BIOGEOCHEMICAL DRIVERS OF MANGANESE CYCLING IN FRESHWATER RESERVOIRS

Elevated manganese (Mn) concentrations in drinking water are an aesthetic and health concern. Present in rocks and minerals, often co-occurring with Fe, Mn is released to water via chemical and biological processes. Mn solubility is highly redox-sensitive; oxidized forms of Mn are generally insoluble and reduced forms are soluble. The cycling of Mn in temperate lakes and reservoirs is strongly impacted by seasonal stratification, which isolates bottom waters (hypolimnion) from oxygen sources. In the absence of oxygen, Mn-reducing bacteria in bottom sediment can use Mn(IV) oxides as an electron acceptor, producing soluble Mn(II) that diffuses upward into the water column. Over prolonged periods of anoxia, soluble Mn can accumulate in the hypolimnion, degrading water quality.

Our decade of research on biogeochemical processes impacting Mn in drinking water reservoirs in southwestern Virginia has focused on patterns of Mn cycling at time scales ranging from hourly to multi-annually. During stratification, reduced Mn is released from sediments to the water column at rates up to 62 mg/m²/day, resulting in Mn concentrations up to 4 mg/L in the hypolimnion. Hypolimnetic oxygenation can help to suppress Mn release from sediments, but once the reduced Mn enters the water column, it is slow to oxidize at circumneutral pH. Our work shows that interaction of reduced Mn with Mn-oxidizing bacteria collected from the upper boundary of the hypolimnion can result in rapid Mn oxidation (0.86 mg/L/d), and subsequent removal from the water column. Although contact between Mn-oxidizers and reduced Mn is limited under stratified conditions, we have evidence that during mixing periods, Mn-oxidizers gain access to reduced Mn, resulting in rapid Mn oxidation and settling of Mn particles. Current work on the impact of pH and alkalinity on Mn oxidation shows rapid Mn removal by oxidation at higher pH (~10) and alkalinity (>80 mg/L as CaCO₃), but increased alkalinity appears to enhance Mn oxidation even at pH 8. We are also examining the nanomineralogy of settling particles in the water column to evaluate the stability of Mn (and Fe) solids. Overall, our results inform water supply managers on optimizing methods for Mn removal and also contribute to a more holistic understanding of Mn cycling in freshwater systems.