Geochemical modeling and isotopic approach for delineating water resources evolution in El Fayoum depression, Egypt

In this study, an attempt was made to understand the hydrogeochemical processes controlling water resources (surface and groundwater) evolution in El Fayoum depression, Egypt. Hydrochemical, inverse geochemical modeling integrated with environmental isotopes was applied. Forty-three surface and groundwater samples were collected and analyzed for major chemistry (cations, anions) and environmental stable isotopes (delta O-18, delta D). The ionic sequence based on relative molar proportions for cations was Na+ > Ca2+ > Mg2+ > K+ and Na+ > Mg2+ > Ca2+ > K+ for surface and groundwater, respectively and for anions was HCO3- > Cl- > SO42- for all water samples. delta O-18 and delta D relationship plot on a typical evaporation line, enhance potential evaporation of recharging water prior to infiltration. Saturation index estimation revealed that water resources were affected by carbonate and evaporites minerals dissolution and supersaturated with Goethite, Gibbsite and Barite minerals. Inverse geochemical modeling using PHREEQC was used to identify water resources evolution in the study area through two flow paths A, B. The inverse model was constrained so the primary mineral phase including calcite, Aragonite, Anhydrite and Gypsum with Carbon dioxide (gas) are constrained to dissolve until they reach saturation, Whereas H-2(g), hydrogen sulphide (gas) and O-2(g) are constrained to out gas in all models for flow path (A). For flow path (B), Dolomite, Anhydrite and Gypsum are tend to dissolve with carbon dioxide (gas), H-2(g) and O-2(g) gases tend to outgassed. Finally, two processes controlling water resource evolution; carbonate and evaporites dissolution and development of cation exchange process through the aquifer system.