Office of
United States Ground Water and
^1 Environmental Protection Agency Drinking Water

CLASS V UIC STUDY FACT SHEET
MINE BACKFILL WELLS
What is a mine backfill well?
Mine backfill wells are used in many mining regions throughout the country to inject a mixture of
water and sand, mill tailings, or other materials into mined out portions of underground mines.
These wells can serve a variety of purposes, including subsidence prevention, filling dangerous
mine openings, disposing of wastes from mine operations, and fire control.
What types of fluids are injected
into mine backfill wells?
The physical characteristics and chemical composition of materials injected into backfill wells
vary widely depending on the source of the backfill material, the method of injection, and any
additives (e.g., cement) that may be included. In general, the materials include slurries of sand,
gravel, cement, mill tailings or refuse, fly ash, or other solids.
Do injectate constituents exceed
drinking water standards at the
point of injection?
Available data from leaching tests (e.g., USEPA Method 1311--TCLP) of backfill materials
indicate that concentrations of antimony, arsenic, barium, beryllium boron, cadmium, chromium,
lead, mercury, molybdenum nickel, selenium, thallium, sulfate, and zinc frequently exceed
primary drinking water standards and health advisory levels. Levels of aluminum, copper, iron,
manganese, total dissolved solids, sulfate, and pH frequently exceed secondary drinking water
standards.
What are the characteristics of
the injection zone of a mine
backfill well?
Mine backfill wells inject into mined out portions of underground mines. The environmental
settings in which the mines are located and injection occurs are diverse.
Are there any contamination
incidents associated with mine
backfill wells?
No incidents of contamination of an underground source of drinking water have been identified
that are directly attributable to injection into mine backfill wells. Although ground water
contamination is not uncommon at mining sites, it is generally difficult to identify the specific
causes. The chance that backfill injection will contribute to ground water contamination is highly
dependent on site conditions, including mine mineralogy, site hydrogeology, backfill
characteristics, and injection practices.
Are mine backfill wells
vulnerable to spills or illicit
discharges?
The vulnerability of mine backfill wells to receiving spills or illicit discharges depends on site-
specific conditions and practices. For example, if coal ash is hauled to a mine site, slurried with
water, and then injected, the likelihood of contamination of the injected material resulting from a
spill or illicit discharge is relatively low. On the other hand, if mill tailings are collected in a
tailings pond along with site runoff and other facility wastes prior to injection, then the likelihood
of contamination of the backfill material by spills would be higher.
How many mine backfill wells
exist in the United States?
There are approximately 5,000 documented mine backfill wells and more than 7,800 wells
estimated to exist in the United States.
Where are mine backfill wells
located within the United
States?
Mine backfill wells are known to exist in 17 states. However, more than 90 percent of the
documented wells reported are in four states: WV (401), OH (3,570), ND (200), and ID (575).
How are mine backfill wells
regulated in states with the
largest number of this type of
well?
Permit by rule: ID, KS, TX, IL, and ND (sometimes general or individual permits are required)
General permit: WY
Individual or area permit: WV, OH, IN, PA
Where can I obtain additional
information on mine backfill
wells?
For general information, contact the Safe Drinking Water Hotline, toll-free 800-426-4791. The
Safe Drinking Water Hotline is open Monday through Friday, excluding federal holidays, from
9:00 a.m. to 5:30 p.m. Eastern Standard Time. For technical inquiries, contact Amber Moreen,
Underground Injection Control Program Office of Ground Water and Drinking Water (mail code
4606), EPA, 401 M Street, SW, Washington, D.C., 20460. Phone: 202-260-4891. E-mail:
lno reen. alliber V/ epa. go\ . The complete Class VUIC Study (EPA/816-R-99-014, September
1999), which includes a volume addressing mine backfill wells (Volume 10), can be found at
http://www.epa.gov/OGWDW/uic/cl5study.html.

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