This paper presents the preliminary results of a long-term study aimed at establishing the regional Quaternary glacial geology of existing and projected landfills in southern Ontario, and the groundwater contamination potential of such landfills. At least 300 landfills in southern Ontario are located in abandoned aggregate or bedrock quarries where little regard was given to groundwater contamination. Many landfills contain toxic liquid and solid industrial wastes. Areas of fine-grained, impermeable glacial sediments, affording some degree of natural protection, have a restricted extent in southern Ontario; silt- and sand-rich tills predominate and, despite their massive, dense character, have significant bulk permeabilities. Projected sites can be fully engineered and protected by "hydraulic traps', thereby minimizing groundwater impacts, but the presence of nearly 1200 sites that lack any engineering poses a significant threat to groundwater supplies. -from Authors