Water treatment plants (WTPs) can produce significant amounts of residual solids (i.e. sludge) as a result of coagulation, flocculation, clarification, and filtration processes to treat raw source water. In North America, alum is a common coagulant used in this process, resulting in the requirement for disposal of significant amounts of alum residual solids. Anticipated improved water quality treatment guidelines for trace metals such as arsenic in North America will result in more alum waste being generated in the future and hence increased pressure on water utilities to examine monofilling of alum-based residual solids to reduce waste management costs. GCLs are a potential cost-effective liner system for this type of application. However, there is currently a paucity of literature related to aluminium migration through GCLs.This paper presents results of GCL hydraulic conductivity, diffusion, and batch testing performed with a simulated WTP monofill leachate. Results of both short term and long term hydraulic conductivity tests show only modest increases in hydraulic conductivity, k, are observed for the tests conditions employed (k< 5x10 /s and a linear distribution coefficient, K d , of 30 mL/g. Batch testing is used to provide additional insight into the sorption behavior of aluminium with the bentonite from the GCL. Based on the limited results presented herein, it appears as if GCLs are suitable at maintaining low hydraulic conductivity values for at least 12 pore volumes of permeation with the high concentration alum residual monofill leachate simulated in this study. The information presented herein also provides WTP monofill designers with estimates of contaminant migration parameters necessary for establishing "alternative" GCL based liner designs.