The Evaluation And Evolution Of Groundwater Quality In Korla City

In recent years,with the rapid urbanization,groundwater has become the main source of water for industrial and agricultural production and main source of drinking water in Xinjiang.However,due to the long-term unreasonable exploitation of groundwater,the groundwater quality deteriorated,the degree of groundwater pollution continued to deepen.It is of great significance to know the quality present,pollution situation and influencing factors of groundwater for rational and effective utilization of groundwater resources and protection of groundwater environment.According to the hydrochemical test results from 45 groups of groundwater sampling points(21 unconfined groundwater samples,24 confined groundwater samples)in Korla City in 2014,combined with groundwater monitoring data from 2003,2011 and 2018,the quality and pollution status of groundwater was evaluated,the chemical characteristics,changes and influencing factors of groundwater quality were analyzed,the main conclusions are as follow:(1)In unconfined groundwater,the dominant cation is Na+and the dominant anion is SO42-in 2014,the groundwater chemical types are SO4·Cl-Na·Ca,SO4·Cl·HCO3-Na·Ca·Mg and SO4·Cl-Na·Mg.From the east to the west,the hydrochemical types of unconfined groundwater evolved from SO4·Cl-Na to SO4·Cl·HCO3Na·Ca·Mg.In confined groundwater,the dominant cation is Na+and the dominant anion is SO42-in 2014,the hydrochemical types of groundwater are SO4·Cl·HCO3-Na·Ca·Mg,SO4·Cl-Na·Ca·Mg and SO4·ClNa·Ca·Mg.The hydrochemical types of confined groundwater in the west,central and east of the study area evolved from SO4·Cl-Na type to SO4·HCO3-Na·Ca·Mg type and then to SO4·Cl-Na·Ca type.(2)In 2014,only 34 groups of groundwater sampling points(15 unconfined groundwater samples,19 confined groundwater samples)were simultaneously tested for the content of seven evaluation indicators(SO42-,TH,TDS,Cl-,Fe,Mn and F-)in the research area.Grey clustering method was used to evaluate 34 groundwater samples quality,the evaluation result shows:in unconfined groundwater,water quality categories of Ⅰ to Ⅴ sampling points are 0,0,three groups,one group and 11 groups,accounts for 0.0%,0.0%,20.0%,6.7%and 73.3%of the total samples respectively.The class Ⅴ sample with the worst water quality are located in the northeast and southern regions,the class Ⅲ water samples with relatively good water quality are located in the central zone of the study area.In confined groundwater,water quality categories of Ⅰ to Ⅴsampling points are 0,six groups,five groups,one group and seven groups,accounts for 0.0%,31.6%,26.3%,5.3%and 36.8%of the total samples respectively.From the south to the north,the quality of confined groundwater has gradually deterioration.(3)Nemerow pollution index method was used to evaluate the pollution status of 45 groups groundwater in 2014,the result shows:in21 groups of unconfined sampling points,14 sample points were not contaminated,sixsample points were slightly contaminated,one sample point was moderately contaminated,accounts for 66.7%,28.5%and 4.8%of the total samples respectively.The pollution degree of unconfined groundwater has gradually increased from the north to the south.Among the 24 groupsconfined groundwater sampling points in the study area,except G17 and G23 are slightly contaminated,all the other sampling points are in unpolluted state.(4)By analyzing the monitoring data of groundwater quality in Korla City,it is found that the average ion concentration varies greatly from 2003 to 2018.According to the characteristics of the inter-annual variation of groundwater quality,the proportion of groundwater sampling points of category Ⅳ to Ⅴ from 2003 to 2018 in unconfined groundwater and confined groundwater increased by 50.0%and 54.2%,respectively,the quality of groundwater showed a deteriorating trend.(5)The mainly influencing factors of groundwater quality include pollution source(industrial and domestic pollution sources,agricultural and domestic pollution sources,domestic pollution sources),land use type,lithology of vadose zone,aquifer characteristic and groundwater over-exploitation.