Study On The Source And Enrichment Mechanism Of Iodine In Groundwater Of Tarim Basin Oasis Zone

Tarim basin is a typical inland arid basin in northwest China.Due to the scarcity of surface water resources,groundwater is an important water source to maintain human survival and development and ecosystem balance in this area.The groundwater quality of oasis in the basin is generally poor,and the ecological environment is very fragile,which seriously threatens to local water security.Previous studies have shown that high iodine groundwater is distributed in this area,which is harmful to human health.Therefore,it is very important to systematically identify the distribution and genesis of iodine in groundwater.The oasis zone of Tarim Basin is taken as the study area,and the high-iodine groundwater in the oasis zone is taken as the research object.The spatial distribution and hydrochemical characteristics of high-iodine groundwater in different regions（recharge zone,transition zone and evaporation zone）and different aquifers（single-structure unconfined groundwater,unconfined groundwater in confined water area,shallow confined water and deep confined water）are analyzed.The formation of high-iodine groundwater is analyzed by correlation analysis,factor analysis and cluster analysis.The sources of iodine in groundwater and the main causes affecting iodine enrichment were determined by combining geological,hydrogeological conditions,groundwater occurrence environment and hydrogeochemical effects of oasis zone.The typical high-iodine groundwater area was selected for intensive sampling to obtain the data of hydrochemistry,deuterium oxygen isotope,iodine morphology,Cl/Br molar ratio,and further reveal the source of iodine in groundwater and iodine enrichment mechanism.（1）The iodine content of groundwater in oasis belt varies from<20.0μg/L to 4000.0μg/L,with an average of 53.7μg/L.7.3%of high iodine groundwater samples（iodine content>100.0μg/L）.The iodine content of groundwater in typical areas ranged from<40.0μg/L to 435.0μg/L,with an average of 123.5μg/L,and high iodine groundwater samples accounted for 50.0%.The main forms of iodine in groundwater in the typical area is mainly I^-（87.5%）,followed by IO₃^-（12.5%）,and there is no coexistence form.Sub-oxidation environment and sub-reduction environment are conducive to the existence of iodine element in IO₃^-and I^-forms,respectively.（2）The groundwater iodine content in oasis belt increases gradually from recharge zone to transition zone and then to evaporation zone,and the hydrochemical types of high iodine water are mainly SO₄·Cl and Cl.The high iodine groundwater is mainly distributed in Ruoqiang County and Yuli County of Bayingolin Mongolian Autonomous Prefecture in the southeast,Kuche County of Aksu Prefecture in the west,Shule County of Kashgar Prefecture in the southwest,and part of it is distributed in Minfeng County of Hetian Prefecture in the south and Yingjisha County of Kashgar Prefecture in the southwest.The groundwater iodine content is highest in shallow confined water,followed by single-structure unconfined groundwater and unconfined groundwater in confined water area,and is generally lower in deep confined water.（3）The potential incidence of high-iodine endemic goiter in the oasis zone ranged from 5.0%to49.1%.The potential incidence of high-iodine endemic goiter was lower in the transition zone than that in the evaporation zone（mean potential incidence 14.2%and 31.5%,respectively）.The potential incidence of high-iodine endemic goiter in shallow confined water was higher than that in single-structure unconfined groundwater and unconfined groundwater in confined water area（mean potential incidence 24.9%,9.5%and 5.7%,respectively）.The potential incidence of high-iodine endemic goitre was higher in Kuche County,Yuli County,Ruoqiang County,Luntai County and Minfeng County（mean potential incidence values were 36.5%,28.8%,24.1%,16.7%and 10.6%,respectively）.（4）Among the Quaternary genetic types,alluvial-lacustrine deposits provide the source of iodine enrichment in groundwater of oasis zone.Alluvial-lacustrine deposits provide a source of iodine enrichment in confined water in typical areas.Groundwater iodine was influenced by reductive dissolution of iron（oxy）hydroxide minerals and microbial degradation of organic matter.Fine-grained rocky soils,gentle topography,shallow groundwater burial conditions and weakly reducing environment are all conducive to groundwater iodine enrichment.Shallow high iodine groundwater is more influenced by evaporation,weathering dissolution of evaporated salt rock,and the dissolution state of rock salt in high iodine groundwater is more equilibrium.（5）The relationship between groundwater iodine content andδD andδ¹⁸O values in typical areas showed that high iodine groundwater was significantly more affected by evaporation than low iodine groundwater,and the Cl^-and Cl/Br molar ratio indicated that groundwater was mainly subject to rock salt dissolution.The overall dissolved organic carbon content of groundwater was low and the correlation between iodine content and dissolved organic carbon content was not significant.（6）The exponential fit between groundwater iodine content and Na⁺,Cl^-,SO₄^2-,TDS and TH in the basins with high iodine content of more than 5%in the oasis belt is better.The critical values of each index corresponding to high iodine content of groundwater（iodine content of 100μg/L）in different basins are determined according to the established fitting equation.