The method of estimating the source origin of the organic matter that consumes dissolved oxygen in an aquatic environment using carbon stable isotope (δ¹³C) of dissolved inorganic carbon (DIC) is effective because the method needs only a small portion of water and a brief duration. In an anoxic water mass in a low-lying depression in a port, δ¹³C-DIC decreases in accordance with the increase in the concentration of DIC (R²=0.93) ; and hence, δ¹³C of the organic matter that consumes dissolved oxygen can be estimated with considerable accuracy using the relationship between the concentration of DIC and δ¹³C-DIC. We demonstrated that the method of using in situ δ¹³C-DIC, which had been applied to lakes, performs effectively under the condition of continually decreasing pH in an anoxic water mass formed at the bottom of the sea. The contribution ratio of the phytoplankton-derived and land-derived organic matter was estimated using their δ¹³C values estimated in previous studies. As a result, 59-88％ of organic matter that consumes oxygen was estimated to originate from phytoplankton. Low-lying depressions in coastal seas, where the water exchange rate is significantly small, is suitable for examining remineralization processes of organic matter in coastal seas.

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A gas chromatography - olfactometry analysis was conducted for biologically treated wastewater samples from seven different plants to clarify the compounds responsible for the odor of biologically treated wastewater. The odor of 2,4,6-trichloroanisole was detected with the highest or second-highest odor intensity for the treated wastewater samples from large-scale plants whose biological reactors are covered with a roof. The treated wastewater from these large-scale plants contained 11.6–21.2 ng L^-1 of 2,4,6-trichloroanisole, and this range of concentration was approximately 100 times as high as the olfactory threshold. Earthy and musty odors, probably caused by geosmin and 2-methylisoborneol, were also detected with high odor intensities in all samples. A large contribution of an unidentified compound with a sweet odor was also observed in the analysis of treated water samples. In addition to earthy and musty odors, sulfuric and septic odors were detected in the samples from small-scale plants.

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