Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
Nevertheless, diffusion and bubbling are two important mechanisms by which carbon dioxide and methane
are emitted into the atmosphere.
Bubbling, diffusion and above water decomposition are not the only ways by which GHG’s are emitted from
reservoirs. Research shows that a major pathway for GHG emissions is diffusion as a result of reservoir
water moving through the turbines of a dam.
This occurs because the turbines create a sudden change in water pressure and temperature, which reduces
the solubility of gaseous methane and carbon dioxide in water. Consequently, the majority of the methane
and carbon dioxide present in the water, prior to moving through the dam, is released into the atmosphere
within a few seconds of the water emerging from the turbines.
Factors that Influence the Amount of Methane and Carbon Dioxide Emitted
There are a variety of factors that influence the amount of methane and carbon dioxide emitted from a
reservoir.
The most important factor seems to be the climate of the surrounding environment. Reservoirs in tropical
environments have been found to have significantly larger emissions than reservoirs in boreal and temperate
climates. One possible reason for this is that the annual water temperature is much higher in tropical
climates. This means that the rate of decomposition is faster, leading to higher carbon dioxide and methane
flux in the water (see below figures from “Tremblay, A., Varfalvy, L., Roehm, C. & Garneau, M. (eds.)
(2004) Greenhouse Gas Emissions: Fluxes and Processes. Environmental Science Series, 732 pp.
Berlin:Springer-Verlag”).
CESI SpA - Mid-Day International Consulting Engineers Page 191