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 Can adian Forces Base Borden, Ontario, Canada. Five liters of a chlorinated sol vent 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 heterog eneously distributed source. The dissolution and dissolved-phase plume deve lopment from this source were studied in detail along a cross-section downg radient 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 o f solvent residuals in the subsurface. Dissolved-phase concentrations excee ded 10% of the effective solubility of the individual components in the por tion of the plume directly downgradient of the source, providing a strong i ndication of the presence of dense nonaqueous-phase liquids (DNAPLs). Disso lved-phase concentrations in several sampling points exceeded 50% of the ef fective solubility. However, even with detailed groundwater monitoring, it was not possible to determine the small-scale distribution of the DNAPL sou rce. Lower dissolved concentrations occurred deeper than the DNAPL source z one, likely as a result of vertical groundwater Row caused by fluctuations in the water table elevation. Spatial delineation of the dissolved-phase pl ume downgradient of the source correlated generally to the lateral and vert ical location of the DNAPL source. The distance between the DNAPL source an d the downgradient sampling cross section could be determined from breakthr ough 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.