Atmospheric CO 2 is naturally sequestered in ultramafi c mine tailings as a result of the weathering of serpen ne minerals [Mg 3 Si 2 O 5 (OH) 4 ] and brucite [Mg(OH) 2 ], and subsequent mineraliza on of CO 2 in hydrated magnesium carbonate minerals, such as hydromagnesite [Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O]. Understanding the CO 2 trapping mechanisms is key to evalua ng the capacity of such tailings for carbon sequestra on. Natural CO 2 sequestra on in subaerially exposed ultramafi c tailings at a mine site near Mount Keith, Australia is assessed with a process-based reac ve transport model. The model formula on includes unsaturated fl ow, equa ons accoun ng for energy balance and vapor diff usion, fully coupled with solute transport, gas diff usion, and geochemical reac ons. Atmospheric boundary condi ons accoun ng for the eff ect of climate varia ons are also included. Kine c dissolu on of serpenne, dissolu on-precipita on of brucite and primary carbonates-calcite (CaCO 3 ), dolomite [MgCa(CO 3 ) 2 ], magnesite (MgCO 3 ), as well as the forma on of hydromagnesite, halite (NaCl), gypsum (CaSO 4 ·2H 2 O), blödite [Na 2 Mg(SO 4 ) 2 ·4H 2 O], and epsomite [MgSO 4 ·7H 2 O]-are considered. Simula on results are consistent with fi eld observa ons and mineralogical data from tailings that weathered for 10 yr. Precipita on of hydromagnesite is both predicted and observed, and is mainly controlled by the dissolu on of serpen ne (the source of Mg) and equilibrium with CO 2 ingressing from the atmosphere. The predicted rate for CO 2 entrapment in these tailings ranges between 0.6 and 1 kg m −2 yr −1 . However, modeling results suggest that this rate is sensi ve to CO 2 ingress through the mineral waste and may be enhanced by several mechanisms, including atmospheric pumping.Abbrevia ons: BET, Brunauer-Emme -Teller; SWCC, soil water characteris c curve; TIC, total inorganic carbon; TSF, tailings storage facility; XRPD, X-ray powder diff rac on.

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[1] Developing a predictive understanding of subsurface contaminant plume evolution and natural attenuation capacity is hindered by the inability to tractably characterize controlling reactive transport properties over field-relevant scales. Here we explore a concept of reactive facies, which is based on the hypothesis that subsurface units exist that have unique distributions of properties that influence reactive transport. We further hypothesize that geophysical methods can be used to identify and spatially distribute reactive facies and their associated parameters. We test the reactive facies concept at a U.S. Department of Energy uranium-contaminated groundwater site, where we have analyzed the relationships between laboratory and field (including radar and seismic tomographic) data sets. Our analysis suggests that there are two reactive facies that have unique distributions of mineralogy, texture, hydraulic conductivity, and geophysical attributes. We use these correlations within a Bayesian framework to integrate the dense geophysical data sets with the sparse corebased measurements. This yields high-resolution (0.25 m  0.25 m) estimates of reactive facies and their associated properties and uncertainties along the 2-D tomographic transects.Comparison with colocated samples shows that the estimated properties fall within 95% uncertainty bounds. To illustrate the value of reactive facies characterization approach, we used the geophysically estimated properties to parameterize reactive transport models, which were then used to simulate migration of an acidic-U plume through the domain. Modeling results suggest that each identified reactive facies exerts a unique control on plume evolution, highlighting the usefulness of the reactive facies concept for spatially distributing properties that control reactive transport over field-relevant scales.Citation: Sassen, D. S., S. S. Hubbard, S. A. Bea, J. Chen, N. Spycher, and M. E. Denham (2012), Reactive facies: An approach for parameterizing field-scale reactive transport models using geophysical methods, Water Resour. Res., 48, W10526,

Deep sedimentary basins are complex systems that over long time scales may be affected by numerous interacting processes including groundwater flow, heat and mass transport, water–rock interactions, and mechanical loads induced by ice sheets. Understanding the interactions among these processes is important for the evaluation of the hydrodynamic and geochemical stability of geological CO2 disposal sites and is equally relevant to the safety evaluation of deep geologic repositories for nuclear waste. We present a reactive transport formulation coupled to thermo‐hydrodynamic and simplified mechanical processes. The formulation determines solution density and ion activities for ionic strengths ranging from freshwater to dense brines based on solution composition and simultaneously accounts for the hydro‐mechanical effects caused by long‐term surface loading during a glaciation cycle. The formulation was implemented into the existing MIN3P reactive transport code (MIN3P‐THCm) and was used to illustrate the processes occurring in a two‐dimensional cross section of a sedimentary basin subjected to a simplified glaciation scenario consisting of a single cycle of ice‐sheet advance and retreat over a time period of 32 500 years. Although the sedimentary basin simulation is illustrative in nature, it captures the key geological features of deep Paleozoic sedimentary basins in North America, including interbedded sandstones, shales, evaporites, and carbonates in the presence of dense brines. Simulated fluid pressures are shown to increase in low hydraulic conductivity units during ice‐sheet advance due to hydro‐mechanical coupling. During the period of deglaciation, Darcy velocities increase in the shallow aquifers and to a lesser extent in deeper high‐hydraulic conductivity units (e.g., sandstones) as a result of the infiltration of glacial meltwater below the warm‐based ice sheet. Dedolomitization is predicted to be the most widespread geochemical process, focused near the freshwater/brine interface. For the illustrative sedimentary basin, the results suggest a high degree of hydrodynamic and geochemical stability.