Geothermal Energy And Water Pollution: What's The Risk?

Geothermal energy is heat contained below the Earth's surface. The most widely developed type of geothermal power plant is known as a hydrothermal plant, which is located near geological hot spots where molten rock produces hot water. Geothermal energy can impact the environment in various ways, including water contamination. Underground geothermal reservoirs often contain high concentrations of harmful substances like sulfur, salts, and other compounds and minerals. Open-loop geothermal power plants can cause these substances to seep into the local environment, potentially affecting the quality of the local water table. Water contamination by spent geothermal fluids is a health hazard.

Open-loop systems can cause water contamination

Open-loop geothermal systems can cause water contamination. Open-loop systems use groundwater from an aquifer, which is piped directly from a well to a building, where it transfers heat to a geothermal heat pump. The water is then expelled back into the same aquifer via a discharge well. This process is often referred to as a "pump and dump" system.

Secondly, open-loop systems release waste gases and steam into the atmosphere, which can lead to air and water pollution. These emissions include hydrogen sulfide, carbon dioxide, ammonia, methane, and boron. While open-loop systems emit less sulfur dioxide and carbon dioxide than fossil fuel power plants, the hydrogen sulfide can turn into sulfur dioxide once in the atmosphere, contributing to the formation of acidic particulates that are harmful to human health and the environment.

Additionally, open-loop systems can affect water quality by discharging dissolved solids and toxic heavy metals into water sources. These include sulfur, chlorides, silica compounds, vanadium, arsenic, mercury, nickel, and other hazardous substances. If these contaminants become concentrated, they can create localized fish and wildlife kills, as well as impact the health of surrounding ecosystems.

Due to the potential for environmental contamination, some municipalities do not allow open-loop systems at all. Closed-loop systems, on the other hand, minimize air emissions by injecting gases removed from the well back into the ground.

Closed-loop systems reduce environmental impact

Geothermal energy is heat contained below the Earth's surface. The most common type of geothermal power plant is a hydrothermal plant, which uses hot water or steam to generate electricity.

However, geothermal power plants can impact water quality and consumption. Hot water pumped from underground reservoirs often contains high levels of sulfur, salt, and other minerals. Most geothermal facilities have closed-loop water systems, in which the extracted water is pumped directly back into the geothermal reservoir after it has been used for heat or electricity production. In such systems, the water is contained within steel well casings cemented to the surrounding rock, and there have been no reported cases of water contamination from geothermal sites in the United States.

Closed-loop systems directly convert geothermal steam or hot water into electricity. Gases or fluids removed from the well are not exposed to the atmosphere and are usually injected back into the ground after releasing their heat. In contrast, open-loop systems expel waste steam and gases into the atmosphere and generally have a greater environmental impact.

Closed-loop geothermal systems offer several advantages over conventional geothermal and enhanced geothermal systems:

Air and Water Quality

• They consume little to no process water.
• They reduce issues with saline and corrosive brines inside the system.
• They have fewer effluent and waste disposal problems and permitting issues.
• They do not interfere with subsurface water.

Public Safety and Environmental Soundness

• They do not cause surface subsidence.
• They do not use hazardous chemicals.
• They pose no risk of induced seismicity.

Closed-loop systems can operate in a much broader range of temperatures and rock compositions than conventional hydrothermal projects. They can also produce power from previously unproductive geothermal wells and played-out oil and gas wells in hot strata. Additionally, closed-loop systems can enhance industrial applications, such as high-value lithium extraction and hydrogen production, while lowering GHG emissions.

Water consumption for cooling and re-injection

Most geothermal facilities use closed-loop water systems, where the extracted water is pumped directly back into the geothermal reservoir after it has been used for heat or electricity production. In such systems, the water is contained within steel well casings cemented to the surrounding rock, and there have been no reported cases of water contamination from geothermal sites in the United States.

Water is also used for cooling in some geothermal plants. All U.S. geothermal power facilities use wet-recirculating technology with cooling towers. The amount of water required depends on the cooling technology used and can range between 1,700 and 4,000 gallons of water per megawatt-hour. Geothermal plants can use either geothermal fluid or freshwater for cooling, with the former being preferable to reduce the overall water impact.

Re-injection of water into the reservoir is a common practice to prevent contamination and land subsidence. However, not all water removed from the reservoir is re-injected, as some is lost as steam. To maintain a constant volume of water in the reservoir, outside water, such as non-potable treated wastewater, may be used. This helps to prevent the reservoir from running dry and ensures the continued operation of the geothermal power plant.

The use of water in geothermal energy production is carefully managed to minimize any potential negative impacts on the environment and ensure the sustainable utilization of this renewable energy source.

Geothermal plants emit harmful gases

Geothermal energy is heat contained below the Earth's surface. The most widely developed type of geothermal power plant is known as a hydrothermal plant, which is located near "hot spots" where molten rock close to the Earth's crust produces hot water.

Geothermal plants emit gases, but the type and amount depend on the technology used. Dry steam and flash steam power plants emit mostly water vapour, while binary-cycle power plants emit virtually no gases because they operate using a closed-loop system. When geothermal power plants do emit gases, it's usually carbon dioxide, which is a greenhouse gas, but in much smaller amounts than fossil fuel power plants.

Open-loop systems, however, do emit harmful gases. These include hydrogen sulfide, carbon dioxide, ammonia, methane, and boron. Once in the atmosphere, hydrogen sulfide changes into sulfur dioxide (SO2), which contributes to the formation of small acidic particulates that can be absorbed by the bloodstream and cause heart and lung disease. Sulfur dioxide also causes acid rain, damaging crops, forests, and soils, and acidifying lakes and streams.

Some geothermal plants also produce small amounts of mercury emissions, which must be mitigated with mercury filter technology.

Land subsidence and seismic instability

Land Subsidence

Land subsidence refers to the gradual sinking of the Earth's surface due to the removal of water from geothermal reservoirs. This phenomenon can lead to changes in ground structure, impacting the stability of the land. The removal of water can be a result of the extraction and reinjection processes involved in geothermal energy production. To mitigate this risk, most geothermal facilities re-inject wastewater back into the reservoirs to maintain stable water volumes. This practice also helps to prevent contamination of water sources.

The consequences of land subsidence can be significant. It can damage buildings, roads, and other infrastructure in the surrounding area. Additionally, it can disrupt natural habitats and ecosystems, affecting wildlife and plant life. Agricultural areas near geothermal power plants are particularly vulnerable to the effects of land subsidence, as it can impact crop growth and land usability. Continuous monitoring and careful planning are vital to minimize the negative consequences of land subsidence.

Induced Seismic Activity

Geothermal energy extraction can also trigger induced seismic activity, especially in areas near fault zones. The injection of water into geothermal reservoirs can increase pressure within the Earth, leading to small earthquakes. These seismic events, known as induced seismicity, are typically localized near the operation sites and are of low magnitude. However, they still pose risks that need to be addressed.

To minimize the impact of induced seismic activity, proper site selection and monitoring are crucial. Geothermal plants should be located at an appropriate distance from major fault lines. Additionally, constant monitoring and transparent communication with local communities are essential when a geothermal system is sited near populated areas.

In summary, land subsidence and induced seismic activity are important considerations in the environmental impact assessment of geothermal energy projects. Proper management techniques, such as wastewater re-injection, careful site selection, and continuous monitoring, can help reduce the negative effects of these phenomena on the environment and surrounding communities.

Frequently asked questions