In 2000, the groundwater in the Xinkailiao area of Lundong Village, Chaozhou Township, Pingtung County, Taiwan, was discovered to contain chlorinated volatile organic compounds. The main chlorinated volatile organic compound in this water was later found to be dichloroethylene, whose level exceeded the maximum permissible limit in the groundwater control standards of Taiwan; therefore, the aforementioned area was declared a groundwater use restriction zone (hereinafter referred to as “the Study Site”). Tetrachloroethylene and trichloroethylene were also detected in the aforementioned area; however, in recent years, the detected levels of these compounds have not exceeded the corresponding maximum permissible limits in the groundwater control standards of Taiwan. The Study Site lies on the eastern side of the Pingtung Plain and upstream of an alluvial plain. The aquifer at this site extends as far as 200 m below the surface. Because of the vastness of the Study Site and the existence of chlorinated organic matter in its groundwater, groundwater remediation at this site is difficult. Recent monitoring data indicate that the contamination zone is moving downstream, which may lead to gradual impacts on densely populated areas, such as neighboring communities and the Chaozhou Township. Consequently, the Pingtung County Environmental Protection Bureau proposed the Monitored Natural Attenuation Plan (hereinafter referred to as “the Plan”) to examine the feasibility of natural degradation at the Study Site and to formulate a zoning-based groundwater management strategy for this site. The contracted tasks and work quantities of the Plan have been completed. According to analysis results, the dichloroethylene level did not exceed the maximum permissible limit in the control standards for Category 2 groundwater in upstream monitoring wells but exceeded this limit in midstream and downstream monitoring wells. The contaminant had not yet reached the monitoring wells at the downstream boundary. A groundwater analysis at three locations with settlements indicated the contaminant levels in the groundwater of these locations did not exceed the corresponding permissible limits for Category 2 groundwater in the groundwater standards. Moreover, the noncarcinogenic health risks of this groundwater were within an acceptable range. The results of a groundwater model simulating the movement of dichloroethylene suggest that over 20 years, this contaminant may move 700 m downstream over time but will not reach the densely populated areas located further downstream. An analysis of groundwater microbes revealed that the Study Site was rich in viable microbial biomass and that the total bacterial count and viable microbial biomass at this site increase considerably after heavy rains; however, the groundwater at the Study Site was found to be aerobic and therefore inconducive to the local microbial degradation of dichloroethylene. A statistical analysis of pollution trends revealed that the dichloroethylene level at the Study Site is stable or weakening, possibly because of dilution, which represents an example of MNA. This study recommends the adoption of MNA practices in upstream regions at the Study Site and the removal of areas exhibiting satisfactory groundwater quality for long periods from monitoring lists. In midstream areas, monitoring wells can be added where required, and on-site methods can be used for reducing contamination. At the boundary of the downstream area, MNA should continue to be practiced at monitoring wells, and the downstream movement of contaminants should still be monitored.

groundwater, MNA , chlorinated volatile organic compounds