Wastewater-irrigation Region In Baiyin, Gansu: Plant Bioaccumulation And Release

Long-term farmland irrigation with wastewater containing high metal concentrations can pose threats to soil environment quality and cause metal accumulation in crops.The geochemical characteristics of metal migration and concentration between soil and vegetables are influenced by multiple factors,and the soil metal contamination can cause health risks to the human beings through the soil to plant translocation and the food chain.This study takes Baiyin City in Gansu Province is presentative wastewater-irrigated area as the research object.Combined with field sampling and laboratory analysis,we analyze metal form,concentration,metal biological utilization of different vegetables during their growth stage and being litter with different irrigation types and two kinds of cultivation modes in medium of soil(Rhizosphere and non rhizosphere soil)and vegetables(aboveground and underground part).Also,we explore the correlation analysis between soil physicochemical properties and metal form,concentration,metal biological utilization,respectively.The following are main conclusions:(1)Comparing the four vegetables in all studied areas,we found that there were large variations depending on the vegetable type,cultivation,and cultivation location.Significantly higher metal pollution severity was found in Dongdagou farmlands than Xidagou farmlands,and in greenhouse farmlands than open field farmlands.All editable vegetable metal concentrations from both cultivation modes commonly exceeded the standard.For each vegetable species,the editable vegetable metal concentrations had differences between different irrigation types and between different cultivation modes.Also,the metal translocation ability as reflected by bio-concentration factor was higher in leafy vegetables than the non-leafy vegetable,and higher in greenhouse than open-field.Overall,the metal concentration depends on vegetable species(leafy vegetables > non-leafy vegetables),tissue type(aboveground > belowground),and the edible biomass metal concentrations at Dongdagou were higher than those at Xidagou.(2)The potential ecological risk index method,comprehensive pollution index method,and the potential ecological risk early warning method were determined to assess the latest metal contamination levels of four types of common vegetables planted in the controlled environments,and we also calculated the health risk index to evaluate the potential human risks via vegetable ingestion.Results showed that potential risk levels for As,Cd were relatively high and As was the largest vegetable health risk contribution factor in Baiyin.For the total risk index,it was found higher in Dongdagou than in Xidagou.(3)We analyzed the soil metal concentration speciation,biological,organic matter,carbon and nitrogen based on greenhouse,open-field in Baiyin.Results showed that both the acid-soluble and reduced metal fractions were higher in greenhouse than open-field,which also agreed with the metal bioavailability.Comparing rhizosphere soil with non-rhizosphere soil,this study revealed that the quantity of metal in all studied soils was higher in rhizospheric soils than non-rhizospheric soils.(4)The in situ vegetable decomposition experiments of rape and leafy lettuce showed that the percent biomass remaining of root and leaf varying with time and agreed with exponential equation model.The whole decomposition process was fast at the beginning and gradually slowed down.The C and N element was released and no immobilization was found throughout the decomposition.Metal release rate was significantly positive correlation with the decomposition of vegetable residual rate.