Influence of CO2 leakage into alluvial aquifers on the mobilization of Trace elements, northern part of Hamadan province

Natural CO2 leakage through water wells into four alluvial aquifers Hamadan, Razan, Komijan and Chardoli in Hamadan province, in addition to decreasing by about one unit of pH and significantly altering physicochemical parameters and soluble constituents in groundwater has led to trace metals enrichment in groundwater. To investigate the effect of CO2 leakage on mobility of trace elements, one hydrothermal spring and 19 water wells were sampled in September 2018. Trace elements, which have increased in gas-rich groundwaters, are classified into two groups based on correlation with Cl. The first group including As, Li, B, Sr, V, Rb and Cs, which are related to mixing of CO2-rich saline hydrothermal water with fresh groundwater. While origin of Al, Ba, Zn, Cr, Ni and Cu is in-situ weathering of aquifer materials. Se, Hg and Sn show no correlation with Cl and their concentration have been decreased in the gas recharged groundwaters. Carbon dioxide gas leakage into the groundwater of the study area does not appear to affect the mobility of these rare elements. The most important factors influencing on the impact of CO2 leakage on the aquifer quality that can attenuate the effects of CO2 leakage on trace elements mobility are high pH buffering capacity of the aquifer materials and CO2 degassing from the CO2-rich groundwaters near the ground surface and consequent precipitation of secondary minerals. Despite all these processes, the groundwater of the study area contains concentrations of several elements (Hg, As, B, Fe and Mn) above the WHO and national thresholds.