Study On Quality Variation And Hydrogeochemical Evolution During The Storage In The Underground With Reclaimed Water

In recent years,with the rapid development of urbanization,the rapid increase of the scale of cities and population has caused the contradiction between water supply and demand.Contamination of surface water and groundwater,uneven distribution of water resources,and frequent droughts caused by extreme global weather patterns have also severely aggravated the phenomenon of water scarcity.To counter this situation,The demand for recycled water recycling is growing stronger.The underground storage with reclaimed water is a method and strategy for water resource management,which is of particular concern.However,in the process of storage,the water quality risk is particularly important due to the imperfection of water quality standard and the uncertainty of water-rock reaction.In this study,laboratory experiments were combined with numerical modeling to study the water quality variation and hydrogeochemical evolution in the process of underground storage with reclaimed water,with theory combining with the actual and in qualitative combined with quantitative way.Reclaimed water from wastewater treatment plant?WWTP?was recharged into a one-dimensional column filled with field sediments.The PHREEQC software was used to establish a numerical model to simulate the parameters of the model,such as dispersion degree,cation exchange capacity,analyze the mechanism of solute transportation.The chemical reactions,soil-soluble species dispersion,mineral dissolution and precipitation,adsorption and cation exchange in the recharge vadose zone were examined.Predict hydrogeochemical evolution under different transportation length conditions which provides practical guidance for practical engineering application.The main conclusions are as follows:?1?During the 159-hr experiment process,the study showed the HCO₃^-content gradually increased to one of inflow by leaching,ion exchange and mineral dissolution/precipitation.It made the system p H based on the background of acidic soil soluble salt gradually changed from 6.31 to 7.77,reaching the alkaline reclaimed water.The soil is composed of about 13.7%of the clay minerals,and the sandy loam is dominated by aluminosilicate mineral.In the initial stage of the whole leaching process,for Al,Zn and Fe,the dissolution of feldspar and hematite is dominant;the adsorption of clay minerals led the concentration of As and Se in water to be always tend toward zero during leaching.In the beginning,the dissolution of minerals made the TN and DOC,which were fixed in the surface by chemical coordination or deposition,re-release into the water and become an"additive"pollutant.Mineral dissolution and cation exchange were dominant for Al,Zn,Fe,and absorption caused As and Se to always tend toward zero during leaching.Dissolution caused TN and DOC to form additional contaminations in the preliminary stage.The composition of dissolved organic compounds?amino acids,proteins,microbial metabolites,and rich acids and humic acids?in leaching solution was determined by the 3-D EEM spectra which illustrated the humus in sediments were potential threat to the groundwater quality by leaching.?2?Hydrogeochemical change is one of the important environmental factors that affect material migration transformation.In this paper,the PHREEQC software is used to establish a numerical model,we get the transportation parameters of the model through the experiment results fitted and verificated:Ion diffusion coefficient and physical dispersion degree are 3×10^-10m/s and 0.015m respectively.The permeability coefficient is 5.6×10^-3m/s;The flow velocity in the pore is 0.25m/hr.The analysis results showed that Na⁺,K⁺,Ca²⁺and HCO₃^-experienced leaching,cation exchange and mineral precipitation,and Na⁺had only leaching effect after 40hours.In contrast,Mg²⁺occurs with mineral dissolution,leaching and cation exchange.Among them,SO₄^2-and NO₃^-ion and Cl^-ions have been subjected to leaching only in the whole experiment due to their stable chemical properties.?3?After determining the numerical model,change migration length that means to change the unit number of subdivision in the migration module and the number of superimposed rotation,to predict the hydrogeochemical evolution under the different migration length conditions.Two different changed forms and patterns were found from the variation of ions content,which was divided into 5m and 10m,15m,20m and 25m based on the migration length.This is determined by the change in the length of the migration resulting in the relationship between the increase in contact reaction time and migration distance and the change in the total content of the mineral medium.Eventually,pH value was 8.05,hardness was 7.7°in reaching the equilibrium.According to the groundwater quality standard issued by China in 2017?GB/t14848-2017?,when the migration length is 25m,the water quality meets the groundwater standard of type I.