Mapping groundwater with geophysics: Locating and characterizing aquifers in the Beaufort Watershed with electrical resistivity surveying
Date
2023
Authors
Yamamoto, Savanna
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Abstract
In the Beaufort Watershed study area on eastern Vancouver Island, groundwater is the primary freshwater supply. More data are needed to improve characterization of the groundwater resource, in order to protect and manage it. Electrical resistivity surveying is a cost-effective remote sensing technique that allows the types of subsurface materials to be inferred, and therefore allow for qualitative and quantitative characterizations of aquifers to be made. Working with the Beaufort Watershed Stewards, I collected, modelled, and interpreted 11 vertical 1D profiles (~83 m deep) and two vertical 2D profiles (81 m long x 17 m deep) in the study area. I was able to distinguish between different types of geological materials such as sand and gravel, till, clay, and various bedrock types and also identify where potential aquifers exist among these materials. Extensive sand and gravel units, which are commonly major sources of groundwater, were only identified throughout the southern part of the study area. Most of these units were determined to be water-bearing, yielding moderate-high quantities of water, and being only partially confined. They had an average thickness of ~17.5 metres, and were all located at ground level. A potential source of groundwater spanning the entire study area was the sedimentary bedrock identified in nearly all profiles, which was determined to be a relatively confined, low-yielding aquifer. This aquifer had an average depth of approximately 32 metres and an undefined thickness. A non-permeable, aquitard unit composed of clay, till, or both was also determined to span the entire region, averaging approximately 30.5 metres thick. The average depth to the aquitard was determined to vary between the northern and southern parts of the study area, where the aquitard in the former averaged roughly 2.5 metres depth, and the aquitard in the latter averaged roughly 18 metres depth. The 2D profiles, on either side of the Ships Point peninsula near Fanny Bay, both indicated the presence of saltwater within a sand and gravel aquifer unit, implying the occurrence of subsurface seawater proximal to shorelines.
Supervisors: Lucinda Leonard and Mike Wei
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Keywords
geophysics, electrical resistivity, aquifer, groundwater, hydrostratigraphy, Beaufort Watershed, Vancouver Island