Numerical Reactive Transport Model Research Articles

A multi-level pore scale reactive transport model for the investigation of combined leaching and carbonation of cement paste

Cementitious materials in underground constructions are exposed to CO2 rich ground waters which leads to combined carbonation and calcium leaching. Complex interplay between leaching, carbonation reaction and changes in transport pathways presents difficulty in parameterizing continuum scale models in a consistent way. Therefore, a novel multi-level pore-scale reactive transport model is presented to capture microstructure changes under combined carbonation and leaching. Model explicitly resolves capillary pores and phases with unresolved porosity as a porous media. Governing equations are solved using a lattice Boltzmann method based reactive transport solver with chemical reaction under thermodynamic equilibrium. The two-dimensional parametric study on idealized microstructures revealed that carbon content and pH of boundary solution strongly affects degradation rates, location and thickness of precipitated calcite layer. Furthermore, reactive surface area plays dominant role and tortuosity of media rather a secondary role. The three-dimensional simulations using virtual cement paste microstructure show that degradation rate exhibit non-linear behaviour with square root of time and time. This implies that simple empirical relations for prediction of progression of reaction fronts are not applicable and use of numerical reactive transport models is inevitable.. The developed model qualitatively captures the development of carbonation, leaching fronts and zonation as observed in experiments. Good quantitative agreement between modelling and experiments is obtained for initial stages. At later times, the modelling result and experimental observations diverge significantly. This discrepancy is likely due to lack of consideration of kinetics of C-S-H dissolution and calcite precipitation.

Influence of physical and chemical hydrology on bioremediation of a U-contaminated aquifer informed by reactive transport modeling incorporating 238U/235U ratios

Microbially-catalyzed reductive immobilization of aqueous uranium (U) as a solid phase has been proposed as a U remediation technique. Both laboratory and field experiments have demonstrated that this reduction reaction alters the 238U/235U ratio, producing a 238U-enriched U(IV) solid. In contrast, other major U reactive transport processes fractionate these isotopes much less. This suggests the potential to quantify the extent of bioreduction occurring in groundwater containing U using the 238U/235U ratio, which would substantially improve upon current practices largely relying on U concentration measurements alone. Current reactive transport models for uranium dynamics include only concentration measurements, which are strongly influenced by highly coupled reactive transformation and aqueous transport processes. The complex physical and chemical behavior of U potentially compromises such quantitative analysis of U storage and release. Here we report the first numerical reactive transport model which explicitly incorporates variations in the 238U/235U ratio of U and demonstrates improved interpretation of the principal chemical reactions and groundwater transport processes affecting the subsurface mobility and distribution of this widespread contaminant.Recent U bioreduction studies performed in a contaminated aquifer in Rifle, Colorado, USA applied Rayleigh distillation models to interpret U stable isotope fractionation observed as a result of acetate amendment. These simplified models were unable to resolve the spatiotemporal pattern of U isotope fractionation recorded in the aqueous solutes. Here, we employ the multi-component, isotope-enabled CrunchTope reactive transport software to interpret these measured U isotope ratios, and demonstrate accurate reproduction of observed trends in both geochemistry and 238U/235U ratios for two consecutive years of field experiments. Our results indicate that accurately modeling both the U concentration and isotope ratio distributions greatly constrains the parameter space of the model. We find that the transport properties of U in the Rifle aquifer are governed by the presence of low-permeability regions, which the isotopes are uniquely sensitive to. When U reduction is spatially constrained by these low-permeability regions, the shift in the 238U/235U ratio becomes more muted. Accurate modeling of observed U isotope ratios thus provides a powerful means to better understand bioremediation, and the current study serves to advance the application of this novel method.

Modelling transport of reactive tracers in a heterogeneous crystalline rock matrix

A numerical reactive transport model for crystalline rocks is developed and evaluated. The model is based on mineral maps generated by X-ray micro computed tomography (X-μCT); the maps used have a resolution of approximately 30 μm and the rock samples are on the cm scale. A computational grid for the intergranular space is generated and a micro-DFN (Discrete Fracture Network) model governs the grid properties. A particle tracking method (Time Domain Random Walk) is used for transport simulations. The basic concept of the model can now be formulated as follows; “when a particle is close to a reactive mineral surface it has a certain probability to get sorbed during a certain time span. Once sorbed it will remain so a certain time”. The model requires a number of input parameters that represent the sorption properties of the reactive minerals. Attempts are made to relate the parameters to traditional distribution parameters. The model is evaluated by comparisons with recent laboratory experimental data. These experiments consider two rock types (veined gneiss and pegmatitic granite) and two radionuclides (cesium and barium). It is concluded that the new reactive transport model can simulate the experimental data in a consistent and realistic way.

