Generally, organophosphorus compounds form their oxones, which usually have stronger choline esterase inhibiting activity than their “originals” by chlorination in water treatment plant. Prothiofos, which is widely being used as organophosphorus insecticide, was recently added to the list of “the complementary items” in Japanese Waterworks act. For the reason, many of water suppliers are monitoring prothiofos in tap water. However, the formation of prothiofos-oxon by chlorination were not reported. In the present study, we have investigated the decomposition of prothiofos and the formation of prothiofos-oxon by chlorination of tap water. At the residual chlorine concentration 0.5 mg/L or more, decomposition of prothiofos and formation of prothiofos-oxon was observed in the chlorine concentration-dependent manner. Further, prothiofos-oxon formed by chlorination was stable over 72 hours in the residual chlorine level of 1 mg/L. These results showed that prothiofos-oxone was transformed from their originals by chlorination on the water purification process and was considerably steady at the actual residual chlorine level (1 mg/L or less) in tap water. Therefore, it is preferable that water suppliers should monitor prothiofos-oxon, rather than prothiofos in tap water.

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Trihalomethane is formed in drinking water primarily as a result of chlorination in water supplies. The extent of trihalomethane formation fluctuates in a variety of situations. The level of trihalomethane can get higher with increasing residence time in water supply systems and in households. Furthermore, heating of water at the point of use increases the concentration of trihalomethane dramatically. Trihalomethane is regulated only in the water which has just come out of taps, prior to any household treatment (heating, filtering, etc.). Therefore, many water suppliers control their water quality according to regulations. However, there is a need to consider the fate of water when controlling water quality. A new method is necessary to evaluate the trihalomethane maximum potential in the water for better management of water quality. In this study, we developed a method for estimating the trihalomethane maximum concentration (THMmax) by heating water in PFA bottles. The THMmax was the total concentration of trihalomethanes (chloroform, dibromochloromethane, bromodichloromethane, and bromoform) after heating at 80°C for 10 hours. The THMmax was suitable for evaluation because it was stable and independent of either residence time or water temperature. We conducted monthly monitoring of the THMmax at Nagasawa water purification plant in Kawasaki city. The average of the THMmax was 49μμ/L, which was 8.3 times as much as that of the total trihalomethanes (5.9μμ/L) in unheated water. The trends showed that compared to trihalomethane in unheated water, the THMmax changed less throughout the year and did not decrease so much in winter.

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