| Due to the shortage of surface water resources,intense high-arsenic(As)groundwater extraction and irrigation activities are widespread.The irrigation process causes the fluctuation of water table,and induces the change of complex redox environment in the unsaturated-saturated zone,and the exogenous substances such as organic matter(OM)and sulfate are input with the infiltration of irrigation return water.The impact of high-As groundwater irrigation on As mobilization cannot be ignored.Therefore,it is of great theoretical and practical significance to explore the mobilization and transformation process of As in the unsaturated-saturated zone under high-As groundwater irrigation,and to study the environmental-geochemical behavior of Asunder irrigation activities.The unsaturated zone contacts with atmosphere directly.The upper interface of the unsaturated zone is the ground,and the lower interface is the groundwater phreatic level.After the surface water flow infiltrates into the unsaturated zone,some of them are temporarily absorbed by the soil and stored in the unsaturated zone as the soil water,and the other part forms underground runoff through infiltration.The material exchange process between the unsaturated zone and the outside is carried out at the upper and lower interfaces.The saturated zone is below the groundwater level and has continuous flow.Materials from the unsaturated zone to the saturated zone will undergo complex interface processes and dynamic changes,resulting in more complex physical and chemical processes of the unsaturated-saturated zone.Previous studies about irrigation activities using high-As groundwater are limited to the enrichment of As in aquifers and the influence of As content in crops and the crop yield.Little research was reported about As mobilization during the whole process of the extraction of high-As groundwater for irrigation activities into the unsaturated zone and then infiltrating into saturated zone.Datong Basin is located in the northern part of Shanxi Province,which is one of the Cenozoic faulted basins with the complex hydrogeological structure and a typical high-As groundwater distribution area in the world.Due to the scarcity of surface water resources,groundwater has become the main source for drinking and agricultural activities.The irrigation return flow and salt washing process as the important sources of shallow aquifer recharge in Datong Basin.The solute transport,hydrogeochemical evolution,and biogeochemical processes caused by strong irrigation activities are potential factors affecting the mobilization of As in unsaturated-saturated zone.In this study,the field simulated irrigation experiments were carried out in the high-As groundwater affected area,Datong Basin,and the column experiments were carried out in the laboratory to explore the mobilization and enrichment of As in the unsaturated-saturated zone under high-As groundwater irrigation activities.The spatial characteristics of As were identified,and the main hydrological and geochemical processes affecting the mobilization and enrichment of As were studied in order to enrich and improve the understanding of the geochemical cycle processes of As under irrigation.At the same time,this study provides scientific basis for rational exploitation and application of groundwater in the high-As affected areas.The main progresses of this study are as follows:1.The field irrigation experiments was conducted in Datong Basin.Combined with hydrological and geochemical simulation,the results revealed that the water level fluctuations induced by irrigation activities caused severe redox environment changes in the unsaturated-saturated zone,and the important influence of Fe minerals on As mobilization and transformation is identified.The transformation of Fe minerals during irrigation activities is quantified.It is proposed that the input of exogenous substances produced by irrigation will drive the geochemical reaction process of As.The fluctuation of water table caused by irrigation may influence the underground redox conditions,thus affecting the geochemical process of As and iron(Fe)minerals in the unsaturated-saturated zone.Therefore,it is necessary to study the mobilization and transformation mechanism of As during irrigation in unsaturated zone and the induced water table fluctuations.(1)Through field irrigation experiments,multi-stage hydrological and geochemical comprehensive monitoring and water chemical composition characteristics,hydrogen and oxygen isotope and As speciation analysis were conducted.The contents of As,Fe and As speciation extraction in sediments and the spatial distribution of As and Fe minerals during high-As groundwater irrigation were identified.The main controlling factors of As mobilization during irrigation were found out,and the process of As mobilization during irrigation was described.Based on the results of hydrogeochemical analysis of water samples and sediment samples,the main controlling factors of As mobilization during irrigation were found out.Both aqueous As and solid As changed during irrigation and irrigation return flow affected the migration and transformation of As and Fe minerals in the unsaturated zone.Irrigation process had different effects on the main ion concentrations of soil water and shallow groundwater.The infiltration of irrigation water into the unsaturated zone leads to the fluctuations of water table,the change of redox conditions and the redistribution of As and Fe contents in topsoil.The geochemical analysis of soil sediments showed that the contents of As and Fe in topsoil increased significantly after irrigation.Before irrigation,the contents of As and Fe in 0.2 m and 0.4 m underground were7.35 mg/kg and 7.03 mg/kg,16.64 g/kg and 20.42 g/kg,respectively.After irrigation,the contents of As and Fe in 0.2 m and 0.4 m underground were 7.88 mg/kg,7.65 mg/kg and22.57 g/kg,respectively.Most As and Fe(hydro)oxides coexist before irrigation,but sediment extraction results confirm that As and Fe minerals are decoupled after irrigation.Irrigation water carries OM and other exogenous substances into the ground,affecting the(bio)geochemical process of the unsaturated-saturated zone,and ultimately affecting the mobilization and transformation pathways of As and Fe.