Injection wells use high-pressure pumps to inject liquid chemicals into under-ground geologic formations (e.g., sandstone or sedimentary rocks with high porosity).
There are five classes for injection wells based on the type of fluid injected and the location of the wells. Class I wells inject hazardous or non-hazardous fluids into isolated rock formations, approximately four thousand feet below the surface, and are strictly regulated under the Resource, Conservation and Recovery Act (RCRA). Their use must demonstrate that underground drinking water sources won't be contaminated. Class II wells are commonly used for the disposal of brine created during oil and gas production. Class III wells inject superheated steam or fluids and then extract them from the geologic formation to remove valuable minerals. Class IV wells were used for injection of hazardous or radioactive wastes, but are currently banned in the United States due to possible contamination of shallow drinking water sources. Class V wells (those not included in Classes I–IV) inject waste into the ground and allow it to drain by gravity into shallow aquifers, providing little or no protection against groundwater contamination. Examples include drainage wells, septic tanks, and cesspools.
In the United States, injection well casings must provide double containment to compensate for any structural failure. Wells are tested every five years for integrity (more frequently for hazardous waste) and are monitored continuously for possible contamination. Because of the threat of contaminating underground drinking water sources, the EPA establishes minimum requirements for the location, construction, operation, maintenance, monitoring, testing, and closure of injection wells. All such wells require authorization or specific permits.