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A lesson on carbon release and sequestration from the past
Goldschmidt Conference Abstracts 2006
Influence of colloids on uranium transport in nuclear waste repositories and abandoned uranium mines—A critical comparison H. ZA¨NKER1, K.-U. ULRICH1, K. OPEL2, V. BRENDLER1 1
FZ Rossendorf, Institute of Radiochemistry, P.O. Box 51 01 19, 01314 Dresden, Germany ([email protected]) 2 Department of Chemistry, University of Oslo, P.O. Box 1033, N0315 Oslo, Norway ([email protected]) It is widely recognized that colloids can influence the migration of toxic and radiotoxic elements such as uranium. There are transport-facilitating and transport-impeding effects caused by colloids. The most common scenario assumed in performance assessment for nuclear waste repositories is contaminant transport through anoxic aquifers. Only transport-facilitating effects of colloids are usually taken into consideration for this case. For abandoned uranium mines, retarding influences of colloids, apart from mobilizing ones, are also of high interest since they have the potential of causing ‘‘natural attenuation’’ of the environmental hazard. The formation of colloids containing U(IV) and U(VI) was studied by laboratory experiments and by comparing the results with field experience. It is shown that there may be transportstimulating effects of colloids on the ‘‘immobile’’ contaminant U(IV) and transport-impeding effects on the ‘‘mobile’’ contaminant U(VI). A key factor in assessing the impact of colloids in a geochemical setting is the timescale that needs to be taken into account. Very long periods of time must be considered for nuclear waste repositories. Here, the point in time at which a certain colloidborne contaminant such as uranium reaches the biosphere depends on the moment at which spent fuel container leakage begins, the water flow velocity, the concentration and mobility of potential carrier colloids, the hindrance of colloidal transport by barrier materials, and the persistence (reversibility vs. irreversibility) of the binding of the contaminant onto the colloids. In the case of mines, the phase critical for the environment occurs when the flood water reaches the level where first connections to unprotected surface waters or underground drinking water resources occur. This phase is characterized by the ‘‘first flush’’ of the mine; the maximum release rate of contaminants such as U from the mine to the environment can be reduced by colloids via the flattening of the release rate profile. Whereas the timescale of critical contaminant release is in the range of centuries for the nuclear waste repositories, it lies in the range of only few years for abandoned uranium mines. It is primarily the different dynamics of the processes behind these two different timescales that causes the different role of colloids for uranium migration in nuclear waste repositories and in abandoned uranium mines. doi:10.1016/j.gca.2006.06.1314
A731
A lesson on carbon release and sequestration from the past R.E. ZEEBE Department of Oceanography, SOEST, University of Hawaii at Manoa, Honolulu, USA ([email protected]) One of the most urgent questions in biogeochemistry is understanding the natural feedbacks that govern the carbon release and sequestration by ocean, atmosphere, bio- and geosphere. Knowledge of the involved mechanisms critically determines our ability to reconstruct and forecast atmospheric CO2 concentrations. A powerful approach to the problem is to study past carbon cycle perturbations, thereby providing information on time scales inaccessible to modern observations. The Paleocene–Eocene thermal maximum (PETM, 55 Ma ago) constitutes a case study for natural, massive methane/carbon release and sequestration, potentially comparable to the release and sequestration of anthropogenic carbon within the next centuries. Here, we use models of various complexities to simulate the carbon cycle perturbation and recovery phase during the PETM and Anthropocene, including terrestrial carbonate and silicate weathering. Our results indicate remarkable similarities between the PETM and future scenarios. However, there are significant differences to consider if the PETM is to be used as a future analogue. This includes initial baseline steady-state conditions as well as transient behavior during perturbation and recovery phase. One lesson to be learned from the past is that despite strong natural restoring feedback mechanisms, the recovery phase from the anthropogenic carbon release may take tens to hundreds of thousands of years. doi:10.1016/j.gca.2006.06.1315
Influence of colloids on uranium transport in nuclear waste repositories and abandoned uranium mines—A critical comparison H. ZA¨NKER1, K.-U. ULRICH1, K. OPEL2, V. BRENDLER1 1
FZ Rossendorf, Institute of Radiochemistry, P.O. Box 51 01 19, 01314 Dresden, Germany ([email protected]) 2 Department of Chemistry, University of Oslo, P.O. Box 1033, N0315 Oslo, Norway ([email protected]) It is widely recognized that colloids can influence the migration of toxic and radiotoxic elements such as uranium. There are transport-facilitating and transport-impeding effects caused by colloids. The most common scenario assumed in performance assessment for nuclear waste repositories is contaminant transport through anoxic aquifers. Only transport-facilitating effects of colloids are usually taken into consideration for this case. For abandoned uranium mines, retarding influences of colloids, apart from mobilizing ones, are also of high interest since they have the potential of causing ‘‘natural attenuation’’ of the environmental hazard. The formation of colloids containing U(IV) and U(VI) was studied by laboratory experiments and by comparing the results with field experience. It is shown that there may be transportstimulating effects of colloids on the ‘‘immobile’’ contaminant U(IV) and transport-impeding effects on the ‘‘mobile’’ contaminant U(VI). A key factor in assessing the impact of colloids in a geochemical setting is the timescale that needs to be taken into account. Very long periods of time must be considered for nuclear waste repositories. Here, the point in time at which a certain colloidborne contaminant such as uranium reaches the biosphere depends on the moment at which spent fuel container leakage begins, the water flow velocity, the concentration and mobility of potential carrier colloids, the hindrance of colloidal transport by barrier materials, and the persistence (reversibility vs. irreversibility) of the binding of the contaminant onto the colloids. In the case of mines, the phase critical for the environment occurs when the flood water reaches the level where first connections to unprotected surface waters or underground drinking water resources occur. This phase is characterized by the ‘‘first flush’’ of the mine; the maximum release rate of contaminants such as U from the mine to the environment can be reduced by colloids via the flattening of the release rate profile. Whereas the timescale of critical contaminant release is in the range of centuries for the nuclear waste repositories, it lies in the range of only few years for abandoned uranium mines. It is primarily the different dynamics of the processes behind these two different timescales that causes the different role of colloids for uranium migration in nuclear waste repositories and in abandoned uranium mines. doi:10.1016/j.gca.2006.06.1314
A731
A lesson on carbon release and sequestration from the past R.E. ZEEBE Department of Oceanography, SOEST, University of Hawaii at Manoa, Honolulu, USA ([email protected]) One of the most urgent questions in biogeochemistry is understanding the natural feedbacks that govern the carbon release and sequestration by ocean, atmosphere, bio- and geosphere. Knowledge of the involved mechanisms critically determines our ability to reconstruct and forecast atmospheric CO2 concentrations. A powerful approach to the problem is to study past carbon cycle perturbations, thereby providing information on time scales inaccessible to modern observations. The Paleocene–Eocene thermal maximum (PETM, 55 Ma ago) constitutes a case study for natural, massive methane/carbon release and sequestration, potentially comparable to the release and sequestration of anthropogenic carbon within the next centuries. Here, we use models of various complexities to simulate the carbon cycle perturbation and recovery phase during the PETM and Anthropocene, including terrestrial carbonate and silicate weathering. Our results indicate remarkable similarities between the PETM and future scenarios. However, there are significant differences to consider if the PETM is to be used as a future analogue. This includes initial baseline steady-state conditions as well as transient behavior during perturbation and recovery phase. One lesson to be learned from the past is that despite strong natural restoring feedback mechanisms, the recovery phase from the anthropogenic carbon release may take tens to hundreds of thousands of years. doi:10.1016/j.gca.2006.06.1315