To reduce the pollution load caused by Combined Sewer Overflow (CSO), which significantly affects the properties of water for public use in the event of rain, effective measures are adopted after the outflow behavior and pollution load are carefully studied. In this research, an area of about 127 ha around the confluence zone located in Nagaoka City, Niigata was targeted, the outflow rate, the water quality of CSO and the amount of rainfall were continuously monitored for 1200 days. The observation was started in June 2004 at two points where chamber has been equipped. As a next step, a simulation model with high reproducibility was designed on the basis of the data obtained by continuous observation using MOUSE. Since the water quality was predicted using the model in the case when it could not be measured, the pollution load of CSO was accurately estimated. It was found that the pollution loads of SS and of BOD for eight months (from April to November) were 50×10³ kg and 28×10³ kg, respectively. In addition, we found that the pollution load basic units of SS and BOD were 59×10³ kg km^-2 year^-1 and 33×10³ kg km^-2 year^-1, respectively.

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We estimated the runoff loadings of nutrients and organic carbon to the Koise River from its catchment area, a major river emptying into Lake Kasumigaura. The catchment was divided into several subwatersheds on the basis of digital elevation data, and extensive surveys of water quality and quantity were conducted in those defined subwatersheds. The runoff loadings were estimated for each subwatershed by multiple regression analysis using land-cover use distributions derived from high-resolution satellite (Quickbird) imagery and the measured loadings. The stepwise method, which cases a nonlinear model requiring no initial values, was introduced in the regression analysis to improve reliability in the estimation of the loadings.
During the nonirrigated period, the unit loading factor from crop lands was highest for nitrogen, phosphorus, and organic carbon in the dissolved phase. The loading factor in the particulate phase from deciduous forests was constant for nitrogen and organic carbon, while that from crop lands was constant for phosphorus. The dominant unit loading factors of total nitrogen, total phosphorus, and total organic carbon were found to differ between the irrigated and nonirrigated periods.

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The floating-leaved plant Trapa japonica, one of the abundant aquatic plants in Lake Izunuma, was used for nutrient uptake and release tests. By comparing the nutrient elution rates from the sediment and the nutrient uptake rates by Trapa japonica, it was estimated that in Lake Izunuma the Trapa japonica communities could take up most of the nutrients eluted from the sediment during summer. It was also confirmed that NH₄-N released from Trapa japonica changed to NO₃-N via NO₂-N. This result was quite similar to the NH₄-N, NO₂-N and NO₃-N concentration changes in Lake Izunuma from autumn to winter. In Lake Izunuma, the dissolved inorganic nitrogen concentration was high in autumn and winter. On the basis of the results of the discharge experiment to determine the effect of the reduction in the water level by tilting the water gate of Lake Izunuma, we consider that the nutrients released by the aquatic plant from autumn to winter can be removed effectively by this water level reduction.

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