Phosphorus Loading Reductions and Initial Signs of Recovery of Hypereutrophic Onondaga Lake, NY

The water quality of Onondaga Lake, a 12 km2, dimictic, water body located in central New York, has been compromised by a 100-year history of municipal and industrial pollution. The Syracuse Metropolitan Wastewater Treatment Plant (Metro), an 85 MGD tertiary treatment facility serving the City of Syracuse and surrounding area, discharges to the lake and accounts for 20% of its annual average inflow and historically between 40% and 50% of the annual total phosphorus loadings. Contravention of the New York State water quality guidance value of 20 ug/L total phosphorus (TP) as well as state standards for dissolved oxygen (DO), ammonia, nitrite, and bacteria led to the development of a Municipal Compliance Plan and Amended Consent Judgment (ACJ) that required Metro treatment plant upgrades and abatement of combined sewer overflow discharges to the lake. The upgrades to Metro included construction of an advanced high rate flocculation system (HRFS) for the removal of phosphorus, put into service in February 2005. The reductions in Metro TP loadings have elicited a response in lake water quality as measured by the ACJ-required ambient monitoring program (AMP) for the lake. In response to reductions in Metro loadings, summer average TP and soluble reactive phosphorus (SRP) concentrations in the epilimnion of Onondaga Lake have declined concurrently with phytoplankton biomass that, in turn, reduced sediment oxygen demand and decreased the volume days of anoxia in the hypolimnion.