Environmental Impact of Geothermal Energy, Traced to the Source

Ask about the environmental impact of geothermal energy and you will get answers about three different technologies, usually without anyone saying which one they mean. Hydrogen sulfide emissions and induced earthquakes belong to utility-scale geothermal power plants. A ground-source heat pump in a suburban backyard has neither — its environmental questions are about the electricity it draws, the refrigerant sealed inside it, the antifreeze circulating underground, and the trench or borehole it took to install.

This article is about that second technology: the ground-source heat pump (GSHP) that heats and cools buildings. Every claim below says which technology it covers and within which boundary — operation only, or the whole system including drilling and refrigerant — because "geothermal is clean" means nothing until both are specified.

Three Technologies Share One Name

"Geothermal" covers three technologies with completely different environmental profiles, and much of what reads as contradiction in this topic is one being quietly swapped for another.

Utility-scale geothermal power plants generate electricity from deep, hot resources. Their environmental file includes hydrogen sulfide emissions, induced seismicity, and brine disposal — real issues, and irrelevant to a house. A power plant also occupies its site permanently.

Direct-use geothermal pipes naturally hot water into district heating systems, greenhouses, and spas. It requires a hot resource under your feet, which most of the world does not have.

Ground-source heat pumps — this article — work anywhere, because they don't need a hot resource at all. A GSHP is an electric appliance that moves heat between a building and the ordinary ground, using the fact that soil temperature a few feet down barely changes across the seasons. Nothing is burned on site, nothing is vented, and in a closed-loop system nothing is drawn from the ground but heat. If the three-way distinction is new to you, our guide to what geothermal energy actually is separates them properly.

So when a critique of geothermal's environmental record cites earthquakes or sulfur smell, it is describing power plants. When a sales page says "zero emissions," it is describing a heat pump's on-site operation — and staying silent about the power plant at the other end of the wire. Attach the technology and the boundary, and the apparent contradiction disappears.

Does Geothermal Energy Cause Pollution?

For a ground-source heat pump in normal operation, the answer breaks into four channels. This is an operational boundary: it covers running the system and putting it in the ground, and it sets aside the manufacturing, material extraction, transport and end-of-life treatment a full lifecycle assessment would add.

1. Electricity. The heat pump burns nothing, but it runs on grid power, and the grid's generation mix determines the operating emissions. Within this operational boundary it is the channel that recurs every day the system runs, and it is covered in the next two sections.
2. Refrigerant. The sealed refrigerant circuit contains a fluid with a global warming potential hundreds to thousands of times that of CO₂, pound for pound, if it escapes. Which fluid is in the box changed by regulation in 2025.
3. Loop fluid. Closed loops in cold climates circulate a water-antifreeze mix through buried pipe. The environmental question is what happens if it leaks, and EPA has published its own answer.
4. Installation. Drilling boreholes or digging trenches disturbs the site once, during construction.

Notice what is not on the list: combustion products at the house. A GSHP has no flue and no on-site CO₂, because there is no fire. Whatever pollution exists sits at the power plant supplying the electricity, in the refrigerant chemistry, or underground at the loop. The rest of this article takes those in order — starting with the number everyone quotes.

The Most-Quoted Number Is From 1999

Search this topic and you keep landing on an EPA finding that geothermal heat pumps cut emissions "up to 44%." Follow it back and you reach a Department of Energy Federal Energy Management Program publication, DOE/GO-10099-727, dated September 1999 — which is itself summarising an EPA study it describes only as "one study," without naming it or dating it. That makes 1999 the date of the DOE document reporting the finding. The underlying EPA work is older still, by an unstated margin. Here is the passage, verbatim:

"In one study of all residential heating, cooling, and water-heating systems, the U.S. Environmental Protection Agency concluded that (1) GHPs can reduce energy consumption and related emissions by 23%-44% in comparison to air-source heat pumps, (2) GHPs generally have lower carbon dioxide emissions than conventional equipment"

— DOE Federal Energy Management Program, DOE/GO-10099-727, September 1999

Read closely, the figure carries three qualifiers that tend to fall away in the retelling:

• It was published in 1999, and the study behind it is older. The finding predates the modern grid, modern heat pumps, and every refrigerant discussed below. The American electricity mix has changed substantially since then, and operating emissions track the grid — so a percentage of at least that vintage cannot describe a system installed today.
• The baseline is air-source heat pumps. Not gas furnaces, not "conventional heating." The 23–44% range measures GSHPs against the other kind of heat pump. Pages that present it as a savings figure versus a furnace are attaching the number to a comparison it never made.
• It covers energy consumption and the emissions related to it. One range, applied to energy use and the emissions that follow from it — not an emissions-only measurement. The separate emissions statement in the same sentence is qualitative — "generally have lower carbon dioxide emissions than conventional equipment" — with no percentage and no named baseline.

None of this makes the finding wrong. It makes it a finding published in 1999 about energy use relative to air-source equipment — a narrower and older claim than the one usually built on top of it. The number this topic leans on is a generation old, and no more recent federal percentage has taken its place.

Operating Emissions Are Your Grid's Emissions

DOE's description of the technology is one sentence long: "Heat pumps move heat from one place to another using electricity." That sentence contains the entire operating-emissions story. The heat pump's own contribution is efficiency — it delivers several units of heat per unit of electricity — but the emissions attached to each of those units of electricity are set by whoever generates it.

So there is no universal answer to "how much does a geothermal heat pump cut emissions?" On a grid heavy with coal, the electricity carries substantial emissions and the heat pump's advantage rests on its efficiency multiple. On a grid dominated by hydro, nuclear, or renewables, operating emissions approach zero. Same machine, different wire, different answer.

