Water: Underground Injection Control
Mining Wells (Class III)
Class III Well Questions
This page describes Class III wells and their uses.
It also describes the various types of Class III wells, explains how their use helps protect drinking water resources, and presents the UIC Program’s requirements that ensure the protection of underground sources of drinking water (USDWs).
Class III Wells
What is a Class III well?
Class III wells inject fluids to dissolve and extract minerals such as uranium, salt, copper, and sulfur. More than 50 percent of the salt and 80 percent of the uranium extraction in the United States involves the use of Class III injection wells. There are about 165 mining sites with approximately 18,500 Class III wells in operation across the nation.
What are the types of Class III wells?
Class III wells are used to mine uranium, salt, copper, and sulfur.
Uranium in-situ leaching (ISL) is the most common method by which uranium is extracted in the United States. A typical uranium mining operation requires injection, extraction, and monitoring wells. The process includes the following steps:
Injection wells are drilled into the formation containing the uranium.
- A solution known as a lixiviant is injected and allowed to remain in contact with the rocks long enough to dissolve the uranium ore.
- When the lixiviant is almost saturated with uranium, the fluid is brought to the surface via a production well.
- At the surface, uranium is separated from the lixiviant.
- The lixiviant is then injected to extract more uranium.
The majority of Class III wells in the United States are uranium ISL wells.
Salt solution mining wells inject clean water to dissolve the salt and the resulting brine (salt water) is pumped to the surface where the salt is extracted. Two methods are used:
- Normal flow, in which water is injected into the well tubing and the saturated fluid is produced through the annulus between the tubing and the casing.
- Bottom injection, in which fluid is injected through the annulus and produced via the tubing.
If the salt is contained in a dome, a single well typically is used. If the salt is contained in multiple, bedded layers, multiple injection wells are used. Salt solution mining wells make up 5 percent of the Class III wells.
- Copper is mined using injection wells only in a few states. A sulfuric acid solution is used to dissolve the copper ore.
- Sulfur may be mined via the Frasch process, in which super-heated steam is injected into the mineral-bearing formation to generate a sulfur solution that can be recovered. Injection wells are not being used to extract sulfur at this time.
Production wells, which bring mining fluids to the surface, are not regulated under the UIC Program.
How do Class III wells protect drinking water resources?
Extraction of metals and salt is necessary to support industrial and economic activities. Solution mining through injection wells is an option that can minimize environmental damage from mining processes. To prevent contamination of ground water (which in many cases is a USDW) by Class III wells, more fluid is extracted than is injected in solution mining processes, thus preventing fluids from moving out of the mining areas.
What are the requirements for Class III wells?
All Class III wells are operated under individual or area permits. Contamination from mining wells is prevented by implementing requirements for mining well operators:
- Before commencing injection, operators must obtain an aquifer exemption if they are injecting into a USDW (which is common in ISL uranium mining), or if the overlying aquifer may subside (which may happen in salt mining operations). The wells must be constructed with tubing made of materials that are appropriate for the injected fluids, which are cased and cemented to prevent the migration of fluids into a USDW. They must also provide financial assurance that resources exist to properly plug the wells when injection operations are complete. Operators must pressure test their wells prior to injection.
- During operation of the well, the operator must monitor injection pressure and flow rate, and they may not inject fluid between the outer-most casing and the well bore. Operators must also monitor USDWs below and above the mining interval if the well is injecting into a USDW of 3,000 ppm (parts per million) total dissolved solids (TDS) or less. Operators of salt solution mining wells must test the well casing for leaks at least once every 5 years.
- When injection operations are complete, Class III operators must properly close (plug and abandon) the wells.
- Visit the Regulations page to read more about the requirements for owners and operators of Class III wells.
- The Guidance page presents the UIC Program's guidance documents.
- The Reporting Forms page provides links to the forms used by Class III well operators and primacy programs to report and demonstrate that injection operations are protecting USDWs.