| Hydrodynamic dispersion coefficient is an important parameter characterizing pollutant migration and distribution in aquifer. A huge study at home and abroad finds that this parameter has scale effect with scope of time and space change during research process. If the hydrodynamic diffusion varying pattern and scale effect of Quaternary loose rock mass pore media can be known exactly, it will be helpful for providing theoretical and data basis for groundwater pollution control and migration simulation in this media. On the basis of summarizing the domestic and foreign literature in this paper, we do a study on the hydrodynamic diffusion varying pattern and scale effect of loose rock mass pore media with lab simulation test, field dispersion test and numerical simulation.(1) The results of lab dispersion show that:①The solute concentration is not the main controlling factor of dispersion coefficient in high-permeable loose mass sediment, when solute concentration increased from 510mg/L to 2040mg/L, dispersion coefficient increased only 1.1～15.7% accordingly;②Adjust the pore media skeleton structure, the dispersity of lab test column decreased with clay mixing proportion increasing, the sand (particle size is 0.25～0.5mm) dispersity decreased from4.48cm to 2.97～1.845cm wih the clay (particle size is 5～50μm) mixing proportion increased from 1/9 to 1/5;③Seepage velocity in a given pore media is the main influencing factor of dispersion coefficient, the seepage velocity of diffuse column increased from 0.026cm/min to 0.051cm/min and the dispersion coefficient increased 2.29～2.32 times;④Scale effect of hydrodynamic dispersion only exists in long distance migration in heterogeneous porous media, the nature of lab dispersion test using even particle size to launch a solute transport simulation in short distance is hydrodynamic dispersion process in homogeneous media, which can not obverse dispersion scale effect. A lab dispersion test in sand(particle size is 0.25～0.5mm)and sand mixing clay (particle size is 5～50μm) was launched and the distance of solute transport distance is only 0.8～1.2m.The result found that dispersity remained stable with transport distance increased.(2) The field dispersion test results show that the longitudinal dispersity are 0.069m (downstream lm position of tracer releasing hole),0.519m (downstream 10m position) and 0.969m (downstream 40m position).With transport distance increase 10～40 times, the dispersity increase 7.5～14 times. Dispersion scale effect is obviously observed in the researched region.(3) Combining with the characteristics of the dispersion coefficient changing with migration distance, the dispersity aL and migration distance meet the function relation aL= 10-1.122 · L0.733 according to fractal formula in the researched region. We can use this formula to estimate dispersity of different distance, which will provide parameter selection basis for the solute transport simulation in the same type media.(4) The writer use Modflow software to establish the numerical model for simulating field dispersion test according to the hydrogeological conditions of the researched region and the results show that: Influenced by dispersion scale effect, dispersity obtained from lm and 40m migration distance selected as model parameter can not simulate the actual concentration change over time of downstream 40m position of tracer releasing hole. Calculate the average dispersity a in 40m migration distance by fractal formula of the researched region, and select a as model parameter to simulate field dispersion test. The simulation results can objectively reflect the actual concentration change over time of downstream 40m position. Thus it can be seen that: When a numerical simulation of groundwater pollution in this aquifer medium is launched, select the average dispersity α calculated by the former fractal formula as the model parameter can improve the accuracy of the simulation results. |
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