Carl I. Steefel

Multicomponent reactive transport in discrete fractures: II: Infiltration of hyperalkaline groundwater at Maqarin, Jordan, a natural analogue site

Abstract A numerical multicomponent reactive transport model described fully in Steefel and Lichtner (1998)[Steefel, C.I., Lichtner, P.C., 1998. Multicomponent reactive transport in discrete fractures, I. Controls on reaction front geometry. J. Hydrol. (in press)] is used to simulate the infiltration of hyperalkaline groundwater along discrete fractures at Maqarin, Jordan, a site considered as a natural analogue to cement-bearing nuclear waste repositories. In the Eastern Springs area at Maqarin, two prominent sets of sub-parallel fractures trending NW–SE are approximately perpendicular to the local water table contours, with the slope of the water table indicating north-westward flow. Extensive mineralogic investigations [Alexander W.R. (Ed.), 1992. A natural analogue study of cement-buffered, hyperalkaline groundwaters and their interaction with a sedimentary host rock. NAgrA Technical Report (NTB 91-10), Wettingen, Switzerland; Milodowski, A.E., Hyslop, E.K., Pearce, J.M., Wetton, P.D., Kemp, S.J., Longworth, G., Hodginson, E., and Hughes, C.R., 1998. Mineralogy and geochemistry of the western springs area. In: Smellie, J.A.T. (ed.), 1998: Maqarin Natural Analogue Study: Phase III. SKB Technical Report TR98-04, Stockholm, Sweden] indicate that the width of intense rock alteration zone bordering the fractures changes from about 4xa0mm at one locality (the M1 sampling site) to approximately 1xa0mm 100xa0m to the north-west in the flow direction (the M2 site), suggesting a lessening of alteration intensity in that direction. Using this information, the dimensionless parameter δ v /φ D ′ (φ=porosity, D ′=effective diffusion coefficient in rock matrix, δ=fracture aperture, and v =fluid velocity in the fracture) and measurements of the local hydraulic head gradient and effective diffusion coefficient in the rock matrix, a mean fracture aperture of 0.194xa0mm is calculated assuming the cubic law applies. This information, in combination with measured groundwater compositions at the Maqarin site, is used as input for numerical simulations of the hyperalkaline groundwater infiltration along fractures. The width of the alteration zones in the rock matrix bordering fractures is also used to constrain mineral dissolution rates in the field. The simulations predict that ettringite [Ca 6 Al 2 (SO 4 ) 3 (OH) 12 ·26H 2 O] with lesser amounts of hillebrandite and tobermorite (hydrated calcium silicates or CSH phases) will be the dominant alteration products forming at the expense of the primary silicates in the rock matrix and fracture, in agreement with observations at the Maqarin site. The simulations also come close to matching the pH of water samples collected along fractures at the M1 and M2 sites, with a fracture aperture of 0.22xa0mm giving the closest match with the pH data (within 13% of the value indicated by the rock matrix alteration widths). The simulations suggest two possible scenarios for the time evolution of the fracture–rock matrix system. Where rate constants for secondary mineral precipitation reactions are the same in both the rock matrix and fracture, the rock matrix tends to become completely cemented before the fracture. This results in a downstream migration of the hyperalkaline plume. In contrast, if rates are as little as one order of magnitude higher in the fracture than in the rock matrix, it is possible to seal the fracture first, thus causing the mineral zones to collapse upstream as a result of the reduction in fracture permeability. Sealing of fractures is observed at Maqarin and the simulations predict a mineral paragenesis in the fracture resulting from this scenario which is broadly compatible with field observations.

Multicomponent reactive transport in discrete fractures: I. Controls on reaction front geometry

A multicomponent reactive transport model with mixed equilibrium and kinetic reactions is presented for a dual porosity system. The model is used to analyze alteration front geometry in discrete fractures and adjacent rock matrix. An analytical solution for a dual porosity system is used to verify the numerical model and to obtain an expression for mineral reaction front geometry under quasi-stationary state conditions. Both the analytical solution and numerical results suggest that the geometry of reaction fronts in a dual porosity system can be characterized by the sum of two dimensionless parameters: φD′/δv (φ=porosity, D′=effective diffusion coefficient in rock matrix, δ=fracture aperture, and v=fluid velocity in the fracture) and λm/λ0f (λm=equilibration length scale in rock matrix and λ0f=equilibration length scale in the fracture in the absence of matrix diffusion). In the case where the system is surface reaction-controlled, the first dimensionless parameter, which is independent of the reaction rate constants, dominates. From an analysis of a system described by linear reaction rates, this parameter can be used to predict quasi-stationary state concentration profiles and the distribution of minerals along the length of a fracture based on the one-dimensional diffusion-reaction profile in the rock matrix bordering the fracture. Numerical simulations of a multi-component problem involving dedolomitization resulting from the infiltration of hyperalkaline groundwater demonstrate that the dimensionless parameter φD′/δv applies in more complicated multicomponent systems as well. This result suggests that field observations of matrix alteration perpendicular to the fracture may be used to predict mineralization along the fracture itself.

