Simulation Study On Hydrogeochemical Process During Recharge Of Reclaimed Water To Groundwater

Hydrogeochemical process can cause changes in water quality and clogging problems during reclaimed water recharge. It can result in certain risks. This work selects the reclaimed water in plain area of Beijing as a case study. Three topics were considered, i.e., change in general water quality indexes during recharge, characteristics of dissolution and precipitation of typical carbonate rocks, change of heavy metal constituents and migration characteristics of reclaimed water. The effects of environmental factors on main hydrogeochemical process were discussed. The main findings are as follows.During the infiltration of reclaimed water through vadose zone, general water quality indexes are affected by effect of leaching and cation exchange. In the medium of vadose zone. Ca2+ and Mg2+ in vadose zone media exchange with Na+ in reclaimed water. In the aquifer, general water quality indexes are affected by physical mixing, mixing water-rock interaction and changing environmental conditions. In physical mixing,anionic type changes at the mixing ratio between 9:1 and 7:3 and cationic type changes at the mixing ratio between 5:5 and 1:9. Mixing water-rock interaction and changing environmental conditions have a great impact on the hydrochemistry type of groundwater. When partial pressure of CO2 decreases from 10-1 to 10-4 bar, transformation process of the hydrochemistry type of groundwater is as follows: HCO3-Naâ†'HCO3·Cl-Na â†'Cl·SO4-Na or Cl-Na. When temperature decreases from 25 ℃ to 5 ℃, the hydrochemistry type of groundwater changes from HCO3 –Na to HCO3-Na·Mg.The dissolution and precipitation of carbonate minerals is one of the most important hydrogeochemical processes which affect the general water quality indexes. The experimental results show that dolomite dissolution is greatly affected by the common ion effect, and it takes 3d to reach equilibrium. The simulation results of PHREEQC indicate that both the mixture ratio of reclaimed water and groundwater and the mineral composition have effects on the dissolution and precipitation of carbonate minerals. When minerals dominated by calcite or dolomite, precipitation gradually increases with the decreased ratio of reclaimed water. The dissolution and precipitation influence each other when two kinds of minerals coexist. The dissolution and precipitation are also related to the mixture ratio. In an approximately closed system near the clogging layer, the thermodynamic calculation results is consistent with the the actual data of calcite precipitation in sediment of Chaobai river.According to the simulation results, As, Cd, Cu and Pb in the reclaimed water mainly exist as HAs O42-, Cd2+, Cu(OH)2 and Pb CO3 form. Fe, Mn, Cr and Zn mainly existt in two or more species, such as hydroxide and carbonate species. The form of Fe, Mn, Cr and Zn are greatly influenced by temperature. When p H varies from 4 to 10, the existence form of the trace metal elements changes not only in the proportion but also in the existential state. The existential state of Fe, As, Cr and Cu are greatly influenced by p E. Temperature has little effect on dissolution and precipitation of heavy metals mineral, and there is a greater effect of p H and p E on dissolution and precipitation of iron manganese minerals.