Experimental Study On The Effect Of Soil Macropore Flow On The Solute Transport Process Under Simulated Rainfall Conditions

The preferential flow formed by the large pores in the soil is the main cause of pollutant migration and groundwater circulation pollution,and it is a common form of soil water movement Due to the ubiquity of macropores in the soil,the flow of water and solutes preferentially forming macropores through these channels has a significant impact on the transport of water and solutes in the soil.Based on soil column tests with seven different macropore configurations,combined with the CXTFIT software developed by the U.S.Salinity Laboratory,this paper systematically studies the preferential flow under rainfall from two aspects:the effective surface porosity of the macropores and the different connectivity of the macropores.The effect of macropore flow on the flow and solute transport under rainfall conditions,and the quantitative evaluation index PFSP of preferential flow is used to clarify the mechanism of the occurrence of preferential flow by macropores.The main research results and conclusions are as follows:1)The response relationship of the soil column with large pores to the rainfall intensity is very distinct.In the case of the same rainfall duration,the greater the rainfall intensity,the shorter the initial outflow time and the end outflow of the bottom pores of relatively developed soil pillars with large pores,and the greater the maximum outflow rate.The water content of the upper soil did not increase with the increase of pore development during the rainfall period,but the lower the water content of the upper soil.The soil water content of the middle layer does not change significantly with the rain fall intensity,and from the time the rainfall intensity starts to decreases to a period of time after the rainfall ends,the soil column with a larger surface porosity results in a lower soil water content.2)The Br-penetration curve of the soil column with large pores has obvious asymmetry and tailing characteristics.With more developed macropores,this characteristic becomes more obvious.Relative to the uniform soil column with equilibrium matrix flow,the soil columns with macroporous surface of 0.27%,1.42%,and 3.47%account for 71.21%,76.8%,and 81.08%of the total leaching amount of Br-cumulative leaching from soil columns,respectively.The preferential flow carrying Br-migration of the soil columns with the upper connected,lower connected,disconnected and penetrating macropores account for 76.8%,52.54%,55.73%and 83.17%of the flow of uniform soil column,respectively.It shows that the contribution of soil preferential flow in the process of Br-migration cannot be ignored.3)The numerical model software CXTFIT is used to simulate the breakthrough curves of different soil columns.The results show that the hydrodynamic dispersion coefficients of the soil columns corresponding to the effective surface porosity of 0.27,1.42,and 3.47 are 2.55 times,3.07 and 4.35 times that of the uniform soil columns,respectively.The retardation factor R decreases with the decrease of surface porosity,and the proportional factor β for the moving water zone and the mass exchange coefficient ω of the two zones increase with the increase of surface porosity.For the connectivity of macropores,the hydrodynamic dispersion coefficients D of the upper,lower,disconnected,and penetrating soil columns of the macropores are 3.07,2.75,2.7,and 7.13 times of the uniform soil column,respectively.The better the development of the macropores,the smaller the retardation factor R,the larger the proportional factor β of the moving water zone,and the larger the mass exchange coefficient ω between the two zones.4)Based on the indoor test results and the parameters obtained by CXTFIT fitting,combined with the method of time moment,the contribution of the preferential flow to the breakthrough curve is distinguished from other contributions.By calculating the PFSP values for different soil columns,it is concluded that the largest contribution rate to the penetration curve extension is the preferential flow effect μ₂^pf,followed by the two-zone effect μ₂^tr,and the diffusion coefficient effect of μ₂^dis on the breakthrough curve the contribution rate is the smallest.At the same time,the PFSP value can be used to quantitatively characterize the contribution of preferential flow.