Variations of methane emissionswithin and between threehydroelectric reservoirs in Brazil

Abstract:

Hydroelectricity is an energy resource which for a long time has been consideredenvironmentally neutral regarding greenhouse gas emission. During the last years this viewhas changed. Studies have shown that reservoirs connected to hydroelectric power plants emitmethane (CH4) and other greenhouse gases to the atmosphere, especially in the tropicalregions where the emission level of CH4 is the highest. The purpose of this thesis was toinvestigate the variations of CH4 emissions in Funil reservoir, Santo Antônio reservoir andTrês Marias reservoir and to identify variables that increase the CH4 emissions.The CH4 emissions were measured by floating static chambers positioned on the surface atseveral locations within each reservoir. A gas sample was collected after 10, 20 and 30minutes from each chamber. The samples were analyzed through gas chromatography toobtain the concentration of CH4 in each sample. Calculations of the change in CH4concentration over time were used to establish the flux of CH4 at each location.The obtained result from Funil reservoir showed CH4 fluxes in the range of -0.04 to 13.16mmol/m2/day with significantly different fluxes between sites (p < 0.05). The CH4 fluxes inSanto Antonio reservoir were within the range of -0.33 to 72.21 mmol/m2/day. In thisreservoir fluxes were not significantly different between sites (p <0.05). The results obtainedfrom Três Marias showed CH4 fluxes in the range of -0.31 to 0.56 mmol/m2/day withsignificantly different fluxes between sites (p < 0.05). The highest fluxes were found in SantoAntônio which were significantly different from the CH4 fluxes in Três Marias (p <0.05).The CH4 flux was positively correlated with CO2 and dissolved organic carbon (DOC) andnegatively correlated with O2 and depth in Santo Antônio. The same correlations were evidentfor the whole data set. In total the measured fluxes from the three reservoirs ranged from -0.33 to 72. 21 mmol/m2/day and the mean flux was 2.31 mmol/m2/day. These fluxes are lowcompared to earlier results. The variation in CH4 flux within and between the reservoirs wassignificantly different in a major part of the comparisons. Even though the majority of thefluxes were different, variables that increase the CH4 emission rate were illuminated. A lowdepth and low O2 concentration increase the CH4 emission rate. A high concentration of DOCand CO2 indicates that a high amount of organic carbon was available for the production ofCH4, leading to an increased CH4 emission rate.