Javier Samper

Modeling of non-isothermal multi-component reactive transport in field scale porous media flow systems

Abstract A general 2-D finite element multi-component reactive transport code, TRANQUI, was developed, using a sequential iteration approach (SIA). It is well suited to deal with complex real-world thermo-hydro-geochemical problems for single-phase variably water saturated porous media flow systems. The model considers a wide range of hydrological and thermodynamic as well as chemical processes such as aqueous complexation, acid-base, redox, mineral dissolution/precipitation, gas dissolution/ex-solution, ion exchange and adsorption via surface complexation. Under unsaturated conditions only water flow is considered, although gas pressures are allowed to vary in space in a depth-dependent manner specified by the user. In addition to the fully iterative sequential approach (SIA), a sequential non-iterative approach (SNIA), in which transport and chemistry are de-coupled, was implemented and tested. The accuracy and numerical performance of SIA and SNIA have been compared using several test cases. The accuracy of SNIA depends on space and time discretization as well as on the nature of the chemical reactions. The capability of the code to model a real case study in the field is illustrated by its application to the modeling of the hydrochemical evolution of the Llobregat Delta aquitard in northeastern Spain over the last 3500 years during when fresh-water flow from a lower aquifer displaced the native saline aquitard waters. Manzano and Custodio carried out a reactive transport model of this case study by using the PHREEQM code and considering water flow, aqueous complexation, cation exchange and calcite dissolution. Their results compare favorably well with measured porewater chemical data, except for some of the cations. Our code is not only able to reproduce the results of previous numerical models, but leads to computed concentrations which are closer to measured data mainly because our model takes into consideration redox processes in addition to the processes mentioned above. A number of sensitivity runs were performed with TRANQUI in order to analyze the effect of errors and uncertainties on cation selectivities.

Numerical modeling of the transient hydrogeological response produced by tunnel construction in fractured bedrocks

Abstract Groundwater inflows into tunnels constructed in fractured bedrocks not only constitute an important factor controlling the rate of advancement in driving the tunnel but may pose potential hazards. Drawdowns caused by tunnel construction may also induce geotechnical and environmental impacts. Here we present a numerical methodology for the dynamic simulation of the hydrogeological transient conditions induced by the tunnel front advance. The methodology is based on the use of a Cauchy boundary condition at the points lying along the tunnel according to which water discharge, Q , is computed as the product of a leakage coefficient, α , and the head difference, ( H − h ), where H is the prescribed head at the tunnel wall and h is the hydraulic head in the fractured rock in the close vicinity of the tunnel. At a given position of the tunnel, α is zero until the tunnel reaches such position when it is assigned a positive value. The use of step-wise time functions for α allows an efficient and accurate simulation of the transient hydrogeological conditions at and around the tunnel during the excavation process. The methodology has been implemented in TRANMEF-3, a finite element computer code for groundwater flow in 3D fractured media developed at the University of A Coruna, Spain, and has been used to simulate the impact of a tunnel on the groundwater system at the Aspo island (Sweden). This tunnel was constructed to access an underground laboratory for research on radioactive waste disposal. The large amount of available data at this site provides a unique opportunity to test the performance of the numerical model and the proposed methodology for tunnel advance. With just minor calibration, the numerical model is able to reproduce accurately the measurements of inflows into the tunnel at several reaches and hydraulic heads at surface-drilled boreholes. These results obtained at the Aspo site lead us to conclude that accurate predictions of the transient hydrogeological responses induced by tunneling works in fractured bedrocks, can be achieved provided that a sound hydrogeological characterization of large-scale fracture zones is available.

