Study On The Influence Of Groundwater Level Raising On The Salt Distribution Of Surface Loess

Salt accumulation in the loess occurred in Heifangtai of Gansu Province and Gutun of Shanxi Province in the Loess Plateau of China which is affected by human engineering activities.This phenomenon is due to a large amount of irrigation and human engineering activities that continuously raise the groundwater level.The soluble salt in the soil will increase the salt content of the groundwater,and the salt will enter the surface of the loess as the groundwater rises,salt keeps accumulating under the influence of evaporation.The increase of surface loess salinity in the study area leads to the decline of cultivated land quality,which directly affects crop yield.Therefore,studying scientific issues such as the distribution characteristics of surface loess salinity,changes in salinity content,and prevention of loess salinity are important to the planning of cultivated land and disaster prevention.This study focuses on the salt accumulation in Heifangtai and Gutun in the Loess Plateau of China,and studies the effects of groundwater raise on the distribution characteristics of surface loess salinity,changes in salinity content,and prevention of loess salinity.Firstly,conducting chemistry experimental analysis of on-site samples,combining with the relationship between salt content and conductivity,and through interpolation calculations,the distribution characteristics and influencing factors of the surface loess salinity were studied.Secondly,ERT was used to detect the groundwater in the middle of terrace.Carrying out indoor physical and chemical experiments to analyse the microstructure and salinity of the loess after soaking,study the relationship between groundwater depth and loess salinity and reveal the reasons for the loess salinity changes.Finally,conducting the experiment of the loess resistivity between the water content and the salt content by using the indoor remodeling loess,which obtained the model of salinity.By verifying the electroosmotic drainage scheme on the spot,the feasibility of reducing the groundwater level in Heifangtai was proposed.The main conclusions are as follows:?1?The components of saline soil and groundwater in the Gutun trench are mainly sodium ions and chloride ions,which are close to each other.The salt content of the loess increases linearly with the increase of the conductivity.GIS spatial analysis shows that the salt in the trench is mainly distributed upstream,the salt content in downstream decreases,and the highest salinity value is 11.4g/kg.The salinity distribution of the trench is affected by groundwater raise,trench reconstruction,and vegetation.?2?The components of saline soil,groundwater and spring in Heifangtai are mainly sodium ions,chloride ions and sulfate ions,which are close to each other.The salt content of the loess increases linearly with the increase of the conductivity.GIS spatial analysis shows that the salinity distribution of the surface loess is consistent with the groundwater depth,and mainly distributed in the central irrigation area,the highest salinity value is 19.3g/kg.The salinity distribution of terrace is affected by irrigation and groundwater raise.?3?The groundwater in the central part of the terrace rises at an average annual rate of 0.5 meters and the rise rate is higher than other parts,and the lowest depth of groundwater is 2 meters.The change of the salt content of the loess at the sampling point corresponds to the groundwater level,the content of salt in loess decreases with a first-order exponent as the groundwater depth increases,groundwater raise is the main cause of salinization.?4?The groundwater raise leads the loess submerged in the groundwater.The permeation test of the loess sample after soaking shows that the permeability of the loess decreases after soaking,and the content of fine sand particles decreases,and the content of silt and clay particles increases during the soaking.The pores also change from large pores to small and medium pores,these changes in permeability and microstructure retard water migration.With the change of soaking days,the soluble salt in the loess continues to dissolve into the groundwater,and the salt will enter the surface of the loess as the groundwater rises.The mineral composition in the surface loess also changes with the groundwater raise.Clay minerals show a decreasing trend with depth,and the content of potassium feldspar?K[Al Si₃O₈]?and calcite?Ca CO₃?increase.?5?The resistivity test shows that the resistivity of the loess decreases with increasing salt content and moisture content,and the increase in water content reduces the degree of resistivity affected by the salt content.The regression analysis of resistivity test data yields the relationship between water content,salt content,and resistivity,and transforms to the salt estimation formula=??+0.51w-12.161?/?0.0987w-2.386?,the formula is verified by remodeling samples,and the salt estimation error ranges from-0.86%to 12.12%.?6?According to the electroosmotic drainage test,the groundwater at the cathode site is discharged from the surface,and a large amount of water accumulation occurred.The apparent resistivity is lower than before in the ERT profile,the range of apparent resistivity is 1.2 meters,which is consistent with the electrode burial depth.This method has good applicability to reduce groundwater.