Nationwide surveys of 1,4-dioxane and methyl-t-butyl ether (MTBE) levels in raw water used for the drinking water supply were conducted at 91 water treatment plants in Japan in 2001 and 2002, prior to the revision of the drinking water quality standards. 1,4-dioxane was widely and continuously detected in raw water samples and its occurrence was more frequent and its concentrations higher in groundwater than in surface water. However, its maximum concentration in raw water was much lower than its new standard value (50 μg/L), which was determined as a level of 10−5 excessive cancer risk to humans. Trace levels of MTBE were also detected in several surface water samples.

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