Detailed measured sections, bedrock aquifer/aquitard facies and potential bedrock aquifer systems of the Upper Cretaceous Nanaimo Group, Nanaimo Lowland, eastern Vancouver Island, British Columbia, Canada
Authors: Hamblin A P 
Published: November 2015

Rapid population growth and expanding commercial developments are, and will continue to, put pressure on the limited groundwater resources of the Nanaimo Lowland. This study focused on the characterization of the bedrock aquifer potential of the Upper Cretaceous
Nanaimo Group as a likely target of importance. This unit is a thick succession of 11 intertonguing sandstone-dominated and shaledominated formations, of which only the lower 8 are present in the defined study area. The field study on eastern Vancouver Island incorporated observations from 61
surface outcrops and 1 subsurface core, comprising a total of 2480 m of measured section from all 8 formations in order to qualitatively evaluate the aquifer potential of these rocks. The Nanaimo Group displays a prominent regional-scale stratigraphic architecture of alternating coarse-grained
(potential regional aquifer zones) and fine-grained (potential regional aquitard zones) units.
The irregular basal unconformity surface, with fracturing, high relief and weathered regolith forms a significantly-enhanced regional aquifer zone in the subsurface, and a recharge zone at surface. The overlying sandstone-and conglomerate-dominated formations (Comox, Extension, Protection, De
Courcy) each represent regional aquifer zones with significant groundwater potential, whereas the intertonguing shale-dominated formations (Haslam, Pender, Cedar District, Northumberland) each represent regional aquitard zones with significant sealing potential. These alternating units are
interpreted to compose a series of 3 stacked regional Bedrock Aquifer Systems. The Lower Aquifer System (up to 200 m thick and heavily fractured) comprises the extensive basal unconformity plus the lower Comox coarse-grained sandstone and conglomerate fluvial facies plus the upper Comox fine- to
medium-grained sandstone shoreline facies. These deposits are overlain by the Haslam Formation shale, up to 200 m thick, which forms a Regional Aquitard Zone and renders this System the highest potential for both subsurface aquifer storage which underlies the entire field area, and for surface
recharge where it outcrops in the west. The Medial Aquifer System (up to 600 m thick) comprises a combination of Extension Formation conglomerate and coarse sandstone facies plus Pender Formation interbedded sandstone and siltstone plus Protection Formation medium- to coarse-grained sandstone. These
units are overlain by the Cedar District shale, up to 300 m thick, which forms a Regional Aquitard Zone that seals this very thick, but heterogeneous, significant Aquifer System which underlies most of the eastern part of the study area. The Upper Aquifer System (up to 300 m thick) encompasses the
De Courcy Formation coarse-grained sandstone which is locally overlain by the Northumberland shale (only 50-100 m thick in the study area) and forms a local potential aquifer zone in one specific area. However, the De Courcy is the only aquifer unit for which we have actual subsurface core data
(sandstone secondary porosity ranges 2-10%, av. 6.8%, permeability ranges 3-105 mD, av. 24 mD) and therefore provides a crucial analogy for the likely characteristics of the others. Additionally, fracture-related porosity and permeability are undoubtedly important in the subsurface setting,
but have not yet been adequately studied.