Study On The Indicator Effect Of Rare Earth Elements On Groundwater Pollution

Wastewater treatment plant（WWTP）effluent is the main way of urban domestic sewage discharge,however,there are still a certain amount of pathogens and organic pollution components in the treated sewage,accompanied by surface water infiltration into the groundwater,posing a threat to the groundwater environment.Rapid identification of the impact of WWTP effluent on groundwater has become an important aspect of prevention and control of groundwater environmental problems.In recent years,anthropogenic rare earth elements（REEs）have been frequently detected in the global water environment,and anthropogenic gadolinium（Gd）has become the most representative REE in WWTP effluent.A series of studies have reported the conservative migration of anthropogenic Gd in the environment,indicating that positive Gd anomaly is a good WWTP effluent tracer.However,the effect of anthropogenic Gd input on the distribution of natural rare earth elements in groundwater and the migration behavior of anthropogenic Gd itself are not fully understood.Therefore,this article takes the Chaobai River upstream WWTP discharge area as the study area,surface water and groundwater as the research object,reveal the anthropogenic Gd input to the effect of natural distribution of rare earth elements in the groundwater,on the basis of the analysis of water chemical characteristics,studied the artificial Gd indicates a role on the groundwater pollution,and ultrafiltration experiment was used to study the occurrence form of anthropogenic Gd,for the study of anthropogenic Gd in surface water to the groundwater in the process of migration behavior provides the basis.Through the analysis of the hydrochemical indexes and the distribution characteristics of rare earth elements,the following understandings were obtained:（1）The chemical characteristics of groundwater indicate that the groundwater in the upper and lower reaches of the study area were polluted to different degrees.The analysis of groundwater chemical indexes showed that the upstream groundwater chemical type were HCO₃·Ca,the downstream groundwater chemical type were HCO₃·Na,HCO₃·Ca.Compared with the background groundwater,the upper and lower reaches of groundwater were polluted to different degrees.The interpretation of groundwater pollution in the study area based on hydrochemical indexes were uncertain,and the identification of pollution sources required more concrete evidence.（2）Artificial positive Gd anomaly can be used as an indicator of the influence of WWTP effluent on groundwater.The positive Gd anomaly in the upstream groundwater were obvious and gradually decreases with the anomaly in the flow direction basically.The positive Gd anomaly in the downstream groundwater were significantly lower than that in the upstream,which is related to the source of WWTP effluent and the hydrogeological conditions.The groundwater pollution based on Gd positive anomaly recognition were consistent with the results based on hydrochemical indexes.There are obvious differences in hydrogeological conditions between the upper and lower reaches.The upstream region had good permeability,and the Gd concentration of WWTP-A effluent were higher than that of the downstream WWTP-B effluent.Although there are significant differences in the upstream and downstream hydrogeological conditions and pollution sources,ultrafiltration experiments showed that anthropogenic Gd were different from the particle reactivity of natural rare earth elements,and it were basically conserved in the environment.Therefore,Gd were abnormal to be used as a tracer.（3）The content of rare earth elements in groundwater were mainly affected by p H value and bicarbonate complexation.The content of rare earth elements were negatively correlated with p H value and positively correlated with HCO₃^-concentration.The groundwater in the upstream region were rich in HREEs,and the correlation analysis showed that it may be due to the superposition of multiple effects such as bicarbonate complexation and Fe/Mn colloid adsorption...