Understanding the hydrological functioning of the scarce freshwater bodies of semiarid regions is crucial, especially in those areas affected by anthropic activities involving land-use changes. In the dry western edge of the Argentina Pampean plains, a system of more than 100 shallow lakes of remarkable stability occurs. These lakes exhibit low salinity compared to those located in the more humid belt. This system has constituted the main water resource for humans from prehispanic times to the present. Stable isotopes were used to establish the seasonal surface-water/groundwater interactions and the hydrological conditions in a lake of the Dry Pampean Plain (DPP), i.e., Lake Los Pocitos, to understand the mechanism that guarantees such a resource. Results indicate that evaporation mainly controls the isotopic composition of lake water, overwhelming the effect of higher rainfall inputs during the wet (but also most evaporative) season. The δ¹⁸O mass balance model indicates greater groundwater inflow to the lake during the dry season (~0.4 m month⁻¹) compared to the wet season (~0.2 m month⁻¹). Lake level decreased in the wet season due to the lowest groundwater inflow and the greatest evaporation rate. Based on the proportion of water entering a lake that leaves through evaporation, Los Pocitos corresponds to a throughflow lake with a short water residence time (~0.47 years). These hydrologic conditions, along with freshwater inputs from a dune located at the western margin of the lake, determine the existence of this relatively stable and freshwater lake in the DPP where high evaporation rates are registered.

Poyang Lake is the largest freshwater lake in China and is well known for its ecological and economic importance. Understanding the contribution of groundwater to Poyang Lake is important for the lake’s protection and management. In this study, stable isotopes (δD and δ¹⁸O), ²²²Rn measurements, and corresponding models (²²²Rn and ¹⁸O mass balance models) were employed to evaluate the groundwater discharge and associated chemical inputs to Poyang Lake. The results showed that the distribution of δ¹⁸O in the lake water reflects the groundwater discharge into the lake. The groundwater discharge estimated using the ²²²Rn mass balance model was in reasonable agreement with the groundwater discharge derived from the ¹⁸O mass balance model. The ²²²Rn mass balance model showed that the groundwater discharge rate was 24.18 ± 6.85 mm/d with a groundwater discharge flux of (2.24 ± 0.63) × 10⁷ m³/d, which accounts for 6.52–11.14% of river-water input in the Poyang Lake area. The groundwater discharge flux estimated using the ¹⁸O mass balance model was 3.17 × 10⁷ m³/d, and the average groundwater discharge rate was 26.62 mm/d. The estimated groundwater discharge was used to estimate the associated chemical fluxes. It was found that groundwater-derived heavy metals such as iron and manganese are potential threats to the lake ecological system because of their large inputs from groundwater discharge.