Hydroelectric Power Plants: Pollution Paradox?

Hydropower is a form of renewable energy that uses the water stored in dams and flowing in rivers to create electricity in hydropower plants. Hydroelectric power plants are significant in electricity production worldwide and are the cheapest way to generate electricity today. However, despite being a low-carbon source of energy, they can still cause pollution and have other environmental impacts. So, do hydroelectric power plants cause pollution?

Hydroelectric power plants do not directly emit air pollutants

Hydroelectric power plants are a significant component of electricity production worldwide. They are a form of renewable energy that uses the water stored in dams and flowing in rivers to create electricity. The falling water rotates the blades of a turbine, which then spins a generator that converts the mechanical energy of the spinning turbine into electrical energy.

Hydropower generators do not directly emit air pollutants. However, this does not mean that hydroelectric power plants have no environmental impact. The construction of dams and reservoirs can affect the environment, and the operation of hydroelectric generators can also have an impact. For example, a dam that creates a reservoir may obstruct fish migration and affect their populations. Similarly, a dam and reservoir can change natural water temperatures, water chemistry, river flow characteristics, and silt loads. These changes may have negative effects on native plants and animals in and around the river.

Reservoir water is usually more stagnant than normal river water, leading to higher than normal amounts of sediments and nutrients. This can cultivate an excess of algae and other aquatic weeds, which can crowd out other river animal and plant life. The water in a reservoir is also typically colder and lower in dissolved oxygen than normal river water. When this water is released, it can negatively impact downstream plants and animals. To mitigate these impacts, aerating turbines can be installed to increase dissolved oxygen, and multi-level water intakes can be used to ensure that water released from the reservoir comes from various levels and not just the bottom, which is the coldest and has the lowest dissolved oxygen.

While hydropower does not directly emit air pollutants, there are still emissions associated with the construction and operation of hydroelectric power plants. The manufacturing of concrete and steel in hydropower dams requires equipment that may produce emissions, especially if fossil fuels are the energy source. Global warming emissions are also produced during the installation and dismantling of hydroelectric power plants, and recent research suggests that emissions during a facility's operation can be significant. Greenhouse gases, such as carbon dioxide and methane, can form in reservoirs due to the decomposition of biomass in the water. The exact amount of greenhouse gas emissions depends on various site-specific and regional factors, and some reservoirs may even act as carbon sinks, absorbing more emissions than they emit.

But they can obstruct fish migration and affect their populations

Although hydropower does not pollute water or air with carbon emissions, it can have large environmental impacts. Hydroelectric power plants can obstruct fish migration and affect their populations. A dam that creates a reservoir or diverts water to a run-of-river hydropower plant may obstruct fish migration. Fish and other organisms can be injured and killed by turbine blades. There are methods to minimise the impact, such as fish ladders and intake screens, but these structures can still change natural water temperatures, water chemistry, river flow characteristics, and silt loads. These changes can harm native plants and animals in and around the river.

Reservoir water is usually more stagnant than normal river water, leading to higher amounts of sediments and nutrients, which can cultivate an excess of algae and other aquatic weeds. These weeds can crowd out other river animals and plant life and must be controlled manually or by introducing fish that eat these plants. The water quality in reservoirs is also affected by eutrophication, the enrichment of water with chemical nutrients such as nitrogen and phosphorus, due to the efficient trapping of particulates and the long residence time of the water. When combined with the lack of movement, reservoirs frequently experience algal blooms, which further degrade water quality.

In addition, reservoir water is typically colder and lower in dissolved oxygen than normal river water due to oxygen stratification, where thermal energy accumulates in the top layers of the reservoir while the rest gets progressively colder. When this oxygen-poor water is released downstream, it can negatively impact the plants and animals in the surrounding ecosystem. Even minor changes in temperature can alter the metabolic rates, reproduction, and growth of animals. The creation of a reservoir can also introduce mercury into the water when elemental mercury found in the rock and soil interacts with bacteria released during the decomposition of submerged plant material.

The construction and operation of hydroelectric power plants can also produce global warming emissions, though the long operating lifetime of a hydropower plant (50-100 years) helps to offset these emissions. The exact amount of greenhouse gas emissions depends on various site-specific and regional factors, such as the reservoir's depth and shape, the amount of sunlight reaching its floor, and wind speed.

