LAUSR

Abstract This study evaluates the potential impact of leaking CO2 gas and CO2-rich waters on shallow groundwater quality by focusing on the mobilization of trace elements. For this study, Group I (acidic CO2-rich waters with low TDS), Group II (slightly acidic CO2-rich waters with high TDS), and Group III (CO2-poor waters with low TDS) were sampled in the study area occurring naturally CO2-rich waters. The carbon isotope data (δ13C and 14C) indicate that the dissolved CO2 in CO2-rich water originates from old and deep-seated CO2. Our results show that Group I is formed by the direct input of CO2 gas into recently recharged shallow groundwater, whereas Group II discharging along fractures or wells occurs by long-term CO2-water-rock interactions in the subsurface. Group I shows higher concentrations of trace elements (Al, Ba, Be, Cr, Cs, Fe, Mn, Ni, Rb, and U) than Group III, and its Al and Mn concentrations slightly exceed the US EPA drinking water standard. However, Group II has significantly higher concentrations of trace elements than Group I. In particular, harmful Be exceeds the US EPA drinking water standard by over 6.5-fold. In addition, Fe and Mn exceed the US EPA drinking water standard by 27.7-fold and 16.1-fold, respectively. The higher concentration of bicarbonate by pH buffering in Group II also increases the mobilization of U6+. Our results show that the mobilization of trace elements in the aquifer having little buffering capacity is mainly determined by the degree of CO2-water-rock interactions. This study indicates that upwelling of deep CO2-rich fluids with long-term water-rock interactions can be harmful to shallow potable groundwaters by supplying high levels of trace elements into shallow aquifers.