographical lows, through a process known as depression-focused recharge, which disagrees with previous studies of the area.