Experimental And Numerical Studies On Solute Transport Within The Capillary Fringe

In recent years,soil and groundwater pollution in the aerated zone caused by human activities has become more serious.Wastewater recharge,landfill leachate penetration,and leakage of toxic liquid chemicals have threatened the safety of soil and groundwater in the aerated zone.Pollutants must pass through Capillary fringe to migrate from the surface to groundwater.According to the traditional theories in soil and hydrogeology,the movement of water in capillary fringe is mainly vertical.However,in reality,it is a basic feature under large-scale conditions.As early as 1932,Wyckoff found the phenomenon of lateral seepage in the capillary fringe.A large number of lab and field tests have found that the lateral movement of water in the capillary fringe has an important impact on the transport of water or related solutes from the unsaturated zone to the aquifer.Water flow and solute transport within capillary fringe are interesting and challenging research topics.Although there are lots of research results from different disciplines on the capillary fringe,the further studies of related processes and factors are still in need.The researches on the mechanisms and factors of the water flow and solute transport in the capillary fringe are the foundation to accurately evaluate the aerated zone and the formation and transformation of groundwater resources.At the same time,it has important theoretical and practical significance for the evaluation and repair of pollution in aerated zone.To this end,this dissertation was based on the theories of soil water dynamics,groundwater solute transport,etc.and on the collection of previous research data and conclusions.Sand box experiments and numerical simulations on saturated-unsaturated groundwater flow were conducted.These studies would elucidate the mechanisms and characteristics on lateral migration of groundwater and solute transport in capillary fringe in order to improve and enrich the saturated-unsaturated solute transport theory in subsurface environment.This dissertation mainly included the following three parts.(1)According to the sand box experiments of solute transport in capillary fringe,lateral seepage characteristics were present and qualitatively analyzed.In addition,a two-dimensional capillary network model was built to reveal the theories of lateral seepage in the capillary fringe.Meanwhile,numerical models of solute transport were also built to quantitatively calculate the hydrodynamic dispersion parameters.At last,HYDRUS software was used to analyze the reliability of different numerical models in solving the problems of capillary water seepage and solute transport.(2)Sand box experiments were conducted by setting different boundary water levels,injecting different concentrations of non-interacting tracer or water insoluble but adsorptive tracer in the capillary fringe.Combined with numerical simulations,the effects of the hydraulic gradient and the amount of discharged pollutants on water seepage and solute transport were analyzed.(3)Taking the advantage of self-designed rainfall device and evaporation device,sand box experiments were carried out at different rainfall intensities and different evaporation intensities.Based on the analysis of the space-time change law of matrix potential during rainfall or evaporation process,pendulum ring model and capillary model were taken into consideration to analyze the mechanisms of capillary water seepage during the process of rainfall or evaporation.Combined with numerical simulation,the effects of rainfall or evaporation on the water potential distribution,flow velocity distribution and solute transport in the capillary zone were comprehensively analyzed.The main conclusions based on above-mentioned researches could be drawn,as follows.(1)The characteristics and mechanisms of lateral seepage and solute transport in capillary fringe.The tracer would transport laterally from the unsaturated zone into the capillary fringe.There was a vertical upward flow velocity component in the recharge area of the capillary fringe.Meanwhile,a vertical downward flow velocity component also existed in the discharge area.Lateral seepage was predominant in the middle area.According to the two-dimensional capillary beam network model and the force analysis of the established model,it was found that there were not only lateral seepage but also vertical capillary velocity resulting from the combination of surface pressure,atmospheric pressure,surface tension,matrix suction and friction near the recharge area.In addition,the velocity gradually decreased during the capillary rise.In the discharge area,the downward flow velocity was caused by the combination of different forces,such as gravity,the friction with capillary boundary's wall,the additional surface pressure,and the pressure exerted by adjacent capillary column.The flow rate in the capillary column gradually increased from top to bottom.Also,the water in the capillary column would flow into the saturated zone rather than flowing out from the column wall.In the lateral seepage zone,the hydraulic conductivity was proportionate to radius of the capillary column.The solute transport in the capillary zone was described by the two-dimensional hydrodynamic dispersion under one-dimensional seepage model.After calculations,the transversal dispersivity was 1.198 cm and the longitudinal dispersivity was 0.286 cm.The experimentally calculated parameters were brought into the HYDRUS model.The comparison between the simulated data and the measured data showed that the HYDRUS simulation results based on the Brooks and Corey models excellently fitted the results of the laboratory experiments.