NATURAL DISSOLUTION OF A HETEROGENEOUS MULTI-COMPONENT DNAPL SOURCE ZONE IN THE "HOMOGENEOUS" BORDEN AQUIFER

Dense nonaqueous-phase liquid (DNAPL) present in the subsurface can form plumes of contaminated groundwater that persist over decades. This study was conducted to improve understanding of how the heterogeneous physical distribution and mixed chemical composition of DNAPLs affect contaminant concentrations and discharges, and source longevity.

A chlorinated solvent mixture (50 L) composed of trichloromethane (TCM, 10%), trichloroethene (TCE, 45%) and perchloroethene (PCE, 45%) were infiltrated below the water table into the hydraulically nearly-homogeneous Borden aquifer under natural groundwater flow conditions. The emanating plume was monitored over a 5 year period using multi-level samplers along transects. Initially a wide plume developed, but solvent concentration maxima became more localized as residual DNAPL trails dissolved. The position of these maxima matched the location of pools identified using ground penetrating radar and was related to minor sedimentological differences within the aquifer. The transient groundwater flow regime caused significant transverse lateral plume direction (>60°). From the onset of the experiment, maximum solvents concentrations down-gradient of the source zone approached initial effective solubility values as predicted by Raoult’s law, indicating the rapid establishment of equilibrium conditions. Preferential dissolution from the DNAPL source zone led to the sequential depletion of TCM (200 days), TCE (2 years) and PCE. Significant spatial differences in dissolution rates within the source zone are illustrated by the concurrent presence of TCE concentrations similar to the initial DNAPL composition and PCE concentrations for depletion to a 100% PCE DNAPL. This large spatial variability in source zone dissolution is a likely cause for the lag period observed after the gradual decrease in mass discharge before a rapid decline in maximum concentrations of each DNAPL component. While the mass discharge decreased after 40% of each of the components was dissolved, maximum concentrations only started to decrease after 80% dissolution.

Overall, the dissolution of the DNAPL source zone was characterized by a high degree of spatial and temporal variability in the aqueous solvent concentrations, even though this aquifer is nearly homogeneous in terms of hydraulic conductivity distribution.