Study On Arsenic Migration And Transformation In The Unsaturated Zone Under The Influence Of Irrigation

The seasonal irrigation of high-arsenic groundwater can cause the accumulation of arsenic in the unsaturated zone,inhibit the growth of agricultural products,and damage the human health through food chain and skin absorption.Therefore,it is of great significance to study the migration and transformation of arsenic in the unsaturated zone under the influence of irrigation.In this paper,a typical high-arsenic groundwater area in the Datong Basin was selected to carry out a field irrigation experiment.The simulation experiment studied the hydrogeochemical processes and geochemical characteristics of sediments in the unsaturated zone under the influence of irrigation.Based on the research results,the migration and transformation mechanism of arsenic in the unsaturated zone under the influence of irrigation was discussed.Through the study,the main conclusions are as follows:1.There was a strong vertical mixing between irrigation return flow and soil pore water in the unsaturated zone during the irrigation,resulting in the increase of ionic strength in the soil pore water and the fluctuation of the oxidation and reduction conditions.The arsenic concentrations in the near-surface soil pore water increased during the irrigation and then decreased after the irrigation.The arsenic concentrations in the pore soil water at depth of 0².0 m during the irrigation were not exceed to 10?g/L,and arsenic in high-arsenic groundwater was fixed to the sediment of surface quickly due to coprecipitation or absorption.During the irrigation,the soil pore water was in a relatively reducing environment,As?V?was largely reduced to As?III?.After the irrigation,the soil pore water was in a relatively oxidizing environment,As?III?was oxidized to As?V?.2.The arsenic migration and transformation in the unsaturated zone under the influence of the irrigation is influenced by multiple geochemical processes.The adsorption/desorption of arsenic by soil particles in unsaturated zone is affected by pH,Ec,and the competitive adsorption between SO₄^2-/HCO₃^-and arsenic.And the fluctuation of redox conditions in the unsaturated zone influence arsenic concentrations in soil pore water.Among them,the fluctuation of redox conditions in the unsaturated zone might be the main controlling factor of the migration and transformation of arsenic in the unsaturated zone during the irrigation.The irrigation caused the conversion of the crystalline iron oxide minerals to the amorphous iron oxide mineral,and the amorphous iron oxide minerals are more easily dissolved in the relatively reducing environment.The dissolution of amorphous iron oxide minerals causes a large number of As?III?to move into the groundwater,resulting in a sharp increase in arsenic concentrations of the depth of 3.0 m of the shallow groundwater.3.The arsenic combined with the crystalline iron oxides is the main arsenic phases of sediments in the unsaturated zone,accounting for 50%to 60%of the total arsenic,followed by the arsenic combined with the ferrihydrite and the weak crystalline goethite,and the arsenic combined with carbonate and phosphate,while the concentrations of the adsorbed arsenic is lowest.After the irrigation,the arsenic combined with the crystalline iron oxides in the unsaturated zone increased significantly.And from the deep to the shallow,the crystalline iron oxides showed a gradual upward trend.The arsenic in high-arsenic irrigation water was rapidly fixed to the surface sediment mainly through co-precipitation with the crystalline iron oxides during irrigation,resulting in a significant increase in arsenic in sediment of the unsaturated zone.The arsenic combined with the ferrihydrite and the weak crystalline goethite and the arsenic combined with carbonate and phosphate were mainly dissolved in the irrigation.The concentrations of the adsorbed arsenic increased slightly after the irrigation.4.The irrigation led to the rise of the phreatic level,causing the unsaturated zone in a relatively reducing environment.The changes of the redox environment results in the conversion of the crystalline iron oxides to the amorphous iron oxide,and the dissolution of the amorphous iron oxides,resulted in the release of arsenic which combined with the amorphous iron oxides due to absorption or coprecipitation.As?V?was reduced to As?III?in the reducing environment and would migrate downward into the shallow groundwater.After the irrigation,groundwater level recovered,and a large amount of O₂ entered into the soil pores water,resulting in the heightening of oxidizing environment,while Fe?II?was oxidized to Fe?III?to form the crystalline iron oxide to fix the arsenic in the end.In conclusion,the cyclical change of the redox conditions induced by the fluctuation of the phreatic water level in the irrigation lead to the transformation of the iron mineral phases in the unsaturated zone,which in turn controls the migration and enrichment of arsenic in the unsaturated zone.The study reveals the mechanism of the migration and enrichment of arsenic in the unsaturated zone under the influence of irrigation.The results are of great significance to the prevention and control of arsenic exposure through the food chain in typical high-arsenic affected areas.