The release of hazardous chemicale,such as dioxins,into the environment has been decreasing since the 1970s,but such chemicals have accumulated in high concentrations in the sediments of lakes and coastal areas.Therefore,evaluation of the biological and ecological risks of chemicals orignating from such sediments is urgently needed in the context of management of chemicals in the water environment.In this study,we construct a mathematical model to reproduce the sedimentary processes and vertical distrbution of chemicals that have accumulated in the sediment over the past several decades.The vertical distribution of chemicals is a reflection of the historical trend of pollution,and is affected by physical and/or biological disturbances of the surface layer,decomposition or decay,and efflux through resuspension. In order to include the consideration of such processes,we constructed a vertical one-dimensional model which has a resusupension layer,a surface strongly-mixed layer,a moderately mixed layer,and a stagnant layer. Chemical loading in the model is divided into a direct input process from the air and an indirect input through the river basin.The latter component includes the residence time within the river basin after the environmental release.
　This model was applied to the basin of Lakes shinji and Nakaumi where precise measurements of radionuclides and the physical properties of sediments have been reported.In the first step of the analysis,we tried to estimate the sedimentary process of each lake by reproducing the vertical distributions of pb-210 and Cs-137 at 29 stations on the lakes.Model parameters were calibrated by using the loading rates of chemicals and the sedimentation rate as the input parameters.In the next step.we considered the verical distribution of dioxins(PCDD/DFs)in the sediment of Lake shinji with the same values for the model parameters as those for the sedimentary process of radionuclides.
　The model gave good reproduction of the vertical distribution of dioxins at a station on Lake Shinji and those of pb-210 and Cs-137 at almost of all the stations on both lakes,except for a few on Lake Nakaumi.The presence of two mixed layers and a resuspension layer in the model was found to be essential for analyzing the sedimentary process of the whole lakes.The resuspension layer had an effect of temporally accumulating chemicals and therefore played a definitive role in forming the vertical distribution of chemicals.Classification of the loading process into a direct input from the air and an indirect process through the river basin was important to analyze the effect of different sources of dioxins such as herbicides and combustion.In order to estimate the sedimentation rate accurately,it was necessary to take into account the mixing process in the surface mixed layer.