Hydrochemical Formation Mechanism And Water Quality Analysis Of Phreatic Water In Yinchuan Plain Under The Influences Of Anthropogenic Activities

The Yinchuan plain is located in the arid semi-arid area of Northwest China.It is one of the oldest irrigation areas in the Yellow River in China.Because of the economic development and the intensification of anthopogenic activities in recent years,the water quality and quantity in this area have been greatly affected,which has caused a series of water environmental problems,such as water quality deterioration,landing funnel,land subsidence,land desertification,Lake atrophy and so on,which are most easily affected by anthopogenic activities.It is particularly obvious.Therefore,on the basis of actual field investigation and data collected in the study area,the chemical formation mechanism and water quality evaluation of phreatic water under the influence of anthopogenic activities are studied.The contents and conclusions are as follows:1.According to the analysis of?¹⁸O and?²H isotope data and hydrochemical data,it can be known that the phreatic aquifer in the area of piedmont acclivitous alluvial and flood plain is groundwater in base rock of Helan Mountain Eastern Region?A₁ zone?is mainly supplied by bedrock fissure water,while the southern Qingkouxia alluvial fan?A₂ zone?and phreatic aquifer in Multi-layered structure zone?A₃ zone?are mainly introduced by Yellow irrigation water supply.Upper confined aquifer mainly comes from precipitation?or Yellow River water?recharge in geological period.The value of tritium in the groundwater gradually decreases as the depth of groundwater increases,indicating that the greater the depth,the longer the time required for groundwater rejuvenation.There are different sources of recharge and there are certain differences in the chemical properties of groundwater.2.Based on the analysis of water chemistry data,isotope data and Gibbs maps,the influencing factors of the chemical composition of the phreatic aquifer were analyzed.The results showed that the water chemistry characteristics were affected by many factors.Divided into anthopogenic activities and natural conditions,anthopogenic activities mainly include groundwater pollution,excessive exploitation or excretion of groundwater,and excessive supplemental groundwater;natural factors mainly include geological and hydrogeological conditions of the Yinchuan Plain,sources of groundwater recharge,soil salinity,evaporation and concentration,mineral dissolution,and ion exchange.3.Using the relationship between TDS and d?surplus?,it can be seen that the contribution of mineral dissolution to phreatic TDS is greater than the contribution of evaporation concentration.In general,the contribution rate of mineral dissolution from the largest to the smallest is A₁ zone,A₃ zone and A₂ zone.The change of the fraction of residual water f from large to small,the rules of the A₁ zone,A₂ zone,and the A₃zone,this rule is consistent with the actual hydrogeological conditions in the study area.4.The analysis results of single-index assessment show that the over-standard rates of TH,TDS,SO₄^2-,Mn,Na⁺,Fe³⁺,F^-and NH₄-N in phreatic aquifer relatively high,except for the potential causes of local hydrogeological conditions,mainly due to anthopogenic activities?Sewage,industrial wastewater,agricultural pollution,etc.?The grey cluster evaluation results show that the groundwater quality in the study area is only?38%?in IV and V,mainly distributed in the northeast and southeast of the study area.The comprehensive evaluation results show that groundwater quality gradually deteriorates along the groundwater flow direction.