Science Inventory

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

Citation:

Mohammadi, Z., M. Gharaat, AND M. Field. The Effect of Hydraulic Gradient and Pattern of Conduit Systems on Tracing Tests: Bench-Scale Modeling. Groundwater. Wiley-Blackwell, Hoboken, NJ, 57(1):110–125, (2019). https://doi.org/10.1111/gwat.12659

Impact/Purpose:

The purpose of the research was to investigate groundwater flow and solute transport in a bench-scale karst aquifer under differing hydrodynamic conditions by means of tracer tests and the main objectives of this research were (1) characterization of the breakthrough curves by investigating representative parameters of its shape and (2) interpretation of the effect of varying hydraulic gradients and conduit patterns on the parameters derived from the shape of the breakthrough curves. The results of the research revealed that velocity and other important paramters increase with increasing hydraulic gradient, but that other paramters tended to decrease in response to increasing hydraulic gradients. With increasing hydraulic gradient velocity increased in both the branchwork- and network-conduit systems. The rate of increasing velocity significantly lessened with a hydraulic gradient greater than 2.6 and 4.1 in the branchwork- and network conduit systems, respectively. This behavior is probably due to change of the flow regime from linear laminar to nonlinear and/or turbulent flow regime.

Description:

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.