Error Analysis and Simulation of Mirex Behaviour in Lake Ontario

Abstract Mirex entered Lake Ontario from two main sources, the Niagara river and the Oswego River. In 1968 a survey showed that about 700 kg were present in the bottom sediments of the lake. Another more limited survey in 1976 produced estimates of 1345 to 1600 kg. Simulations show that if loading estimates are correct then about 1750 kg (1648 to 1784 kg) of Mirex are present in 1981 in Lake Ontario sediments. Since 1968 most Mirex has entered the lake from the Niagara river and only a small percentage from the Oswego River. Simulation models are employed to quantify the behaviour of a persistent chemical, Mirex, in the water column, in the biota and in the bottom sediments of Lake Ontario. The purpose of the study was threefold: 1) to verify whether the persistence of Mirex in sediments could be reproduced by a model which used information on the chemical properties of Mirex and the chemical, biological, physical, geological and meteorological characteristics of Lake Ontario; 2) to identify and quantify our confidence in such a model; but mainly 3) to identify important processes, whose detailed knowledge would improve our confidence in the model. Thus, error analysis was used to indicate areas which need to be investigated with field experiments, hereuntofore not performed. Once these data exist a comprehensive model validation can be done. Error analysis has indicated environmental and chemical characteristics which control Mirex behaviour in Lake Ontario. The most important are the bulk density of the sediments and the water-sediment exchange processes, i.e., sedimentation and resuspension. Adsorption on sediments is also important but because the adsorption constant is so large, variations (even 100%) make relatively little difference. Over the long term, one thousand years and longer, volatilization is an important process of Mirex removal from the lake. Mirex is bioconcentrated. Living organisms in-fluence the medium term distribution (100 years or less) of Mirex. Mirex is very persistent in the lake and most likely will not be removed but only buried in the sediments of loadings stop. Otherwise it will continue to accumulate. An exposure analysis has shown that even if heavy loading of Mirex from US sources has stopped or will be soon stopped, the exposure of living organisms, particularly in the benthos, to Mirex will remain high. Mirex is not acutely toxic, even at high concentrations, but given its long term predicted persistance in Lake Ontario, it will remain a chronic hazard. The risk to environmental health will therefore continue for a period much longer than was foreseeable when heavy doses of Mirex were released into Lake Ontario over a relatively short period of time. Coverage of Mirex contaminated bottom sediment by clean sediments, if inputs are completely stopped, will not be a solution for at least the next 200 to 600 years. A bioconcentration exposure is definitely evident, whereas bio-magnification in the food chain has not been proven. The decision to ban Mirex in Canada was therefore appropriate, even if ecologically perhaps too late to avoid long term low level exposure effects.