Hydrogeochemistry of a small saline lake: Assessing the groundwater inflow using environmental isotopic tracers (Laguna del Plata, Mar Chiquita system, Argentina)

Major ion and stable isotopic compositions of a saline lake linked to the SW margin of Laguna Mar Chiquita, Argentina (30 degrees 55'S, 62 degrees 51'W) were studied. The groundwater inflow into the Laguna del Plata was quantified for the first time using a stable isotope mass balance. Shallow groundwater inflow is expected in the Laguna Mar Chiquita system since it represents the terminal water body of the catchment. Lake waters are alkaline and exhibit total dissolved solids contents > 30 g L-1. The cationic relative abundance is Na+ >> Mg2+ > Ca2+ > K+, whereas that of major anions is Cl- >> SO42- > HCO3- + CO32-, which are mainly present as free aqueous ions. PHREEQC simulations indicate that lake waters are oversaturated in Ca2+ and Mg2+ carbonates (calcite, huntite, aragonite, dolomite, magnesite), which are expected to be the first minerals to precipitate due to evaporation processes. The main tributary of the lake (Suquia River) and the phreatic aquifer exhibit delta O-18 and delta H-2 values consistent with the isotopic signature of rainfall, whereas lake waters define an evaporation line (delta H-2 (%o) = 5.4 delta O-18 (parts per thousand) + 1.8). The confined aquifer shows a more depleted isotopic composition, reflecting an allochthonous recharge from the mountain ranges (Sierras Pampeanas). The delta O-18 mass balance indicates a groundwater inflow of similar to 4.7 m year(-1) to the lake, confirming the hydrological conceptual model for the catchment. However, the groundwater contributions do not affect the chemical composition of lake waters, which mainly results from the evaporation processes as shown by the Spencer diagram.