Improperly dumped oil or grease can leach into soil and contaminate the groundwater supply for years. Because of this, the proper storage, transfer, and disposal of oil is regulated by both the US Environmental Protection Agency, and state-specific environmental agencies.
With the typical use of oil, contaminates such as dirt, metal scrapings, water, or chemicals can cause impurities in the oil that eventually lead to degradation so it no longer performs well. Eventually, that oil must be disposed of or recycled in order to be replaced with virgin or re-refined oil.
Examples of used oil include:
Oils that do not meet EPA's definition of used oil can still pose a threat to the environment when disposed of and could be subject to the RCRA regulations for hazardous waste management. It is important to work with a waste vendor that specifically lists oils or used oils as one of their accepted wastes.