Abstract
Hydrogeophysics is increasingly used to understand groundwater flow and contaminant transport, essential basis for groundwater resources forecast, management and remediation. It has proven its ability to improve the characterization of the hydraulic conductivity (\textit{K}) when used along with hydrogeological knowledge. Geophysical tools and methods provide high density information of the spatial distribution of physical properties in the ground at relatively low costs and in a non-destructive manner. Amongst them, the Electrical Resistivity Tomography (ERT) has been widely used for its high spatial coverage and for the strong theoretical links between electrical resistivity ($\rho$) and key hydrogeological parameters such as \textit{K}. Historically, ERT data processing was based on isotropic hypothesis. However, the unconsolidated aquifers in Canada reveal in most cases a strong anisotropic behaviour for \textit{K} both with \textit{in situ} or laboratory measurements. Recently, electrical anisotropy has been considered model-wise, but it is seldom considered as an interpretation tool or in the characterization process of the anisotropy of \textit{K}. In order to evaluate the potential of ERT to assess the anisotropy of electrical resistivity, we developed a forward and inverse modeling code. These codes have been validated and tested on a realistic synthetic case reproducing the behaviour of a real aquifer extensively characterized, the site of Saint-Lambert-de-Lauzon in Quebec (Canada). On this site, innovative \textit{in situ} hydraulic tomography has revealed a strong anisotropy, with up to three orders of magnitude between horizontal and vertical \textit{K} components. In order to confirm the link between \textit{in situ} \textit{K}- and $\rho$-anisotropies, an ERT survey has been performed, using the same wells as for the hydraulic tomography. The inversion confirms a strong link between \textit{K}- and $\rho$-anisotropies. It demonstrates the suitability of the anisotropic ERT approach coupled with well measurements to provide better estimates of \textit{K} and its anisotropy at the scale of a site.
Highlights
Understanding groundwater flow and contaminant transport in the subsurface for water management and aquifer remediation generally requires a good knowledge of the spatial distribution of hydraulic properties within the aquifers
K can vary over several orders of magnitude within a same geological unit, which highlights the importance of having accurate high-resolution and high-coverage estimates to reduce errors in groundwater flow and mass transport and improve groundwater management
Hydraulic anisotropy has a major influence on the groundwater flow and mass transport
Summary
Understanding groundwater flow and contaminant transport in the subsurface for water management and aquifer remediation generally requires a good knowledge of the spatial distribution of hydraulic properties within the aquifers. While previous hydraulic tests were shown to provide invaluable estimates of K-anisotropy in real field conditions, these methods are time consuming to operate and can only provide very local information. We propose using geophysical data to complement hydraulic tests as geophysical methods can provide broad pictures of the subsurface in a considerably shorter amount of time than hydraulic methods. There is a theoretical equivalence between K-anisotropy and ρanisotropy in unconsolidated sediments were the electric current flows in the conductive saturated pores (Hubbard and Rubin, 2005). The methodology is applied for a real case study known to be highly heterogeneous, and ρ-anisotropy estimated through anisotropic inversion is compared to Kanisotropy obtained with hydraulic tests at the study site to strengthen the reliability of the proposed approach (section 5). This study exposes the capacity of DC surveys to improve hydrogeological characterization
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