Arsenic chemistry in a ground water aquifer from the eastern province of Saudi Arabia

Groundwater samples were collected from shallow aquifers underneath an industrial complex in the Eastern Province of Saudi Arabia. Arsenic (As) concentrations in the groundwater samples varied between 10^-8.6 and 10^-6.8 M (0.18 and 11.14 μg L^-1), with an average of 10^-7.5 M (2.19 μ L^-1). The analysis of variance for the analytical data showed that sampling locations had significantly affected As concentrations in the groundwater samples. Analytical and thermodynamic calculations showed that H₂AsO₄^- was the most predominant As species in acidic groundwater samples, and HAsU₄^2- was the most abundant species in alkaline groundwater samples. Concentrations of HaAsO₄ and AsO₄^3- were too low to be important in this study. Reduced As chemical forms were also expected to be very low. All the groundwater samples were undersaturated with respect to the thermodynamic solubility isotherms of Ca₃(AsO₄)₂(c), Fe₃(AsO₄)₂(c), and Mn₃(AsO₄)₂(c) minerals. Lack of reliable thermodynamic data for these arsenates could be responsible for differences between the theoretical and measured concentrations of As in the shallow groundwater samples. The general trend in the distribution of HAsU₄^2- activities in the groundwater samples was parallel to that of the Ca₃(AsO₄)₂ solubility isotherm but different from those of Fe₃(AsO₄)₂(c), and Mn₃(AsO₄)₂(c). These data suggest that As concentrations in the groundwater samples were probably controlled by the precipitation and dissolution of Ca₃(AsO₄)₂ type mineral. A three step hypothesis for As interactions in groundwater/soil system is proposed that combines both solid phase formation and adsorption of As onto the solid colloidal surfaces. This hypothesis is expected to better represent As behavior in groundwater/soil environment.