A Review of Greenhouse Gas Emissions by Hydropower Reservoirs

Hydroelectric reservoirs have environmental impacts as many other sources of energy. Regarding hydropower, these effects include flooding cultivated and forest areas, changes in water quality, negative impacts on water biodiversity, conflict with indigenous people and fish migration. In the nineties, researchers put in evidence of another important impact of dam construction: the greenhouse gases generated by flooding organic matter by reservoir flooding. Scientists argue that like natural human water bodies, the hydropower reservoirs emit biogenic gases into the atmosphere. The diffusive gas flux is associated with the difference between gas partial pressure of each chemical substance considering the aquatic system and the atmosphere. Ebullition is a process where some chemical substances are not soluble in water and bubbles are formed in the sediment at the bottom of the reservoir. Ebullition is often the dominant pathway of CH4 that is released from aquatic ecosystems. The phenomenon is episodic and irregular and depends mainly on hydrostatic pressure and other physical influences, such as currents, temperature gradients and the bathymetry of the water body. At hydropower reservoirs, other pathways for gas emanation to the atmosphere are the degassing by water passing through turbines of the powerhouse and the gas diffusion across the river downstream dam. This paper gives a review of the state-of-the-art and advances in the research of greenhouse gas emissions and removals from hydropower reservoirs.