Identifying geochemical processes by inverse modeling of multicomponent reactive transport in the Aquia aquifer

Modeling reactive geochemical transport in the subsurface is a powerful tool for understanding and interpreting geochemical processes in aquifer systems. Different conceptual models can include different combinations of geochemical processes. A limitation of current inverse models is that they are based only on one conceptual model, which may lead to statistical bias and underestimation of uncertainty. We present a stepwise inverse modeling methodology that can include any number of conceptual models and thus consider alternate combinations of processes, and it can provide a quantitative basis for selecting the best among them. We applied the inverse methodology to modeling the geochemical evolution in the Aquia aquifer (Maryland, USA) over 105 yr. The inverse model accounts for aqueous complexation, acid-base and redox reactions, cation exchange, proton surface complexation, and mineral dissolution and precipitation; identifi es relevant geochemical processes; and estimates key reactive transport parameters from available hydrogeochemical data. Inverse modeling provides optimum estimates of transmissivities, leakage rates, dispersivities, cation exchange capacity (CEC), cation selectivities, and initial and boundary concentrations of selected chemical components. Inverse modeling with multiple conceptual models helps to identify the most likely physical and chemical processes in the paleohydrology and paleogeochemistry of the Aquia aquifer. Identifi cation criteria derived from information theory are used to select the best among ten candidate conceptual models. In the fi nal model, both proton surface complexation and methane oxidation are identifi ed as relevant geochemical processes.

Inverse modeling of multicomponent reactive transport through single and dual porosity media

Compacted bentonite is foreseen as buffer material for high-level radioactive waste in deep geological repositories because it provides hydraulic isolation, chemical stability, and radionuclide sorption. A wide range of laboratory tests were performed within the framework of FEBEX (Full-scale Engineered Barrier EXperiment) project to characterize buffer properties and develop numerical models for FEBEX bentonite. Here we present inverse single and dual-continuum multicomponent reactive transport models of a long-term permeation test performed on a 2.5 cm long sample of FEBEX bentonite. Initial saline bentonite porewater was flushed with 5.5 pore volumes of fresh granitic water. Water flux and chemical composition of effluent waters were monitored during almost 4 years. The model accounts for solute advection and diffusion and geochemical reactions such as aqueous complexation, acid-base, cation exchange, protonation/deprotonation by surface complexation and dissolution/precipitation of calcite, chalcedony and gypsum. All of these processes are assumed at local equilibrium. Similar to previous studies of bentonite porewater chemistry on batch systems which attest the relevance of protonation/deprotonation on buffering pH, our results confirm that protonation/deprotonation is a key process in maintaining a stable pH under dynamic transport conditions. Breakthrough curves of reactive species are more sensitive to initial porewater concentration than to effective diffusion coefficient. Optimum estimates of initial porewater chemistry of saturated compacted FEBEX bentonite are obtained by solving the inverse problem of multicomponent reactive transport. While the single-continuum model reproduces the trends of measured data for most chemical species, it fails to match properly the long tails of most breakthrough curves. Such limitation is overcome by resorting to a dual-continuum reactive transport model.

Stochastic analysis of transport and multicomponent competitive monovalent cation exchange in aquifers

Most stochastic analyses of reactive transport in physically and geochemically heterogeneous aquifers have focused on the analysis of a single reactive species. Here we conduct the stochastic analysis of multicomponent competitive monovalent cation exchange. Transport equations for dissolved cations are coupled with nonlinear cation exchange terms, which, for chemical equilibrium, are described by mass-action law expressions. These equations can be effectively decoupled by assuming that the weighted sum of cation concentrations is constant. The weight of each cation is equal to the reciprocal of its selectivity. Randomness of cation exchange capacity (CEC) leads to random retardation factors. Analytical expressions for effective retardation factors, longitudinal macrodispersivities, and concentration spatial moments are derived for a chemical system made of three monovalent cations (Na1 ,K 1 , and Cs 1 ) using the stochastic analytical solution of Miralles-Wilhelm and Gelhar (1996). Our results indicate that effective retardation factor, RC,i, spatial moments, and macrodispersivities of K1 are significantly different from those of Na1 . Effective retardation factors asymptotically attain their mean values after a transient phase of cationdependent duration. They strongly depend on the correlation between log-permeability (log K) and CEC. Pre-asymptotic effective retardation factor values for a negative correlation are smaller than the mean value, regardless of the value of the coefficient of variation of CEC (CVCEC). The smaller (larger) the variance of log K, , the great2 sf er (smaller) the effective retardation factor for a negative (positive) correlation. Cation

