Geothermal energy harnesses the Earth’s internal heat to generate electricity. This process typically involves drilling wells to access the heat, which is used to spin turbines. While this power source is consistently available and often promoted as a clean alternative to fossil fuels, it is not without environmental trade-offs. The environmental consequences of geothermal power are highly dependent on the specific geologic characteristics of the site and the technology employed at the power plant.
Comparison of Atmospheric Emissions
Geothermal power plants offer a substantial reduction in greenhouse gas emissions compared to coal or natural gas facilities. Conventional coal-fired plants can emit around 1,000 grams of carbon dioxide equivalent (\(\text{CO}_2\text{e}\)) per kilowatt-hour of electricity generated. In contrast, geothermal facilities typically range from 6 grams to about 79 grams of \(\text{CO}_2\text{e}\) per kilowatt-hour, depending on the reservoir’s chemistry and the plant’s design.
These lower emissions arise because geothermal operations do not rely on combustion. However, geothermal reservoirs naturally contain dissolved gases that are released. These non-condensable gases (NCGs) primarily include carbon dioxide, but also small quantities of hydrogen sulfide (\(\text{H}_2\text{S}\)), methane, and ammonia.
The release of \(\text{H}_2\text{S}\) is of particular concern for localized air quality, as it is a noxious gas with a detectable odor even at low concentrations. Older flash steam plants, which vent steam and gases to the atmosphere, have higher localized emissions compared to modern designs. Newer closed-loop binary cycle plants are designed to circulate the geothermal fluid in a sealed system, transferring heat to a secondary working fluid.
The geothermal fluid and its dissolved NCGs are reinjected back into the reservoir. Reinjection systems significantly mitigate the release of NCGs, making the atmospheric emissions of these modern plants negligible. For plants that still release some gases, abatement technologies are often used to strip the \(\text{H}_2\text{S}\) out of the steam before it is vented.
Impact on Local Water Resources
Geothermal power generation requires significant water resources, especially for cooling, which can create localized conflicts in arid regions. Binary cycle plants, while having lower air emissions, require substantial amounts of make-up water for their cooling towers. This water consumption can raise concerns in areas where existing water supplies are already under stress from agriculture or municipal use.
The primary water-related environmental challenge involves the geothermal fluid itself, known as brine. This brine is often highly saline and contains elevated concentrations of salts and toxic heavy metals. Trace elements such as arsenic, mercury, and boron are naturally present in the geothermal fluid.
Improper disposal of this geothermal brine poses a significant risk of contaminating shallow aquifers and local surface water bodies. For sustainable operation, the cooled brine is typically reinjected deep underground to maintain reservoir pressure and prevent surface discharge. If reinjection wells are poorly managed or if the fluid migrates, the toxic elements can leach into usable groundwater sources.
A less common but still relevant water impact is thermal discharge, which occurs when cooled geothermal fluids or cooling water are released into nearby streams or lakes. Even if the water is chemically safe, the introduction of warmer water can disrupt aquatic ecosystems. This thermal pollution can alter dissolved oxygen levels and affect the reproductive cycles and behavior of local fish and wildlife.
Land Footprint and Geologic Stability
The physical infrastructure required for a geothermal project includes the power plant, extensive piping networks, and multiple well pads. While the total area required per unit of energy produced is generally smaller than that of large solar or wind farms, the impact on the local landscape is permanent. Construction and drilling activities can lead to visual disturbances and habitat fragmentation in the immediate vicinity.
Localized noise pollution is also a factor, particularly during the drilling and construction phases of a project. Once operational, the continuous noise from turbines, pumps, and venting steam requires mitigation efforts, especially for plants located near residential areas. These localized effects are highly impactful on the surrounding environment.
A more complex geological impact is the potential for induced seismicity, which refers to small, human-caused earthquakes. This can occur when fluids are injected into the deep subsurface, a practice common in both reservoir maintenance and the development of Enhanced Geothermal Systems (EGS). The injection of fluid increases the pore pressure within the rock formation.
This pressure increase can effectively lubricate pre-existing faults that are already under tectonic stress, reducing the friction that holds them in place. While most induced seismic events are micro-earthquakes too small to be felt, some operations have been associated with felt earthquakes, occasionally reaching magnitudes that cause public concern or minor damage. To manage this risk, geothermal operators employ rigorous monitoring protocols to track seismic activity and adjust injection rates to maintain geological stability.