Experimental Study And Numerical Modeling On One-Dimensional Solute Transport In Low Permeability Media

With the development of economy, society, industry and agriculture, the quality of groundwater is getting worse. The migration of contaminant in the aquifer is affected not only by physical, chemical, biological, and other activities, but also by the texture, structure, spatial variation of the media. Up to now, a series of research on solute transport have been conducted. However, these studies mainly focused on the solute transport in high permeability media. Research on pollutants migration and transformation regulation is still limited in low permeability media. Besides, seepage flow is non-Darcy in low permeability media, so the Convection-Dispersion Equation (CDE) which is based on Darcy is not suitable in describing solute transport. In order to know something about solute transport in low permeability, to find the best mathematical model to describe solute transport process, it is necessary to conduct some further research on the solute transport in low permeable media.In order to make the study more comprehensive, in this study, we carried out one-dimensional solute transport experiments in small columns in low permeability media. Three scenarios were considered:they are(1) same diameter with different length of the column, (2) same length of the column with different diameters, and (3) with thread in the inner wall of the column. The flow rate was measured in the experiment, then the flow velocity can be obtained subsequently. In addition, the electrical conductivity was also measured during the experiment. With the experiment data, the flow velocity and the transport processes can be analyzed..Four solute transport models have been used to analyze the experiment data, they are Convection-Dispersion Equation (CDE), Two Region Model (TRM), Stochastic Convection-Dispersion Model (SC) and Continuous Times Random Walk (CTRW). Both of the breakthrough process and the leaching process were simulated by these four models. The agreements between the experiment data and the simulated data were discussed, the best mathematical model is proposed based on the study. Several conclusions can be drawn from this study:1.During the experiment, the seepage velocity of solute transport process in low permeability media can be divided into three stages:steady-drop-steady. Because NaCl solution should be changed to clean water from the transport process to the leaching process. During this period, the NaCl solution above the media in the inlet should be pumped out. Consequently, the pressure in the inlet dropped and the structure of the pore may be changed and the connectivity of the pore may also become smaller than that of the original media. Thus, hydraulic conductivity will be smaller. When the soil structure approaches a new steady state, seepage velocity will be stabilize.2. The diameter and the length of the soil column will affect velocity, as well as the roughness of the inner wall of the column. For the breakthrough process:a lager diameter of the column result in a larger flow velocity; the flow velocity in the column with roughness inner wall is significantly less than that in the smooth column. In the leaching process:the impact of diameter and length of soil column are opposite to that of the breakthrough process, the impact of roughness can be ignored.3.All of these four mathematical models can be used to describe the breakthrough process, which indicated that the breakthrough process is Fickan. The scale effect of the dispersity was found for the CDE and TRM. The Vψ in CTRW model shows that the size of the soil column will affect the relationship between the average speed of particles and the average flow velocity, the average speed of particles is less than the average flow velocity (v in CDE) in small size. A larger diameter results in the average speed of particles is greater than the average flow velocity. Compared to other mathematical models, the scale effect of the dispersity in the CTRW model is subtle.4.Significant tailing phenomenon of solute transport occurred during leaching procedure in low-permeable medium. CDE, TRM and SC show bad agreement between the observed data and the simulated data for the leaching process. Different prediction precision was obtained for the TRM model for different scenario. The prediction precision of TRM mainly depends on the change of the content of the moving water before and after leaching at same diameter in different lengths of the column. For scenario (2), a better agreement of TRM occurred as the soil column diameter increased. SC showed a little better prediction than CDE and TRM methods which indicates that partial flow-tube pattern existed in small scale when solute transport at low-permeable media under leaching procedure.5.It seems that the CTRW might be the best tool to describe solute transport in the leaching process. Small diameter of the column showed better prediction precision than that of the larger one for the leaching process. The prediction results of CTRW may be affected by preferable flow, it will be a bad prediction result when preferable flow exist.