Office of
United States	Ground Water and
Environmental Protection Agency	Drinking Water
What is a heat pump/air
conditioning return flow well?
What types of fluids are injected
into heat pump/air conditioning
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 heat
pump/air conditioning return
flow well?
Are there any contamination
incidents associated with heat
pump/air conditioning return
flow wells?
Are heat pump/air conditioning
return flow wells vulnerable to
spills or illicit discharges?
How many heat pump/air
conditioning return flow wells
exist in the United States?
Where are heat pump/air
conditioning return flow wells
located within the United
How are heat pump/air
conditioning return flow wells
regulated in states with the
largest number of this type of
Where can I obtain additional
information on heat pump/air
conditioning return flow wells?
Heat pump/air conditioning return flow wells are Class V underground injection control (UIC)
wells used to return ground water, which has been circulated through open-loop, heat pump/air
condition (HAC) systems, to the subsurface. These HAC systems heat or cool buildings by
extracting heat energy from ground water or using ground water as a heat sink when cooling.
Ground water that usually has the same quality as the original source ground water, except that it
may be cooler or wanner. Therefore, salts and other dissolved solids may either precipitate into
suspension or dissolve into solution. HAC injectate can also contain: metals leached from the
pipes and pumps; bacteria; precipitated ferric iron solids; and chemical additives.
Available data indicate that HAC injectate has in some cases exceeded the primary drinking water
standard for lead and copper and the secondary drinking water standard for chloride and total
dissolved solids.
HAC systems most commonly re-inject ground water into the same formation from which it is
withdrawn. The aquifer must be relatively porous in order to provide adequate ground water flow
to source wells and to accept fluids from return wells.
Relative to the number of HAC wells that exist in the United States, the number of documented
contamination incidents is small. Due to a leaking well casing in one well in NY, chloride and
total dissolved solid levels were found to be above secondary drinking water standards in one
underground source of drinking water (USDW). In MN, two different water samples indicated
high levels of lead and copper. This was attributed to leaching of metals from the HAC system
pipes and pumps. In NC, well samples have been reported to contain high levels of iron and
colifonn attributed to poor HAC well construction and operation.
HAC return flow wells are generally part of systems that are completely closed above ground, and
are generally located on private property. Therefore, the likelihood of USDW contamination by
illicit discharges from HAC return flow wells is very low.
There are 27,921 documented HAC return flow wells in 34 states and it is estimated that more
than 32,804 wells (but probably not more than 35,000) exist in over 40 states.
Approximately 88 percent of all documented wells are in four states: TX (46 percent), VA (28
percent), FL (11 percent), and TN (4 percent). Another 30 states collectively account for the
remaining 11 percent of the total documented United States inventory.
Permit by rule: AZ, IL, KS, MI, MN, NE, ND, NY, OH, PA, SC, TN, TX, VA, WV, WY
General permit WI (for open-loop discharge to shallow subsurface soil absorption field in the
unsaturated zone above the uppermost drinking water aquifer)
Individual permit. DE, FL, MD, MO, NV, NC, OR (unless individually exempted), VT, WA
Banned: WI (for open-loop discharge directly back into an aquifer)
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 heat pump/air conditioning return flow wells (Volume
19), can be found at http://www.epa.gov/OGWDW/uic/cl5study.html.