| The emission of the industrial waste water, gas and residue, the running off of the excess agricultural fertilizer and pesticides, all of these result in serious pollution of groundwater, and also in threatening people's health. In order to estimate the degree of groundwater pollution, the pollutants' transport and dynamic distribution in porous medium need to be studied, and the mathematical models should be developed to describe solute transport in different circumstances and determine the parameters in the models. A simple approach is provided by the boundary layer method for estimating the parameters, but the feasibility are still under study in practice. In addition, the problem of how to determine the position of boundary layer remains unsolved in this method. Using visible tracer to indicate the solute movement and TDR to measure the concentration change of solute in soils, the suitability of tracing and detecting solute front using brilliant blue FCF and time domain reflectometry (TDR) were studied, and at the same time the accuracy of boundary layer method in the parameter's determination was analyzed. The result as follows:(1)The transport parameters of cr indicated by the dye, which were obtained by boundary layer method, are greater than those of cr by fitting break-through curve with CXTFIT. The brilliant blue FCF is in negative ion state in water, but it is strongly absorbed in soil; the transport of brilliant blue FCF slower than cr in soil, which shows that the brilliant blue FCF cannot be used to precisely indicate the movement of the boundary layer of CF. The brilliant blue FCF has its own chemical property and the characteristic of absorption in soil, so it is difficult that its movement characteristic is directly used to indicate other chemicals' movement, and the position of solute front indicated by the dye should be revised.(2)According to boundary layer method, the depth of boundary layer theory has high linear-Iv-relationship with time, and also the depth of boundary layer theory of CF indicated by the dye has a high linear relationship with time, so the depth of CF indicated by the dye can be revised using the kno~iaparameters of CF estimated by fitting break-through curve. The revised depth of CF shows as follows:In the column of sandy loam with bulk gravity of l.3gIcm3: d~1 = 5.3 9d~ -6.12. In the columns of bess soil with bulk gravity of I .3gIcm3, 1.25 g/cm3 and 1.20 g/cm3, respectively:dCI=9.96ddye~22o7l~ d~1=9.41d~ -12.89, da=5.51d~~8.33.(3)The accuracy of TDR of the FAM/MTS type used in this study is 0.OO1N in measuring salinity, and it can measure the solute concentration; from the process figure of solute front and concentration change with time we can see that the salinity is a constant before the solute front arrived at the detecting position. When the solute front arrived at the position, salinity began to gradually increase, the process curve of the concentration change obtained by the TDR and the break-through curve are extremely similar. It is shown that the feasibility of using TDR to detect the solute front is applicable, and it provided a simple and reliable approach of monitoring solute front in the determination of parameters of solute transport by the boundary layer method.(4)By comparing the parameters obtained by the transient boundary layer method and those obtained by the traditional fitting break-through curve method, we can see that the values of hydraulic dispersion coefficient (D) obtained by the two methods are very close, but all values of retardation factor (R) obtained by the boundary layer method are greater than those obtained by the curve-fitting method, the reason resulting in the greater values of R is that the boundary layer monitored by TDR lagged than the actual position. In fact, the boundary layer if gradually formed, it is very difficult to monitor in practice because TDR has a limitation of sensitivity. When TDR captured the boundary layer, the actual boundary layer had already p... |
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