Influence Of Salt-isolation Layer On Water And Salt Tansport In Coastal Saline Soil And Numerical Simulation Study

The coastal saline soil in the Yellow River Delta region of China is an important reserve land resource for expanding the town area and ecological environment construction.Due to the shallow groundwater depth,high mineralization and severe soil desalinization in coastal saline soils,the utilization of land resources in this area is restricted.In this study,peat and biochar were used as soil conditioners to form a salt-isolation layer,and soil moisture sensors were placed on the upper and lower sides of the salt-isolation layer through an indoor soil column infiltration test to monitor the changes of soil moisture and soil salinity at this location,and to record the transport of wetting fronts in the test column.Then,the one-dimensional water infiltration process was numerically simulated using the Hydrus-1D model.The measured soil water content and soil salinity during infiltration were used to numerically invert and optimize the soil hydraulics parameters for each treatment,and the fitting ability of the model was evaluated.Finally,the optimized Hydrus-1D model was used to investigate the effects of different salt-isolation layer materials(peat and biochar)and different peat thicknesses(0 cm,3 cm and 6 cm)on the infiltration of soil water and the dynamic distribution of water and salinity,and to briefly analyze the transport mechanism.The main findings are as follows.(1)The changes of moisture front transport,cumulative infiltration amount and soil salt distribution by different treatments of salt-isolation layer materials and thicknesses were investigated through an indoor soil column one-dimensional infiltration test.It was shown that both peat and biochar salt-isolation layer had the effect of reducing infiltration and salinity.The effect of the peat layer was the most obvious,and the effect was proportional to the thickness of peat,while the effect of the biochar layer was not so effective.(2)Calibration and validation of Hydrus-1D model.The actual water content,soil salinity,saturated water content(θr)and saturated hydraulic conductivity(Ks)measured by partial indoor soil column water infiltration tests were used to obtain soil hydraulic parameters such as residual water content(θr),soil shape factor(n)and suction value correlation coefficient(α)by inversion.The inversion parameters were compared with the measured values and good results were obtained in the validation stage.The Hydrus-1D model can accurately describe and analyze the dynamic changes of moisture and the distribution of moisture and salinity during the infiltration phase of soils set with a salt-isolation layer.(3)Among the inversion parameters using Hydrus-1D,parameter α(soil suction value coefficient)and parameter n(soil shape coefficient)vary most significantly,and the buried salt-isolation layer has a certain degree of influence on the sensitivity of both parameter α and parameter n.Among them,the peat salt-isolation layer has a certain degree of influence on the sensitivity of parameter α and parameter n.Among them,the sensitivity of the peat salt barrier was greater than that of the biochar salt-isolation layer,and the effect on the sensitivity of parameter α was lower with the increase of peat thickness,and the effect on the sensitivity of parameter n was higher and higher.(4)The Hydrus-1D model was used to investigate the effect of the salt-isolation layer on soil moisture and salt transport.The results showed that compared with homogeneous soils,the installation of peat and biochar salinity compartments in the soil changed the spatial distribution pattern of soil water and salt,and water accumulation occurred above the salinity compartments.This increases the residence time of water in the upper soil layer during infiltration,allowing sufficient time for the salt in the soil to dissolve,thus improving the efficiency of salt washing.The salt washing effect of the peat salt-isolation layer was outstandingly better than that of biochar,and the thicker the peat salt-isolation layer,the more obvious the effect was.The Hydrus-1D model can accurately simulate the soil water and salt transport in the presence of a salt-isolation layer,and a peat barrier with a thickness of 6 cm is a more ideal treatment method.It can improve the water content of the soil on the upper side of the salt-isolation layer and reduce the soil salinity in the process of water infiltration,which can provide a theoretical basis for the improvement of coastal saline soils.