GM defect of the composite liner,such as holes and wrinkles,can cause the leachate to leak to the groundwater collection system,and therefore influence the quality of the under-GM effluent.Through exhumation and reconnaissance of the local damage zone and long-term investigation into the construction and operation of the landfill,GM defects information was acquired,and the COD concentration and flow rate of the under-GM effluent were monitored.Based on the field survey,monitoring data and the analytical solution for leakage through defective composite liner,back-analysis was performed on the leakage of local damage zone and the whole site,and parameters for evaluating leakage through composite liner were obtained,and the whole site leakage and its effect on the quality of under-GM effluent were predicted.