Using GIS to Assess Risk of Contaminant Release and Migration at Industrial Facilities Where Subsurface Conditions Have Been Modified by Site Operations

Characterization and remediation activities at contaminated sites are subject to uncertainty. However, this uncertainty is even greater at industrial facilities like the Department of Energy, Y-12 National Security Complex where subsurface hydrogeologic conditions have been modified by plant construction and operations. Site alterations create ill-defined conduits and preferred pathways for contaminant migration. A systems analysis approach was employed at the Y-12 site in which the bounds of known and suspected transport pathway features are documented using geographic information system (GIS) technology (ARCGIS 9.2™), and assigned a value associated with their risk of migration. Preferred pathway layers that were determined to have an effect beyond their boundaries (e.g., sumps) were buffered with a multi-part buffer tool and assigned a rating for vulnerability and a weight for the buffer distance. Layers were combined using an overlay tool (union).  The union overlay tool computes the geometric intersection of the polygon features. All polygons from the features are split at their intersections and preserved in the output layer. Each new polygon has all the attributes of all the preferred pathway layers and their buffers that contributed to the new polygon. For each polygon in the union overlay layer, migration vulnerability was calculated by multiplying the rating for each contributing preferred pathway layer by its distance weight and then summing all the vulnerabilities for each polygon. The result is a map of the facility showing the risk of migration due to the presence of the multiple pathway features. Existing soil and groundwater contamination information was similarly processed in GIS, overlain and compared with the pathway maps to further refine the assessment of likely contaminant migration pathways. The pathway analysis results can be used directly to make decisions on additional site investigation and remedial activities or as input to more sophisticated 3-D flow and transport models.