GEOCHEMICAL PROCESSES AFFECTING GROUNDWATER QUALITY IN WADI SIDRI BASIN, SOUTH SINAI, EGYPT

The chemistry of groundwater in arid and semi-arid environments is typically influenced by a set of physical and hydrogeochemical processes that are commonly observed, including precipitation, interaction between water and rocks, and evaporation. It is important to accurately assess the amount and quality of groundwater recharge, as it affects domestic and agricultural activities. This study used traditional diagrams and correlation analysis to understand the hydrochemical processes in the Wadi Sidri basin, Sinai of Egypt and develops a method to estimate rainfall recharge. The trilinear diagram showed that Na and Ca predominate over Mg and K, while strong acids (SO₄ + Cl) predominate over weak acids (CO₃ + HCO₃). The Gibbs plot revealed that rock-water interaction is the primary factor influencing groundwater chemistry. The ion correlations between multiple selected ion ratios, including (Na/Cl)─(Ca/Mg), (Mg/Ca+Mg)─(HCO₃/SiO₂), (HCO₃+SO₄)─(Ca+Mg), and (Ca/Ca+SO₄)─(Na/Na+Cl) indicated that additional mechanisms, such as reverse ion exchange, direct ion exchange, as well as silicate and carbonate weathering and leaching of interbedded marine deposits influenced groundwater chemistry. The chemical activity diagram revealed that cation exchange processes involving an increase in Na at the expense of Mg control the cation chemistry of groundwater. This diagram, together with the saturation indices obtained by the PHREEQC program, indicated that groundwater is in equilibrium with kaolinite and gibbsite, but is only partially equilibrated with illite. The chloride mass balance method (CMB) was used to estimate the net groundwater recharge by rainfall. It was found that the overall estimate of the recharge for the whole areas was1.78 mm/y for 1981 to 2021 period with a total percentage of the fallen precipitation of 8.06%.