Table of Contents
Overview
Geothermal energy is often described as a relatively low impact renewable source, especially when compared to fossil fuels. However, it is not impact free. Its environmental footprint depends on the type of geothermal system, depth and temperature of the resource, technologies used, and site conditions. This chapter focuses on the specific environmental impacts, both negative and positive, that are most relevant to geothermal projects, and how these impacts can be assessed, minimized, and managed in practice.
Land Use And Landscape Changes
Geothermal power plants and direct use facilities occupy land for wells, pipelines, power plant buildings, cooling systems, and access roads. Compared with many other power technologies, the land footprint per unit of electricity can be relatively small, especially for high temperature fields where many megawatts can be produced from a compact site. Even so, land occupation and landscape change matter locally.
Surface installations can alter the visual character of an area, particularly in scenic or protected landscapes. Drilling rigs, steam plumes, and pipelines may be considered intrusive by nearby communities or visitors. For low temperature direct use and geothermal heat pumps, land use impacts are usually modest since installations can often be integrated into existing buildings or buried underground.
The planning phase is critical to avoid sensitive areas such as protected habitats or culturally important landscapes. Careful layout of pipelines and well pads, use of existing roads, and design choices that reduce visible structures all help to limit land and visual impacts over the project life.
Water Use, Thermal Pollution, And Scaling
Geothermal systems interact closely with water. Water is present as the geothermal fluid, and additional water may be required for cooling in some plant designs. If cooling towers use fresh water in arid regions, competition with other water users can become a concern. Closed loop cooling or air cooling can greatly reduce this pressure, but may impose higher costs or slightly lower efficiency.
Produced geothermal fluids are typically hot and often contain dissolved minerals and gases. If these fluids are discharged at the surface into rivers or lakes, they can raise local water temperature. This thermal pollution can affect aquatic life by altering oxygen levels and species composition. For this reason, modern plants increasingly favor reinjection of fluids back into the reservoir to maintain pressure, minimize surface discharges, and preserve water quality.
The high mineral content of geothermal fluids can cause scaling and corrosion in pipes and equipment. Scaling materials, such as silica or carbonates, may need to be periodically removed and managed as solid waste. Where these wastes contain harmful trace elements, appropriate treatment and disposal are essential to prevent contamination of soil or water bodies.
Air Emissions And Non-Condensable Gases
While geothermal energy does not involve combustion, it can still result in air emissions. Some geothermal reservoirs, especially high temperature systems, naturally contain non condensible gases that are released when steam and hot water reach the surface. Common gases include carbon dioxide, hydrogen sulfide, methane, and others in smaller quantities.
Carbon dioxide emissions from geothermal power are much lower on average than from coal or gas fired plants, yet they are not always negligible. The magnitude depends on the geology of the reservoir. Reinjection strategies and gas treatment technologies can further reduce these emissions.
Hydrogen sulfide is of particular concern because it has a strong odor and can be toxic at higher concentrations. Without control, it can contribute to local air pollution and acid deposition. Modern geothermal plants typically use abatement systems to capture and convert hydrogen sulfide into solid or less harmful forms before releasing cleaned air.
Air emissions from geothermal also include small quantities of trace substances, such as mercury or ammonia, that may be naturally present in the fluids. These are managed through gas treatment systems and by favoring technologies that keep fluids in closed loops, particularly in binary cycle plants where the geothermal fluid does not come into direct contact with the atmosphere.
Key point: Geothermal plants can emit non condensible gases that originate from the reservoir, especially CO$_2$ and H$_2$S, but modern designs and abatement technologies significantly reduce these emissions compared with fossil fuel plants.
Subsurface Impacts: Reservoirs, Induced Seismicity, And Subsidence
Using geothermal resources alters conditions in the subsurface. When large volumes of hot fluids are extracted, reservoir pressure and temperature change. If not carefully managed, this can lead to reservoir depletion and a gradual decline in output. Reinjection of cooled geothermal fluids back into the reservoir is a standard practice to maintain pressure, improve long term sustainability, and reduce surface discharges.
Changes in pressure and fluid movement can also lead to induced seismicity. Small earthquakes may occur when existing faults are reactivated by injection or extraction activities. High temperature conventional fields and especially enhanced geothermal systems are more prone to this issue because they involve deep drilling, stimulation, or high injection rates. Most induced events are very small and only detectable by instruments, but in rare cases they may be felt at the surface.
To manage seismic risk, developers perform detailed geological and geophysical studies to map faults, monitor microseismic activity during drilling and operation, and adjust injection pressures and rates if needed. Regulatory frameworks often define thresholds for ground motion or event magnitude that trigger changes in operation.
