Genesis Of High Arsenic Groundwater In The Plain Along The Yangtze River (Anqing Reach)

Anqing,which located in the southwest of Anhui Province,is an important node of the Yangtze River Economic Belt.Due to economic development,the pollution of surface water in this region has become more and more serious,and local residents rely more on groundwater.In recent years,it has been found that there is high arsenic groundwater in some areas along the Yangtze River,but no relevant research has been carried out on the genesis of high arsenic groundwater.The reductive dissolution of iron(hydrogen)oxides with microorganisms participation is the key mechanism for the formation of high-arsenic groundwater.In this biogeochemical process,different kind of dissolved organic matter has distinct effect on arsenic enrichment because of the disparate in bioavailability.Therefore,it is necessary to think about the function of dissolved organic matter when study the high arsenic enrichment mechanism.This paper takes the Yangtze River Plain(Anqing Section)as the study area.Based on the research of the regional groundwater cycle,the chemical characteristics of high-arsenic groundwater are identified,and the mechanism of arsenic enrichment with dissolved organic matter participation is analyzed in order to in-depth understanding of the causes of high-arsenic groundwater and ensuring the safety of groundwater quality.The main research of this study are as follows:1.Groundwater in the study area is mainly recharged by precipitation and tributaries of the Yangtze River such as the Wanhe River,and is discharged into the Yangtze River from northwest to southeast.The ¹⁴C age of groundwater from the exit to the plain increased to 9542,forming a relatively stagnant area,laying the foundation for the formation of high-arsenic groundwater.2.The chemical type of groundwater in the study area is HCO₃-Ca,and the As content in unconfined groundwater is significantly higher than that in confined groundwaterwater.High-arsenic groundwater exists in a weak alkaline reducing environment,and mainly distributes in the sand and gravel aquifer within 10-40m near Wanhe Farm.3.The content of effective arsenic in sediments in high-arsenic groundwater area is significantly higher than that in low-arsenic groundwater area.The arsenic in sediments mainly exists in the form of iron-manganese oxide combined state,which is an important source of groundwater arsenic.4.From the recharge area to the discharge area,denitrification,iron(hydrogen)oxide reduction and sulfate reduction take place sequentially.The reductive dissolution of iron(hydrogen)oxide causes arsenic which adsorbed on the surface of the sediment to enter groundwater.5.The content of microorganism-derived humus(C3)and protein-like(C4)in the deep unconfined groundwater is higher and has more active microbial activity.It can provide energy for microorganisms to promote reductive dissolution of arsenic-containing iron(hydrogen)oxides.The contribution of DOM reductive dissolution to arsenic release from sediments is greater than that of DOM complexation.