Study Of Reactive Transport Of Reclaimed Water Through Vadose Zone And Its Impact On The Groundwater Environment With Prevention Measures Of Seepage In Lakes And Rivers

Water resource scarcity in Beijing city leads to restricting the economic growth and deteriorating the ecological environment of the city. Reclaimed water is characterized by being easily accessible, having stable source, and substantial quantity, so it becomes a major source to maintain eco-environmental water use. However, there is concern that reclaimed water in river channels may infiltrate and put groundwater at risk. For this reason, exploring the response of the local groundwater environment to reclaimed water discharging into rivers is significant for the choice of liner materials, and protection and exploitation of groundwater.This study takes the ecological restoration of Yongding River in Beijing city as a background. The urban and suburban area of Beijing city containg a partial section of Yongding River is chosen as a typical area. A multi-speices reactive transport model considering preferential flow has been established based on laboratory soil columns experiments with different liners installation. A borehole scale flow and solute transport model has also been established based on soil columns experiments, collected data and field study. A groundwater flow and solute model has been developed and loosely coupled with the borehole scale model. The coupled model was further adopted to predict the impact of reclaimed water discharging into rivers on the local groundwater environment. The main research contents and results are as follows:(1) For a typical layered soil system with a covered liner, the Mobile Immobile Model-based multi-species reactive solute transport model that accounts for preferential flow was establishes by using PHREEQC-2. Results showed that the model could simulate the transport of typical nutrients in reclaimed water through the layered soil columns. The results showed that neglecting fingering flow may underestimate pollution risk. For the ecological liner materials with three different mass ratio of bentonite, a higher proportion of bentonite material BE19can not only reduce leakage but also enhance the degradation of NH4and orgaica matter (OM). However, BE19may conduce to discharge large amount of total phosphate (TP) and NO3-stored in soils, leading to their concentrations of outflow increase.(2) The applicability of method of minimum flux in saturation layer (MFSL) in estimating the seepage of reclaimed water through a vadose zone was validated. MFSL was further applied to predict the river leakage under the conditions without liners, with ecological liners and with geomembranes installation. The borehole scale reactive transport model was established based on the reavtive parameters obtained by soil columns experiments. The range of leachate concentration of typical solutes was then obtained by numerical experiments using the larger scale model. Simulation results showed that the stable infiltration rate with the BE19material installation is10-100folders lower than no lining stallation condition under planned water level. The stable infiltration rate can reduce6-7order of magnitudes with the BEi9ecological liner paving on the river channel, comparied without liner. Lining installation may help decrease the concentration of NH4+and OM, but increase the concentration of NO3-and TP into groundwater.(3) A groundwater flow and solute model in the typical area was developed based on FEFLOW. The model was then calibrated and validated using observed data of groundwater levels and concentration of typical pollutants from June2009to December2010. The root mean square error (RMSE) of groundwater levels in observation wells was0.69m during both the calibration and validation periods. R2was0.69and0.59, respectively. The observed and simulated groundwater water level distributions also agreed well. The results indicated that the model and input parameters were reasonable, and can be applied to predict scenarios.(4) A vadose zone-groundwater joint model was adopted to predict the response of groundwater environment to reclaimed water discharging into the rivers with different channel liners installation. Under the condition with consecutive years of normal precipitation, paving geomembranes on river channels can slightly increase groundwater supply per year, compared with the previous dried-up condition, and decrease groundwater supply4.11×10m3per year in comparison with paving ecological liners. Geomembranes tend to retard the travel time of leaky reclaimed water into groundwater. Even if geomembranes are broken or water pollution accidents happen, installating geomembranes on river channels can extend the transport time through vadose zone, and shrink the range of transport and diffusion in groundwater within a same period, compared with paving ecological liner on river channels.