Effects of Oxygenation on Metals Cycling in Lakes

Stephen R. Dent

This dissertation summarizes three different evaluations of oxygenation treatments on metals in freshwater lakes conducted from 2007 to 2011. The goal of this work was to evaluate the effectiveness of hypolimnetic oxygenation in repressing methylmercury accumulation in the water column to add to the body of knowledge on how to ultimately reduce the accumulation of mercury in the aquatic food web. Methylmercury, the mercury species of concern in aquatic food webs, is formed by the methylation of inorganic mercury by anaerobic sulfate reducing bacteria. Maintaining oxic conditions should, therefore, repress anaerobic methylation of mercury. The first evaluation covered in this dissertation is a sediment-water interface (SWI) chamber study presented in Chapter 2. There was little difference between the oxic and anoxic chambers, and after examination, it became apparent that the anoxic chambers do not have an oxic-anoxic interface (OAI), an important parameter for the dynamic of mercury methylation. To avoid these problems associated with the confining environments of chambers, the remaining assessments were conducted on full lake water columns. In Chapter 3 and 4, a full-scale oxygenation system was evaluated at North Twin Lake, WA. The first evaluation followed a late season oxygenation test conducted after the onset of hypolimnetic oxygenation with iron, manganese, and methylmercury already accumulated in the bottom waters. Injection of oxygen dropped iron and manganese by 70% within 8 hours of operation and dropped methylmercury 95% by the end of the two week test. Full-scale operation of oxygenation had mixed results in regards to oxygen delivery to the bottom waters. Iron, manganese, and methylmercury accumulation was repressed during times when dissolved oxygen was kept above 4 mg/L. However, considering the system was designed to deliver dissolved oxygen higher in the water column, there were many times throughout the 3 year study period where bottom dissolved oxygen dropped below 1 mg/L. Although this system was not optimal for repressing methylmercury accumulation in this application, it did demonstrate a promising treatment effect and shows potential for the optimization of future applications of hypolimnetic oxygenation for repressing methylmercury accumulation in aquatic ecosystems in the future.

Effects of Oxygenation on Metals Cycling in Lakes

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