Convergence of DNAPL source strength functions with site age.

Dissolution of dense nonaqueous phase liquid (DNAPL) source zones can be accurately predicted based on appropriate characterization of the source zone architecture, which controls the rate of mass discharge or source strength function. However, the architecture changes temporally as the source zone mass is depleted by dissolution. To generalize comparisons between contaminated sites with different porewater velocities or contaminant solubilities, site age is defined in terms of the fraction of contaminant mass that has been eluted from the source zone by aqueous dissolution. Here changes in DNAPL architecture during dissolution of a source zone were measured by light transmission visualization in laboratory flow chambers. Architectures measured at ages corresponding to initial conditions, 20, 50, and 90% mass removal were used in an equilibrium streamtube (EST) model to accurately predict subsequent dissolution. It is shown both experimentally and theoretically that as DNAPL contaminated sites age, fractional reductions in contaminant discharge and mass converge to become equal, regardless of the initial architecture. This behavior is a consequence of convergence from log-normal to exponential behavior. Analysis of errors in dissolution predictions suggests that the age of many contaminated sites is likely sufficient that architecture and source strength function characterization may not be necessary as it can be assumed with reasonable accuracy that future dissolution will follow an exponential decay model.