Abstract
Assessment of the karst network geometry based on field data is an important challenge in the accurate modeling of karst aquifers. In this study, we propose an integrated approach for the identification of effective three-dimensional (3D) discrete karst conduit networks conditioned on tracer tests and geophysical data. The procedure is threefold: i) tracer breakthrough curves (BTCs) are processed via a regularized inversion procedure to determine the minimum number of distinct tracer flow paths between injection and monitoring points, ii) available surface-based geophysical data and borehole-logging measurements are aggregated into a 3D proxy model of aquifer hydraulic properties, and iii) single or multiple tracer flow paths are identified through the application of an alternative shortest path (SP) algorithm to the 3D proxy model. The capability of the proposed approach to adequately capture the geometrical structure of actual karst conduit systems mainly depends on the sensitivity of geophysical signals to karst features, whereas the relative completeness of the identified conduit network depends on the number and spatial configuration of tracer tests. The applicability of the proposed approach is illustrated through a case study at the Hydrogeological Experimental Site (HES) in Poitiers, France.
Highlights
Karst conduits in carbonate aquifers provide low-hydraulic resistance paths, which exert a strong control on the spatiotemporal propagation of pressure-head perturbations and the transport of solutes in groundwater 20 (Goldscheider and Drew, 2007; Worthington and Ford, 2009; Kresic, 2012; Ronayne, 2013)
5 Discussion and conclusions 315 The approach outlined in this paper aims to improve the assessment of conduit network geometry in karst aquifers
The prerequisite data for the application of the method are i) a set of breakthrough curves (BTCs) corresponding to tracer experiments conducted between different points in the karst network whose geometry is to be specified, and ii) a 3D model of subsurface heterogeneity derived from geophysical methods sensitive to karst features
Summary
Karst conduits in carbonate aquifers provide low-hydraulic resistance paths, which exert a strong control on the spatiotemporal propagation of pressure-head perturbations (e.g., pumping-induced drawdowns) and the transport of solutes in groundwater 20 (Goldscheider and Drew, 2007; Worthington and Ford, 2009; Kresic, 2012; Ronayne, 2013). Given the possibly complex conduit network patterns in karst aquifers, the tracer may follow different routes between the injection and monitoring points. 135 the relationship between both parameters does not need to be explicitly modeled, which is of particular interest given the notoriously complex and site-specific nature of the problem, e.g., Pride (2006), Hyndman and Tronicke (2006) and Brauchler et al (2012) It is beyond the scope of this paper to examine the potential strengths and weaknesses of the different existing geophysical methods for the characterization of hydrogeological heterogeneity. Borghi et al (2012) applied the fast marching algorithm of Sethian (1996) 155 to simulate karst conduit networks as least-resistance paths based on 3D scalar grids of pseudovelocity fields empirically derived from different geological indicators. In the case study presented below, we adopted an empirical threshold of 0.5
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