Abstract
Solute transport in low-permeability media such as clay has not been studied carefully up to present, and we are often unclear what the proper governing law is for describing the transport process in such media. In this study, we composed and analyzed the breakthrough curve (BTC) data and the development of leaching in one-dimensional solute transport experiments in low-permeability homogeneous and saturated media at small scale, to identify key parameters controlling the transport process. Sodium chloride (NaCl) was chosen to be the tracer. A number of tracer tests were conducted to inspect the transport process under different conditions. The observed velocity-time behavior for different columns indicated the decline of soil permeability when switching from tracer introducing to tracer flushing. The modeling approaches considered were the Advection-Dispersion Equation (ADE), Two-Region Model (TRM), Continuous Time Random Walk (CTRW), and Fractional Advection-Dispersion Equation (FADE). It was found that all the models can fit the transport process very well; however, ADE and TRM were somewhat unable to characterize the transport behavior in leaching. The CTRW and FADE models were better in capturing the full evaluation of tracer-breakthrough curve and late-time tailing in leaching.
Highlights
Low-permeability porous media (LPPM) exist extensively in natural sedimentary deposits such as shale and clay
The Darcian velocity is an essential element in solute transport and is calculated from the following [58]: q = Q, (1)
The soil samples during the flushing period were not the same as those for the tracer introducing period, one did not change the actual soils in the column. This implied that at least for tracer tests in LPPM, the transport processes for the tracer introducing test and tracer flushing test could be different because the properties of the soil had been inevitably altered during the switch from tracer introduction to tracer flushing
Summary
Low-permeability porous media (LPPM) exist extensively in natural sedimentary deposits such as shale and clay. They have played critical roles in protecting groundwater resources, affecting the accumulation of petroleum and ore deposits, and controlling geological processes such as structural evolution of the crust [1,2,3,4,5]. It is a little hard to understand the solute transport mechanism in LPPM using actual experiments This is partially due to the challenges of designing adequate experimental apparatus for acquiring accurate data of flow and transport in such media, and partially due to the extremely long time needed for carrying out such experiments.
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