Distribution And Genesis Mechanism Of High Arsenic Groundwater In The Guide Basin, Qinghai

Arsenic poisoning caused by high arsenic?As?groundwater has become one of the most serious environmental geological problems for international community.Source and genesis mechanism of high arsenic groundwater in the Guide Basin is currently unclear.Based on combination of field research and laboratory experiments,this paper explores the source,migration and enrichment mechanism of arsenic in high arsenic groundwater in the Guide Basin,which would not only help local government to find a safe alternative water source and reduce the threat of drinking water As poisoning,but also help improve our understanding of genesis and evolution of high As groundwaterin the basin and other regions with similar geological conditions.The main conclusions are shown as follows.Precipitation is the main recharge source for unconfined and confined groundwater in the study area.Chemical compositions of unconfined groundwater mainly comes from weathering of silicate and dissolution of carbonate in the aquifer,while those of confined groundwater is mainly controlled by dissolution of evaporite,which increases along the flow path.Arsenic concentrations in unconfined groundwater are low.High arsenic groundwater is mainly distributed in confined aquifer at depths from 100 to 300m,which show an increasing trend with depth and along the flow path.Chemical compositions of unconfined groundwater are variable seasonally,while chemical compositions of confined groundwater are temporally constant.Arsenic contents in sediments of the Guide Basin is higher than the average As content in the global sediments.The amount of As coprecipitation with amorphous and crystalline iron-manganese oxide minerals accounts for more than half of t total As in sediments,and there is a significantly positive correlation between total As contents and iron/manganese contents.They indicate that iron-manganese oxide minerals are the main carriers of As in sediments and the main source of As in high-As groundwater.There are significant correlation between TOC,NH₄-N,humification index?HIX?,fluorescence index?FI?,?¹³C_DIC-?¹³C_DOC and As concentrations in confined groundwater,indicating that release of As is closely related to degradation of organic matter.The degradation of organic matter continuously drives redox reactions in the aquifer,which makes reducing conditions,and results in reductive dissolution of iron-manganese oxide minerals and consequently As release into groundwater.In addition,the humus-like components produced by degradation of organic matter promote the migration of arsenic.Although the geothermal waters have low As concentrations,diffusion of their heat into confined aquifer promotes reductive dissolution of iron-manganese oxide minerals and increases As desorption,which release As into the groundwater.The hydrogeological conditions of confined aquifer are conducive to accumulation of As along the flow path,thus forming high As groundwater near the centre of the basin.