Legacy pollution

Brownfield refers to land that is abandoned or underutilized due to pollution from industrial use. The specific definition of brownfield land varies and is decided by policy makers and/or land developers within different countries. The main difference in definitions of whether a piece of land is considered a brownfield or not depends on the presence or absence of pollution. Overall, brownfield land is a site previously developed for industrial or commercial purposes and thus requires further development before reuse.

Many contaminated post-industrial brownfield sites sit unused because the cleaning costs may be more than the land is worth after redevelopment. Previously unknown underground wastes can increase the cost for study and clean-up. Depending on the contaminants and damage present adaptive re-use and disposal of a brownfield can require advanced and specialized appraisal analysis techniques.

In mining, tailings or tails are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are different from overburden, which is the waste rock or other material that overlies an ore or mineral body and is displaced during mining without being processed.

Tailings can be dangerous sources of toxic chemicals such as heavy metals, sulfides and radioactive content. These chemicals are especially dangerous when stored in water in ponds behind tailings dams. These ponds are also vulnerable to major breaches or leaks from the dams, causing environmental disasters. Because of these and other environmental concerns such as groundwater leakage, toxic emissions and bird death, tailing piles and ponds are often under regulatory scrutiny. There are a wide range of methods for recovering economic value, containing or otherwise mitigating the impacts of tailings. However, internationally, these practices are poor, sometimes violating human rights. To mitigate risks of harm, the first UN-level standard for tailing management was established 2020.

An abandoned mine refers to a former mining or quarrying operation that is no longer in use and has no responsible entity to finance the cost of remediation and/or restoration of the mine feature or site. Such mines are typically left unattended and may pose safety hazards or cause environmental damage without proper maintenance. The term incorporates all types of old mines, including underground shaft mines and drift mines, and surface mines, including quarries and placer mining. Typically, the cost of addressing the mine's hazards is borne by the public/taxpayers/the government.

An abandoned mine may be a hazard to health, safety or environment.

Orphan, orphaned or abandoned wells are oil or gas wells that have been abandoned by fossil fuel extraction industries. These wells may have been deactivated because of economic viability, failure to transfer ownerships (especially at bankruptcy of companies), or neglect and thus no longer have legal owners responsible for their care. Decommissioning wells effectively can be expensive, costing millions of dollars, and economic incentives for businesses generally encourage abandonment. This process leaves the wells the burden of government agencies or landowners when a business entity can no longer be held responsible. As climate change mitigation reduces demand and usage of oil and gas, its expected that more wells will be abandoned as stranded assets.

Orphan wells are an important contributor of greenhouse gas emissions causing climate change. Wells are an important source of methane emissions through leakage through plugs, or failure to plug properly. A 2020 estimate of US abandoned wells alone was that methane emissions released from abandoned wells produced greenhouse gas impacts equivalent of 3 weeks US oil consumption each year. The scale of leaking abandoned wells are well understood in the US and Canada because of public data and regulation; however, a Reuters investigation in 2020 could not find good estimates for Russia, Saudi Arabia and China—the next biggest oil and gas producers. However, they estimate there are 29 million abandoned wells internationally.

Abandoned wells also have the potential to contaminate land, air and water around wells, potentially harming ecosystems, wildlife, livestock, and humans. For example, many wells in the United States are situated on farmland, and if not maintained could contaminate important sources of soil and groundwater with toxic contaminants.