A SEDIMENTOLOGICAL VIEW OF HETEROGENEITY IN THE BORDEN AQUIFER, ONTARIO, CANADA

Over the past 20 years numerous experimental studies of subsurface organic contaminant behaviour have been conducted in the unconfined sand aquifer at CFB Borden, Ontario. The Borden aquifer structure has previously been determined statistically from detailed measurements of grain size and falling head permeameter conductivity values obtained from vertical cores. The aquifer has not, however, been examined in the context of sedimentological structure and origin. In this study, conducted in the DNAPL experiment area at Borden, a 2.5m X 3.5m sheet pile cell was installed and excavated for detailed examination of perpendicular vertical faces.

Previous research suggests that the aquifer sands were deposited in glacial Lake Algonquin (early phase) and subsequently reworked by shallow water and beach processes during the main phase of the lake. Excavation of the cell revealed five sedimentary facies within the aquifer, which are interpreted as follows: 1) horizontal to low angle laminated sands with abundant heavy minerals deposited in the swash zone of the beach foreshore environment; 2) high angle laminated sands interpreted as offshore bar sediments; 3) silt / clay laminations deposited during quiescent conditions; 4) wave-generated ripples deposited in a transitional zone between the surf and swash zones in the lower foreshore environment and 5) current-generated ripples deposited by a stream or rivulet crossing the beach face. Paleocurrent data obtained from these ripples indicate that flow across the beach face was in a NNE direction. Collectively, the observed facies suggest deposition in a fluctuating, though generally prograding, nearshore to foreshore environment, periodically interrupted by erosion and deposition of offshore bars.

The use of vertical faces allowed geological unit identification and correlation, critical in the establishment of the facies architecture of the site. This architecture was not discernible from cores. The resulting geological model for the aquifer heterogeneity provides the conceptual framework for source zone DNAPL characteristics and plume formation and attenuation.