Dissolution and mass transfer of multiple organics under field conditions:The Borden emplaced source

The process that transfers mass from a subsurface source zone of residual d ense nonaqueous phase liquid (DNAPL) to the flowing groundwater is a contro lling factor in determining the time required to dissolve the source by non invasive means. While mass transfer can be kinetic or equilibrium under lab oratory conditions, aqueous concentrations in the field are generally found to be below equilibrium levels. To gain insight into the mass transfer pro cess under field conditions, we simulated the dissolution of the emplaced D NAPL source at the Canadian Forces Base Borden, Ontario, which contains a m ixture of three DNAPLs. The simulations clearly show that mass transfer at this site is equilibrium-controlled during the 1000-day observation period and that apparent tailing of one of the organic components is due to its de clining solubility, rather than mass transfer kinetics. Flow lines passing through the source are focused in a narrow streamtube downstream of the sou rce, and equilibrium concentrations are therefore observed only at the cent er of the effluent plume. Since the concentration peaks can be easily misse d in the sampling, streamline focusing can explain the low concentrations o bserved in the field.