Solving the problem at the source: Controlling Mn release at the sediment-water interface via hypolimnetic oxygenation

One of the primary goals of hypolimnetic oxygenation systems (HOx) from a drinking water perspective is to suppress sediment-water fluxes of reduced chemical species (e.g., manganese and iron) by replenishing dissolved oxygen (O₂) in the hypolimnion. Manganese (Mn) in particular is becoming a serious problem for water treatment on a global scale. While it has been established that HOx can increase sediment O₂ uptake rates and subsequently enhance the sediment oxic zone via elevated near-sediment O₂ and mixing, the influence of HOx on sediment-water fluxes of chemical species with more complicated redox kinetics like Mn has not been comprehensively evaluated.This study was based on Mn and O₂ data collected primarily in-situ to characterize both the sediment and water column in a drinking-water-supply reservoir equipped with an HOx. While diffusive Mn flux out of the sediment was enhanced by HOx operation due to an increased concentration driving force across the sediment-water interface, oxygenation maintained elevated near-sediment and porewater O₂ levels that facilitated biogeochemical cycling and subsequent retention of released Mn within the benthic region. Results show that soluble Mn levels in the lower hypolimnion increased substantially when the HOx was turned off for as little as ∼48 h and the upper sediment became anoxic. Turning off the HOx for longer periods (i.e., several weeks) significantly impaired water quality due to sediment Mn release. Continual oxygenation maintained an oxic benthic region sufficient to prevent Mn release to the overlying source water.