INFLUENCE OF MULTICOMPONENT NAPL DISSOLUTION ON CONTAMINANT-PLUME DYNAMICS

Multicomponent NAPL dissolution is typically not considered when assessing contaminant plume movement from NAPL source zones. However, considering mixture dissolution is critically important for proper assessment of contaminant-plume dynamics. Typically, the source is considered to not be a NAPL source when the plume's center of mass is not stationary, which assumes that the plume's center of mass will always be co-located at the NAPL source. However, depending on the composition of the mixture and the magnitude of the ground-water velocity, a NAPL source can produce a plume with a moving centroid as the NAPL mixture dissolves. To demonstrate this phenomenon, a model is used that combines a numerical solution for NAPL- mixture source zone dissolution with an analytical solution of a 3-dimensional advection- dispersion equation. The model is used to simulate plume movement from a NAPL-mixture source at the Hanford Site in Washington state, USA. The purpose of this work is to show the potential importance of considering NAPL-mixture dissolution when assessing plume movement using realistic Hanford site data (groundwater velocities and NAPL-composition) for model input to ensure a realistic scenario. The NAPL source zone at the site is a complex mixture comprised of chlorinated solvents, lard oil, and butyl-phosphate compounds. At this site, a moving plume centroid has been used as evidence that no NAPL source exists even though other evidence suggests NAPL is present. The uncalibrated model demonstrates that the centroid of a carbon tetrachloride plume would be expected to move approximately 200 m under the higher flow rates associated with pump and treat, although insignificant movement of the centroid occurred during natural ground-water flow velocities prior to pump and treat. The model result is consistent with plume observations at the site.