Another potential subsurface impact is land subsidence. When large volumes of fluid are withdrawn without sufficient reinjection, the rocks can compact and the land surface may gradually sink. This can damage infrastructure such as buildings, roads, and pipelines, especially in urban or agricultural areas. Reinjection strategies and carefully designed production rates are central tools to avoid or minimize subsidence.
Solid And Liquid Wastes, Chemical Handling
Geothermal development generates several types of waste that must be managed responsibly. During drilling, drilling muds and cuttings are produced. These materials may contain natural minerals and, depending on local geology, trace amounts of metals. Proper storage and disposal of drilling wastes are required to prevent contamination of soils and water bodies.
During plant operation, geothermal fluids can precipitate minerals that form scale in pipes and heat exchangers. Periodically, this scale is removed and handled as solid waste. The composition of these solids varies; in some cases, valuable minerals can be recovered, while in others, hazardous constituents must be carefully contained.
Liquid wastes may include separated brines, condensate, and cleaning solutions from equipment maintenance. Reinjection of brines into suitable geological formations is the preferred approach, as it returns fluids to depth and limits surface exposure. When treatment is needed, technologies such as neutralization, filtration, and evaporation are applied before discharge or reuse.
Plants also use various chemicals for corrosion inhibition, scaling control, and cleaning. Storing, using, and disposing of these chemicals safely is essential. Secondary containment for storage tanks, spill prevention, worker training, and adherence to environmental regulations all contribute to minimizing the risk of accidental releases.
Noise, Odors, And Local Nuisances
Geothermal development introduces new sources of noise and potential odors into the local environment. During drilling and well testing, noise levels can be high, sometimes comparable to heavy construction activities. This phase is temporary but can disturb nearby residents or wildlife. Once the plant is operating, noise from turbines, cooling towers, and pumps is usually lower, but still needs to comply with local regulations.
Hydrogen sulfide, when present, has a characteristic rotten egg odor even at very low concentrations. Although modern gas abatement significantly reduces emissions, occasional smells may still occur near vents or during maintenance. Good design of exhaust points and continuous monitoring help to keep concentrations below nuisance levels.
Community engagement plays an important role. Informing residents about the nature and duration of noise, scheduling the loudest activities at less sensitive times, and implementing sound barriers or improved insulation in equipment can all reduce the perceived impact.
Biodiversity And Ecosystem Considerations
Geothermal projects can affect ecosystems, especially in locations with unique geological and biological features. Many geothermal areas coincide with hot springs, geysers, or volcanic landscapes that host specialized species and have high conservation or tourism value. Surface installations, land clearing, and altered hydrothermal flows can disturb habitats or change water chemistry.
Direct use applications that involve discharging warm water into rivers or wetlands can change local temperature regimes, favoring some species and disadvantaging others. This can reduce biodiversity if sensitive species are displaced. Reinjection, appropriate siting of discharge points, and cooling of effluents before release help to limit such effects.
Baseline ecological studies before project development identify sensitive habitats and species. With this information, designers can adjust the footprint of infrastructure, maintain buffer zones, and plan restoration activities after construction. Where necessary, some areas may be designated off limits to preserve especially valuable geothermal features.
Positive Environmental Contributions
Despite its specific impacts, geothermal energy provides several environmental benefits that are central to a sustainable energy system. By displacing fossil fuel based electricity and heat, geothermal projects reduce greenhouse gas emissions, air pollutants such as particulate matter, sulfur dioxide, and nitrogen oxides, and the health impacts associated with them.
Geothermal plants usually have a small land footprint relative to their continuous output, and they deliver firm, reliable energy that can support a higher share of variable renewables in power systems. In the heating sector, geothermal district heating and heat pumps can replace oil or gas boilers and significantly improve local air quality in cities and towns.
When well designed and managed, geothermal systems can operate for decades with stable output and limited surface disturbance, creating a long term, low emission source of energy that complements other renewables.
Mitigation, Monitoring, And Best Practices
The overall environmental performance of a geothermal project depends on how impacts are anticipated and managed across its life cycle. Environmental impact assessments help identify key risks and inform siting decisions, technology choices, and operational plans. Reinjection is a central strategy to reduce water contamination, maintain reservoir pressure, and avoid subsidence. Gas abatement systems and closed loop plant designs significantly cut air emissions. Careful control of injection pressures and locations reduces the likelihood of problematic induced seismicity.
Ongoing monitoring of ground motion, seismic activity, air and water quality, and noise is crucial. Monitoring allows operators to detect early signs of environmental stress and adjust operations. Transparent reporting and engagement with local communities and regulators builds trust and allows concerns to be addressed quickly.
Geothermal projects that apply these best practices demonstrate that it is possible to harness subsurface heat while respecting environmental limits. They provide valuable experience for the further development of geothermal resources, including more advanced systems that will be explored elsewhere in the course.