| Rainfall infiltration is the process by which rainwater forms soil water.Studying the process of rainfall soil water leakage is an effective way to understand the mechanism of soil water cycle,which will contribute to the rational regulation of water resources.The process of soil water leakage will carry soil solutes into groundwater.As indicators for monitoring groundwater quality,Cl-and NO3-have strong mobility in the soil and are prone to leaching with water,resulting in pollution of groundwater quality.Therefore,studying the interaction between rainfall soil water leakage-Cl-and NO3-loss,exploring the characteristics and connections of soil water leakage and Cl-and NO3-migration processes,is of great significance for the prevention and control of groundwater pollution.In view of this,this study focuses on undisturbed soil columns and conducts research on soil leakage and Cl-,NO3-leaching and migration processes based on natural rainfall experiments.The research results provide theoretical support for the optimization and control of water resources and pollution source prevention.The main conclusions of the paper are as follows:（1）The analysis of soil leakage process and the response of Cl-and NO3-to rainfall shows that in unsaturated soil,the flow rate in the flow hydrograph is dominated by soil moisture content during the slow rising period,the flow rate is dominated by rainfall intensity during the fast rising period,and the flow rate is dominated by soil water release capacity during the slow fading period.When the maximum rainfall intensity occurs,the effect of preferential flow is obvious and water is continuously replenished to the soil matrix.After the rainfall intensity weakens,the soil matrix continuously releases water to the dominant area.In the later stage of outflow,the matrix flow begins to dominate and produces a"trailing"effect of growth time.As the rainfall intensity increases,the leaching concentration of soil Cl-and NO3-decreases,indicating the dilution effect of rainwater on soil Cl-and NO3-.（2）The experimental results of the influence of different soil type on the seepage process show that during rainfall,the water transfer mode of mountainous brown soil will change from preferential flow to preferential flow and substrate flow;The water migration mode of purple soil is first dominated by preferential flow,followed by supplementary outflow of old water from the soil matrix,and later dominated by matrix flow;The water migration mode of dark brown soil in mountainous areas is dominated by matrix flow.（3）The influence of different soil type on the dynamic process of Cl-and NO3-leaching concentration showed that the Cl-leaching concentration in mountain brown soil was mainly dominated by rainwater dilution,hydrodynamic dispersion,and soil transportable Cl-content at different stages;The concentration of Cl-leaching in purple soil is mainly dominated by the replenishment of old water,hydrodynamic dispersion,and the content of Cl-that can be transported in the soil at different stages.There is a clear phenomenon of soil old water replenishment,which is caused by clay particles causing soil swelling and reducing the preferential flow path;The Cl-leaching concentration in mountainous dark brown soil is mainly influenced by soil Cl-content and water migration mode at different stages.The NO3-leaching concentration in mountainous brown soil is the most sensitive to flow changes,while the NO3-leaching concentration in purple soil has no obvious pattern.The NO3-leaching concentration in mountainous dark brown soil is not sensitive to flow changes.（4）The assessment of water quality in terms of Cl-and NO3-in different soil type shows that the average concentration of Cl-leaching in the three soil type is less than 10mg/l,the risk of Cl-leaching is small,and all can play a role in purifying Cl-;The risk of NO3 leaching concentration in three types of soil type showed different situations.The average NO3-leaching concentration in mountainous brown soil within 90 cm soil layer was21.3-28.8 mg/l,which could not fully play the role of water purification;Purple soil at a depth of 60 cm can fully exert its water purification effect,while purple soil at a depth of 90cm carries a risk of NO3-leaching;The NO3-leaching concentration in mountainous dark brown soil within a 90 cm soil layer range is between 0.3 and 3.6 mg/l,which can fully exert the water purification effect.（5）The analysis results of different soil layers on the leakage process and Cl-,NO3-leaching concentration indicate that there is a lag process of water migration from top to bottom.The deeper the soil layer is,the stronger the weakening effect of the soil layer on rainfall,and the stronger the soil’s water storage capacity.The variation trend of Cl-leaching concentration in the 90 cm soil layer of purple soil is mainly led by the supplementation of old soil water and hydrodynamic dispersion,showing an upward trend,while the variation trend of Cl-leaching concentration in the 60 cm soil layer is mainly led by the dilution effect of rainwater,showing a fluctuating downward trend;The trend of Cl-leaching concentration in the 60 cm soil layer of mountainous dark brown soil changed from the dominant role of hydrodynamic dispersion to the dominant role of rainwater dilution,showing a stable fluctuation,while the trend of Cl-leaching concentration in the 90 cm soil layer decreased due to the dominant role of rainwater dilution.In summary,in unsaturated soil,the process of soil water transport under different rainfall periods is dominated by soil moisture content,transitioning to dominated by rainfall intensity,and then to dominated by soil water release capacity.The migration of Cl-and NO3-is more affected by the dilution effect of rainwater;In addition,the difference of soil internal properties under different soil type and soil layer thicknesses significantly affected the flow hydrograph shape and solute transport process,resulting in the change of the dominant factors over time.The above results provide theoretical support for the rational use of water resources and the prevention and control of groundwater pollution. |
