Analysis Of Hydrochemical Evolution Characteristics Of Groundwater In A Channel Infiltration Area Of Reclaimed Water In Chaobai River

The utilization of reclaimed water to replenish river area may pose environmental risks due to the significant difference between reclaimed water and natural groundwater.The water diversion project from Wenyu River to Chaobai River has been running for more than 10 years.To investigate the characteristics and mechanism of groundwater hydrochemical evolution under long-term river infiltration of reclaimed water,the monitoring data of surface water and groundwater in 11 years were collected.Meanwhile,cluster analysis was conducted to divide the surface water into different regions and typical groundwater monitoring points and monitoring section were selected to reveal the hydrochemical evolution characteristics of groundwater.The infiltration volume of reclaimed water and the mixing ratio of reclaimed water to groundwater was calculated,and PHREEQC,environmental indicators such as sediment,hydrogeological conditions,Eh,dissolved oxygen(DO),etc.,were used to analyze the chemical evolution mechanism of shallow groundwater under reclaimed water infiltration conditions.The main achievements are as follows:(1)After the reclaimed water entered the Chaobai River from the Jian River,the p H value and the concentration of chlorophyll a,potassium permanganate index(COD_Mn)and CO₃^2-were obviously increased,while the concentration of TDS,TH,TN and TP were obviously reduced,and the concentrations of Fe and Mn did not change significantly.The main reason was that the reclaimed water was rich in nitrogen and phosphorus,the water body became eutrophication after being reused in the river.Phytoplankton reproduces in the river section,which was conducive to the formation of an anaerobic micro-environment,resulting in photosynthesis and denitrification.This process increased the p H value,and changed the carbonic acid balance,so that the water chemical indicators HCO₃^-,Ca²⁺and TDS concentration were greatly reduced.(2)Under the influence of reclaimed water infiltration,the p H value of the monitoring well near the river increased significantly,TDS and TH gradually approached the reclaimed water,and the concentration of Fe and Mn decreased significantly;The hydrochemical type of groundwater gradually tended to be reclaimed water,which evolved from HCO₃-Ca-Mg type to HCO₃-Cl-Na-Ca type.With the increase of the distance from the river,the influence of reclaimed water infiltration on groundwater gradually decreased.However,the evolution trend of nitrogen components and hydrochemical types of groundwater in different monitoring wells were different,which were mainly affected by reclaimed water quality,redox environment and water rock interaction.(3)In the process of reclaimed water infiltration,nitrogen was obviously attenuated by adsorption,nitrification and denitrification,but the reach with thin aeration zone,strong oxidation and low total organic carbon(TOC)concentration in groundwater was not conducive to denitrification,so the higher NO₃-N concentration in reclaimed water would cause the higher NO₃-N concentration in groundwater;If the decomposition rate of sediment was much lower than the net photosynthetic rate of phytoplankton,the long-term infiltration of reclaimed water would increase the content of nitrogen and organic matter in sediment,and the mineralization of organic nitrogen would occur after reaching the appropriate C/N ratio,which led to the increase of NH₄-N concentration in groundwater.(4)After reclaimed water infiltration,the hydrochemical evolution of groundwater was affected by mixing;Dissolution and precipitation of calcite and montmorillonite,dissolution of albite,and precipitation of quartz and chlorite;Denitrification,mineralization of organic nitrogen,desulfuric acid and cation exchange were all important factors.Among them,mixing,calcite dissolution and precipitation,and desulfurization had great influence on groundwater.When the river reach was mainly affected by mixing and calcite dissolution and precipitation,the hydrochemical type of groundwater tended to be reclaimed water gradually.However,if the groundwater environment had strong reducibility and sufficient carbon source,the reclaimed water infiltration would produce desulfurization,which increased the amount of calcite precipitation,reduced Ca²⁺,SO₄^2-,TH and TDS,and significantly changed the evolution trend of groundwater.