The Santa Olalla pond is the only permanent pond located in the Doñana National Park (Southern Spain). Hydrochemistry of both surface and groundwater samples was analyzed during 2017 and 2018. Those samples were representative of the wet and dry seasons, respectively. In total, 22 samples were analyzed. Samples were taken in alignment with the general flow path from the dunes to the pond. Sampling points were: one medium-depth piezometer (21 m depth), three shallow piezometers (3 m) and surface water. Groundwater samples were taken using a pump and conserved for subsequent analysis. Physical-chemical parameters and iron analysis were measured in situ. Salinity values ranged from 45 mg/l at the beginning of the flow path, close to the dunes, to 8,850 mg/l in the pond itself. Facies varied from calcium-bicarbonate to sodium-chloride facies depending both on seasonality and location. The three inverse model scenarios performed with Phreeqc 3.0 and the database Wateq4f reveal sulfate reduction, redox reactions (Fe-S), mineral dissolution-precipitation, evaporation and mixing with deep water. These processes change seasonally, in accordance with hydrodynamic modifications in the flux directions in this pond. Minerals that were found to play a role in the pond hydrodynamics were calcite, gypsum, halite and pyrite. Evaporation was also found to be one of the main driving factors controlling the ionic composition in the surface water.