The long-term monitoring of organophosphoric acid triesters started in 1976 in the Yodo River basin; Lake Biwa, Yodo River and the rivers in Osaka City. An extensive survey was also conducted in the Yamato River and Osaka Bay, Japan. This report presents the current levels and distributions of OPEs as well as the historical trend of TDCPP. Some or all of OPEs studied here have been detected in the entire water samples over the monitoring period. It was clear that OPEs were ubiquitous pollutants in the river and coastal waters. Total OPE levels ranged from 0.4 to 2.4 µgl−1 in the Yodo River basin, 2.4 to 12 µgl−1 in the rivers in Osaka City, 18 to 28 µgl−1 in the Yamato River basin and 0.1 to 1.3 µgl−1 in Osaka Bay. An apparent difference was found in the individual OPE level among the different water areas surveyed. In the Lake Biwa and the Yodo River, the predominant compound was TBXP followed by TCEP and TDCPP. The higher levels were observed for TBXP, TEP, TCPP and TCEP in the rivers in Osaka City. The highest TCPP level, which occupied more than 70% of OPEs' composition, was observed and the mean level reached to 13.1µgl−1 in the Yamato River. In Osaka Bay, 6 kinds of OPEs, except for TPP, TEHP and TCP, were found and the most prevalent compounds were TCEP and TBP. TDCPP had gradually increased in concentration in the Yodo River basin until 1987 from 1976 year by year. However, the reverse trend of TDCPP has been noticed during the last several years.

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