Removal of COD(Cr) and nitrogen in severely polluted river water by bank filtration

Laboratory column experiments on the colmation layer and a three-year successive field observation at the Xucun and Huangqiao sections of the Kui River--a severely polluted river in the Huai River basin in China--have been carried out. The transport and removal ratios for COD(Cr), NH4- -N, and NO3- -N in river water infiltrated through the riverbed colmation layer, the unsaturated zone, and the saturated aquifer, as well as to the pumping well, were investigated. The results of the column tests show that the colmation laver can effectively clog the infiltration of water and contaminants and the non-conservative contaminants can be effectively adsorbed or degraded by riverbed microbes in the colmation layer and its sludge. The removal ratios were approximately 48-51% for COD(Cr) and NH4- -N, and 23.5-25.5% for NO3- -N. Field observations indicate that the colmation layer and the unsaturated zone possess an obvious capacity to adsorb and degrade contaminants. A significant accumulation could be identified at the interface between the unsaturated and saturated zones. In the saturated zone of groundwater, contaminant concentrations declined sharply and were very low at the pumping well. Additionally, one- and two-dimensional numerical models were used to simulate the transport and transformation processes of water and contaminants filtered through the unsaturated zone and the saturated zone, and the model equations were solved numerically with a fully implicit difference method. The good agreement between calculated and measured values suggests the feasibility and reliability of the simulation method.