Responses of typical metal ions in groundwater to changes in redox conditions under long-term recharge of reclaimed water: a case study of Chaobai River in Beijing

During infiltration through the riverbank, the recharge of reclaimed water inevitably poses a threat to groundwater quality. Based on monitoring data from 2007 to 2019, the responses characteristics of typical metal ions (i.e., Fe, Mn, and Ba) in the phreatic aquifer of the recharge area were investigated. After two years of recharge, the groundwater within 150 m from the riverbank was completely replaced by reclaimed water. With increasing groundwater level and the input of organic carbon, a reductive zone was formed within 50 m from the riverbank and the intensity of sulfate reduction became steady in this zone after 2010. Within the reductive zone, reductive dissolution of Mn in minerals and the desorption of adsorbed Ba in the solid phase resulted in the concentrations of both being significantly higher than those in reclaimed water during the period 2008–2010. After 2010, the concentrations of both decreased significantly and tended to stabilize. With regard to Fe, the combined effects of reductive dissolution of Fe in minerals and FeS precipitation caused the Fe concentration to approach that of reclaimed water. Beyond the reductive zone, Fe, Mn, and Ba concentrations exhibited no significant spatial variations along the groundwater flow path. In the optimized electron trapping capacity (ETC), the variation of sulfate makes the greatest contribution. In the reductive zone, ETC effectively indicated the concentration changes of Fe, Mn, and Ba in groundwater. These findings are helpful for a better understanding of the impacts of reclaimed water recharge on groundwater quality. Effects of reclaimed water recharge on groundwater included increasing and steady stages. Concentrations of Fe, Mn, and Ba dominated the quality of groundwater in riverside. Sulfate reduction significantly affected the contents of Fe, Mn, and Ba in riverside groundwater. The optimized electron trapping capacity can well reflect the sulfate reduction intensity in groundwater.