Mechanism And Modeling Of Clogging During Groundwater Recharge With Stormwater In Pinggu Of Beijing

Pinggu district, as one of the four emergency water sources in Beijing, is facing a series of issues, such as a prolonged drought, serious shortage of water resources and a continually widening gap between the water supply and its actual demand. Stormwater Harvest has both a good prospect for the development and an applicative value to alleviate water resources tension of Beijing. Groundwater Artificial Recharge (GAR) has been proved to be a promising method for regulating and storing stormwater in underground space. However, Clogging Problem was recorded to cause many recharge systems out of use in GARs all over the world. Thus, it is necessary to analyze and evaluate the degree of clogging before GAR construction in the study area, which will be of great theoretical and practical significance to maintain the stormwater GAR systems stable and safe.This thesis focuses on the possible clogging problem that may occur during groundwater recharge with stormwater in Pinggu. Several methods were combined in this study, such as field investigation and observation, lab experiments, computer modeling, and etc. The main work includes analysis of possible clogging types, modeling of the clogging process due to suspended solids, calculation of the precipitation in chemical reactions, and bioclogging evaluation. Based on the above research, discussions on integrated clogging were furthered, and some prevention and redevelopment techniques were recommended as well. Finally, such conclusions were draw as follows:(1) Analysis of groundwater recharge conditions, including the recharge sites, recharge water quality, and etc, represents that possible clogging is induced by suspended solid, chemical precipitation and biological organisms.(2) To reveal the development of suspended solids clogging, a lab column experiment was conducted. In this experiment, hydraulic conductivity of the porous media increased at the beginning, followed with steady decline. The range of clogging influence is the area of 52cm below the surface, of which the section between 0cm and 10cm clogged heavily. Moreover, the clogging rate fell with depth, so the fastest rate appeared in the layer of the upper 1cm. While for the whole sand column, the clogging rate is less than any clogging layers.(3) The mathematic model for suspended solid clogging was derived from the theories of groundwater dynamics, suspended solids migration and etc. Comsol software was applied to solve this model. Then, the comparison figures of the simulations with the experimental data illustrated modelled results had a reasonably good agreement with the observation data within the process of experiment.(4) Clogging extention under five fixed conditions was simulated by the above model respectively. From the modeled results, we discussed the the effects of concentration of SS in recharge water and the depth of water table on clogging development. First, specific discharge will fall down to less than half of the initial value when recharging with the max concentration of SS, 273mg/L. Second, the infiltration rates between two degrees of groundwater table with 10m interval, were narrow, only 5% difference after continuously recharge of 30d.(5) Another column experiment was carried out to simulate the water chemical evolution in vadose zone. The experimental data represented that total dissolved solid (TDS) and saturated index (SI) of recharge water increased during infiltration. Whereas, the TDS and SI of the outflows decreased with time, and went towards those values of the inflows.(6) Based on the results of filtration experiments, Phreeqc was introduced to calculate three typical calcium precipitations after outflow mixed with groundwater in different times. The calculated results shows CaSO4 will precipitate from water, with the max mass of 0.97mol per liter water at the beginning.(7) Bioclogging degree assessment employed microorganism growth theory and the quality relationship between the amount of biofilm and hydraulic conductivity of porous media. We evaluated Water Recourses at 8 sites, of which water in Xiyu Reservoir has the worst effect, and it can reduce the permeability of surface media down to 97.8 percent of the virgin at most. Overall, the degree of bioclogging is little in the study area. (8) Integrated analysis was made from three aspects, including clogging area, evolution and degree. Suspended solid clogging was proved to be the major type of concern. Consequently, preventive and redevelopment measures should be taken in terms of water quality and engineering. However, there is no effective technique to redevelop the recharge system at prevent.