An on-site release is a discharge of a pollutant to the environment within the boundaries of the reporting facility. This includes emissions to air, discharges to surface waters, on-site releases to land and deepwell underground injection. Surface water releases do no include discharges to municiple wastewater treatment plants. These are reported under off-site transfers for treatment. Landfills are sites in which wastes containing NPRI pollutants are buried.
Most landfills found in Canada are provincially approved waste-disposal sites.
Some landfills are classified as hazardous waste only. Regardless of classification, all landfills are required to have appropriate permits and be specifically designed under strict guidelines for use as final disposal sites for waste. In most cases, permitted landfills are managed to minimize the release of pollutants to the natural soil. A leak differs from a spill in terms of the time required for an event. Spills normally occur over a period of hours to days, whereas leaks occur over periods of days to months.