Study On Environmental Hydrogeochemical Characteristics And Selenium Enrichment Mechanism Of Groundwater In Handan

Selenium（Se）has provoked much concern on account of its double-edges sword effect on human health.Selenium enrichment has been found in environmental media in Handan area that is located in central North China where is generally recognized as Se-deficit region.However,concentration characteristics,distribution pattern and enrichment mechanism of Se in shallow groundwater in Handan Se-enriched belt（HSEB）still remain unclear.Specific research in terms of hydrogeochemistry is incomplete up to now in spite of complex lithology of outcrops,intensive mines and heavy industries as well as agricultural activities in HSEB.This study systematically explores for the first time on hydrochemical classification,hydrogeochemical processes,spacial evolution of component,source and influencing factors of shallow groundwater in HSEB based on analysis on major ions,trace elements,rare earth elements and Se.Quantitative evaluation on groundwater quality and health risk are conducted as well.This study not only facilitates understanding enrichment mechanism of Se in groundwater in localized Se-enriched area in typical Se-deficit regions but also offers a scientific guidance on rational and adequate utilization of Se-enriched groundwater resource.The main achievements are provided as follows:（1）Evident differences of hydrochemical types and controlling factors of groundwater among each zone in HSEB can be identified.Chemical composition of fissure groundwater collected fromⅠzone mainly inherits from the characteristics of atmospheric precipitation,resulting in its classification of Ca（Mg）-HCO₃type.Karst groundwater collected fromⅡzone can be classified as Ca（Mg）-HCO₃type as well on account of the weathering and dissolution of carbonate rocks.The main type of karst groundwater collected fromⅢzone is mixed type because oxidative dissolution of metallic sulfide minerals leads to significant elevation of theSO₄^2-component.The type of phreatic water collected fromⅣzone turns into Na（K）-SO₄（Cl）type because of multiple influences from evaporation,inverse cation exchange and adsorption and agricultural activities.（2）Multiple statistical methods combining composition and distribution pattern of trace elements as well as actual condition of research area were employed for source apportionment of 16 selected trace elements.The results reveal various major factors controlling the trace elements in groundwater,including weathering and dissolution of outcropped rocks,industrial contamination,mining,evaporation,agricultural activities and transportation.（3）Selenium concentrations in groundwater collected from HSEB average at 1.294μg/L with variable coefficient（CV）of 143.7%,reflecting significant spacial difference that the highest concentration level of Se occurs inⅢzone with an average of 2.218μg/L.Compared with the mean of Hebei Plain（0.84μg/L）,enrichment of Se in groundwater collected fromⅢzone can be confirmed.Outcropped strata of Archean to Permian in age provide basic material of Se in groundwater in HSEB.Selenium-enriched metallic sulfide minerals occurred in coal-bearing dark plastic rock series of Carboniferous to Permian play the most important role in Se enrichment in groundwater in HSEB.Besides,heavy industrial contamination also generate a small portion of Se enrichment of which spacial distribution is patchy.Migration of Se in the environment compartments is also positively affected by certain stretch of Minghe River.Oxidative condition coupled withNO₃^-andSO₄^2-is recognized as advantage of Se enrichment while Fe and Mn are unfavorable for Se enrichment in groundwater in HSEB.（4）Groundwater quality of groundwater in HSEB was evaluated by integrated water quality index（IWQI）,heavy metals pollution index（HPI）and a series of irrigating water quality indexes.Generally,the results suggest good quality of groundwater in HSEB.Coal mining,industrial contamination,agricultural activities and evaporation,however,lead to water quality deterioration in certain areas inⅢandⅣzone.（5）Health risks induced by groundwater in HSEB were quantitatively evaluated by discriminating the age group and exposure pathway.The area with high risk was delineated by using spacial interpolation afterwards.The results indicate that the risk of child is higher than that of adult while the risk induced by ingestion is higher than that induced by dermal contact.In terms of trace elements,non-carcinogenic risk is mainly caused by As,Ba,Cr and Pb while carcinogenic risk originates from As and Cr primarily.Overall,HI of most groundwater in HSEB is lower than the permissible limit,indicating low non-carcinogenic risk.In contrast,nearly all of groundwater represent high TCR values exceeding permissible limit,which reveals high carcinogenic risk to local residents.