A coupled THC model of the FEBEX in situ test with bentonite swelling and chemical and thermal osmosis

The performance assessment of a geological repository for radioactive waste requires quantifying the geochemical evolution of the bentonite engineered barrier. This barrier will be exposed to coupled thermal (T), hydrodynamic (H), mechanical (M) and chemical (C) processes. This paper presents a coupled THC model of the FEBEX (Full-scale Engineered Barrier EXperiment) in situ test which accounts for bentonite swelling and chemical and thermal osmosis. Model results attest the relevance of thermal osmosis and bentonite swelling for the geochemical evolution of the bentonite barrier while chemical osmosis is found to be almost irrelevant. The model has been tested with data collected after the dismantling of heater 1 of the in situ test. The model reproduces reasonably well the measured temperature, relative humidity, water content and inferred geochemical data. However, it fails to mimic the solute concentrations at the heater-bentonite and bentonite-granite interfaces because the model does not account for the volume change of bentonite, the CO(2)(g) degassing and the transport of vapor from the bentonite into the granite. The inferred HCO(3)(-) and pH data cannot be explained solely by solute transport, calcite dissolution and protonation/deprotonation by surface complexation, suggesting that such data may be affected also by other reactions.

Groundwater flow and solute transport in fracture zones: an improved model for a large-scale field experiment at Äspö (Sweden)

157-172 Groundwater flow and solute transport in fracture zones: an improved model for a large-scale field experiment at Äspö (Sweden) Jorge Molinero E.T.S. Ingenieros de Caminos Canales y Puertos, Campus de Elvina Universidadde A Coruña 15192 A Coruña Spain E-mail: molinero@iccp.udc.es Javier Samper E.T.S. Ingenieros de Caminos Canales y Puertos, Campus de Elvina Universidad de A Coruna, 15192 A Coruna, Spain. E-mail: jsc@iccp.udc.es Several countries around the world are considering the final disposal of high-level radioactive waste in deep repositories located in fractured granite formations. Evaluating the long term safety of such repositories requires sound conceptual and numerical models of groundwater flow, solute transport and chemical and radiological processes. These models are being developed from data and knowledge gained from in situ experiments carried out at deep underground laboratories such as that of Äspö, Sweden, constructed in fractured granite. The Redox Zone Experiment is one of such experiments performed at Aspö in order to evaluate the effects of the construction of the access tunnel on the hydrogeological and hydrochemical conditions of a fracture zone intersected by the tunnel. Hydrochemical and isotopic data of this experiment were interpreted by Banwart et al. (Appl. Geochem., 14, 1999, 873) using a mass-balance approach based on a qualitative description of groundwater flow conditions. Such an interpretation, however, is subject to uncertainties related to an oversimplified conceptualization of groundwater flow. Here we present finite element numerical models of groundwater flow and solute transport for this fracture zone. The first model is based on Banwarts conceptual model. It presents noticeable inconsistencies and fails to match simultaneously observed drawdowns and chloride breakthrough curves. To overcome its limitations, a revised flow and transport model is presented which relies directly on available hydrodynamic and transport parameters, is based on the identification of appropriate flow and transport boundary conditions and uses, when needed, solute data extrapolated from nearby fracture zones. A significant quantitative improvement is achieved with the revised model because its results match simultaneously drawdown and chloride data. Other improvements are qualitative and include: ensuring consistency of hydrodynamic and hydrochemical data and avoiding inconsistencies in the hydrodynamic model. These results enable us to conclude that quantitative analyses of hydrochemical data should rely on sound conceptual and numerical flow and transport models which provide an appropriate framework for checking the consistency of hydrodynamic and hydrochemical data. This framework can be subsequently used for coupling groundwater flow, solute transport and chemical processes in the future.

