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Langenhagen, Germany

The hypothetical leakage of methane gas caused by fracking a 1,000-m deep Cretaceous claystone horizon at Damme, Germany, is simulated in a TOUGHREACT reactive-transport model with 5,728 elements. A hypothetical leakage zone connects the Cretaceous horizon with a Quaternary potable-water aquifer (q1). Methane gas rises up to the q1 horizon in less than 2 days in all calculated scenarios. The simulations include the major constituents of groundwater as well as the seven most hazardous trace components that are natural constituents of groundwater (As, Cd, Cr, Ni, Pb, Se and U). The general trend is characterised by depletion of the natural hazardous components with decreasing acidity and oxygen fugacity in the relevant pH range (7–9). Nevertheless, the concentrations of elements whose dominant aqueous species are negatively charged in this pH range (Cr and Se) rise against the general trend due to desorption reactions. Slight enhancement effects are produced by the dissolution of contaminant-bearing oxides such as Cr-bearing goethite. In summary, the geological risks of a fracking operation are minor. The technical risks are more important. This is especially the case when rising methane gas gets into contact with fracking fluid that accidentally escapes through faulty well seals. © 2014, The Author(s). Source

Schwartz M.O.,MathGeol
International Journal of Greenhouse Gas Control | Year: 2014

The hypothetical injection of CO2 into the ≥1250m deep Triassic Volpriehausen sandstone (smV1) at Birkholz-Beeskow, Germany, is simulated in a TOUGH2-MP flow model with 510,607 elements. A hypothetical leakage zone connects the Triassic smV1 horizon with a Miocene potable-water aquifer (tmi). The leakage zone has a transmissibility (product of width and permeability) of 5×10-14m3 to 5×10-11m3. Carbon dioxide rises up to the tmi horizon if the transmissibility of the leakage zone is ≥1.7×10-13m3. The output of the TOUGH2-MP flow model is used for defining flow conditions for a TOUGHREACT reactive-transport model with 28,409 elements. The models does not only consider the chemical effects produced by intruding CO2 but also the intruding aqueous liquid phase, which accompanies the rise of CO2. The concentrations of hazardous As and Se in the tmi groundwater remain below the regulatory limits (10μg/L) in all calculated pollution scenarios. The concentrations of U remain below the limit (2μg/L) if adsorption is neglected and the dissolution of uranium oxide is assumed to be slow. The concentrations of Ni remain below the limit (20μg/L) if adsorption is neglected. The concentrations of Pb remain below the limit (10μg/L) only under the unrealistic assumption that the carbonates are Pb-free provided that adsorption is neglected. © 2014 Elsevier Ltd. Source

The planned high-level nuclear waste repository at Forsmark, Sweden, will accommodate 6,824 containers with a total of 13,920 tonnes of uranium in burnt fuel at approximately 400 m depth in a fractured-granite aquifer. The transport of radionuclides, which may be released from the disposed waste, is simulated with the TOUGHREACT code for a three-dimensional model with 305,571 elements. The model performs coupled flow-transport simulations. It aims to achieve more realistic simulations of contaminant transport than the commonly used decoupled procedure consisting of three-dimensional flow and one-dimensional transport simulations. The model has a relatively small problem size because it is designed as a double-porosity model (one matrix continuum) that is the parameterised equivalent of a much larger multiple-interacting continua (MINC) model, i. e. a model with a finely discretised matrix (several matrix continua). The parameterisation is performed with two-dimensional models. Only one or two variables among three variables (diffusive transport distance between fracture and matrix, retardation factor and effective diffusivity) have to be parameterised. The results obtained with the parameterised three-dimensional model are very close to those that can be obtained with a much larger MINC model but may be quite different from those that can be obtained with the conventional decoupled procedure. © 2012 Springer-Verlag. Source

The candidate repository for high-level nuclear waste in the Gorleben salt dome, Germany, is expected to host 8,550 tonnes of uranium in burnt fuel. It has been proposed that 5,440 waste containers be deposited at a depth of about 800 m. There is 260-280 m of siliciclastic cover sediments above the proposed repository. The potential groundwater contamination in the siliciclastic aquifer is simulated with the TOUGHREACT and TOUGH2-MP codes for a three-dimensional model with 290,435 elements. Two deterministic cases are simulated. The single-phase case considers the transport of radionuclides in the liquid phase only. The two-phase case accounts for hydrogen gas generated by the corrosion of waste containers and release of gaseous C-14. The gas release via a backfilled shaft is assumed to be steady (non-explosive). The simulation period is 2,000,000 years for the single-phase case and 7,000 years for the two-phase case. Only the radioactive dose in the two-phase case is higher than the regulatory limit (0.1 mSv/a). © 2012 Springer-Verlag. Source

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