United States
Environmental Protection Agency
Office of
Ground Water and
Drinking Water
CLASS V UIC STUDY FACT SHEET
GEOTHERMAL DIRECT HEAT RETURN FLOW WELLS
What is a geothermal direct heat
return flow well?
What types of fluids are injected
into geothermal direct heat
return flow wells?
Do injectate constituents exceed
drinking water standards at the
point of injection?
What are the characteristics of
the injection zone of a
geothermal direct heat return
flow wells?
Are there any contamination
incidents associated with
geothermal direct heat return
flow wells?
Are geothermal direct heat
return flow wells vulnerable to
spills or illicit discharges?
How many geothermal direct
heat return flow wells exist in
the United States?
Where are geothermal direct
heat return flow wells located
within the United States?
How are geothermal direct heat
return flow wells regulated in
states with the largest number of
this type of well?
Where can I obtain additional
information on geothermal
direct heat return flow wells?
Geothermal direct heat return flow wells are Class V underground injection control (UIC)
wells that return geothermal fluids, which have been used to heat individual homes and/or
communities or to provide heat to greenhouses, aquaculture, and other commercial and
industrial processes, to the subsurface.
The temperature and chemical characteristics of geothermal fluids used for heating vary
substantially from site to site. In general, the composition of the geothermal fluids normally
does not change appreciably as a result of use for heating, although traces of pump lubricating
oil may be added in some cases.
While some geothermal fluids are of drinking water quality, it is more common for
concentrations of some constituents to exceed drinking water standards. Available data
indicate that arsenic, boron, sulfate, and fluoride exceed primary drinking water standards or
health advisory levels and that total dissolved solids, chloride, iron, manganese, and sulfate
exceed secondary drinking water levels.
When geothermal fluids used for heating are reinjected into the subsurface following use
(rather than discharged to surface water or used for drinking, irrigation, or livestock watering),
they typically are reinjected into the same hydrothennal formation from which they were
produced.
No documented cases of underground sources of drinking water contamination by geothermal
direct heat return flow wells have been reported.
Typically, geothermal direct heat return flow wells are not vulnerable to illicit discharges
because the geothermal fluids are handled in closed piping systems.
There are 31 documented geothermal direct heat return flow wells and another 17 wells
estimated to exist nationwide.
These wells exist in as many as 11 states, although more than 80 percent of the documented
wells are in only five states: OR (8), NV (7), UT (4), NM (4) and ID (3). The 17 estimated
wells are in OR.
Permit by rule: ID (<18 ft deep)
Individual permit: CA, NM, NV, UT, OR, ID (>18 ft deep)
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 V UIC Study (EPA/816-R-99-
014, September 1999), which includes a volume addressing geothermal direct heat return flow
wells (Volume 18), can be found at http://www.epa.gov/OGWDW/uic/cl5study.html.

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