Whole-lake nitrate addition for control of methylmercury in mercury-contaminated Onondaga Lake, NY

Methylmercury (MeHg) strongly bioaccumulates in aquatic food webs resulting in exposure to humans and wildlife through consumption of fish. Production of MeHg is promoted by anaerobic conditions and the supply of inorganic Hg (Hg2+), sulfate (SO42−), and labile organic carbon. The anaerobic sediments of stratified lakes are particularly active zones for methylation of Hg2+ and can be an important source of MeHg to the water column during summer anoxia and fall turnover. Nitrate (NO3–) addition has recently been proposed as a novel approach for the control of MeHg accumulation in the hypolimnia of Hg-contaminated lakes. In 2011, a whole-lake NO3– addition pilot test was conducted in Hg-contaminated Onondaga Lake, NY with the objective of limiting release of MeHg from the pelagic sediments to the hypolimnion through maintenance of NO3––N concentrations >1 mg N/L. A liquid calcium-nitrate solution was added to the hypolimnion as a neutrally buoyant plume approximately three times per week during the summer stratification interval. Maximum hypolimnetic concentrations of MeHg and soluble reactive phosphorus (SRP) decreased 94% and 95% from 2009 levels, suggesting increased sorption to Fe and Mn oxyhydroxides in surficial sediments as the regulating mechanism. Increased MeHg concentrations in the upper waters during fall turnover, which had been a generally recurring pattern, did not occur in 2011, resulting in decreased exposure of aquatic organisms to MeHg. Over the 1992–2011 interval, the hypolimnetic NO3– supply explained 85% and 95% of the interannual variations in hypolimnetic accumulations of SRP and MeHg, respectively.