Geometry of the modelled freshwater/salt-water interface under variable-density-driven flow (Pétrola Lake, SE Spain)

Pétrola Lake in southeast Spain is one of the most representative examples of hypersaline wetlands in southern Europe. The rich ecosystem and environmental importance of this lake are closely associated with the hydrogeological behaviour of the system. The wetland is fed by the underlying aquifer with relatively fresh groundwater—1 g L −1 of total dissolved solids (TDS)—with a centripetal direction towards the wetland. In addition, the high evaporation rates of the region promote an increase in the concentration of salts in the lake water, occasionally higher than 80 g L −1 TDS. The density difference between the superficial lake water and the regional groundwater can reach up to 0.25 g cm −3 , causing gravitational instability and density-driven flow (DDF) under the lake bottom. The objective of this study was to gain an understanding of the geometry of the freshwater–saltwater interface by means of two-dimensional mathematical modelling and geophysical-resistivity-profile surveys. The magnitude and direction of mixed convective flows, generated by DDF, support the hypothesis that the autochthonous reactive organic matter produced in the lake by biomass can be transported effectively towards the freshwater–saltwater interface areas (e.g. springs in the lake edge), where previous research described biogeochemical processes of natural attenuation of nitrate pollution.