Intense rainfalls trigger nitrite leaching in agricultural soils depleted in organic matter

Nitrate and ammonium are common inorganic contaminants of anthropogenic origin in many shallow aquifers around the world, while nitrite is less common, but it is most harmful than nitrate and ammonium due to its high reactivity. This paper presents evidence of nitrite accumulation after intense rainfalls in soil samples collected in an agricultural field characterized by organic matter chronic depletion. Moreover, an intact core from the same site was also collected to perform an unsaturated column experiment (60 cm long and 20 cm outer diameter) mimicking heavy rainfalls (230 mm in 2 days). Results from the field site showed nitrite accumulation (up to 0.45 mmol/kg) at 50–70 cm depth, just below the plough layer. The column experiment showed very high initial concentrations of nitrate and nitrite in the leachate and a progressive decrease of nitrate due to denitrification. The numerical flow model was calibrated versus the observed volumetric water contents and leachate flow rates. The numerical reactive transport model was calibrated versus the leachate concentrations of six dissolved species (ammonium, nitrate, nitrite, dissolved organic carbon, chloride and bromide). The optimized model resulted to be robustly calibrated providing insights on the kinetic rates driving the production, accumulation and leakage of nitrite, showing that incomplete denitrification is the source of nitrite. As far as the authors are aware, this is the first study reporting a clear link between high nitrite leaching rates and extreme rainfall events in lowland agricultural soils depleted in organic matter. The proposed methodology could be applied to quantify nitrite cycling processes in many other agricultural areas of the world affected by extreme rainfall events.

Forecasting evolution of formation water chemistry and long-term mineral alteration for GCS in a typical clastic reservoir of the Southwestern United States

Groundwater chemistry and rock properties can change dramatically following CO2 injection in a geologic sequestration system. A favored target for subsurface sequestration is clastic reservoirs, due to their limited tendency to impact water quality or porosity and permeability due to dissolution or precipitation compared to carbonate reservoirs. However, most clastic reservoirs will exhibit geochemical changes, especially during the injection phase and over the long term. And, in most oil reservoirs targeted for enhanced recovery and concomitant CO2 storage, water-alternating-gas, or so-called “WAG” injection schemes are preferred to maximize CO2 mobility and minimize viscous fingering of CO2. Under WAG schemes, reactive transport processes and resulting water quality changes and rock property changes may differ when compared to continuous CO2 injection (CCI) schemes.The purpose of this paper is to analyze and quantify the extent of geochemical changes to both water chemistry and rock properties, specifically for the “low hanging fruit” of CO2 storage targets: a sandstone formation using a WAG injection scheme. Specifically, the objectives of this study are: (1) to evaluate the evolution of formation water chemistry and mineral alteration induced by WAG injection in a typical southwestern U.S. sandstone reservoir; (2) to quantify CO2 trapping mechanisms and associated porosity and permeability evolution over the long term following injection; (3) to investigate whether different injection schemes (WAG vs. CCI designs) may affect the evolution of water chemistry and mineral alteration during the injection phase. Because it is not just a candidate formation, but rather is already undergoing CO2 injection for enhanced oil recovery (EOR) and sequestration, the Morrow Sandstone Formation in the Anadarko Basin of Texas was selected as representative of a typical clastic CCS candidate. A numerical reactive transport model of a 5-spot well pattern in the Morrow Formation was developed and used to simulate WAG injection and subsequent geochemical processes. Initial conditions of flow and geochemistry were based on actual measurements from the Morrow Formation within the Farnsworth EOR field in northern Texas. The simulation design period included WAG injection for 25 years (injection phase) followed by 975 years of post-injection monitoring (arbitrary post-injection phase). Simulation results suggest that formation water chemistry (pH, aqueous species Ca2+, Mg2+, Fe2+, HCO3−) dramatically changes after CO2 arrival, and mineral dissolution (with an increase in porosity and permeability) is greatest near the injection well during the injection phase. The simulated increase in porosity is approximately 2.7%, with a maximum permeability increase of almost 8.4% at the end of the injection phase. The possibility of halite mineral precipitation, a phenomenon observed in many injection scenarios, was specifically examined. However, no simulations yielded halite precipitation. Mineral precipitation increases in the long term (hundreds of years), resulting in both increased CO2 trapping by mineralization as well as decreased porosity and permeability.Finally, the analysis of impacts of WAG injection during the injection phase suggests that the extent of CO2-rock geochemical interactions following WAG increases compared to CCI scenarios. Specifically, the extra water injected (in WAG) facilitates aqueous reactions compared to CCI, which “dries out” the formation. Changes in porosity and permeability for CCI schemes are much less than those for WAG schemes, a factor to consider with respect to how these different schemes may impact injectivity.

