Mother Nature Makes the Case for Adaptive Water Quality Management of Impaired Water Bodies

The unexpected response of phosphorus cycling in Onondaga Lake, NY to changes in treatment plant nitrate and phosphorus loadings underscores the need to manage our nation's recalcitrant water quality problems in an adaptive manner. The Onondaga County, NY Syracuse Metropolitan Wastewater Treatment Plant (Metro), an 85 million gallons per day (MGD) tertiary treatment facility serving the City of Syracuse, NY, discharges to Onondaga 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 μg/L total phosphorus as well as State standards for dissolved oxygen, ammonia, and nitrite led to the development of an Amended Consent Judgment (ACJ) that required a staged implementation for both ammonia and phosphorus treatment at Metro based on a Total Maximum Daily Load (TMDL) developed in 1997. With an investment of over 120 MM, the County constructed a year-round ammonia treatment facility and the first stage of an advanced phosphorus treatment system. Brought on-line in 2004, the ammonia treatment system brought the Lake into compliance with ammonia standards, but also substantially increased the nitrate concentrations in the water column. In 2005, an advanced phosphorus treatment system was completed which treated effluent to concentrations of 0.12 mg/L, nearly six times higher than the 0.02 mg/L treatment levels required by the ACJ by 2012. Onondaga Lake responded to these reduced phosphorus loadings with decreased water column productivity that was much more dramatic than anticipated during development of the Phase 1 TMDL. This decline was primarily due to changes in the internal cycling of phosphorus. Indeed, the Lake met the State's summer average water column guidance value for phosphorus in 2008 and has remained below this threshold in 2009. Although more monitoring is needed to verify that these conditions represents a stable state for the system, it suggests that additional, and expensive, treatment of the Metro effluent may not be necessary to meet the States' phosphorus guidance value. As the Onondaga Lake experience shows, the implementation of highly uncertain TMDLs with correspondingly large implementation costs should be staged to allow for additional data collection, which reduces the uncertainties in loading estimates, but also allows time to gage the system response to changes in loadings. Mother Nature can be very unpredictable; we need to develop water quality management strategies that allow us to learn from this timeless teacher.