(2)Geochemical model results suggest desorption,leaching and cation exchange processes are responsible for the temporal changes of As concentrations in the pore water,groundwater,and sediments during irrigation.Before irrigation,the unsaturated zone is in full contact with air(oxidizing conditions)and the phreatic water table is located?1.7 m below the ground surface.The correlation between As and Fe in sediment shows As co-exists with crystalline Fe(III)(hydro)oxides,and the anoxic conditions prevalent in the saturated zone have a minor effect on As immobilization.Arsenic in irrigation water is directed adsorbed by Fe(III)(hydro)oxides due to the oxidation of Fe(II)present in the irrigation water in the initial stages of irrigation.The oxidation of the small amounts of Fe(II)in the irrigation water to Fe(III)(hydro)oxide results in the adsorption saturation being reached quickly and fast vertical infiltration to the saturated zone;As and Fe contents in the topsoil noticeably increase but the aqueous As concentration does not conspicuously change.With pores gradually occupied by irrigation water concurrent with a rise in the water table,redox conditions in the unsaturated zone change from oxidizing to moderately reducing.As such,the dominant As species changes from As(V)to As(III),and the weaker association between As(III)and Fe(III)minerals causes the release of As from solid phases to the aqueous phase.With irrigation water infiltration,other materials and OM are carried from the surface to the subsurface.OM can enhance redox reactions,resulting in the reduction of Fe(hydro)oxides that,in turn,releases dissolved As.Due to the gradual dissolution of salts of sediments,the conductivity of soil water changed obviously,accompanied by cation exchange process.When irrigation stopped,the residual irrigation water on the land surface is in full contact with the air.Therefore,the infiltration of the residual oxidizing water decreasing results in the gradual increase of Eh values detected from the soil water.The stronger oxidizing conditions strengthen the absorption of As onto Fe(III)minerals,as reflected in gradually decreasing concentrations of aqueous As.The results indicate that flood irrigation using As-contaminated groundwater should be controlled and gradually replaced by drip irrigation,sprinkler irrigation or irrigation using non-contaminated water resources to mitigate As accumulation in the unsaturated zone and shallow groundwater.2.Hydrogeochemical processes of OM affecting the mobilization of As and Fe minerals under high-As groundwater irrigation were characterized by three-dimensional fluorescence spectroscopy(EEM),Fourier-transform infrared(FTIR)spectra and hydrogeochemical methods.The existence of As-Fe-OM ternary complex was found,and the effect of OM on the formation of As-Fe-OM complex during high-As irrigation was revealed.High-As groundwater irrigation can also lead to the input of OM.How to control the mobilization of As in the unsaturated-saturated zone under irrigation conditions has not been studied.Therefore,it is urgent to explore the OM components of the unsaturated-saturated zone and study the geochemical process of As under the influence of OM during irrigation.(1)Spatial distribution of As,Fe minerals and OM during high-As groundwater irrigation was identified by a smaller scale filed irrigation experiment.Fluctuations of redox conditions and complex geochemical processes during irrigation led to the release,mobilization and redistribution of As.Irrigation activities led to the accumulation of aqueous As in the saturated zone and increasing As content in the topsoil.Arsenic concentrations in the water samples varied from 0.75 to 228?g/L;all the water samples with high As concentrations being from a depth of 3.0 m.The total As content in sediment samples varied from 5.81 to 7.72 mg/kg pre-irrigation to 5.36 to 7.86mg/kg post-irrigation.As content from 0 to 1.20 m below the ground surface increased after irrigation,while As content from 1.25 to 1.40 m decreased after irrigation.(2)The effect of OM on As and Fe minerals mobilization under high-As irrigation was revealed combing with the changes of As,Fe minerals,and OM and the analysis of EEM,FTIR spectra during the whole process.Analysis of parallel factor analysis(PARAFAC)of EEM could better describe the source and composition of water extractable OM(WEOM)in sediments.The result reveals that WEOM of sediments comprises three major components,two terrestrial humic-like components(C1 and C2)and one protein-like component(C3).The better correlations between As,Fe content,and the relative abundance of C2 after irrigation clearly reflect the critical role of terrestrial humic-like components with high molecular weight,which promotes As-Fe-OM complex reactions.FTIR spectra suggest the existence of As-Fe-OM ternary comple and the disassociation of As-Fe-OM ternary complex after irrigation is confirmed.Before irrigation,As(V)is the dominant As species,and As in sediments co-exists with both Fe minerals and OM.The unsaturated zone is characterized by oxidizing conditions and the phreatic water table is 1.51 m below ground level.When irrigation activity begins,As,OM,some Fe(II),and CO2 in the irrigation water are transported to the subsurface.The oxidation of Fe(II)happens suddenly,with As in the irrigation water then being adsorbed by Fe(III)minerals.With continued irrigation,As(III)gradually becomes the main As species in the relatively reducing conditions and demonstrates greater mobility,as demonstrated through hydrogeochemical parameters that change with irrigation infiltration.The input of labile OM from the irrigation water could affect the redox conditions and mediate Fe(III)-bearing minerals undergoing reductive dissolution and disassociation with As,thus contributing increasing