Surface passivation model explains pyrite oxidation kinetics in column experiments with up to 11 bars p(O2)

Despite decades of research in numerous experimental and field studies, the reaction kinetics of pyrite oxidation is still not characterized for high partial pressures of oxygen and near-neutral pH-levels. These conditions potentially exist in aquifers where oxidative site remediation, temporary water storage, or a leakage from a compressed air energy storage facility is present. For planning and monitoring of such field operations, their potential side effects on protected natural resources like groundwater have to be characterized. Thereby, site-scale assessments of such side effects of subsurface use by numerically modeling geochemical changes caused by the presence of oxygen need parametrization. Also, a function transferring results from simple, low pressure experiments to high pressure environments requires experimental bases. Pyrite oxidation can be the main consequence of oxygen intruding reduced aquifers. In this study, pyrite oxidation kinetics was examined at oxygen partial pressures from 0 to 11 bars, corresponding to an air intrusion in up to 500 m depth, at neutral pH-levels in high and low pressure flow-through column experiments representing aquifer conditions. A reaction rate equation was developed and evaluated with 1D PHREEQC numerical reactive transport models using experimental data as transfer function between high pressure and low pressure experiments. This model development included an improvement of established rate laws with a passivation term, which is, in contrast to previously published functions, dependent on the partial pressure of oxygen. The resulting model on passivated oxidation kinetics of pyrite at high oxygen partial pressures was able to reproduce independent experimental results acquired using different experimental set-ups. This assessment found the passivation to overcome the theoretical increase in pyrite oxidation kinetics caused by elevating oxygen partial pressure. These findings contribute to future experimental and modeling efforts for risk assessment and monitoring of oxygen-rich plumes in the subsurface.

Modeling the hydrogeochemical evolution of brine in saline systems: Case study of the Sabkha of Oum El Khialate in South East Tunisia

We studied the effects of evaporation and groundwater flow on the formation of salt minerals in the Sabkha of Oum El Khialate in South East Tunisia, which contains large amounts of sulfate sodium mineral deposits. Due to the fact that there are no important surface water bodies present in this sabkha, transport of solutes is dominated by advection rather than mixing in lakes. For our study we used both analytical conservative and numerical reactive transport models. Results showed that salinity varies with distance and may reach very high levels near a watershed where the groundwater flux is zero. As a consequence, reactive transport simulations results showed that more minerals precipitate and water activity decreases values near this watershed. Model results also showed that a sequence of precipitating minerals could be deduced after 140,000years. From the boundary of the sabkha towards the watershed the mineral sequence was dolomite, gypsum, magnesite, bloedite, halite and mirabilite. It was found that the amounts as well as the mineral precipitation distribution strongly depend on salinity and rates of inflowing water.

Model based evaluation of a contaminant plume development under aerobic and anaerobic conditions in 2D bench-scale tank experiments

The influence of transverse mixing on competitive aerobic and anaerobic biodegradation of a hydrocarbon plume was investigated using a two-dimensional, bench-scale flow-through laboratory tank experiment. In the first part of the experiment aerobic degradation of increasing toluene concentrations was carried out by the aerobic strain Pseudomonas putida F1. Successively, ethylbenzene (injected as a mixture of unlabeled and fully deuterium-labeled isotopologues) substituted toluene; nitrate was added as additional electron acceptor and the anaerobic denitrifying strain Aromatoleum aromaticum EbN1 was inoculated to study competitive degradation under aerobic /anaerobic conditions. The spatial distribution of anaerobic degradation was resolved by measurements of compound-specific stable isotope fractionation induced by the anaerobic strain as well as compound concentrations. A fully transient numerical reactive transport model was employed and calibrated using measurements of electron donors, acceptors and isotope fractionation. The aerobic phases of the experiment were successfully reproduced using a double Monod kinetic growth model and assuming an initial homogeneous distribution of P. putida F1. Investigation of the competitive degradation phase shows that the observed isotopic pattern cannot be explained by transverse mixing driven biodegradation only, but also depends on the inoculation process of the anaerobic strain. Transient concentrations of electron acceptors and donors are well reproduced by the model, showing its ability to simulate transient competitive biodegradation.

