The Asahi River Dam reservoir is one of the important freshwater resources for Okayama City, Japan. The eutrophication of the reservoir has been warned of since the 1980s. In this study, we discuss the relationship between hydraulic retention time and increase of phytoplankton, and the influence of wind-driven currents on the spatial distribution of phytoplankton based on the observations and numerical simulations. Observations were carried out from 1993 to 1995. The numerical simulation of hydraulics in the reservoir was carried out by applying an orthogonal curvilinear finite difference method. Judging from phosphorus and nitrogen concentrations, the water quality of the Asahi River Dam reservoir is usually eutrophic. However, high concentrations of chlorophyll-a are not always observed. The observed dependence of chlorophyll-a concentration on hydraulic retention time is reproduced fairly well by a simple relationship derived from the balance of phytoplankton based on the assumption of complete mixing. The hydraulic retention time is a limiting factor of an increase in phytoplankton in the Asahi River Dam reservoir. It takes a retention time of 2 weeks for the sufficient increase of phytoplankton. The results obtained by the simulations show that the wind-driven currents play an important role in the spatial distribution of phytoplankton.

The eutrophication of Lake Biwa, which is the largest lake in Japan and one of the most important fresh water resources, has been warned since 1960s. In order to improve the water quality of the lake, it is inevitable to reduce the pollutant loads from the river basins. The Hino river and the Soma river basins are considered in the present study and models which predict the influence of pollutant load generation on the discharged pollutant loads through the rivers. These basins mainly consist of forests and rice paddy fields. The identified models reproduce the observed water quality relatively well. Evaluation of loads from point-sources and non-point sources indicates that the loads from non-point sources occupy the most of pollutant loads from the basins. Especially, the loads from rice paddy fields occupy most of the pollutant loads from the basisn. Therefore, controlling non-point source pollutant loads in the Hino and Soma river basins is an important step in checking the eutrophication of Lake Biwa.

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Genotoxic chemicals are increasingly being used in our daily life. 180-chemicals were screened to identify the priority of hazardousness from the point of view of their genotoxicity. Umu-test was used to evaluate the genotoxicity of these chemicals. Some chemicals exhibited high genotoxic response. A scoring algorithm was developed to evaluate the hazardousness level of these chemicals based on their genotoxicity level and four other characteristics: biodegradation probability, soil adsorption coefficient, (KOC), bioconcentration factor (BCF), Henry's Law constant (HLC). Then, these chemicals were sorted in a descending order of their hazardousness probability.

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