Controlling reduced iron and manganese in a drinking water reservoir by hypolimnetic aeration and artificial destratification.

ABSTRACT

The concentrations of iron (Fe) and manganese (Mn) in the water column have extremely important effects on the water quality of drinking water reservoirs; however, reservoirs often experience problematic Fe and Mn levels during seasonal stratification and rainfall events. Water-lifting aerators (WLAs) were deployed in the Jinpen Reservoir to control these issues with Fe and Mn at the source via bottom aeration and artificial destratification. In this study, variations of Fe and Mn concentrations in the water column, porewater, and sediments, were used to characterize behaviors of reduced Fe and Mn under the conditions of hypolimnetic aeration and artificial destratification during periods of hypolimnion hypoxia and rainfall events. The results showed that replenishing aquatic oxygen levels by aeration can effectively decrease the dissolved Fe and Mn in the water column thereby increasing the sedimentation rate and the diffusive flux of Fe and Mn at the sediment-water interface (SWI). The dissolved Fe was significantly chemically oxidized and the concentration remained relatively low in the water column during WLA operations, while dissolved Mn persistently accumulated in the near-sediment regions because of its complex kinetics. Our in situ profiles of labile Fe and Mn in the sediments demonstrated that the diffusive flux of Mn (JMn) was largely increased by the increased concentration gradient at the SWI, while the diffusive flux of Fe (JFe) decreased. The sediments were observed to rapidly become anoxic and release Fe and Mn after WLA deactivation; this emphasized the importance of appropriate operations linking the artificial and natural mixing periods to prevent SWI hypoxia and the release of reduced substances.