(2)Influences of hydraulic gradient,tracer flow rate and water insoluble but adsorptive pollutants on water flow and solute transport in the capillary fringe.(1)Different flow rates of tracer would have complex impacts on the water potential distribution and flow velocity distribution in the capillary zone and saturated zone.Thus the distribution of solute in these zones would be affected as well.When the tracer flow rate was small,the capillary fringe could hinder the vertical migration of the tracer into the groundwater.The pollution halo would not pass through the capillary fringe into the saturated zone.But,when the flow rate was increasing,the velocity of capillary water would increase and the range of pollution halos would expand.As the flow rate continued to increase,the pollution plume would pass through the surface of the diving surface and into the saturated area.(2)Based on laboratory tests at different hydraulic gradients and numerical simulations,it is found that the ratio between hydraulic gradient in lateral direction and that in lateral direction would increase when water head differences between the recharge and discharge zone were larger during the process of water movement in the capillary fringe.As a result,the tracer was in lateral migration.However,with the decreasing water head differences at both sides in seepage zone,the effects of the capillary fringe on the vertical migration of pollutants were weaker.It was easier that pollution took place in the saturated area.(3)The water insoluble but adsorptive pollutants could be adsorbed on the surface of porous media during the process of migration in the capillary fringe.Thus,the diameter of capillary column in porous media was reduced.As a result,rising height of capillary water increased and percolation velocity in the capillary column reduced.Moreover,the larger the adsorption distribution coefficient of the pollutants,the smaller the area of the pollution halo,and the slower the migration of the capillary water were supported.Since the HYDRUS model did not consider the tracer adsorbed on the surface of the porous medium and the changes of pore structure of the porous medium,the experiment results were lagging comparing with results from the HYDRUS simulation.For the pollutants with larger adsorption partition coefficient,the simulation was less accurate.(3)Effects of rainfall and evaporation on water flow and solute transport in the capillary fringe.(1)During the initial stage of the rain,when the water content of the porous medium was lower than the residual water content,the pore angle of the porous medium began to absorb water rapidly.The pendulum ring model was used for generalizability analysis.When the matrix suction at the interface of the pendulum ring was smaller than the gravity of the pendulum ring,the capillary water of pore angle was no longer increased.And the small capillary channels began to be filled with water.The water content continued to increase.When the gravity of the water column in the capillary was greater than the combined force of the water column matrix suctions,the increase of water content stopped.During the process of rainfall,the matrix potential of each point in capillary fringe would increase.If rainfall intensities increased,the increasing amount of matrix potential would be increased too.The differences in the increasing amount of each point led to the changes in total water potential of each point resulting in a vertical hydraulic gradient.The greater the rainfall intensity,the weaker the effect of the lateral seepage of the capillary zone on the vertical migration of pollutants.(2)During the evaporation process,the bigger capillary channel in the capillary fringe would evaporate preferentially.Thus,the water content would gradually decrease.When the water content of the large capillary channel was lower than the critical value of the moisture of the capillary bond disruption,the position of the evaporation surface would continuously move downward.For the evaporation of the stable water table,the water content decreased led to reduction of the matrix potential at the upper end of the capillary column.The difference in water potentials at both ends of the capillary fringe increased,and the capillary ascending velocity increased.When the evaporation intensity was equal to the capillary ascending velocity,the water content did not decrease any more.So the evaporation became stable.Under different potential evaporation conditions,there was a significant difference in migration speed of pollution plume.The greater the potential evaporation intensity,the slower the tracer lateral migration.The effects of evaporation on the capillary water seepage and solute transport were the result of the coupling of heat,moisture and salt migration.The main innovations are:(1)According to the sand box experiment combined with the two-dimensional capillary beam network model,we reveal the mechanisms of the lateral seepage in the capillary fringe;(2)Factors,like the hydraulic gradient,the pollutant discharge flow rate and the non-water-soluble adsorption pollutants,are taken into consideration to comprehensively elucidate mechanisms of water flow and solute transport in the capillary fringe;(3)The effects of evaporation and rainfall on water flow and solute transport in the capillary fringe are discussed.