They can also change water temperatures, river flow, and silt loads

While hydropower generators do not directly emit air pollutants, dams, reservoirs, and the operation of hydroelectric generators can affect the environment. Hydroelectric power plants can change water temperatures, river flow, and silt loads.

Reservoir water is usually colder than normal river water. When this water is released, it can negatively impact downstream plants and animals. Water drawn into the turbine from the bottom, oxygen-poor layer of the reservoir, flows downstream, reducing the overall oxygen content of the river beyond the dam. Even minor changes in temperature can alter the metabolic rates, reproduction, and growth of animals.

Reservoirs also undergo eutrophication, the enrichment of water with chemical nutrients such as nitrogen and phosphorus, due to how efficiently they trap particulates. The long residence time of the water in the reservoir also contributes to eutrophication, and reservoirs are frequently sites of algal blooms. When algae populations explode, water quality suffers as the algae die and are decomposed by microorganisms in the water.

The creation of a reservoir can also lead to the emission of greenhouse gases due to the decomposition of flooded organic material. The exact amounts of GHG that form in and are emitted from hydropower reservoirs depend on many site-specific and regional factors.

The construction and operation of a reservoir can cause emissions

Reservoir water is usually more stagnant than normal river water, resulting in higher than normal amounts of sediments and nutrients, which can cultivate an excess of algae and other aquatic weeds. These weeds can crowd out other river animal and plant life and must be controlled through manual harvesting or by introducing fish that eat these plants. The long residence time of the water in the reservoir also contributes to the frequent eutrophication, which is the enrichment of water with chemical nutrients such as nitrogen and phosphorus. When combined with the lack of movement of the water, reservoirs frequently become sites of algal blooms, which negatively impact water quality.

In certain conditions, a reservoir will release greenhouse gases due to the decomposition of flooded organic material. The exact amount of GHG that forms in and is emitted from hydropower reservoirs is uncertain and depends on many site-specific and regional factors. Emissions from reservoirs tend to be highest in the first 10 to 20 years immediately following their creation and then decrease over time. In other cases, a reservoir may act as a carbon sink, absorbing more emissions than it emits.

The operation of a reservoir can also affect the temperature of the water downstream. Thermal energy accumulates in the top layers of the reservoir, closest to the sun, while the rest of the reservoir gets progressively colder. Water is then drawn into the turbine from the bottom, oxygen-poor layer of the reservoir, and flows downstream, altering the temperature of the water. Drastic enough temperature changes can cause direct casualties in marine populations, but even minor changes can alter metabolic rates, reproduction, and growth.

Overall, hydroelectric power plants can have large environmental impacts

One of the main concerns is the impact on aquatic ecosystems and wildlife habitats. Dams and reservoirs may obstruct fish migration and affect their populations. The operation of hydroelectric generators can also change natural water temperatures, water chemistry, river flow characteristics, and silt loads. These changes can have negative effects on native plants and animals in and around the river. Additionally, the water in reservoirs is typically colder and has lower dissolved oxygen levels than normal river water. When this water is released downstream, it can negatively impact the plants and animals there.

The creation of reservoirs can also lead to water quality degradation. Reservoirs often undergo eutrophication, the enrichment of water with chemical nutrients, due to the efficient trapping of particulates and the lack of water movement. This can lead to algal blooms, which negatively affect water quality. The accumulation of thermal energy in reservoirs can also cause thermal pollution, altering the temperature of water downstream and impacting marine populations.

The construction and operation of hydroelectric power plants can also contribute to greenhouse gas emissions. The manufacturing of concrete and steel for dams may produce emissions, and the flooding of land for reservoirs can result in the release of greenhouse gases due to the decomposition of organic material. However, over their long operating lifetimes, the emissions associated with hydroelectric power plants are offset by the emissions-free electricity generated.

Furthermore, the construction of dams and reservoirs can result in the displacement of people living in settlements under the reservoir and the loss of important natural areas, agricultural land, and archaeological sites. The controlled release of water downstream can also impact plant and animal life, and influence the precipitation patterns of the regional climate.

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