Formulation of the inverse problem of non-isothermal multiphase flow and reactive transport in porous media

A methodology for solving the inverse problem of non-isothermal multiphase flow and multicomponent reactive solute transport is presented. Different types of data can be taken into account including heads or pressures, temperature, concentrations of dissolved components, total (dissolved plus sorbed) concentrations, water fluxes, water content, as well as parameter prior information. The inverse problem is solved by minimizing a generalized least-squares criterion with a Gauss-Newton-Levenberg-Marquardt method. Approximate confidence intervals are computed from the parameter covariance matrix. All these features are incorporated in INVERSE-FADES-CORE, a general code which solves both the forward and inverse problems. The code can estimate a wide range of different types of parameters including: intrinsic permeability, parameters of the relative permeability curve, parameters of retention curve, tortousity of vapor, thermal conductivity, dispersivity, distribution coefficient, molecular diffusion coefficient, initial and boundary concentrations, pH and pE, selectivity coefficients, cation exchange capacity, specific surface of minerals, and initial volume fraction of minerals. Synthetic data have been used to verify the formulation and study the convergence, uniqueness, and stability of the algorithm. The methodology has been applied to the estimation of transport and chemical parameters of a heating and hydration laboratory experiment performed on a sample of a compacted bentonite, which is planned to be used as buffer and sealing material for the disposal of high-level radioactive waste in several European countries.

Conceptualizing a mountain hydrogeologic system by using an integrated groundwater assessment (Serra da Estrela, Central Portugal): a review

Mountains are often considered as the world’s water towers. This paper presents a critical review on the research concerning the integrated assessment of groundwater resources of the mountain hydrogeologic system of Serra da Estrela Natural Park (central Portugal). The study area is the Zêzere river basin upstream of Manteigas village located at the Serra da Estrela Mountain in Central Portugal. It provides the source of strategic water resources for the Portuguese mainland, including normal groundwaters, thermomineral waters and surface waters. An integrated approach has been used to formulate a conceptual model for this complex mountain hydrogeological system by integrating the geological, morphotectonic, hydroclimatic, unsaturated soil zone, hydrogeological, hydrogeophysical, hydrogeochemical and isotopic data. This model has been useful to: i) evaluate the water resources; ii) provide the basis for a sustainable management of water resources, iii) design measures for groundwater exploitation and contamination control; and iv) set up land-use policies.

Dual-continuum multicomponent reactive transport with nth-order solute transfer terms for structured porous media

The dual-continuum model (DCM) is a type of modeling approach often used to interpret anomalous and non-ergodic solute transport in which the breakthrough curve cannot be explained by the classical advection–dispersion equation. Experimental studies show that bentonites have a macro-porous domain containing free water and a micro-porous domain containing double-layer and interlayer water. Therefore, a DCM could be needed to describe water flow, solute transport, and chemical reactions through compacted bentonite. In most DCMs, the mass exchange between domains is based on a lumped first-order solute transfer term which is not always accurate. An nth-order solute transfer term for dual-continuum flow and reactive transport model for structured porous media is proposed here. The value of n is derived from the approximation of the analytical solution of diffusion through a thin slab. The parameters of DCMs are obtained by an inverse methodology. Solute transfer terms for compacted bentonite have been evaluated for 1-D and 2-D synthetic cases by solving the inverse problem for several values of n. The best results are obtained with an n of 0.72 and a scale term of 2.5. The reactive transport DCM has been tested with data from a permeability test conducted on a sample of full-scale engineered barrier experiment (FEBEX) bentonite. By accounting for the solute flux across the micro–macro interface, the DCM overcomes the limitations of the single-continuum model (SCM) which fails to reproduce the long tails of the breakthrough curves of most chemical species of this test. The exponent n and the scaling factor were estimated from Cl− data. They are similar to those obtained from the 2-D synthetic case, thus indicating that the exponent and the scaling factor derived for the FEBEX bentonite could be used for other compacted bentonites.