ARSENIC AND OTHER INORGANIC NATURALLY OCCURRING CONTAMINANTS IN GROUNDWATER OF THE MAIN ETHIOPIAN RIFT: A HUMAN EXPOSURE ASSESSMENT

This study investigates the mechanisms of arsenic (As) and other naturally occurring contaminants (F⁻, U, V, B, and Mo) mobilization from Quaternary sedimentary aquifers of the Main Ethiopian Rift (MER) and their enrichment in the groundwater, which is the predominant source of drinking water for local communities. The study is based on measurements of major and trace elements in groundwater, coupled with geochemical and mineralogical analyses of the aquifer rocks. The Rift Valley aquifer is composed of rhyolitic volcanics and lacustrine sediments. Total As content in the sediments (mean=6.6 mg/kg) exceeds those of the rhyolites rocks (mean: 2.5 mg/kg). The average concentration of As in groundwater was 22.4 ± 33.5 (n=50) and 54 % of samples exceeded the WHO drinking water guideline value of 10 µg/L. The dissolution of volcanic glass characteristically produces groundwater with near-neutral to alkaline pH (range 6.9–8.9), predominance of Na-HCO₃ ions, and high concentration of SiO₂ (mean: 85.8 ± 11.25 mg/L). Groundwater data show positive correlation of As with Na, HCO₃¯, F^„Ÿ, U, B, V, and Mo. Chemical modeling of the groundwater indicates that Fe-oxides and oxyhydroxides minerals were saturated in the groundwater, suggesting that the As reactivity is controlled by adsorption/desorption processes with these minerals. The data show that most of As including U, B, Mo and V were released particularly at pH > ~8, reflecting pH-dependent co-occurrence. We posit that the oxidizing state of the aquifers, Na-HCO₃ composition, and pH>~8 lead to adsorption-desorption relationships with Fe-oxides and consequently groundwater contamination by oxyanion-forming elements. Our measurement of trace metals in urine in exposed populations revealed direct relationships of these contaminants between urine and drinking groundwater. These urinary biomarkers enable us to evaluate the burden of the short-term exposure of contaminants in the human body. Findings from this study apply for groundwater contamination along the Eastern African Rift that pose adverse health effect to larger populations of Eastern Africa.