The importance of conceptual models in the reactive transport simulation of oxygen ingress in sparsely fractured crystalline rock.
ABSTRACT Redox evolution in sparsely fractured crystalline rocks is a key, and largely unresolved, issue when assessing the geochemical suitability of deep geological repositories for nuclear waste. Redox zonation created by the influx of oxygenated waters has previously been simulated using reactive transport models that have incorporated a variety of processes, resulting in predictions for the depth of oxygen penetration that may vary greatly. An assessment and direct comparison of the various underlying conceptual models are therefore needed. In this work a reactive transport model that considers multiple processes in an integrated manner is used to investigate the ingress of oxygen for both single fracture and fracture zone scenarios. It is shown that the depth of dissolved oxygen migration is greatly influenced by the a priori assumptions that are made in the conceptual models. For example, the ability of oxygen to access and react with minerals in the rock matrix may be of paramount importance for single fracture conceptual models. For fracture zone systems, the abundance and reactivity of minerals within the fractures and thin matrix slabs between the fractures appear to provide key controls on O(2) attenuation. The findings point to the need for improved understanding of the coupling between the key transport-reaction feedbacks to determine which conceptual models are most suitable and to provide guidance for which parameters should be targeted in field and laboratory investigations.
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ABSTRACT: Reactive solute transport modeling was utilized to evaluate the potential for natural attenuation of a contaminant plume containing phenolic compounds at a chemical producer in the West Midlands, UK. The reactive transport simulations consider microbially mediated biodegradation of the phenolic compounds (phenols, cresols, and xylenols) by multiple electron acceptors. Inorganic reactions including hydrolysis, aqueous complexation, dissolution of primary minerals, formation of secondary mineral phases, and ion exchange are considered. One-dimensional (1D) and three-dimensional (3D) simulations were conducted. Mass balance calculations indicate that biodegradation in the saturated zone has degraded approximately 1-5% of the organic contaminant plume over a time period of 47 years. Simulations indicate that denitrification is the most significant degradation process, accounting for approximately 50% of the organic contaminant removal, followed by sulfate reduction and fermentation reactions, each contributing 15-20%. Aerobic respiration accounts for less than 10% of the observed contaminant removal in the saturated zone. Although concentrations of Fe(III) and Mn(IV) mineral phases are high in the aquifer sediment, reductive dissolution is limited, producing only 5% of the observed mass loss. Mass balance calculations suggest that no more than 20-25% of the observed total inorganic carbon (TIC) was generated from biodegradation reactions in the saturated zone. Simulations indicate that aerobic biodegradation in the unsaturated zone, before the contaminant entered the aquifer, may have produced the majority of the TIC observed in the plume. Because long-term degradation is limited to processes within the saturated zone, use of observed TIC concentrations to predict the future natural attenuation may overestimate contaminant degradation by a factor of 4-5.Journal of Contaminant Hydrology 01/2002; 53(3-4):341-68. · 2.89 Impact Factor
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ABSTRACT: Reactive transport modeling was used to evaluate the performance of two similar column experiments. The experiments were designed to simulate the treatment of acid mine drainage through microbially mediated sulfate reduction and subsequent sulfide mineral precipitation by means of an organic carbon permeable reactive barrier. Principal reactions considered in the simulations include microbially mediated reduction of sulfate by organic matter, mineral dissolution/precipitation reactions, and aqueous complexation/hydrolysis reactions. Simulations of column 1, which contained composted leaf mulch, wood chips, sawdust, and sewage sludge as an organic carbon source, accurately predicted sulfate concentrations in the column effluent throughout the duration of the experiment using a single fixed rate constant for sulfate reduction of 6.9 x 10(-9) mol L(-1) s(-1). Using the same reduction rate for column 2, which contained only composted leaf mulch and sawdust as an organic carbon source, sulfate concentrations at the column outlet were overpredicted at late times, suggesting that sulfate reduction rates increased over the duration of the column experiment and that microbial growth kinetics may have played an important role. These modeling results suggest that the reactivity of the organic carbon treatment material with respect to sulfate reduction does not significantly decrease over the duration of the 14-month experiments. The ability of the columns to remove ferrous iron appears to be strongly influenced by the precipitation of siderite, which is enhanced by the dissolution of calcite. The simulations indicate that while calcite was available in the column, up to 0.02 mol L(-1) of ferrous iron was removed from solution as siderite and mackinawite. Later in the experiments after approximately 300 d, when calcite was depleted from the columns, mackinawite became the predominant iron sink. The ability of the column to remove ferrous iron as mackinawite was estimated to be approximately 0.005 mol L(-1) for column 1. As the precipitation of mackinawite is sulfide limited at later times, the amount of iron removed will ultimately depend on the reactivity of the organic mixture and the amount of sulfate reduced.Environmental Science and Technology 07/2004; 38(11):3131-8. · 5.26 Impact Factor
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ABSTRACT: The rate and stoichiometry of biotite dissolution were studied in the pH range 2–10 using thin-film continuous flow reactors. The release of interlayer K is relatively fast and becomes diffusion-controlled within a few days. The release rates of framework ions (Mg, Al, Fe, Si) are much slower and reach an apparent steady-state within ten days. The stoichiometry and rate of dissolution vary greatly with pH. Consistent with surface reaction control of release rates, an empirical rate law, R + kH[H+]m + k0 + kOH[H+]n (moles m−2 h−1) describes proton- and hydroxyl-catalysed dissolution for each ion. SiFeMgAllog kH−4.45−5.10−4.93−5.31m0.910.510.570.40log k0−7.31log kOH−11.29−15.36−10.57−12.58n−0.48−0.81−0.29−0.54Rapid K+ release provides a tracer for the extent of the hydrated reacting layer on the biotite surface and within interlayers. An altered reaction layer composition, calculated from mass balances for released ions, results from preferential leaching of some ions and is consistent with that of vermiculite. X-Ray powder diffractometry confirmed the formation of both vermiculite and kaolinite during the weathering reaction. The pH dependence of release rates, normalised to the corresponding ion concentrations in the reacting layer, correlate with those for the respective binary oxides (SiO2, Al2O3, Fe2O3, MgO).Release rates for Al, Mg, and Fe at neutral pH are much slower when the mineral has been previously reacted at low pH where these ions are released rapidly. Model simulations suggest that, for ions that initially dissolve rapidly, release rates will decrease as the ion is depleted in the reacting layer. Rates will eventually approach those of the most slowly dissolving ion. At 25°C and pH 7, this process would lead to stoichiometric dissolution within 50 y.Geochimica Et Cosmochimica Acta - GEOCHIM COSMOCHIM ACTA. 01/1997; 61(14):2779-2799.