Origin And Chemical Evolution Of Groundwater In The Northern Tacheng Basin

The contradiction between water supply and demand in the northwest arid and semi-arid inland basin is very prominent.The plain area is dry and has little rain.The groundwater resources are mainly formed in the mountains and piedmonts.The recharge sources are atmospheric precipitation and snow melt water.The long-term runoff of groundwater from recharge area to discharge area determines the complexity of the evolution of groundwater hydrochemistry in arid and semi-arid basins.For example,the intense exploitation of groundwater changes the groundwater cycle pattern and the pollution of groundwater,which may affect the hydrochemistry evolution.Therefore,understanding the origin and evolution mechanism of groundwater hydrochemistry has important scientific and practical significance for the division of regional groundwater resources function and management,and could promote the coordinated development of the economy and ecological environment in arid regions.This paper took the groundwater system in the northern Tacheng Basin as the research area,and used traditional hydrogeology,isotope hydrology,hydrogeochemistry,and mathematical statistics to conduct multidisciplinary cross-research methods to explore the characteristics of groundwater recharge,runoff,and discharge.Based on this,the distribution characteristic of groundwater chemistry with groundwater flow paths were analyzed,and the main geochemical effects of groundwater chemistry are analyzed in order to reveal the spatial evolution mechanism of groundwater chemistry.This thesis can provide scientific basis for sustainable development of groundwater resources in the research area and prevention of groundwater quality deterioration.The main conclusions reached are as follows:?1?Based on the long-term dynamic monitoring data of groundwater levels and the ages calculated from ³H and ¹⁴C dating of the groundwater,it is concluded that the single cycle belt of groundwater?the area to the west of the loess plateau?has better groundwater runoff conditions,faster groundwater recharge replacement rate and better renewable capacity compared with the double cycle belt of groundwater?the area with extended loess plateau?.?2?Freshwater with TDS less than 1.0 g/L is widely distributed in the aquifer of the study area,and high TDS brackish water and salty water are formed only in the topographic depression of the discharge area.The groundwater TDS increased from the recharge area to the discharge area,and showed a good horizontal zoning law.The groundwater in the study area has the horizontal zoning law of common hydrochemical types in inland basins.The groundwater from the supply area to the drainage area,its hydrochemical type transitions from HCO₃–Ca to HCO₃?SO₄–Ca?Mg,SO₄?HCO₃–Ca?Na and SO₄–Na?Ca.According to the analysis of pollution characteristics indicators such as"three nitrogen"and COD,it is concluded that human activities have not yet caused significant pollution of unconfined groundwater,nor have they changed the chemical composition of unconfined groundwater.The large-scale exploitation of confined groundwater has not changed the recharge and discharge relationship between the confined and unconfined aquifer,and has not significantly affected the chemical composition of confined water.?3?Through ion ratio method,mathematical statistics method,and evaporation fractionation simulation,it is concluded that the chemical characteristics of groundwater in the study area are mainly controlled by mineral leaching,and secondly by the cation exchange,and minimally affected by evaporation.In the recharge area,the dissolution of carbonate minerals?calcite and dolomite?controls the chemical composition of the groundwater.In the runoff and discharge areas,the effects of gypsum and halite dissolution on the groundwater chemical composition gradually exceed that of carbonate minerals becoming the main leaching effect.In addition,positive cation exchange generally occurs throughout the runoff path,but overall,the effect on the chemical composition of groundwater gradually decreases with the runoff path.It is worth noting that,on individual runoff paths,the effect of cation exchange on groundwater chemical composition exceeds that of mineral leaching.?4?The results of inverse hydrogeochemical modeling show that groundwater in the single cycle belt compared with groundwater in the double cycle belt,the contribution of dissolution and cation exchange to the hydrochemistry of groundwater is smaller,and halite dissolution does not occur in the single cycle belt.In addition,in the respective cycle belt,hydrogeochemical processes contribute more to hydrochemistry of unconfined groundwater rather than confined groundwater.