D is the hydrodynamical dispersion factor, and it can be repre sented by two items. (2) The al is called dynamical dispersion factor, and D'is molecule dis persion factor. Since the D"is very small, we can neglect it For convenience, we consider the density of the pollutants source is o before the new pollution occurring, and become Co just after that time timc e Figure 1 step-change source This model of pollution source has pracical value because the pollution that lasts for a long time does more harm to people than that lasts for a short period, and we must locate it and manage to eliminate it. So in this model, we put the emphasis on the step change source and try to locate it Use the step-change source to represent the boundary cond tions C(l,0)=0 >=0 (3) C(0,l)=C (4) (+∞,t)=0 >=0 (5) Then use these boundary conditions, we can get the analytic solution of the equation (1) C_1 Croc( tl 0 2√D)(6) 234 PDF文件使用" pdffactory"试用版本创建w, fineprint,com,cn
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Here I is the distance along the path of the groundwater stream. In the practical condition, Dt is very large, so the second item of the formuia could be ignored This formula can be used to calculate the density curves of the section plane C 1 erfc( t Determine the pollutants arrival time At any position in the polluted area, use the solution of the e- quation(1), we can get the density-time curve. It is a erfc func- (C/C)0.5 Lime t Figure 2 density-time curve We may think the pollutants come to this position at t.We can determine the t, by the solution At ts time, C/C0=0.5 Because erfc(0)=1, so we can get 0 u could be calculated by the Darcy Principle, it shows that dh (8) where k is the hydraulic conductivity In this small area, we think the streamline should be approxi- 235 PDF文件使用" pdffactory"试用版本创建w, fineprint,com,cn
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