Marco Aurélio dos Santos

Carbon dioxide and methane emissions and the carbon budget of a 10-year old tropical reservoir (Petit Saut, French Guiana)

emissions, 0.07 ± 0.01) the first 3 years after impounding (1994–1996) and then decreased to 0.12 ± 0.01 Mt yr 1 C( CO2, 0.10 ± 0.01; CH4, 0.016 ± 0.006) since 2000. On average over the 10 years, 61% of the CO2 emissions occurred by diffusion from the reservoir surface, 31% from the estuary, 7% by degassing at the outlet of the dam, and a negligible fraction by bubbling. CH4 diffusion and bubbling from the reservoir surface were predominant (40% and 44%, respectively) only the first year after impounding. Since 1995, degassing at an aerating weir downstream of the turbines has become the major pathway for CH4 emissions, reaching 70% of the total CH4 flux. In 2003, river carbon inputs were balanced by carbon outputs to the ocean and were about 3 times lower than the atmospheric flux, which suggests that 10 years after impounding, the flooded terrestrial carbon is still the predominant contributor to the gaseous emissions. In 10 years, about 22% of the 10 Mt C flooded was lost to the atmosphere. Our results confirm the significance of greenhouse gas emissions from tropical reservoir but stress the importance of: (1) considering all the gas pathways upstream and downstream of the dams and (2) taking into account the reservoir age when upscaling emissions rates at the global scale.

Greenhouse gases and initial findings on the carbon circulation in two reservoirs and their watersheds

the overlying water. Bubbles containing mainly CO2, and CH4 also rise from this fresh sediment layer. We draw the lower boundary at the surface below which all carbon is permanent (i.e. not susceptible to mobilization and on its way to fossilization). We estimate that this boundary is somewhere between 5‐20 cm below the water-sediment interface. At this depth humic substances are already resistant to further carbon decomposition, as can be seen from the constant C/Si ratio starting at about this depth. In fact, the lower boundary could be described as lying in the two-dimensional region where this ratio is constant from this point down. Reservoir carbon inputs into the black box come through rivers, underground water, rainfall, and occasional diffusive absorption. Carbon outputs are effluent outflow, permanent sedimentation, and diffusive and bubbling emissions such as CH4 and CO2.

Gross Greenhouse Gas Emissions from Brazilian Hydro Reservoirs

This paper presents the results of gross carbon dioxide and methane emission measurements in several Brazilian hydro reservoirs. The term ‘gross emissions’ means gas flux measurements from the reservoir surface without correcting for natural pre-impoundment emissions by natural bodies such as the river channel, seasonal flooding and terrestrial ecosystems. The net emissions result from estimating pre-existing emissions by the reservoir. Measurements were carried in the Miranda, Barra Bonita, Segredo, Tres Marias, Xingo, Samuel and Tucurui reservoirs, located in two different climatological regimes. Additional data were used here from measurements taken at the Itaipu and Serra da Mesa reservoirs. Emissions of carbon dioxide and methane in each of the reservoirs selected, whether through bubbles or diffusive exchange between water and atmosphere, were assessed by sampling, with subsequent extrapolation of results to obtain a value for the reservoir. A great variability was found in the emissions, linked to the influence of various factors, including temperature, depth at the point of measurement, wind regime, sunlight, physical and chemical parameters of water, the composition of the local vegetation and the operational regime of the reservoir.

A model for the data extrapolation of greenhouse gas emissions in the Brazilian hydroelectric system

Hydropower reservoirs are artificial water systems and comprise a small proportion of the Earths continental territory. However, they play an important role in the aquatic biogeochemistry and may affect the environment negatively. Since the 90s, as a result of research on organic matter decay in manmade flooded areas, some reports have associated greenhouse gas emissions with dam construction. Pioneering work carried out in the early period challenged the view that hydroelectric plants generate completely clean energy. Those estimates suggested that GHG emissions into the atmosphere from some hydroelectric dams may be significant when measured per unit of energy generated and should be compared to GHG emissions from fossil fuels used for power generation. The contribution to global warming of greenhouse gases emitted by hydropower reservoirs is currently the subject of various international discussions and debates. One of the most controversial issues is the extrapolation of data from different sites. In this study, the extrapolation from a site sample where measurements were made to the complete set of 251 reservoirs in Brazil, comprising a total flooded area of 32 485 square kilometers, was derived from the theory of self-organized criticality. We employed a power law for its statistical representation. The present article reviews the data generated at that time in order to demonstrate how, with the help of mathematical tools, we can extrapolate values from one reservoir to another without compromising the reliability of the results.

Estimate of degassing greenhouse gas emissions of the turbined water at tropical hydroelectric reservoirs

Biogenic gases generated underwater in hydroelectric reservoirs come from organic matter decomposition by microorganisms. There are 3 different pathways of emitting greenhouse gases (GHG). Gas emissions by impoundments are mainly methane (CH4), carbon dioxide (C02) and nitrous oxide (N20) transported through bubbles formed by decomposing organic matter on the lake bottom, as well as rising up the lake gradient through molecular diffusion. Another gas emission pathway is the degassing of these gases at turbined water sites and along the river below the dam. These different gases (C02, CH4, and N20) have distinct warming effects in the atmosphere; their instantaneous radiative forcing are different and so are their atmospheric life spans. This is particularly true in the case of power generation. The use of fossil fuels in thermal power plants and bacterial decomposition of biomass in hydroelectric reservoirs produce greenhouse gases. Debate continues regarding the amount of GHG emitted from freshwater reservoirs, especially the question of effects of impoundment on degassing emissions downstream of the power house. These emissions were observed for the first time by French researchers at Petit Saut reservoir in French Guiana in 1995 (GALY-LACAUX et al. 1997). The water coming out of the turbine was anoxic (O% o f oxygen); to allow the survival o f fish in the ri v er between the dam and the estu-

Thermo-power in Brazil: diagnosis of control and monitoring of gas emissions

In parallel to Brazils recent supply crisis, the privatization process of its power sector has drastically reshaped the nations energy matrix. From a profile based mainly on hydro-power generation, this sector is being reshaped through a thermo-power plant construction program whose environmental repercussions will certainly be felt over the next few years. This paper offers a description of the thermo-power segment currently in operation, under construction and on the drawing board in Brazil, followed by the results of a diagnosis of the control and monitoring of the gas emissions by this segment. The methodology used for the exploratory analysis and to prepare the diagnosis consists of surveys through questionnaires completed by companies owning the thermo-power plants. After consolidating, processing and analyzing the findings reached through the replies sent in by the companies, it is concluded that thermo-power plants currently in operation lack control systems that would help reduce atmospheric pollution, and are not equipped with monitoring systems for these emissions. The thermo-power plants currently under construction and on the drawing board indicate a trend towards including these systems in their project designs, due to more stringent licensing processes.