Hydraulic conductivity heterogeneity of a local deltaic aquifer system from the kriged 3D distribution of hydrofacies from borehole logs, Valcartier, Canada

Summary The deltaic aquifer system of the Valcartier sector in Quebec, Canada, is part of a quaternary valley fill contaminated by dissolved trichloroethene (TCE). The objective of our study is to define the aquifer system heterogeneity that should influence TCE transport. Heterogeneity is defined by the distribution of both hydrofacies and hydraulic conductivity ( K ). Hydrofacies are defined as lithologic facies with distinctive hydraulic conductivity ranges. Our approach was developed to take advantage of the abundant stratigraphic and lithologic data provided by borehole logs (7000 m logged from 430 locations). Four site-specific deltaic hydrofacies were defined on the basis of lithologic descriptions, supported by data from grain size analyses, slug tests and cone penetration tests. Each hydrofacies includes a group of geologic facies found in borehole log descriptors to which an initial mean horizontal conductivity K H is associated based on slug tests. Borehole logs were converted to hydrofacies proportions over 5-m intervals to provide 1350 data points. The spatial distribution of hydrofacies was interpolated by three successive interconnected 3D kriging steps using the new technique of “imbricated kriging”. Global dual kriging is directly carried out on the 3D grid of a numerical model. Finally, the proportions of hydrofacies were used to estimate horizontal ( K H ) and vertical ( K V ) hydraulic conductivity fields using generalized means for layered media. Final K H values assigned to the hydrofacies are calibrated by comparison with 2D trends in K H shown by slug tests. This approach also provides an estimated vertical K V field with a spatially varying proportion to K H , rather than a fixed anisotropy ratio. Imbricated kriging does not preserve the statistical variability of fine scale hydrofacies distribution representative of geological variability. However, the approach provides K H and K V estimates over a 3D numerical grid that are coherent with hydrofacies distribution, which in this case control K H and K V .