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Spatial and temporal patterns of nitrate (NO3(-)) and bisulfide (HS(-)) are documented in mercury-polluted, culturally eutrophic, Onondaga Lake, New York, following implementation of year-round nitrification treatment at a domestic wastewater treatment plant (WWTP). Measurements of NO3(-) and HS in the lake were made with a rapid-profiling, high-resolution, in situ ultraviolet spectrophotometer (ISUS) and were validated by standard laboratory wet chemistry analyses. A nearly 2-fold increase in epilimnetic NO3(-) concentrations, prolonged presence of NO3(-), and delay of the onset of HS(-) accumulations in the hypolimnion by approximately 1 month are demonstrated. Detailed vertical patterns resolved within the anoxic hypolimnion first depict operation of the thermodynamically favored NO3(-) reduction process(es) and, subsequently, sulfate (SO4(2-)) reduction and the localization of these processes in the lake's sediments. Variations in the effective depth of entry of WWTP discharge into the lake's water column, ranging from surface waters to metalimnetic depths, are demonstrated. Two- and three-dimensional patterns of NO3(-) from ISUS profiles depict substantial spatial structure mediated primarily by hydrodynamic processes. In situ ultraviolet spectrophotometer measurements of NO3(-) and HS(-) will play an important role in ongoing rehabilitation programs for the lake.

Responses of polluted Onondaga Lake, New York, to early stages of a phased program to rehabilitate the lake from the effects of domestic waste inputs are documented. The analysis is based on more than 10 years of paired monitoring of the effluent (total ammonia and total phosphorus) of a wastewater treatment plant (WWTP) that discharges to the lake as well as the lake itself (including total ammonia, nitrite, total and dissolved forms of phosphorus, plankton biomass and composition, Secchi disc transparency, and zebra mussel density). Major reductions in total ammonia and total phosphorus loading relative to the preceding decade are reported for the WWTP for the November 1998 through October 1999 interval. Dramatic and, in some cases, unanticipated changes in the lake's water quality and biota in response to the reductions in loading are documented for the April to October interval of 1999 including: (1) major decreases in total ammonia concentrations and improved status with respect to ammonia toxicity standards, (2) development of dense populations of zebra mussels, (3) decreases in fall concentrations of nitrite and improved status with respect to the related toxicity standard, (4) decreases in total phosphorus and total dissolved phosphorus concentrations, and (5) a severe Microcystis (phytoplankton) bloom that caused nuisance conditions and poor clarity. The zebra mussel invasion is attributed to the reductions in total ammonia concentrations to below toxic levels. The Microcystis bloom was probably related to the abrupt increase in the zebra mussel population. Additional reductions in phosphorus loading from the WWTP will be required to limit phytoplankton production and avoid the potential for continued nuisance conditions. Potential complications in resolving lake responses to future reductions in loading associated with the zebra mussel invasion are considered.

The recent history of loading of total ammonia (T-NH3) and organic nitrogen (N) from a pharmaceutical manufacturing facility to a municipal treatment plant (Metro) in Syracuse, New York, and the discharge of these constituents from Metro to N-polluted Onondaga Lake is documented. Further, the benefit of the implementation of pretreatment at the pharmaceutical plant, and the effect of an upset event at this treatment facility on loading to Metro and the lake and inlake concentrations are also documented. Models are used as analytical tools to couple loading and in-lake concentrations, to delineate the role that this pharmaceutical facility has played in the lake's ammonia pollution problem, and to evaluate the potential implications of future pretreatment upset events for the success of a rehabilitation program that is underway for the lake. The responsiveness of the lake to reductions in external loading is established by the lower T-NH3 concentration observed in the upper waters of the lake in the spring of 1999. Model analysis demonstrates this reduction was primarily (approximately 75%) because of the decrease in loading from the pharmaceutical facility achieved by pretreatment. An abrupt increase in loading in May 1999 associated with an upset event at the pretreatment facility caused a corresponding increase in the T-NH3 concentration of the lake of approximately 0.5 mg N/L. Model projections demonstrate that the load from the pharmaceutical plant before construction of the pretreatment facility exacerbated the lake's ammonia problems by increasing the occurrence and margin of violations of the toxicity standard. Continued upset events at the pretreatment plant could compromise the lake rehabilitation program.