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The groundwater pollution by volatile organochlorines has been increasingly becoming a major environmental issue in many developed nations. In particular, trichloroethylene and tetrachloroethylene were widely detected in regional groundwaters of Japan. The paper describes the characteristics of groundwater pollution of Japan on the basis of the nationwide survey results, and introduces an incident in industrial site and subsequent counter-measures. In this site detailed investigations of 26 monitoring wells surrounding the pollutant source have continued to date since remedial operation in 1984, and cis-l,2-dichloroethylene was discovered to be created as intermediate product from trichloroethylene during the biodegradation process. Furthermore, the long-term continuous surveys reveal two types of organochlorine behavior in ground-water, that is, (l)cis-l,2-dichloroethylene concentration is proportional to that of trichloroethylene and (2)cis-l,2-dichloroethylene concentration is nearly constant in no relation with trichloroethylene behavior, in seasonal variation of individual well-water.

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