Geochemical modeling and isotope analysis of the shallow groundwater aquifer of Baghdad Area

Abstract

Baghdad City is characterized by high population density and wide variation in land use. It is covered by Quaternary flood plain deposits of variable nature where silt is the predominant component. The shallow aquifer is unconfined to semi-confined at some locations. The hydraulic properties of the aquifer are highly variable in the study area. A study of this groundwater shallow aquifer and the hydrochemical relation with the Tigris River were conducted using geochemical modeling approach. Baghdad Meteoric Water Line (BMWL) was also constructed using data of stable isotopes of hydrogen and oxygen. The hydrochemical parameters of the Tigris River show significant differences at high and low flow conditions, and there are clear difference among the selected stations. Groundwater parameters show also significant spatial and temporal variations in major and minor elements concentrations. Geochemical modeling results indicate that dissolution of dolomite, gypsum, chlorite, siderite, chalcedony, cation exchange of Ca²⁺/Na⁺ and precipitation of calcite, illite, kaolinite, and hematite are the main chemical reactions in the Rasafa side, whereas no specific reactions can be shown in the Karkh side. Mixing models of the shallow groundwater and Tigris River water show various patterns affected by other factors such as the aquifer recharge and evaporation, especially at the most shallow parts. The BMWL has been defined by the equation ${\delta^2}\mathrm{H} = 8.6\ {\delta^{18}}\mathrm{O} + 17.48$ and the stable isotopes of hydrogen and oxygen reveal different signatures in the Karkh and Rasafa sides, where clear zonation at Rasafa can be observed. We conclude that recharge water undergoes significant evaporation through its transit to the aquifer.