Groundwater Pollution Assessment In Reclaimed Water Recharging Zone Based On Process Simulation

With the continuous improvement of urban sewage and industrial wastewater treatment technology,reclaimed water,as an important water resource,has been paid more and more attention.However,as the main source of reclaimed water after treatment of urban sewage,industrial wastewater and atmospheric precipitation,water quality is relatively poor,and there are large different between groundwater,in the process of covering groundwater can produce a series of hydrogeochemical action,to break the original state of equilibrium of aquifer,makes the components in the groundwater must be changed,even lead to some components exceed the standard,causing some pollution to groundwater,and then affect the development and utilization of groundwater.The identification of the hydrogeochemical process of reclaimed water and the evaluation of the pollution caused by groundwater in the process of reclaimed water recharge are the key and difficult points in the study of rational reclaimed water recharge.This paper chose Yang Dike Section of Bei Yun Canal as the research field.Firstly,a series of hydrogeochemical reactions occurred in the process of reclaimed water recharge were formulated by PHREEQC reverse simulation,Secondly,the ORTI3D software was used to simulate the hydrogeochemical process,so as to establish a two-dimensional multi-component reaction migration model in line with the actual situation,Finally,on the basis of former multi-component reaction migration model of typical section,the single index pollution standard index method was used to evaluate the groundwater pollution and the threshold value of reducing the corresponding pollution risk factors was given.The results show as follows:（1）The average concentrations of Na⁺,Cl^-,K⁺,SO₄^2-,NO₂^-and NO₃^-in reclaimed water are higher than the initial values of groundwater,while the average concentrations of Ca²⁺,Mg²⁺,HCO₃^-,Fe²⁺and NH₄⁺are lower than the initial values of groundwater.In the process of reclaimed water recharging groundwater,Na⁺,K⁺,Ca²⁺,Mg²⁺,NH₄⁺,SO₄^2-,Fe²⁺showed upward trends with time,and HCO₃^-showed a firstly downward and then upward trend with time.The concentration of NO₃^-kept low with time,and NO₂^-showed an upward trend with time.（2）The identification results of hydrogeochemical processes show that in addition to mixing,Na⁺,K⁺,Mg²⁺and NH₄⁺are mainly controlled by cation exchange adsorption,and Ca²⁺is mainly affected by mineral dissolution and precipitation.NO₃^-and NO₂^-are mainly controlled by denitrification.HCO₃^-is mainly controlled by calcite dissolution.Fe²⁺is mainly controlled by pyrite REDOX.SO₄^2-is affected by gypsum dissolution and precipitation and desulfurization.（3）The simulation of hydrogeochemical process showed that HCO₃^-was not only affected by calcite dissolution and precipitation,but also affected by organic carbon mineralization.The rapid decrease of SO₄^2-concentration is mainly affected by desulfurization.（4）The average pollution risk thresholds of Na⁺,Cl^-and NO₂^--N areⅡlevel,Ⅱlevel andⅢlevel respectively,and the thresholds are:105 mg/L,110 mg/L,0.46 mg/L.