Geothermal energy harnesses the Earth’s internal heat, typically by tapping into underground reservoirs of hot water and steam to generate electricity. This resource is recognized as a renewable power source with a significantly lower carbon footprint than fossil fuels. However, extracting this heat is not entirely benign and can result in specific, localized environmental consequences. Understanding geothermal power’s impact requires examining the chemical pollutants brought to the surface, the physical demands placed on local ecosystems, and the geological risks associated with fluid management. These potential downsides demonstrate that even renewable energy requires careful site management to minimize negative environmental effects.
Release of Subsurface Gases and Minerals
Bringing geothermal fluids to the surface releases materials previously trapped deep within the Earth’s crust. A significant concern is the emission of non-condensable gases (NCGs), which are dissolved in the geothermal steam or hot water. These gases primarily include carbon dioxide (\(\text{CO}_2\)), hydrogen sulfide (\(\text{H}_2\text{S}\)), methane (\(\text{CH}_4\)), and ammonia.
Geothermal plants emit less \(\text{CO}_2\) per megawatt-hour than gas or coal-fired power stations, but the release still contributes to atmospheric greenhouse gases. \(\text{H}_2\text{S}\) is a more immediate problem, as it is often the most abundant NCG and is toxic, possessing a distinct rotten-egg odor even at low concentrations. This gas must be treated or abated before release to protect human health and surrounding ecosystems. Closed-loop binary cycle plants can virtually eliminate these atmospheric emissions because the geothermal fluid never directly contacts the atmosphere, unlike open-loop flash steam systems.
Beyond gases, the extracted geothermal fluid, known as brine, is often hypersaline and rich in dissolved solids and heavy metals. This fluid can contain high concentrations of elements such as arsenic, mercury, boron, lead, and zinc. If this spent brine is not properly handled, these toxic substances pose a serious risk of contaminating local surface water and groundwater systems. Effective environmental management requires the prompt and complete reinjection of the spent brine back into the deep reservoir, preventing the leaching of these dissolved minerals into the shallower water table.
The potential for chemical pollution is tied to the plant’s design and the specific chemistry of the geothermal reservoir. For instance, highly saline brines found in the Salton Sea region of California are particularly enriched in heavy metals. Without reinjection, the volume of mineral-laden wastewater from open-loop systems can overwhelm local disposal capacity, leading to ecological damage. Responsible operation depends on robust monitoring and engineering controls to ensure these subterranean contaminants remain sequestered.
Impacts on Local Water and Land Resources
Geothermal power generation places demands on local water resources, which is challenging in the arid regions where many geothermal fields are located. Water consumption depends heavily on the cooling system, with water-cooled systems requiring significant volumes for makeup fluid. Water-cooled geothermal facilities can consume between 1,700 and 4,000 gallons of water per megawatt-hour (\(\text{MWh}\)) of electricity produced.
This high rate of consumption places considerable stress on local aquifers and surface water bodies, competing with the water needs of agriculture and residential communities. Dry steam and flash plants use less freshwater, and binary plants that utilize air-cooling technology consume virtually none. Even when geothermal fluid is re-injected, some water is lost as steam during the power generation cycle, requiring outside water sources to maintain the reservoir’s volume and pressure.
Another concern is thermal pollution, which occurs when cooled, but still warm, wastewater is discharged into nearby rivers or lakes. Even a small temperature increase can disrupt the balance of aquatic ecosystems. Such thermal changes alter the dissolved oxygen levels and negatively affect the spawning cycles and biodiversity of local fish and plant life. Operators must manage the thermal discharge carefully to mitigate impacts on sensitive habitats.
Geothermal development requires a physical footprint on the land, leading to habitat disruption and fragmentation. Construction involves drilling sites, power plants, access roads, and extensive pipeline networks. This activity can destroy or fragment natural habitats, displacing local wildlife populations. Furthermore, long-term extraction of geothermal fluids without adequate reinjection can lead to land subsidence, where the ground surface sinks as the pressure in the underground reservoir drops.
Risk of Induced Seismic Activity
A specific geological risk associated with certain geothermal technologies is induced seismicity, commonly known as human-caused earthquakes. This phenomenon is tied to the injection of fluids deep underground, a practice used for both wastewater disposal and reservoir enhancement. The risk is significantly higher in Enhanced Geothermal Systems (EGS), which deliberately involve injecting high-pressure fluid into hot, dry rock formations.
The EGS process aims to fracture the rock, creating an artificial reservoir with the necessary permeability for heat exchange. This high-pressure fluid injection increases the pore pressure within the rock, which acts as a lubricant on pre-existing fault lines. By reducing the effective normal stress holding the fault together, the fluid allows the fault to slip, triggering a seismic event.
While many induced seismic events are micro-earthquakes too small to be felt, the potential for larger tremors remains a public concern. The injection of fluids, even in conventional geothermal systems, alters the subsurface stress field, meaning careful monitoring is necessary. Geothermal projects must employ robust seismic monitoring networks to track pressure changes and fault responses in real-time, allowing operators to adjust injection rates to mitigate the risk of triggering earthquakes.