Study On The Causes Of High-fluorine Groundwater In Dali County,Shaanxi Province

High fluoride groundwater is widely distributed around the world.Fluoride concentration in groundwater is closely related to human physiological health.Long-term drinking of high fluoride groundwater will cause people to suffer from endemic fluorosis.Therefore,it has important theoretical and practical significance to reveal the mechanism of high fluoride groundwater,to ensure drinking water safety,to prevent diseases and improve water quality.Based on the project"Disaster investigation of ground fissure in Weinan area of Guanzhong Basin"(NO.121201001000150122)from China Geological Survey(CGS),a1:50000 map of Xuzhuang(NO.I49E007008)selected as the research area,by means of a combination between field investigation and laboratory analysis,and between dynamic field and chemical field of groundwater,the spatial distribution regularity of high fluoride groundwater was revealed,the characteristics of chemical composition and hydrochemical types of high fluoride groundwater were analyzed,the factors affecting the formation of high fluoride groundwater were discussed,the Principal Component Analysis(PCA)model and hydrochemical composition model were constructed,and the applicability of the two models were tested in this article.Overall,the research results have been followed.Firstly,the spatial distribution characteristics of fluoridated groundwater are obtained.Fluoride concentration in most of the groundwater in the area exceeded the standard values of the third class in GB/T14848-2017,and its ratio was 93.67%.Along the groundwater flow direction,the fluoride concentration in the groundwater gradually decreased from Sunzhen Loess Plateau to Luohe River valley.However,it gradually increased from Weizhuang Loess Plateau to Weihe River valley.On the vertical direction,the fluoride concentration in the groundwater of different geomorphic types showed a decreasing trend with the increase of buried depth of groundwater table.Secondly,characteristics of chemical composition and hydrochemical types of high fluoride groundwater were obtained.Statistical analysis showed that the groundwater in the area is alkaline,higher Total Dissolved Solids(TDS),higher total hardness,higher Na⁺+K⁺and HCO₃^-,and lower Ca²⁺,lower Mg²⁺,lower SO₄^2-and Cl^-.The hydrochemical types of groundwater were bicarbonate types,or transitional types related to bicarbonate,then sulphate and chloride types.It was a significantly positive and negative relativity between fluoride concentration and p H and between it and Ca²⁺concentration,respectively.It was also a significantly positive relativity between fluoride concentration and HCO₃^-concentration.However,it was not significant relativity between fluoride concentration and concentration of the other chemical components.Thirdly,influence factors of high fluoride groundwater were analyzed.PCA and hydrochemical analysis showed that high fluoride groundwater was affected by natural factors and human activity.Among the natural factors,strong evaporation concentration under arid and semi-arid climate was a background factor,dynamic field and chemical field of groundwater were core control factors.In addition,topographic and geological factors were positive.Between human activity factors,irrigation was positive.Fourthly,fluoride concentration models in groundwater were established and the applicability of the models were tested.Based on the groundwater quality test data,PCA model and the hydrochemical composition model based on the principle of dimensional harmony are established.Their applicability was tested by means of the adjacent map data.The results showed that PCA model had a better accuracy when fluoride concentration is bigger than 1mg/L.However,the hydrochemica composition model was relatively better when it is smaller than 1mg/L,but was slightly worse when bigger than 1mg/L.The above-mentioned research results can provide evidences for the mechanism of high fluoride groundwater,the safety of drinking water,the prevention of diseases and improvement of water quality in this area.These also can provide a reference for fluoride concentration estimation in similar areas.So it has important theoretical and practical significance to improve the cause researches of high fluoride groundwater.