SCALES OF GEOLOGIC HETEROGENEITY INFLUENCING DNAPL DISTRIBUTION IN THE BORDEN AQUIFER

Four chlorinated solvent DNAPL releases have occurred below the water table into the unconfined sand aquifer at C.F.B. Borden, Ontario, since 1990. In these experiments the infiltrated free-product typically migrates considerable distances laterally at multiple depths; however the nature and scale of the controlling heterogeneity has not previously been investigated. In the most recent DNAPL experiment, 50 liters of a three-component DNAPL [TCM, TCE and PCE] were infiltrated into the natural groundwater flow system where the sand aquifer is 3.5 m thick. Even though the input volume was relatively small, a suspended layer of DNAPL migrated laterally more than 7 m from the infiltration point. Previous sampling of continuous vertical cores at 5-cm increments showed hydraulic conductivity varied within only one order of magnitude. The goal of this investigation is to determine the physical characteristics that control DNAPL behavior in this aquifer at spatial scales not previously employed. The aquifer is comprised of deltaic sands reworked by glacial Main Lake Algonquin between 10,500 and 11,500 ybp. The facies architecture was determined through observations in a dewatered excavation at the site. Detailed [cm-scale] sampling for thin sections and grain size distributions from specific geologic units and layers in the excavation faces and continuous vertical cores allowed characterization of the physical properties within the geologic framework. Profiles of the second moment [sorting] from grain size data correlate well with VOC concentration profiles taken from source zone cores and sampled at the same scale. The frequency of geologic units being defined by changes in facies are commensurate with the number of distinct layers of DNAPL spreading laterally in the aquifer. These results suggest multiple scales of characterization, tied to the sedimentological framework, are required to explain the observed DNAPL distributions.