Long-term fluxes of reactive species in macrotidal estuaries: Estimates from a fully transient, multicomponent reaction-transport model

A coupled, fully transient, multicomponent reaction-transport model has been developed to estimate long-term fluxes of reactive compounds in strong tidal estuaries. The model is applied to a preliminary analysis of the carbon cycle in the Scheldt estuary in Belgium and The Netherlands. The model provides a realistic description of the residual circulation in a strong tidal estuary and includes the essential feedback mechanisms between interdependent chemical species. The model has been used to analyze the fundamentally transient nature of strong tidal estuaries and, in particular, the effect of these non-steady state conditions on the long-term fluxes of chemical species out of the estuary. The results indicate that flux estimation techniques based upon steady-state assumptions may result in significant errors. The model has also been used to investigate biogeochemical interactions characterized by a large spectrum of time scales, which it does by including simultaneous equilibrium reactions and kinetically-mediated processes. Simulations carried out with the model suggest that a formulation based upon microbially-mediated, kinetically-controlled reactions provides a superior description of solute profiles in the Scheldt estuary than does a global equilibrium redox formulation. The mixed equilibrium-kinetic formulation also makes it possible to track simultaneously two master variables: the redox state of the system and the pH. By providing strong constraints on the system, these two master variables can be used to test the models self-consistency. The simulations carried out with the model suggest the pH profile in the Scheldt estuary is the result of a balance of biogeochemical reactions which produce H + and degassing which consumes H + and not the result of simple mixing between seawater and freshwater.

Multidimensional, multicomponent, subsurface reactive transport in nonuniform velocity fields : code verification using an advective reactive streamtube approach

Abstract High performance computing has made possible the development of high resolution, multidimensional, multicomponent reactive transport models that can be used to analyze complex geochemical environments. However, as increasingly complex processes are included in these models, the accuracy of the numerical formulation coupling the nonlinear processes becomes difficult to verify. Analytical solutions are not available for realistically complex problems and benchmark solutions are not generally available for specific problems. We present an advective reactive streamtube (ARS) transport technique that efficiently provides accurate solutions of nonlinear multicomponent reactive transport in nonuniform multidimensional velocity fields. These solutions can be compared with results from Eulerian-based advection–dispersion-reaction models to evaluate the accuracy of the numerical formulation used. The ARS technique includes mixed equilibrium and kinetic complexation and precipitation–dissolution reactions subject to the following assumptions: (1) transport is purely advective (i.e., no explicit diffusion or dispersion), and (2) chemistry is described by a canonical system of reactions that evolves with time and is unaffected by position in space. Results from the ARS technique are compared with results from the massively parallel, multicomponent reactive transport model MCTRACKER on a test problem involving irreversible oxidation of organic carbon and reaction of the oxidation products with two immobile mineral phases, gypsum and calcite, and fifteen aqueous complexes. Truncation error, operator splitting error, and the nonlinear transformation of these errors in the high-resolution reactive transport model are identified for this problem.

Impact of microstructure on anion exclusion in compacted clay media

The sensitivity of ion concentration distribution models to three key model assumptions, the pore-size distribution of clay media, the distance of closest approach of ions to the clay surface, and the accessibility of sub-nanometer-wide clay mineral interlayer spaces to anions, was explored by solving the Poisson-Boltzmann equation for swelling and non-swelling clay materials. Our calculations show that all three model assumptions significantly impact values predicted for the anion accessible porosity. As a consequence, macroscopic measurements of anion exclusion in clay media cannot be used to test any of the three model assumptions independently of the two others. Information gained at the nanoscale,in particular, a detailed characterization of pore size distribution, is necessary to develop accurate predictive models of the anion accessible porosity of clay media.

Investigation of Coupled Processes and Impact of High Temperature Limits in Argillite Rock

Author(s): Zheng, Liange; Rutqvist, Jonny; Steefel, Carl; Kim, Kunhwi; Chen, Fei; Vilarrasa, Victor; Nakagawa, Seiji; Houseworth, James; Birkholzer, Jens