Hydrogeological assessment for a large aggregate extraction facility on the BC coast

Willy Zawadzki, Don Chorley, Mark Johannes, Michel Mailloux, Leslie Smith

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Mineral and Gas II

ABSTRACT: A hydrogeological assessment was carried out in support of a feasibility study for a proposed aggregate extraction facility on the British Columbia coast. The site is located in southwestern British Columbia approximately 50 km northwest of Vancouver and adjacent to the coast of Howe Sound. Current project plans include mining of the aggregate resources from an approximately 77 hectare portion of the property situated about 500 m from the marine foreshore and extending northward approximately 600 m toward the southern banks of McNab Creek. Development of this resource is expected to reduce the transportation distance for aggregate to the Lower Mainland. The geology consists of over 100 m of alluvial fan deposits of predominantly sands and gravels filling a U-shaped trough that was formed within igneous bedrock. Hydrogeological investigations were undertaken at the project site to provide data required to assess the influence of aggregate extraction on site groundwater and surface water conditions, in support of project planning and permitting. This included installation and testing of multi-level monitoring wells, monitoring of hydraulic heads, stream flows, and groundwater and surface water quality and temperature over a period of three years. This information was then used to develop a conceptual understanding of site groundwater conditions that formed the basis for the development of the site numerical hydrogeological model. Following calibration, the numerical model was used to simulate future groundwater conditions during the 16 years of aggregate extraction. Figure 1 presents a schematic representation of the conceptual hydrogeological model that was developed at the conclusion of site investigations. This model consists of unconsolidated sediments underlying the site that form a relatively permeable unconfined aquifer. This valley fill aquifer is bounded at depth and laterally by bedrock and locally north of the site by glacial till, both of which are much less permeable than the aquifer. The hydraulic conductivity of the aquifer to a depth of approximately 20 m is inferred to be in the mid 10-4 m/s range; whereas, at greater depth somewhat lower values are anticipated, likely in the low 10-4 m/s range. Considering observations made during drilling and general depositional setting, the aquifer is likely anisotropic with vertical hydraulic conductivity less than horizontal. Water table elevation and groundwater flow conditions in the valley fill aquifer are controlled by several hydrogeological boundaries. The dominant recharge source for this aquifer is McNab Creek, which appearsFigure 1. Site conceptual model

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