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.
Underground injection is a method of waste disposal.
Subject to provincial regulation, wastes are injected into known geological formations at great depths. The NPRI requires that only the quantity of the listed pollutant in the waste be reported. Waste materials, such as sludge, are often a mixture of many compounds associated with water and other inert material. As a result, the total quantity reported to the NPRI may be smaller than that reported in other emission inventories. Off-site treatment does not necessarily constitute an environmental release because the pollutant may be altered chemically or physically, and may not be ultimately released in its original form.