Conceptual and Numerical Modeling of Radionuclide Transport and Retention in Near-Surface Systems

Scenarios of barrier failure and radionuclide release to the near-surface environment are important to consider within performance and safety assessments of repositories for nuclear waste. A geological repository for spent nuclear fuel is planned at Forsmark, Sweden. Conceptual and numerical reactive transport models were developed in order to assess the retention capacity of the Quaternary till and clay deposits for selected radionuclides, in the event of an activity release from the repository. The elements considered were carbon (C), chlorine (Cl), cesium (Cs), iodine (I), molybdenum (Mo), niobium (Nb), nickel (Ni), radium (Ra), selenium (Se), strontium (Sr), technetium (Tc), thorium (Th), and uranium (U). According to the numerical predictions, the repository-derived nuclides that would be most significantly retained are Th, Ni, and Cs, mainly through sorption onto clays, followed by U, C, Sr, and Ra, trapped by sorption and/or incorporation into mineral phases.

Reactive transport modeling of the clogging process at Maqarin natural analogue site

The Maqarin site in Jordan has been investigated for three decades as a natural analogue for the long term changes of materials in contact with hyper-alkaline solutions. Similar processes are expected in radioactive waste disposal sites, where cement based materials are in contact with natural rocks or other e.g. clay based materials. In this context, a numerical reactive transport model was used to study local geochemical alterations and induced porosity changes for the Maqarin marl rock in contact with the hyper-alkaline solution. The geochemical setup for the rock mineralogy and the pore water was calibrated to match measurements from the Maqarin site. The setup includes several clay and zeolite minerals, considers cation exchange processes, and a state-of-the-art model for cement phases. Similar to earlier calculations by Steefel and Lichtner (1998) who used a much simpler geochemical model, the pore clogging occurred after several hundred years at a distance of 5–10mm from the contact to the hyper-alkaline solution. In our calculations, this was caused by a massive precipitation of ettringite and C–S–H minerals. We performed a sensitivity study by varying the intrinsic diffusion coefficient, the Archie’s law exponential factor, and the mineral surface area available for dissolution and precipitation. We found that the dissolution of clay minerals controls the availability of Al, which is needed for ettringite and C–S–H phase precipitation. Thus, the amount and kinetically controlled dissolution of clay minerals controls the spatial and temporal evolution of porosity changes. The simulations reveal that neither cation exchange processes nor the formation of zeolite minerals strongly influence the geochemical evolution of the system.

Dual Carbon–Chlorine Stable Isotope Investigation of Sources and Fate of Chlorinated Ethenes in Contaminated Groundwater

Chlorinated ethenes (CEs) are ubiquitous groundwater contaminants, yet there remains a need for a method to efficiently monitor their in situ degradation. We report here the first field application of combined stable carbon and chlorine isotope analysis of tetrachloroethene (PCE) and trichloroethene (TCE) to investigate their biodegradation in a heavily contaminated aquifer. The two-dimensional Compound Specific Isotope Analysis (2D-CSIA) approach was facilitated by a recently developed gas chromatography-quadrupole mass spectrometry (GCqMS) method for δ(37)Cl determination. Both C and Cl isotopes showed evidence of ongoing PCE transformation. Applying published C isotope enrichment factors (ε(C)) enabled evaluation of the extent of in situ PCE degradation (11-78%). We interpreted C and Cl isotopes using a numerical reactive transport model along a 60-m flow path. It revealed that combined PCE and TCE mass load was dechlorinated by less than 10%, and that cis-dichloroethene was not further dechlorinated. Furthermore, the 2D-CSIA approach allowed estimation of Cl isotope enrichment factors ε(Cl) (-7.8 to -0.8‰) and characteristic ε(Cl)/ε(C) values (0.42-1.12) for reductive PCE dechlorination at this field site. This investigation demonstrates the benefit of 2D-CSIA to assess in situ degradation of CEs and the applicability of Cl isotope fractionation to evaluate PCE and TCE dechlorination.

