Preemptive activation of hypolimnetic oxygenation improves control of Fe and Mn in drinking water reservoirs

Seasonal anoxia in the hypolimnion of freshwater reservoirs can degrade water quality by promoting the release of iron (Fe) and manganese (Mn). To address these issues, hypolimnetic oxygenation (HOx) is an engineered treatment that aims to prevent upward diffusion of reduced Fe and Mn from anoxic sediments and to oxidize reduced Fe and Mn in the water column, enabling sedimentation and removal. However, quantification of the effect of HOx on Fe and Mn concentrations is challenging without a reference ecosystem for comparison and multiple years of monitoring data. In this study, we monitored dissolved Fe and Mn in the water column and Fe and Mn removal through sedimentation for six years in two temperate reservoirs - one with a HOx system and one without. Our results show that HOx significantly enhances Fe and Mn sedimentation in freshwater reservoirs, but sedimentation of Fe was much greater than Mn. When activated before the onset of anoxia, HOx substantially decreased the rates of Fe release into the water column. In contrast, although early HOx activation delayed Mn release, it did not decrease the Mn release rate into the water column. However, early HOx activation increased the Mn sedimentation rate. When HOx was activated after the onset of anoxia, maximum total Fe and Mn concentrations in the hypolimnion were approximately 4x and 1.6x greater than concentrations in years with preemptive activation. Our study indicates that HOx improves water quality with respect to Fe and Mn, especially when activated prior to the onset of anoxia.