In this paper, the statistical properties of surface water quality in the Asahi River estuary are discussed based on the frequency distributions and the power spectra. The concentrations of COD(Mn), SS, chlorine ion and chlorophyll-a are mainly dealt with.
The frequency distributions of COD(Mn) and SS during several tidal cycles are in accord with normal distributions and logarithmic normal distributions, respectively.
Not only the tide but also the seiche or the secondary undulation affects the variations of the surface water quality. In the Asahi River estuary, the tide has the greatest effect on each water quality item.
The strength and periods of the variations in the water quality items are somewhat different from each other.

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In order to obtain a better understanding of the situation on trihalomethane precursors and halogenated organics in a watershed, it is essential to evaluate the trihalomethane precursors loads caused by the discharge of secondary effluents into the receiving water as well as the natural loads originated from humic substances.
For each unit process of a municipal wastewater treatment, the daily variations of formation potential of halogenated organics, such as THM (Trihalomethane), TPTHM (Total Potential THM) and TOX (Total Organic Halide), and those of the concentrations of volatile halogenated organics were investigated. Volatile halogenated organics are reduced to 14% of the initial concentration by activated sludge treatment and precursors of halogenated organics are reduced to 23% of TOX, 46% of TPTHM and 72% of THM. Formation potential of THM, TPTHM, and TOX formed from the chlorination of the secondary effluent are 25.5±4.1(μg·mg C^-1), 54.1±10.0(μg·mg C^-1) and 166+ 33(μg·mg C^-1), respectively. Although TOX formation potential of the secondary effluent is 9% lower than that of the sewage, THM formation potential of the secondary effluent is 2.6 times as much as that of the sewage, and TPTHM formation potential is 1.6 times as greater as that of the sewage. However, there is no significant difference between these values and those obtained from the surface water.

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