amounts of aqueous As that is transported to greater depths.The dissolution and recrystallization of salts in the unsaturated zone caused by irrigation activities lead to the mobility of cations,which further results in the disassociation of As-Fe-OM ternary complex and the formation of As-metal-OM complex.When irrigation ends,the residual irrigation water on the land continues to infiltrate after prolonged contact with air.Due to the stronger oxidizing conditions,As(V)concentrations increase,and more As can be immobilized onto Fe minerals,as reflected in the increasing As content in sediments in the unsaturated zone.The frequent oxic-anoxic fluctuations introduced by the irrigation activity enhance the mobility of As and increase concentrations in the topsoil.3.By methods of X-ray diffraction(XRD),FTIR,scanning electron microscopy(SEM),Raman spectroscopy,X-ray photoelectron spectroscopy(XPS)and other comprehensive analyses,the interaction process of As,Fe and OM under irrigation conditions was described through laboratory column experiments.The effect of OM on As mobilization and transformation under different irrigation conditions was explored,and the process of As desorption induced by OM and water flow under the influence of irrigation was revealed.OM often combines with Fe minerals and affects the mobilization of As.Different C/Fe molar ratios can make Fe(hydro)oxides exhibit different properties.Humus is a common type of OM.The functional groups of humic acid(HA)have a significant impact on metal oxides,with high reactivity and strong binding ability,affecting the enrichment of metal oxides.In order to understand how different reaction pathways and OM concentrations affect Fe(hydro)oxides and further regulate the geochemical process of As,the ferrihydrite coated sand,sodium humate and As(III)were selected for column experiments to reveal the reaction mechanism of As-Fe-OM under different reaction pathways.Experiment 1:The columns were filled with ferrihydrite-coated quartz sand.Irrigation water of the reaction column(column A)was containing HA,As(III),and Na Cl,and that of the control column(column B)was Na Cl solution.Experiment 2:The columns were filled with As-ferrihydrite-coated quartz sand.Irrigation water of the reaction columns(column C and D)were containing two different concentrations of HA,As(III),and Na Cl,and that of the control column(column E)was Na Cl(2 m M)solution.(1)Through the concentrations of As,Fe,and OM in liquid phase and the contents of As and Fe in solid phase during the experiment,the mobilization of As under irrigation was identified,and the influence of OM and water flow on the adsorption and desorption process of As was revealed.Arsenic mobilization is strongly controlled by the adsorption of Fe minerals,and which is affected by the presence of OM.Irrigation simulated experiments were conducted under different scenarios by columns.In the pure ferrihydrite-coated sand column,the outlet As and HA concentrations gradually increased and reached a stable state.In the ferrihydrite-As-coated sand columns,the release of As happened with HA infiltration,and the HA concentrations influenced the release rate of As from the ferrihydrite.The rapid decrease of desorption capacity indicates that some As(III)fixed on the surface of the ferrihydrite is in a highly unstable state,so it can respond quickly to the fluctuation of environmental conditions.The rapid stabilization of the release amount indicates that the residual solid As may be adsorbed and form a more stable inner-sphere complexation.As(III)is bound to Fe(hydro)oxide via inner layer complex with different strengths,fixed with outer layer complex and hydrogen bonds,resulting in the formation of weak complex with low strengths,which are more sensitive to the disturbance of liquid concentrations introduced by the flow.(2)The existence of As-Fe-OM ternary complex was revealed by methods of microscopic mineral characterization.The transformation of Fe minerals promoted by OM during irrigation and the adsorption and desorption of As were analyzed.Changes of Fe minerals and the ternary complex of As-Fe-OM were detected through mineral characterization.The inhibition of As adsorption in the presence of HA occurs mainly because the HA can compete with As for the available binding sites on Fe(hydro)oxide surfaces,and the changes of Fe minerals decreased the binding sites with As.The results suggest that OM interacts with As and Fe minerals promote As mobility and the concentrations of OM influence the release amount of As.Although competitive adsorption occurred in the presence of OM,the ternary complex of As-Fe-OM affected the transformation and mobilization of As.Clarifying the main factors affecting As mobilization during the high-As groundwater irrigation process and revealing the mechanism of As mobilization in the unsaturated-saturated zone during the irrigation process are the important theoretical basis for improving the rational development and utilization of groundwater in the high-As groundwater affected area.Therefore,this study focuses on the spatial distribution characteristics of As under high-As groundwater irrigation,revealing the mobilization and enrichment mechanism of As,which can provide scientific basis for the management of As or As,OM contaminated groundwater under agricultural activities,and provide new ideas for the geochemical process of As in the unsaturated-saturated zone.The new understanding has reference significance for the mobilization and enrichment mechanism and exposure pathways of As in the unsaturated-saturated zone of high-As groundwater affected areas in China and abroad,and has important theoretical and practical significance for the rational development and utilization of groundwater,water supply and food safety in these areas.The present study revealed the mechanism of the interactions among As-Fe-OM in the unsaturated-saturated zone affected by high-As groundwater irrigation–induced water level fluctuation. |
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