&EPA

Office of

United States	Ground Water and

Environmental Protection Agency	Drinking Water

CLASS V UIC STUDY FACT SHEET
GEOTHERMAL ELECTRIC POWER WELLS

What is a geothermal
electric power well?

What types of fluids are
injected into geothermal
electric power wells?

Do injectate constituents
exceed drinking water
standards at the point of
injection?

What are the characteristics
of the injection zone of a
geothermal electric power
well?

Are there any
contamination incidents
associated with geothermal
electric power wells?

Are geothermal electric
power wells vulnerable to
spills or illicit discharges?

How many geothermal
electric power wells exist in
the United States?

Where are geothermal
electric power wells located
within the United States?

How are geothermal electric
power wells regulated in
states with the largest
number of this type of well?

Where can I obtain
additional information on
geothermal electric power
wells?

Geothermal electric power wells are Class V underground injection control (UIC) wells used
to dispose of spent (meaning cooled) geothermal fluids following the extraction of heat for the
production of electric power.

Geothermal fluids extracted from subsurface hydrothennal systems. The characteristics of
these fluids vary, with some having high concentrations of total dissolved solids (TDS) and/or
metals. In some cases, power plants add supplemental water from other sources, such as
surface waters, storm waters, ground water, and wastewater treatment effluent

Available sampling data show that concentrations of some metals (e.g., antimony, arsenic,
cadmium, lead, mercury, strontium, zinc) and other constituents in the injected geothermal
fluids routinely exceed primary drinking water standards or health advisory levels at one or
more geothermal fields. The data also indicate that sulfate, chloride, manganese, iron, pH,
and TDS frequently exceed secondary drinking water standards.

Geothermal fluids used for electric power generation are normally injected into the same
subsurface hydrothennal system from which they were produced. A majority of geothermal
injection wells were drilled as production wells and subsequently converted to injection wells.

One monitoring investigation showed that at a geothermal power plant site in HI, the
temperature, chloride concentrations, and chloride/magnesium ratios in the surrounding
ground waters increased following a well failure during the drilling of an injection well.

Geothermal electric power wells are not vulnerable to receiving spills or illicit discharges
because geothermal fluids are handled in closed piping systems that are managed as an
integral part of the power plant system.

There are a total of 234 documented geothermal electric power wells. The number of
geothermal power injection wells is not expected to increase substantially in the foreseeable
future.

CA, UT, HI, and NV have all of the 234 documented wells, with most of the wells reported in
CA (174, or 74 percent) and NV (53, or 23 percent).

Individual permit'. CA, HI, NV, UT

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: moreen.amber(@epa.gov. The complete Class VUIC Study (EPA/816-R-99-
014, September 1999), which includes a volume addressing geothermal electric power wells
(Volume 17), can be found at http://www.epa.gov/OGWDW/uic/cl5study.html.


-------