| Unsaturated zone is the required path of various pollutants from the surface to the ground water.Therefore,it is of great significance to study the law of solute transport in unsaturated zone for regional ecological and agricultural development.In this paper,the distribution characteristics of the unsaturated zone of the alluvial and diluvial fan in Dali are taken as an example.Based on the sedimentary characteristics of the alluvial and diluvial fan,a physical model of the unsaturated sandbox is established.The seepage and solute transport laws in the unsaturated zone of the alluvial and diluvial fan are studied using electrical resistivity tomography(ERT).The purpose of this study is to provide a basis for the prevention and management of groundwater pollution in heterogeneous aquifers.The main findings of the study are as follows:(1)Field investigation of different parts of alluvial fan and indoor Darcy test show that:There are more gravel in the top and fan of Dali alluvial-proluvial fan,the lithology is mixed,the separability is poor.The particle size and saturated permeability coefficient from the top of the fan to the edge of the fan decreased gradually,showing high heterogeneity and heterogeneity.(2)The dominant channel in the unsaturated zone can form"finger flow".The large size particles such as boulders and boulders in the unsaturated zone block the unsaturated seepage,and the heterogeneity of the media in the alluvial and diluvial fan zone controls the heterogeneity of water flow and solute transport.(3)The experiment of rainfall infiltration in sand box was carried out.The results show that the wetting front and moisture content of water in three parts of alluvial fan vary nonlinearly with time,and the size and arrangement of medium can affect the process of saturation in unsaturated zone.The wetting front in the top of the fan moves slowest,takes the longest time,but is fastest in the fan edge.The water flow obstacle in unsaturated zone is divided into capillary obstacle and water conduction obstacle,since the hydrodynamic conditions are gradually reduced during deposition,capillary barriers are more common in alluvial fan unsaturated.(4)Solute transport tests show that solute(Na Cl)can greatly increase the transport time and saturation time of wetting peak.The smaller the particle size and the smaller the pore size,the more obvious the solute effect.(5)The monitoring results of variable saturation process of sand box by ERT show that the resistance value decreases gradually from shallow layer to deep layer and has obvious heterogeneous characteristics with the process of variable saturation.In the ERT inversion image,the water flow and solute transport in the unsaturated zone can be observed directly,and the variation of wetting peak and moisture content with time during the process of variable saturation can be observed,and the variation law is in good agreement with the sand box observation test.(6)By analyzing the relationship between resistivity and moisture content,it is found that the fitting degree is high in the form of y=ax~b function.The maximum fitting degree(R~2)is 0.99139 and the minimum is 0.96622.It is proved that the modified Archie formula is also suitable for this kind of highly heterosexual and heterogeneous porous media.In the fitting function y=ax~b,the solute(Na Cl)reduces the coefficient a value by increasing the additional conductivity of the medium and increases the absolute value of the index b. |
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