| In this paper,a one-dimensional solute transport experiment with preferential flow is carried out,and the early breakthrough and tailing analysis and numerical simulation of the solute transport curve are carried out based on the experimental data.Physical properties such as porosity and permeability coefficient of the sand sample were measured before the experiment was officially started.The average porosity of sand trough is 0.293 and the average permeability coefficient is 46.14m/d.Then a one-dimensional solute transport experiment was carried out in a horizontal glass trough with a length of 14.5m,a width of 10 cm and a height of 10 cm.Leaching experiments were carried out by changing clean water after the concentration of all sampling points reached the peak value in the sand trough.In the experiment,river sand with a particle size range of 0.15mm~0.3mm was selected as the medium,and there is a certain gap between the surface of sand sample and the top glass.The average height of preferential flow at 0-750 cm is greater than 750-1400 cm.The first experimental breakthrough stage lasted 18720 minutes and the leaching stage lasted 11280 minutes.The total time was 30000 minutes.The second experimental breakthrough stage lasted 18450 minutes,and the leaching stage lasted 24660 minutes,with a total time of 43110 minutes.Thirteen sampling ports with equal spacing were set on one side of the sand trough.The sampling interval was initially 6-12 hours,then 8-14 hours.The conductivity of each sampling point was measured and the measured concentration was calculated from the standard curve.In the first experiment,each sampling point was sampled 47 times,totally 611 times.In the second experiment,each sampling point was sampled 42 times,totaling 546 times.The experimental results were given in the form of C-t diagrams.In addition,according to the experimental conditions,the model was constructed and solved by FORTRAN language,and each solute transport curve was fitted.The model was constructed according to the experimental conditions,and each breakthrough curve was fitted.According to the fitting results,the simulation effect of the model was analyzed.The specific conclusions are as follows:1)The sand flow rate of the two experiments was measured and the results were as follows: In the first experiment,the average flow velocity of the sand trough from 0-750 cm in the inlet was 0.0054 cm/min,and the average flow velocity in the 750-1450 cm section was 0.092 cm/min.In the second experiment,the average flow velocity of the sand trough from 0-750 cm in the inlet was 0.0054 cm/min,and the average flow velocity in the 750-1450 cm section was 0.0882 cm/min.The experimental results show that the flow velocity difference between the front part and the back part of the sand trough is quite different.The flow rate of the sand trough from 0-750 cm in the inlet is much smaller than the flow rate of the sand trough in the range of 750-1450 cm,thus dividing the sand trough into two sections for analysis.2)The solute transport curve obtained from the experiment was analyzed: In the two experiments,the concentration breakthrough rate of the sampling point at the front end of the second section of the sand tank is faster than that at the back end of the first section of the sand tank.In the two experiments,the concentration leaching rate of the sampling point of the second section of the sand tank was faster than that of the first section of the sand tank.The above results indicate that early breakthrough and tailing occurred during the solute transport phase,and these phenomena are obvious features of preferential flow.3)The construction model was simulated for the transport of solute containing preferential flow: The model fits well the overall trend of the measured C-t curve,and it is reflected in the early breakthrough and the tailing phenomenon,but these phenomena cannot be completely fitted.The model fitting sand trough dispersion is 2cm,porosity is 0.3,the exchange coefficient of the first slot and the priority flow in the penetration phase is 0.003,the second exchange coefficient is 0.5,and the exchange coefficient in the leaching phase is 0.00005.The second exchange coefficient is 0.5.The exchange coefficient of the former part and the latter part of the model has a great influence on the simulation result,but the dispersion degree is the opposite.The model simulation did not find the existence of a diffusion scale effect.4)The error of the model simulation was analyzed: In this paper,the preferential flow is generalized into two segments,and the height of each preferential flow is constant,but in the actual loading the height of the preferential flow is not completely uniform.And the average cross-sectional flow rate of the model is not exactly the same as the actual flow rate.In addition,due to the large span of experimental time,which has a certain influence on the viscosity coefficient of water,and may cause the change of the flow rate of the sand tank. |
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