Solvent release into a sandy aquifer. 1. Overview of source distribution and dissolution behavior

This paper describes some of the results from a field experiment at the Canadian Forces Base Borden, Ontario, Canada. Five liters of a chlorinated solvent mixture (2.0 L of trichloroethylene, 0.5 L of chloroform, and 2.5 L of tetrachloroethylene) was released into a sandy aquifer to create a heterogeneously distributed source. The dissolution and dissolved-phase plume development from this source were studied in detail along a cross-section downgradient of the source for a period of approximately 1 yr. At the conclusion of the experiment, the site was excavated to map the actual distribution of solvent residuals in the subsurface. Dissolved-phase concentrations exceeded 10% of the effective solubility of the individual components in the portion of the plume directly downgradient of the source, providing a strong indication of the presence of dense nonaqueous-phase liquids (DNAPLs). Dissolved-phase concentrations in several sampling points exceeded 50% of the effective solubility. However, even with detailed groundwater monitoring, it was not possible to determine the small-scale distribution of the DNAPL source. Lower dissolved concentrations occurred deeper than the DNAPL source zone, likely as a result of vertical groundwater Row caused by fluctuations in the water table elevation. Spatial delineation of the dissolved-phase plume downgradient of the source correlated generally to the lateral and vertical location of the DNAPL source. The distance between the DNAPL source and the downgradient sampling cross section could be determined from breakthrough curves for some of the sampling points but not for others because slow expansion of the DNAPL zone continued for some time during the dissolution experiment.