Nitrate and bisulfide: monitoring and patterns in Onondaga Lake, New York, following implementation of nitrification treatment.

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.