The peer-reviewed literature treats it the same way. A study in Environmental Research Letters (2007) evaluated heat-pump emission reductions explicitly "for different electricity generation methods, heat pump efficiencies, and heating loads" — three variables, none of them constant across households. A single nationwide reduction percentage for today's installations is not a figure the federal sources support.

What you can control is the efficiency side of the equation. How much electricity a system actually draws, and what determines it, is covered in our guide to geothermal electricity usage.

The Refrigerant Inside the Cabinet

Every heat pump — ground-source or air-source — contains a sealed refrigerant circuit, and refrigerants are potent greenhouse gases if they escape during a leak, a repair, or disposal. It is an easy channel to overlook, and it is where the rules changed most recently.

The measuring stick is global warming potential (GWP): how much warming a pound of the gas causes relative to a pound of CO₂. Per EPA's GWP reference table (implementing 40 CFR 84.64, last updated April 2, 2026):

• R-410A, the refrigerant in legacy residential geothermal units and much of the installed base, has a GWP of 2,088.
• R-454B, the common low-GWP replacement, has a GWP of 465 — roughly a 78% reduction per pound released.

The regulatory driver is EPA's Technology Transitions program. Under the final rule of October 24, 2023 (88 FR 73098), residential and light-commercial stationary air conditioning and heat-pump systems are capped at a GWP of 700, with a compliance date of January 1, 2025.

Two details of that rule are easy to get wrong:

The transition is visible in manufacturer catalogs: ClimateMaster's Tranquility TS series lists R-410A while its Tranquility SZ "utilizes R-454B low Global Warming Potential (GWP) refrigerant," and Bosch's TW series is sold in an R-454B version. For a homeowner, the practical question at quote time is simply which refrigerant the proposed unit contains — the answer now separates a GWP of 2,088 from a GWP of 465 sitting in your basement for the next two decades.

One boundary note: refrigerant GWP is a potential, realized only if the charge escapes. No reliable leak-rate figure for residential ground-source units is published, so treat the sealed charge as a liability proportional to its GWP and to how it is handled at service and end of life.

Antifreeze and Groundwater

A closed-loop system circulates fluid through buried plastic pipe, and in cold climates that fluid is water plus antifreeze. The reasonable homeowner question — what happens to the ground if it leaks? — has a primary federal source: EPA's Manual on Environmental Issues Related to Geothermal Heat Pump Systems (EPA 430-B-97-028), published in September 1997. It remains the most direct federal treatment of the subject. It is also nearly three decades old, so its specifics describe practice as of 1997 and should be read as reference points, not a survey of today's job sites.

What the 1997 manual lays out:

• The fluids. Closed loops circulate water, or water mixed with potassium acetate, sodium chloride, calcium chloride, ethanol, methanol, ethylene glycol, or propylene glycol.
• The quantities. Its example 5-ton residential loop holds about 60 gallons of fluid total, roughly 15 gallons (25%) of it ethanol, providing freeze protection to 14°F. That is the full inventory at stake in a worst-case total release — comparable in volume to a few fuel tanks, not a tanker truck.
• The testing. Installation practice it describes includes a pressure test at 100 psi held for four hours, against normal operating pressures of 20 to 30 psi.
• The leak risk. Once properly installed and operating, the manual puts the probability of a leak at "small to none."

EPA is specific about what makes a release matter: the impact depends primarily on the antifreeze's toxicity and the volume released, with exposure possible if fluid reaches groundwater and migrates to wells or lakes. The same manual identifies a second, less obvious risk that has nothing to do with antifreeze: an improperly constructed borehole can itself become a channel between the surface and groundwater, or between aquifers. That is a construction-quality issue — it exists even with pure water in the pipe, and it is a reason borehole grouting practice matters as much as fluid choice.

The current industry reference on loop fluids is PPI MS-7 (May 2023), hosted by IGSHPA. It names propylene glycol, ethanol, and methanol — with approved corrosion inhibitors and environmental stabilizers — and requires other fluids to comply with ANSI/CSA/IGSHPA C448 or local codes. It also flags ethanol and methanol as flammable, which is a handling issue for the installer more than an in-ground one. MS-7 describes itself as a model specification, adopted at a project's or local authority's discretion, so which fluids are permitted where you live is a local-code question — no current nationwide table of those rules exists, and your installer and permitting office are the source of record.

The pipe, fittings, and grout that keep all of this contained have their own standards, covered in our guide to ground loop components. And note this whole section is about closed loops; open-loop systems use groundwater itself as the heat-exchange fluid, which trades the antifreeze question for water-supply, water-quality, and discharge-permit questions of its own.

Land Use: Digging Up the Yard

Installing a ground loop disturbs the site once. DOE's GSHP overview (February 2009) gives the footprint for the two main configurations:

For a typical 3-ton home system, a vertical installation touches roughly 600 to 1,200 square feet of yard during drilling; a horizontal one opens 6,000 to 10,500 square feet of trench area. EPA's 1997 manual adds the construction-phase obligations: horizontal trenches run about 5 feet deep and potentially several thousand feet long, and excavated soil has to be managed so it doesn't migrate into surface waters or sewers, then regraded or removed. The same 1997 document describes vertical bores reaching depths up to 400 feet, spaced typically 10 to 25 feet apart.

The distinction that matters for an environmental accounting: this disturbance is temporary and restorable. Once the loop is buried and the surface regraded, the yard goes back to being a yard — lawn, garden, driveway — with the pipe doing its work out of sight below. That is a different kind of footprint from a geothermal power plant, which occupies its site for its operating life. What the buried half of the system actually consists of is covered in our guide to what a geothermal system is.

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