Study On Evolution Of Chemical Characteristics Of Shallow Groundwater In Pinggu Plain Of Beijing

Pinggu District is an ecological conservation development zone in Beijing,with abundant groundwater resources.The two emergency water sources of Wangduzhuang and Zhongqiao in the district have played an important role in ensuring the safety of water supply in the capital.The increasing degree of groundwater exploitation has caused problems such as land subsidence in the Pinggu urban area and the southwest,and shallow groundwater pollution has also become more serious.Iron,manganese,nitrate nitrogen,and ammonia nitrogen components have exceeded the standard for many years.Based on the hydrogeochemical theory,this paper uses Arcgis geographic information processing technology,Phreeqc reverse simulation technology,and hydrochemical graphic method to identify the characteristics of the main hydrochemical components in shallow groundwater in the Pinggu Plain,analyze the evolution of typical pollution components,and reveal The evolution mechanism of shallow groundwater chemical composition.The research results can indicate the direction of groundwater environment treatment and maintenance in the Pinggu Plain area,and provide preliminary data and basic theoretical understanding of hydrogeology for the groundwater pollution prevention and control work in Beijing,Tianjin and Hebei.The main conclusions are summarized as follows:1.Characteristics of main hydrochemical components of shallow groundwaterDuring the transition from the groundwater recharge area to the groundwater discharge area,the lithological particles change from coarse to fine,the water flow slows down,the water-rock reaction becomes more complete,and the content of soluble components increases significantly.The groundwater is mostly HCO₃-Ca type,HCO₃-Ca·Mg type,HCO₃·Cl-Ca·Mg type,which is low-mineralized water formed by leaching.The evaporation and concentration effect is not strong.Cl^-and SO₄^2-are the main variables causing the evolution of the chemical composition of shallow groundwater.2.Evolution of typical polluted components of shallow groundwaterThe spatial distribution of trinitrogen in shallow groundwater is very uneven,and the pollution during dry season is more serious.Nitrate nitrogen is stable in nature and widely distributed beyond the standard;nitrite nitrogen and ammonia nitrogen are relatively concentrated in the distribution area.The mutual conversion process of"trinitrogen"is related to groundwater p H,groundwater burial depth,aeration zone thickness,and iron-manganese ion content:increased groundwater alkalinity and increased aeration zone thickness will inhibit the progress of nitrification;enrichment of iron and manganese ions shows that groundwater is in a reducing environment,which is conducive to the denitrification of strongly oxidizing NO₃^--N,the content of NH₄⁺-N increases,and the concentration of NO₃^--N decreases.The iron content in shallow groundwater has continued to decrease in the past 10years;the downward trend of manganese content is not obvious,and the situation of exceeding the standard is serious.Iron and manganese pollution is even more serious during the dry season.In the piedmont plain area,the iron and manganese content is low;the central river valley plain has a high iron and manganese excess rate.High manganese water is more widely distributed than high iron water.The formation of highiron-manganese groundwater is closely related to the redox environment.NO₃^-and SO₄^2-have strong inhibitory effects on the content of iron-manganese ions;the anions/cations in the groundwater will also remove high-valent manganese ions from the soil through ion exchange The ions in the adsorbent are released into the groundwater.The F^-concentration in the shallow groundwater is generally low,and the content in the high water period is higher than that in the low water period.In the past ten years,the fluoride pollution in the groundwater is weak and the distribution is concentrated.The formation of"fluorine"-containing groundwater is related to the groundwater p H,the type of water chemistry,and the depth of groundwater:the alkaline environment and high concentrations of Na⁺and HCO₃^-will promote the dissolution of fluorine-containing minerals;when the groundwater depth is in a shallow range,the F^-content increases with its increase.3.Evolution mechanism of shallow groundwater chemical componentsThe weathering dissolution of carbonates and sulfates is the main water-rock reaction of shallow groundwater.The shallow groundwater has the strongest leaching effect on rock salt,followed by gypsum,and the lysis effect of calcite and dolomite is relatively Weak;the central river valley plain has relatively strong ability to dissolve minerals.The transition from the groundwater recharge area to the discharge area,the water-rock reaction at the shallow water sample point changed from rock weathering to evaporative concentration.The alternating cation adsorption process is an important hydrogeochemical process in shallow groundwater,and the intensity and direction of the cation exchange process of groundwater changes along the runoff.Rainfall,excessive exploitation of groundwater,and groundwater pollution are all important factors affecting the evolution of shallow groundwater chemical components in the study area.4.Shallow groundwater hydrogeochemical simulationIn the Cuohe groundwater system,the evolution of hydrochemical components during the high water period is dominated by saturated precipitation of rock salt,accompanied by the occurrence of sodium-rich salinization hydrogeochemical processes such as silicate mineral dissolution;solubility of sulfate minerals in dry season significantly enhanced,the solubility of sodium-soluble salts and calcium-containing minerals weakened,and the water-rock interaction changed from leaching to precipitation.In the Juhe groundwater system,the evolution of water chemistry components during the high water period is dominated by the leaching and filtration of soluble salts?gypsum,rock salt?.During the dry season,the content of groundwater chemical components increased during the migration process,and the water salinization became more serious.