THE IMPORTANCE OF CONCEPTUAL MODEL RE-EVALUATION - A FIELD EXAMPLE

Re-examining a site conceptual model throughout the investigation and remediation process is necessary for successful site cleanup. One good illustration of this concept is provided by work performed at a former fire training site in Myrtle Beach, South Carolina. The site was used as a practice area to conduct fire fighting exercises from 1965 through 1969, and consisted of a 100-foot-diameter, sandy, earthen burn ring with a low-relief berm. The RCRA Facility Investigation at the site identified impacted soils and groundwater from activities at the former burn area. An interim corrective measure (e.g. excavation and disposal of contaminated soil) was performed to remove soils that were acting as a source for the dissolved phase contaminants in groundwater. Subsequent groundwater sampling indicated decreasing contaminant concentration in groundwater in the vicinity of the former burn area. In the downgradient direction, the distribution of dissolved volatile organic compounds was interpreted as a zone of more highly contaminated groundwater that had traveled by advection from the former source area. As additional wells were added on the site for remediation purposes, the working conceptual model seemed valid, until one of the new wells intersected a small zone of highly contaminated soil. This spurred a re-examination of the conceptual model that resulted in a brief field effort to delineate the extent of these soils, and a follow-up source removal excavation. If the original conceptual model had remained unchanged as more data became available during well installation for the corrective measure, the planned remediation technique (potassium permanganate injection) would not have been successful.