Hydrochemical Characteristics And Formation Mechanism Of Groundwater In Oasis Area Of Hotan River Basin

Groundwater resource is an important part of water supply in arid areas of northwest China,and it is also a key factor to restrict the ecological security and economic development of oasis.In recent years,due to the influence of global climate change and human activities on groundwater,a variety of adverse ecological environmental problems have emerged in the oasis.The spatial distribution of groundwater defines the pattern of the oasis,while water quality and quantity limit its scale.In order to make groundwater resources better serve the economic development of oasis and the health of residents,it is necessary to understand groundwater environmental problems from the perspective of interaction between groundwater and surrounding environment.Based on the analysis of the natural geographical conditions,formation lithology and hydrogeological conditions in oasis area of Hotan River Basin,this paper studies the quality,hydrochemical characteristics and influencing factors of groundwater,and reveals the formation mechanism of the chemical composition of groundwater by the methods of hydrogeochemical diagram,multivariate statistical analysis and hydrogeochemical simulation according to the chemical detection data of groundwater.The main results are as follows:(1)The results of single index evaluation show that quality of shallow groundwater in the oasis area is generally poor.In 1980,2014 and 2017,61.8%,91.1%and 72.5%of groundwater samples were classified as grade ? or above.According to the contribution rate of super ?water index,the key indexes affecting the quality of groundwater in oasis area are not only the major components such as Total Dissolved Solids(TDS),Total Hardness(TH),Cl-,Na+,SO42-,but also the micro components such as F,Fe,Mn,BAP.(2)The chemical types of groundwater in the oasis area are complex,mainly composed of HSL-NCM.The oasis area are basically brackish and high-hardness groundwater,and the content of common cations are significantly different,the order from largest to smallest is Na+>Ca2+>Mg2+>K+.There is little difference in the content of common anions,the order from largest to smallest is HCO3->Cl->SO42-.Under the control of physical geography and hydrogeological conditions,the spatial distribution of groundwater hydrochemical composition changes regularly.Generally speaking,the contents of TDS,TH,Na++K+,Mg2+,Cl-,SO42-,HCO3-,F,Fe and Mn in groundwater in the horizontal direction from south to north are increasing continuously,and the closer the desert area is,the higher the content is.Hydrochemical type change from mixed type to C-N type and CH-NC type.In vertical direction,the content of chemical components in shallow phreatic water is generally higher than that in deep phreatic water.From 1980 to 2017,TDS and TH of groundwater showed a slight increase trend,and the content of soluble ions(strong acid radical and alkali metal)also increased,while the content of insoluble ions(weak acid radical and alkali earth metal)decreased.(3)High fluoride,low iodine,high iron and manganese are the important characteristics of groundwater chemistry in oasis area.The fluorine content in groundwater is relatively high,and the exceeding rate of fluorine is 40.0%and 36.0%respectively in 2014 and 2017.The main occurrence forms of fluorine are free F-and MgF+,accounting for 86.8%-97.4%and 1.9%-11.5%of the fluorine content respectively.When the fluorine content is larger than 1.5 mg/L,the changes of F-and MgF+ contents show an opposite trend.The exceedance rates of Fe and Mn in groundwater are 57.8%and 46.7%respectively in 2014.The rock in the oasis area are rich in iron-manganese and fluorine containing minerals,which provide the material foundation for the formation of high-fluorine and high iron-manganese groundwater.The lack of iodine sources and the oxidation environment are the main reasons for the low iodine content in the groundwater of oasis.(4)The "three nitrogen" pollution of groundwater in oasis area is light,but from 1980 to 2017 it tends to increase.Nitrate nitrogen is the main form of "three nitrogen" in groundwater.The excessive areas of nitrate nitrogen(NO3--N)and ammonia nitrogen(NH4+-N)are distributed sporadically and do not constitute a large area of pollution.Under the influence of hydrochemical environment,thickness of aeration zone and lithology,the content of NO3--N and NO2--N in groundwater in oasis area is higher in the south and lower in the north,while the distribution area of NH4+-N in the north is wider than that in the south.The degree of organic pollution of groundwater in the oasis area is slight,but BaP pollution of groundwater in local area is more serious in 2014.The risk value of carcinogenesis under the way of drinking groundwater exposure exceeds the acceptable level,which is harmful to human health.(5)Under the control of topography and formation lithology,the runoff and discharge mode of groundwater in oasis area are changing constantly,which affects the degree and type of hydrogeochemical action.The hydrogeochemical graphic method,factor analysis method and reverse hydrogeochemical simulation method reveal that the main hydrogeochemical process controlling the formation and change of groundwater chemical composition in oasis area is evaporation,which is more significant in weak runoff area.The second is the dissolution and filtration of rock and soil.To a large extent,the chemical composition in groundwater inherits the chemical composition in the upstream groundwater.Along the way,carbonate,gypsum,rock salt and minerals containing fluorine and iron and manganese are dissolved and filtered in the water-bearing medium.Meanwhile,surface water and soil irrigation water are also continuously replenished.Then,cation is alternately adsorbed,which is more obvious in the weak runoff area,changing the content of cation in groundwater.