Geochemical and Hydrological Controls on Manganese in a Weakly-Stratified Drinking Water Reservoir

Reservoir stratification can lead to high concentrations of dissolved manganese (Mn) in the raw water intake of water treatment facilities, complicating treatment processes. Pathways for Mn release to and precipitation in stratified lakes are not fully understood. We monitored Mn and other water quality parameters in a transiently stratified reservoir in New York State in 2021 and 2022. Moderate drought conditions in 2022 increased the duration of stratification, leading to Mn concentrations up to 35 μM in the raw water intake. Mixed Mn-calcium (Ca) carbonates, but not rhodochrosite (MnCO₃), were supersaturated in the epilimnion, and (Ca,Mn)CO₃ minerals were identified in suspended particulates using X-ray absorption spectroscopy. However, it was unclear whether Mn-bearing carbonates precipitated in the water column or were suspended from lake-bottom sediments, which consisted of 31% (Ca,Mn)CO₃ based on EXAFS analysis. The prevalence of Mn(II) in lake-bottom sediments suggests that reductive dissolution of sedimentary Mn(III/IV) oxides is not the only trigger for Mn release into stratified water columns and that pH-driven effects may also influence Mn release. Warmer temperatures and drier weather under climate change are expected to increase stratification in lakes and reservoirs, likely exacerbating problems associated with high Mn concentrations in surface drinking water sources.