Saline lakes are common features of arid and semi-arid regions, where the excess of evaporation over precipitation resulted in the accumulation of salts at the land surface. Among them, terminal saline lakes represent the end-point of groundwater flow systems, where water contribution derives from regional groundwater flow (RGF) as well as from perimeter recharge areas. Terminal lakes may be responsible for increasing the salinity of the shallow groundwater, due to the formation of wedge-shaped high salinity zones underneath these terminal saline lakes. Simultaneously, solutes can be transported by the density-driven flow (DDF) from the saline lake to the freshwater-saltwater interface, where they can exert an important role in the natural attenuation of pollutants. Therefore, a deeper understanding of surface-water/groundwater interaction in saline lakes is essential to know how the transport of solutes occurs. For this purpose, stable isotopes of the water molecule (δ18OH2O and δDH2O) have been widely used as tracers to provide information about hydrogeological processes. In lakes, the isotopic composition of surface water is closely connected to climate, reflecting meteorological conditions of the region, but also to the balance between inputs and outputs.
In this context, Pétrola Lake is a terminal saline lake located in the discharge zone of an endorheic basin in La Mancha region (High Segura Basin, SE Spain), an important area of distribution of saline lakes in the Iberian Peninsula. The anthropogenic pressure (agriculture and wastewater spills) over this saline wetland modifies the mass balance and the intensity of biogeochemical processes. The purpose of this work was to evaluate the interaction between groundwater and saline water from Pétrola Lake in order to improve the knowledge of groundwater recharge processes by DDF in terminal lakes. To reach this goal, hydrochemical (chloride concentration) and stable isotope (δ18O and δDH2O) data were used.
The isotopic composition (δ18OH2O and δDH2O) of 190 groundwater and surface water samples collected between September 2008 and July 2015 were determined. The results showed that groundwater recharge in the basin is mainly produced by Atlantic-derived precipitation. Moreover, data provided a regression line (δDH2O = 5.0·δ18O – 14.3‰, R2 = 0.95) consistent with dominant evaporation processes. Isotope data from lake samples suggested that the loss of water occurred at humidity values between 60% and 75%. Then, a saline boundary layer is formed, producing a leakage from the lake to the underlying aquifer by means of the DDF. The findings reported in this study has provided a deeper insight into groundwater recharge processes by DDF in terminal saline lakes using stable isotopes.