Solute transport in dual conduit structure: effects of aperture and flow rate

We built 11 lab-scale dual-conduit structures by varying the apertures of the two conduits and we conduct solute transport experiments consisting of step tracing. We investigated how the transport process can be influenced by the following two factors: flow rate and aperture width of both conduits. We found that, as the flow rate increases, the dual-conduit structures more likely presents a breakthrough curve (BTC) with double-peak effect. When the shorter conduit has smaller aperture than the longer conduit, the dual-conduit structure may lead to either single-peaked BTCs or to dual-peaked BTCs with a much lower early peak. When the shorter conduit has larger aperture than the longer conduit, the dual-conduit structure may lead to double-peaked BTCs or to single-peaked BTCs with a bump on the falling limb.

We then compared the ability of three different numerical models in fitting the experimental BTCs: Weighted Sum Advection–Dispersion Equation (WSADE), Mobile Immobile Model (MIM), and Dual Region Mobile Immobile Model (DRMIM). MIM does not reproduce the double-peaked or bump-tailed BTCs, but it captures the overall shape of the experimental curves. The WSADE reproduces some of the double-peaked BTCs except the experiment of 4-6, 200 rpm. The DRMIM exhibits better performance than the other two models, and it captures the observed behaviors of all the experimental BTCs: the second peak, the bump, and the tailing. We finally showed that parameter estimation of the DRMIM model can be improved by restricting the contrast between the parameter pairs: u_m1 and u_m2, D_m1 and D_m2, k₁ and k₂, w_m1 and w_m2.