Classifications of wells
The number of injection wells operating in the United States is estimated to approach 850,000, according to 2012 data released by the EPA. Divided into six categories based on use, the wells are used to inject a broad range of substances into the ground.
Class I: Inject industrial non-hazardous liquids or municipal wastewater below the aquifers used for drinking water. (680 wells nationwide)
Class II: Inject brines and other fluids associated with oil and gas production, and hydrocarbons for storage. (172,068 wells nationwide)
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Class III: Inject fluids associated with solution mining of minerals beneath aquifers used for drinking water. (22,131 wells nationwide)
Class IV: Inject hazardous or radioactive wastes into or above drinking water aquifers. These wells are banned unless authorized under a federal or state ground water remediation project. (33 sites nationwide)
Class V: In general, inject non-hazardous fluids into or above drinking water aquifers. Some are built to inject below drinking water aquifers. (400,000 to 650,000 wells nationwide)
Class VI: Inject carbon dioxide for long-term storage, or geologic sequestration. (6 to 10 wells expected to come online by 2016)
Source: United States Environmental Protection Agency