Characterization of full 3D hydraulic conductivity tensors of hydrostratigraphic units applied to Innisfil Creek watershed, Ontario

Abstract

Variations of hydraulic conductivities (K) in hydrostratigraphic systems may significantly affect the flow velocity field and mass dispersion. Field data used for assessment of K are generally representative at a local scale only and for one or a few hydrofacies forming a specific hydrostratigraphic unit (HSU). Regional groundwater flow systems encompass a multitude of HSUs, where a given HSU can have K-range variability spanning several orders of magnitude. Therefore, for regional systems, blocks with respective K-equivalent have to be defined as part of each HSU. A major challenge under such conditions is to determine the spatial distribution of the full 3D K-tensor blocks (Kb) considering the effect of local scale variability in K. In this study, an efficient method is developed for regional characterization of the HSUs with 3D Kb. The method was tested for the Innisfil Creek watershed. For each HSU, it consists of the following major steps: (i) assessment of K from measured data; generation of the probability density function of K; definition of the spatial covariance of K; and geostatistical simulation of K at local scale; (ii) upscaling of the local scale realizations of K into full 3D Kb; definition of the spatial covariance of 3DKb; and definition of spatial distribution of the 3DKb. The preliminary results include a K database built using 1694 grain size analyses, 32 HSU borehole samples and 1086 transmissivity measurements in public wells. The grain size samples were collected by Ontario Geological Survey (OGS) from 15 boreholes in the South Simcoe area. Between 28 and 301 samples/HSU were analysed covering all of the 14 HSUs observed in the study area. K from grain size analyses were in good agreement with K based on laboratory permeability test measurements of field core. The database was completed with K assessments from specific capacity analyses in public wells, which have a strong bias from the high permeability formations. To obtain the local scale variability of K, only the results from grain size analyses were used mainly due to their sufficient quantity to define the probability density function of each HSUs and their results reflecting the expected strong variability of hydrofacies within a given HSU. Local scale K fields were simulated with non-conditional turning band simulation. These results were then upscaled to the block regional scale. In that regards, we revisited the Zhou et al. (2010) methodology for non-local 3D hydraulic conductivity full tensor upscaling using flow simulator. Preliminary results suggest no correlation between upscaled blocks within the study area. The upscaling methodology is still in development, upscaling parameters and validation criteria need to be tested. The final outputs of this study will be an ensemble of hydrostratigraphic models with equivalent 3D K-tensor parameters, which will be used to assess the impact of K and HSU uncertainties on groundwater flow modeling. The proposed methodology is appropriate for characterizing the uncertainty of groundwater flow and transport. For example, it can be used for aquifer vulnerability assessment and the delineation of wellhead protection areas.