Mercury pollution in the coastal Urmia aquifer in northwestern Iran: potential sources, mobility, and toxicity

The concentration of total dissolved mercury (Hg_T) in surface and groundwater resources in the coastal parts of Urmia aquifer (NW of Iran) was investigated to identify the possible sources and sinks of mercury and the geochemical mechanisms controlling its mobilization. The distribution of water samples on the Piper diagram demonstrates that most samples have the Ca-Mg-HCO₃ facies. From 62 water samples collected in this area, one sample contained Hg_T concentrations exceeding the maximum contaminant level recommended by the WHO (6 μg/L). The principal component analysis (PCA) produced five principal components. The positive moderate correlation of Hg_T with EC, Cl, K, Mg, and Na indicated that the weathering of geological formations was one of the main sources of mercury in groundwater samples. Position of water samples in Eh-pH regions where microorganisms involved in mercury methylation and mineralization were potentially active demonstrated that the aquifer had undergone sulfate reduction and had reached the final stage of the terminal electron accepting process (TEAP) sequence in the methane production processes which are limited to only 37% of the water samples that have anaerobic conditions. Some Hg-bearing species are in nonequilibrium geochemical conditions. The supersaturation of water samples with magnetite and goethite indicated that these Fe-bearing minerals could act as the strong reducing agents for the reduction of Hg(II) to Hg(0).