Study On Migration Mechanism Of Lead In Undulating Zone Of Groundwater

Groundwater is an important resource for human survival.The changes of natural conditions such as meteorology,hydrology,groundwater exploitation and human activities make the groundwater level rise and fall,forming a water level fluctuation zone.Water level fluctuation not only changes the hydrodynamic conditions,but also changes the environmental conditions and hydrogeochemical processes,affecting the migration and transformation process of pollutants.However,there are few reports on the migration and transformation of heavy metals under water level fluctuation.Through field sampling,laboratory experiments,theoretical analysis and numerical simulation,the characteristics of lead adsorption and desorption in different media were analyzed,the migration mechanism of lead under the condition of groundwater level fluctuation was revealed,and the numerical model of lead migration in groundwater level fluctuation zone was constructed,and the temporal and spatial distribution of lead under different fluctuation scenarios was predicted.The main research contents and achievements are as follows(1)The kinetic and thermodynamic experiments of lead adsorption / desorption were carried out.The results showed that the maximum adsorption capacity of fine sand,medium sand and coarse sand were 2366.6mg/kg,1847.6mg/kg and 1543.8mg/kg respectively.The second-order kinetic equation,elovich equation and parabolic equation were used for the adsorption kinetic equation.Elovich equation had the best fitting effect,and fine sand,medium sand and coarse sand had the largest desorption capacity 18 mg / kg,62.38 mg / kg and 81.6 mg / kg respectively The results of thermodynamic experiments show that the thermal mechanical properties of lead adsorption desorption in three media can be described by Freundlich equation,and the desorption thermodynamic equations of fine sand,medium sand and coarse sand are Freundlich equation,Temkin equation and Freundlich equation respectively The equation fitting effect is the best.(2)The effects of p H and organic matter on the adsorption and desorption of lead were studied.The results showed that: with the increase of p H of pollutant solution,the adsorption capacity of lead increased,the desorption capacity decreased,and finally tended to equilibrium;with the increase of p H,the difference between adsorption capacity and desorption capacity decreased;the average adsorption rate was coarse sand < medium sand <fine sand,and the average desorption rate was coarse sand > medium sand > fine sand The adsorption rate and desorption rate of fine sand change little with p H value,while the adsorption rate and desorption rate of medium sand and coarse sand change with p H value.The presence of organic matter will increase the adsorption capacity of the media.After removing organic matter,the adsorption capacity of the three media generally decreases.The adsorption capacity of fine sand and medium sand decreases significantly.The contribution rates of organic matter adsorption of coarse sand,medium sand and fine sand are 2.26%,3.37%and 12.06% respectively.(3)The results show that the p H value in the soil column is inversely proportional to the distance from the pollutant.From the top to the bottom,the p H value first decreases and then increases.The p H value at the 35 cm position is the lowest,which is consistent with the position of the pollutant.The average p H value of the same sampling port in the three soil media is coarse Sand < medium sand < fine sand,the stronger the physical adsorption capacity is,the smaller the content of free lead ion is,the weaker the ability to capture Oh-in solution is,which makes the p H of three media change with time: Coarse Sand > medium sand > fine sand;under the action of groundwater level rising and falling and capillary water,the lead in soil enters into water through desorption,and moves up and down with the fluctuation of water level And the stronger the adsorption capacity of soil medium,the weaker the migration capacity of lead in water and soil medium.The migration distance of coarse sand is greater than that of medium sand and fine sand.With the rise of water level,the upward migration distance is only about 5cm,and the downward migration distance is less than 10 cm The lower the p H,the greater the activity of lead,the more desorption,the higher the concentration of lead in the solution,the higher the concentration of lead in the groundwater at 35 cm,the farther the distance from the pollutants,the smaller the maximum content of lead in the groundwater,the interaction of groundwater level,p H and the concentration of lead in the pollution medium affect the migration process of lead in water and soil.(4)By using Hydrus software,a numerical model of lead migration in vadose saturated zone coupled with water and solute transport is established.The model is identified according to the experimental results of lead migration process under the condition of groundwater level fluctuation.The results show that the fitting effect is good,indicating that the model can well express the lead migration process.(5)This paper simulates the migration of lead in four different scenarios: the fluctuation of water level has a significant impact on the migration of lead.In the same medium,reducing the fluctuation range of water level and increasing the number of fluctuations will increase the concentration of lead in the solution in the same period of time,but the migration ability of lead is weak.Increasing the fluctuation range and reducing the number of fluctuations will increase the concentration of lead in the solution in the same period of time The migration ability of lead is strong.When the initial concentration is increased or decreased,the concentration of lead in the solution will increase or decrease,and the variation range is approximately the same as the multiple of increase or decrease.