The fish death accident occurred in Miyagi Prefecture in the winter of 1999/2000. It was considered that urea compound deicer used on the road flowed into the stream, and microorganisms in the fluffy mud of the site decomposed urea, then the strong toxicity, which was caused by the synergism of ammonia and carbonic acid, led to the fish death.
We tested the possibility of decomposing urea compound deicer by the fluffy mud in the stream where accident occurred. We carried out experiments of decomposition of the urea solution using the fluffy mud samples of the accident spot of the stream at various temperatures. The experimental results showed that the fluffy mud strongly decomposed urea into ammonia and carbonic acid even at the low temperature, and one fungus was recognized in the mud : This was identified as Trichosporon cutaneum by the biological and chemical tests. None have reported that this fungus decomposes urea in the river, the natural water world.

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Removal characteristics and removal improvement in sewage treatment plant, and batch-type laboratory test were examined for organic chemicals such as bisphenol A (BPA), a suspected endocrine disruptor.
The BPA removal efficiencies were 33-72% in a sewage treatment system, and the majority was carried out in the aeration tank. In the laboratory test, gross removal efficiency was 71%, of which about 40% was due to initial adsorption to sludge (IAS). Moreover, it was found that removal rate of BPA in the solution after IAS was almost equal to its degradation rate in the sludge.
IAS was enhanced by increasing MLSS concentration and by reducing the BPA concentration, however, it was decreased as increasing frequency of usage in batch type activate sludge.
IAS was restored by the modified reaeration process (MRP) applied to the sludge. It was thought that the appropriate time of reaeration was at least ten hours. MRP was also effective in the removal of other organic chemicals tested in this study. However, it was not effective for DIO (1,4-dioxane), a substance whose octanol/water partition coefficient (Log Pow) and biodegradation ratio were low.

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For grouping organic carbon in sediment, organic carbon hydrolyzed in HCl solution was defined as the acid decomposable organic carbon, and organic carbon in fulvic acid, humic acid and kerogene which resist the hydrolysis, was defined as the hardly acid decomposable organic carbon. Protein, carbohydrate and lipid were hydrolyzed, and the released amino acids, sugars and fatty acids were analyzed respectively by the developed method using GC/MS. These proposed indexes applied to the sediment in Hakata bay.
The following results were obtained from a microbiological treatment of the sediment and analyses of 6 points of the sediments in Hakata bay.
(1) The acid decomposable organic carbon ratio (C%) in each the sediment of Hakata bay decreased from bay mouth to interior part, however, ratio of the hardly acid decomposable organic carbon increased. From this result, the organic substances in the sediment of interior part of Hakata bay were found to be more stable microbiologicaly than those of bay mouth.
(2) The both amounts of the acid decomposable carbon and hardly acid decomosable organic carbon in the sediment of interior part of Hakata bay did not decrease by the treatment using Neanthes japonica, the organic substances were found to be fairly stable and further microbial decomposition was difficult. The amounts of amino acids and sugars did not decrease by this treatment, while fatty acids were decreased ca. 40%.

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In order to grasp the eutrophication potential of nonpoint sources, we performed a detailed investigation on the soils of paddy field, dry field and forest in the Koto basin of Yamaguchi Prefecture. It was found that the vertical distributions of TN, TP and IL in the soils decreased exponentially with increasing the depth of the soils. The amount of accumulated N and P in the layer from 0 to 50cm was calculated. The accumulated N was 72 times of the used N-fertilizer per year in the paddy field and 51 times in the dry field. The accumulated P was 56 times of the used P-fertilizer per year in both paddy field and dry field. The percentages of soluble N in the surface soils were 2.5% for dry field and 0.1% for paddy field and forest. The percentages of soluble P were 0.8% for paddy field and dry field, and close to zero for the forest. The fraction of soluble N increased with increasing depth of the soil, whereas soluble P was mainly remarkable in the surface soil. The present investigation gives insight to the actual situation of the eutrophication potentials of nonpoint sources in agricultural land.

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For comprehensive hazard assessment of environmental water samples with lower toxicity, simple bioassay using human cell lines with brief vacuum condensation have been applied to samples from Lake Kasumigaura, Lake Suwa, and Lake Biwa. The samples from Lake Kasumigaura both in 1999 and 2000 showed a similar seasonal variation in cytotoxicity: the cytotoxicity was detected significantly during summer season and peaked in August. The samples from Lake Biwa and Lake Suwa also showed weak but significant cytotoxicity during summer season of 2000.
Based on behavior during pretreatment with various cartridge columns, toxic substances existing in the samples were suggested to be hydrophobic with molecular weight less than 3,000. No significant correlation between the cytotoxicity and water quality data except phycocyanin level was observed among samples from Lake Kasumigaura. No specific agricultural chemicals or heavy metals were detected at the level sufficient to show the cytotoxicity alone.
These results indicate that this bioassay system is quite useful for comprehensive assessment of toxicity existing in environmental water. However, to integrate this kind of indices in environmental management process, systematic and continuous accumulation of bioassay data on water samples with authentic water quality data must be essential.

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