To evaluate the loads of nitrogen, phosphorus and COD constituents from the bottom sediment in Lake Sanaru, one of the most eutrophic lakes in Japan, their release fluxes from the sediment were measured in a laboratory experiment using sediment cores. The cores were collected from two sites in August 2005. The release flux of nitrogen was underestimated due to the volatilization of ammonia in water during the experiment. The estimated release fluxes of phosphorus and COD constituents of the entire lake corresponded to approximately 6- and 0.6-fold their respective inflow fluxes. In addition, the nutrients are transferred from the sediment in dissolved forms that phytoplankton can use. Owing to the shallowness of the lake (mean depth:∼2 m), the nutrients are used by phytoplankton immediately after their release from the sediment. These suggest that nutrient release from sediment has a large contribution to primary production in Lake Sanaru. This nutrients release is probably due to the rapid decomposition of large amounts of dead phytoplankton deposited at the lake's bottom.

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The research center for Chemical Risk Management (CRM) of the National Institute of Advanced Industrial Science and Technology (AIST) has developed a watershed model for chemical risk assessment in river basins and for evaluating emission reduction measures, which was released as an AIST-Standardized Hydrology-based AssessmeNt tool for chemical Exposure Load (AIST- SHANEL). This model is used to estimate chemical exposure concentrations in water systems using a spatial resolution of 1×1 km grid and a time resolution of 1 day by inputting of simple data such as Pollutant Release and Transfer Register (PRTR) release data, river basin information, and data on the basic physical properties of the chemical substances of interest. Various types of information can be easily derived from calculations made using this tool, including the location of high-release areas within a river basin, the correlation between exposure load and areas with high releases, and the relationships between river flow rate and exposure load. The calculated values were fitted with the measured data for nonylphenol ethoxylates (NPnEOs), nonylphenol (NP), linear alkylbenzene sulphonate (LAS) and bisphenol A (BPA) by a factor of 10, which indicates the validity of this model.

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The application of a model of long-term runoff of herbicide mefenaset from paddy fields to a river was investigated. The model of long-term runoff of mefenaset to the river was constructed as follows. The change in the concentration of mefenaset in ponding water provided a pseudo-first-order rate, and a flow model provided the amount of the surface runoff in the paddy field of the tank model. For the model of long-term runoff, although there were little gaps, the characteristic of mefenaset from paddy fields of the runoff to a river was reproduced. From the simulation results, it is considered that it is possible to use it for a sufficiently precise forecast of the movement of applied pesticides in the environment and use it for runoff load reduction.

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A distributed hydrological model that simulates water, nutrients and organic matter flow in a watershed including a paddy field area was developed to analyse the effects of paddy farming on water environment. The model considered biochemical reactions, by simplifying them to be of 1st kinetics, in ponding water in a paddy field and in water in a river. This model enabled estimates to be made of the contributions of paddy agriculture, irrigation water intake, domestic sewage and rainfall to the amount of nutrients discharged as a result of each land use. The model was applied to the basin of the highly eutrophicated Lake Aburaga-fuchi. The calculated values agreed well with the observed data. The calculated results indicate that river water quality was improved by irrigation water intake and paddy farming, and that nitrogen removal in paddy fields plays an important role in watershed nutrient circulation.

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We examine the collection method for phosphate from volcanic ash soil type phosphorus adsorbent used in secondary effluent. The phosphorus in volcanic ash soil type phosphorus adsorbent is desorbed by 68.9% using 0.5 mol · l^-1 H₂SO₄ solution at a space treatment speed (SV) of 2.5 for 6 h. Phosphorus in the extraction solution can be completely collected by adjusting the pH of the H₂SO₄ solution to 4.8∼8.7. The major components of the collected phosphorus extract are sodium, aluminum, iron and phosphorus. The total phosphate amount in the collected phosphorus extract is approximately 43.0 gP · kg^-1, which is equal to that of a phosphate rock. The citrate-soluble phosphorus in the collected phosphorus extract is estimated to be 80% of the total amount of collected phosphate. Moreover, when the collected phosphorus extract is applied to the subsoil of andosol to grow Brassica campestris cv. Komatsu, its effect is identical to that of superphosphate application. As for the level of application of the collected phosphorus extract, it is found that 6% is suitable. It is proved that the collected phosphorus extract is very promising for soil reduction and reuse.

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We used the diatom tracer method to survey the area around the Isahaya Bay to evaluate the influence of suspended particles discharged from the Isahaya reservoir. This method allowed the frustules of Skeletonema subsalsum and S. costatum, the predominant diatoms in the Isahaya reservoir and the bay, respectively, to remain even after the decomposition of cells of these phytoplanktons. We found that the greatest amounts of the suspended particles transferred within the area 1.5 km from the gates of the reservoir, at the head of the bay. We also found that most particulate organic matter in the bottom sediment originated from the primary production areas at the center and mouth of the bay. Moreover, we confirmed the reliability of the diatom tracer method from the finding that the distribution of the frustules of the diatoms in the sediment showed good agreement with that in the water column of the bay.

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The populations of Japanese bitterlings and unionid mussels and the land use of the watershed were investigated in 36 sampling sites in the Tatara river system in northern Kyushu, Japan. Five bitterling species were found in 11 sites in the system. Although Rhodeus ocellatus kurumeus was found to be distributed in 23 sites in 1983, this species was found in three sites in this study. The population of the bitterlings decreased in the sites with a high urbanization rate, although the populations of several other fish species showed no dependence on the urbanization rate. The population of the mussels showed a negative correlation with urbanization rate. In addition, the mussels populations showed positive relationship with the bitterling populations. These three relationships indicate that the decrease in the bitterling population due to the urbanization of the watershed was responsible for the decrease in the mussel population.

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