The Effect of Hydraulic Gradient and Pattern of Conduit Systems on Tracing Tests: Bench‐Scale Modeling

Abstract

Tracer breakthrough curves provide valuable information about the traced media, especially in inherently heterogeneous karst aquifers. In order to study the effect of variations in hydraulic gradient and conduit systems on breakthrough curves, a bench scale karst model was constructed. The bench scale karst model contains both matrix and a conduit. Eight tracing tests were conducted under a wide range of hydraulic gradients from 1 to greater than 5 for branchwork and network-conduit systems. Sampling points at varying distances from the injection point were utilized. Results demonstrate that mean tracer velocities, tracer mass recovery and linear rising slope of the breakthrough curves were directly controlled by hydraulic gradient. As hydraulic gradient increased, both one half the time for peak concentration and one fifth the time for peak concentration decreased. The results demonstrate the variations in one half the time for peak concentration and one fifth the time for peak concentration of the descending limb for different sampling points under differing hydraulic gradients are mainly controlled by the interactions of advection with dispersion. The results are discussed from three perspectives: different conduit systems, different hydraulic-gradient conditions, and different sampling points. The research confirmed the undeniable role of hydrogeological setting (i.e., hydraulic gradient and conduit system) on the shape of the breakthrough curve. The extracted parameters (mobile-fluid velocity, tracer-mass recovery, linear rising limb, one half the time for peak concentration, and one fifth the time for peak concentration) allow for differentiating hydrogeological settings and enhance interpretations the tracing tests in karst aquifers.

Dynamics of the Flow Exchanges between Matrix and Conduits in Karstified Watersheds at Multiple Temporal Scales

“…In case of the existence of a hydraulic gradient between the conduit and the surrounding matrix part of the contaminant may be temporally stored in so-called "dead space" [52]. This has been investigated at a laboratory scale [53,54] but remains badly constrained in real karst systems. Then, a well-constrained flow partition between different flow compartments can be useful for solute transport modeling.…”

Review of Laboratory Scale Models of Karst Aquifers: Approaches, Similitude, and Requirements

“…The common feature of these LSMs summarized in Table S5 is the consideration of hydraulic and transport exchange between conduits and matrix. Conduit flow is simulated with a single pipe (Li et al 2008; Faulkner et al 2009; Gallegos et al 2013;Castro 2017 ; Xiao et al 2018) or a series of interconnected pipes (Florea and Wicks 2001; Mohammadi et al 2019). The pipes are designed to be active inside (Florea and Wicks 2001; Li et al 2008; Castro 2017; Mohammadi et al 2019) or outside (Faulkner et al 2009; Gallegos et al 2013; Xiao et al 2018) of the matrix (Figure 6).…”

Coupling SKS and SWMM to Solve the Inverse Problem Based on Artificial Tracer Tests in Karstic Aquifers

“…Besides, the RTD shape may be directly linked to the spatial distribution of flow and transport. Investigations at bench-scale [16,[19][20][21] enabled the identification of the effects of different flow patterns on the RTD measured at the outlet of the system. Also, changes in boundary conditions should be considered properly, since they may have a strong influence on the RTD shape [11,12] even on a flood event scale [13].…”