Assessing groundwater-surface water interactions in shallow lakes under anthropogenic stress (Pampa plain, South America): A hydrochemical and isotopic approach

Human interventions, such as land conversion, climate change and water use, significantly impact the global water cycle. Lakes and wetlands -the most abundant inland water bodies- are particularly affected. This study assessed the impacts of anthropogenic interventions on the hydrological dynamics and surface water-groundwater interactions in artificially connected shallow lakes of the Southern Pampa plain (La Chanchera-La Brava system). A small, unconnected lake (Médano de Castro lake) was also analyzed. The canal-connected lakes and associated groundwater are alkaline (mean pH 8.6), brackish (TDS from 1.6 to 13.2 g L⁻¹), and of the sodium-chloride/sodium-non-dominant type, exhibiting minimal seasonal variations. In contrast, the unconnected Médano de Castro lake has sodium bicarbonate-type waters, with salinity increasing from 6.7 g L⁻¹ in the dry (austral winter) to 12.3 g L⁻¹ in the wet (austral summer) seasons. Isotopic modeling (δ¹⁸O) applied in both La Chanchera and Médano de Castro lakes indicates that groundwater inflow occurs year-round, at rates approximately seven times higher in the wet season due to water table rising. Both lakes are throughflow systems with short water residence times (<1 year), indicating a constant water exchange. The hydrochemical and isotopic evidence indicates that the Médano the Castro lake receives local groundwater recharge from the surrounding dunes, imparting a bicarbonate signature and preventing its desiccation. In contrast, the canal-connected La Chanchera lake receives the input from regional groundwater with a chloride-sulfate composition. This study highlights the importance of understanding lake-groundwater dynamics in regions facing agricultural expansion and climate pressures.