Mirex in Lake Ontario: Photolysis rate constant and assessment of residence time in the water

Mirex is a fully chlorinated pollutant entering Lake Ontario from the Niagara and Oswego Rivers. In the lake, mirex photolyzes to 8-monohydromirex, or photomirex, through a natural organic matter-mediated reaction. This reaction occurs in the surface water, and the mirex and photomirex concentrations in the lake's sediments record the average extent of this reaction. The first order rate constant for mirex photolysis in the lake can be combined with the ratio of photomirex to mirex (P/M) in the sediments to calculate the average residence time of mirex plus photomirex in the water column. This residence time can be used to independently test the accuracy of mass-balance models.; Monthly photolysis rate constants were calculated from the quantum yield coefficient spectrum and the spectral solar irradiance reaching the lake. Quantum yield coefficients, which describe the efficiency of the photoreaction for a specific wavelength, were determined under laboratory conditions for fourteen lake samples. Solar irradiance was determined for cloudless conditions using a radiative transfer model. Attenuation of light due to clouds was calculated using a regression model based on meteorological parameters.; The annual average whole-lake photolysis rate constant was 0.61 year −1 with uncertainty limits from 0.46 to 0.82 year−1 . The magnitude of this rate constant rivals that of other loss processes. Monthly rate constants ranged from 0.18 year−1 in December to 1.1 year−1 in July. Combining the annual rate constant with P/M in the sediment, the residence time for total mirex species (mirex + photomirex) was 0.24 year and ranged from 0.13 to 0.46 year. A mass balance model yielded a residence time of 0.52 year. The values for these independent approaches agree well considering the large uncertainties in mass-balance models. As more detailed models for Lake Ontario become available, the photolysis rate constant can continue to serve as an independent test, utilizing P/M in sediments and biota as boundary conditions.