Implications of redox processes for the rehabilitation of an urban lake, Onondaga Lake, New York

A synthesis of the impacts of domestic and industrial wastes on oxygen resources and redox conditions of polluted Onondaga Lake, Syracuse, N.Y., is presented based on a long-term (1978-2005) monitoring program. Insights from this retrospective analysis are used to evaluate management alternatives for the remediation of oxygen resources and redox conditions. Reduced byproducts of anaerobic metabolism accumulated annually in the hypolimnion, causing low dissolved oxygen (DO) concentrations in the upper waters during fall turnover. The high natural sulfate (SO(4)(2-)) concentration of the lake promoted DO depletion and the production of methylmercury (CH(3)Hg(+)). Severe depletions of DO occurred annually in the upper waters during fall mixing, representing violations of water quality standards for extended intervals from 1978 to 1996. Depletions of DO during fall were less severe from 1997 to 2004. The improvement is reported to be in response to: ( 1) more routine occurrence of spring turnover following closure of an industry; ( 2) reduction in primary productivity; ( 3) return of large bodied Daphnia from closure of the industry; ( 4) satisfaction of historic debt from earlier higher primary production levels; and (5) year-round nitrification at a contributing domestic wastewater facility. Additional improvements in oxygen resources and decreases in SO(4)(2-) reduction are anticipated based on mandated future upgrades of phosphorus and ammonia treatment at the wastewater facility. Prevailing DO conditions during fall turnover, particularly within the context of anticipated improvements from the mandated upgrades, indicate hypolimnetic aeration or oxygenation is not required to meet water quality goals. Amendments to the hypolimnetic pool of NO(3)(-) are recommended instead of aeration or oxygenation to inhibit production of CH(3)Hg(+) by SO(4)(2-) reducing bacteria.