The trapped sediments in reservoirs is a matter of concern in the decommissioning of dams: the flux of sediment during and after removal, increase in availability of potential toxic contaminants, the influence of deposited sediments on downstream habitat and other site-specific considerations. The significant volume of sediments accumulated over the years and the carbon content can also lead to methane emissions, potentially contributing to climate change and influencing the life cycle greenhouse gas emissions from hydroelectric power. Many factors can affect the greenhouse gas (GHG) emissions from trapped sediments, such as sedimentation rate, rate of mineralization, CO2:CH4 ratio, carbon content, among others. The behavior of these parameters for different conditions are still unknown, especially in a dam removal scenario. However, by using a Monte-Carlo analysis applied to published studies evaluating the life-cycle impacts of hydropower plants and methane emission from decomposition of sediments, we found that even in an optimistic scenario with low rates of sedimentation and mineralization, the carbon emissions from the decommissioning of dams may represent 20% of the GHG life-cycle of hydropower plants; in the worst scenario it may account for up to 90%. Such variation is the consequence of a wide range, 10-60 million tons of CO2e, in the dam decommissioning stage. These results demonstrate the need for further analysis to identify the conditions that drive the methanogenesis process post-removal and for strategies to reduce this impact. Best practices for sediments management could be a strategy to reduce the GHG emissions from dam removal, especially when anoxic conditions may be dominant.

Simone P Souza: Currently, I am postdoctoral researcher at Environmental Research Group, University of New Hampshire, working on a project regarding "The Future of Dams". My duties include apply an integrated analysis in different scenarios of dam removal. I have also worked on sustainability science of bioenergy systems focused on identifying pathways to reduce the GHG emissions and the energy consumption by applying quantitative analysis.