Water Chemistry Characteristics Of Groundwater In Heihe River Basin

The Heihe River Basin,the second largest inland river basin in China,is one of the major ecological critical areas in China,and it's also a scarce area of water resources.The rapid deterioration of the water environment has seriously affected the sustainable development in the basin.In this paper,focusing at the contents of main anions(Cl^-,SO₄^2-,HCO₃^-,F^-and NO_x^-)and main cations(Na⁺,Ca²⁺,Mg²⁺and K⁺)in the Heihe River Basin,by using correlation analysis method such as ion ratio method,piper diagram method and chloralkaline index method to analyze the hydrochemical characteristics and the ion exchange effect of groundwater in the Heihe River Basin.In this way,to reveal the source and distribution characteristics of the main hydrochemical ions of groundwater,and to provide a scientific basis for groundwater quality assessment in the Heihe River Basin.The results are as follows:1.In the upper reaches of the Heihe River,the pH,EC?electric conductivity?and TDS?Total Dissolved Salts?of groundwater varied from 7.4 to 8.3,5 to 3186?S/cm and 3 to1752 mg/L,respectively.The pH,EC and TDS of the groundwater of the middle reaches varied from 6.2 to 9.4,256 to 12400?S/cm and 111 to 6160 mg/L,separately.The pH,EC and TDS of the groundwater of the lower reaches ranged from 5.6 to 9.3,344 to 7592?S/cm and 118 to 5833 mg/L,respectively.Namely,pH,EC and TDS in the middle and lower reaches were larger than that of the upper reaches due to the middle and lower reaches are plain areas with frequent human activities,which has a great impact on groundwater.2.The results of the Piper diagram show that the groundwater hydrochemical type in the upper reaches of the Heihe River was dominated by HCO₃^--Ca²⁺·Mg²⁺and SO₄^2-·Cl^--Na⁺.In the middle reaches of the Heihe River was dominated by SO₄^2-·HCO₃^-·Cl^--Mg²⁺.In the lower reaches,it was dominated by SO₄^2-·Cl^--Na⁺·Mg²⁺.The reason for the differences may be the different control effects of groundwater hydrochemical components in the Heihe River Basin.Combined with the Gibbs diagram,the hydrochemical components of groundwater in the upper reaches of the Heihe River were mainly affected by the interaction of water and rock.The hydrochemical components of groundwater in the middle and lower reaches were affected by the water-rock interaction and the interaction of evaporation and concentration.3.If the chloro-alkaline indexs?CAI1 and CAI2?are positive values,it indicates that ion exchange mainly occurs in high salinity groundwater.If it is a negative value,it means that the chloro-alkali is unbalanced,and a cation-anion exchange reaction occurs.In this study,the groundwater in the upper and middle reaches of the Heihe River has a negative CAI1and a positive CAI2,indicating that the chlor-alkali imbalance has occurred in the upper and middle reaches of the Heihe River.However,CAI1 and CAI2 of the groundwater in the lower reaches of the Heihe River are positive,indicating that ion exchange mainly occurs in high salinity groundwater and is also an important mechanism affecting the chemical composition of groundwater.4.The content of F^-in groundwater in the middle reaches of the Heihe River Basin decreased with increasing depth,but the trend was not obvious;in the downstream basin,the content of F^-increased significantly with the augment of depth,it can be attributed to the evaporation in the middle and downstream areas of the Heihe River is strong,and the groundwater stays for a long time.The average value of nitrite nitrogen and nitrate nitrogen in groundwater in the upper reaches of Heihe River is 0.160mg/L and 2.261mg/L,respectively.The average value of nitrite nitrogen and nitrate nitrogen in the middle reaches was 0.188mg/L and 20.662mg/L,separately.the average value of nitrite nitrogen and nitrate nitrogen in the lower reaches was 0.913 mg/L and 13.201 mg/L.It shows the content of nitrite nitrogen and nitrate nitrogen of groundwater in the middle reaches of Heihe River was the highest,that in the lower reaches is the second and the lowest in the upper reaches.It shows that the water quality of the upstream groundwater is favorable.Due to the large-scale used of chemical fertilizer nitrogen fertilizer and high-intensity irrigation,the groundwater nitrogen pollution in the middle reaches of the Heihe River is serious.Recharged by surface water with high nitrogen content in the middle reaches,so the groundwater in the lower reaches of Heihe River is also subject to more serious nitrogen pollution.