A plume is stabilized if it remains within the "existing area of contaminated groundwater." A plume of contaminated groundwater could remain in its existing area if it is no longer expanding above levels of concern in the vertical or horizontal dimensions due to, for example, natural attenuation or engineered controls such as hydraulic containment and/or physical barriers. Alternatively, the plume of groundwater contamination might not be expanding within the geologic formation, but it might be discharging into a hydraulically connected surface water body. In such a situation, the plume of contaminated groundwater is not getting any bigger (i.e., the plume has "stabilized"), but it might or might not be "under control." The environmental indicator determination in such a setting would be based on whether or not the continued discharge of groundwater represented an unacceptable impact to the receiving surface water body.
For more information, see http://www.epa.gov/epawaste/hazard/correctiveaction/eis/index.htm.
What do we mean by a stabilized plume?
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