Fast semi-analytical approach to approximate plumes of dissolved redox-reactive pollutants in heterogeneous aquifers. 1. BTEX

Various numerical reactive transport models were developed in the last decade to simulate plumes of pollutants in heterogeneous aquifers. However, these models remain difficult to use for the non-specialist, and the computation times are often long. Users who need to fit several model parameters to match predictions with field data in heterogeneous aquifers may be discouraged by the time needed to run the simulations. The objective of this paper is to provide a set of approximations that allow performing almost instantaneous calculations for transport of redox-reactive pollutants, the most common examples being benzene, toluene, ethylbenzene and xylenes (BTEX). The approach relies on two major tools: (i) the use of flux tubes (FT), a variant of stream tubes that include dispersion, and (ii) sequential superposition of the reactions (Mixed Instantaneous and Kinetics Superposition Sequence (MIKSS)). The calculation of transport is uncoupled from the calculation of reactions. The superposition principle has been used previously for the analytical solution of a bimolecular reaction of an electron donor with an acceptor and is here extended to more than one dissolved electron acceptor reacting with more than one donor. The approach is furthermore improved by including limitations of the kinetic reactions according to the availability of the reactants and by combining kinetic and instantaneous reactions. The results computed with this approach are compared to three well known numerical models (RT3D, PHT3D, PHAST) for various test cases including uniform, slightly diverted or highly irregular flow fields and several reaction schemes for BTEX. The FT-MIKSS solution gives nearly the same results as the other models and proved to be very flexible. The major advantage of the FT-MIKSS solution is fast computation times that are generally 100 to 1000 times faster than other numerical models. This approach might be a useful tool during the long fitting procedure of field data, which may be followed by one single run of a classical numerical model using the best-fit parameters.

Devolatilization‐induced pressure build‐up: Implications for reaction front movement and breccia pipe formation

AbstractGeneration of fluids during metamorphism can significantly influence the fluid overpressure, and thus the fluid flow in metamorphic terrains. There is currently a large focus on developing numerical reactive transport models, and with it follows the need for analytical solutions to ensure correct numerical implementation. In this study, we derive both analytical and numerical solutions to reaction‐induced fluid overpressure, coupled to temperature and fluid flow out of the reacting front. All equations are derived from basic principles of conservation of mass, energy and momentum. We focus on contact metamorphism, where devolatilization reactions are particularly important owing to high thermal fluxes allowing large volumes of fluids to be rapidly generated. The analytical solutions reveal three key factors involved in the pressure build‐up: (i) The efficiency of the devolatilizing reaction front (pressure build‐up) relative to fluid flow (pressure relaxation), (ii) the reaction temperature relative to the available heat in the system and (iii) the feedback of overpressure on the reaction temperature as a function of the Clapeyron slope. Finally, we apply the model to two geological case scenarios. In the first case, we investigate the influence of fluid overpressure on the movement of the reaction front and show that it can slow down significantly and may even be terminated owing to increased effective reaction temperature. In the second case, the model is applied to constrain the conditions for fracturing and inferred breccia pipe formation in organic‐rich shales owing to methane generation in the contact aureole.

Modeling Dehalococcoides sp. Augmented Bioremediation in a Single Fracture System

A numerical reactive transport model was developed to simulate the bioremediation processes in a perchloroethene (PCE) contaminated single fracture system augmented with Dehalococcoides sp. (DHC). The model describes multispecies bioreactive transport processes that include bacterial growth and detachment dynamics, biodegradation of chlorinated species, competitive inhibition of various reactive species, and the loss of daughter products because of back‐partitioning effects. Two sets of experimental data, available in the study by Schaefer et al. (2010b), were used to calibrate and test the model. The model was able to simulate both datasets. The simulation results indicated that the yield coefficient and the DHC maximum utilization rate coefficient were the two important process parameters. A detailed sensitivity study was completed to quantify the sensitivity of the model to variations in these two parameter values. The results show that an increase in yield coefficient increases bacterial growth and thus expedites the dechlorination process, whereas an increase in maximum utilization rate coefficient greatly increased dechlorination rates. The proposed model provides a mathematical framework for simulating remediation systems that employ DHC bioaugmentation for restoring chlorinated‐solvent contaminated groundwater aquifers.

A modified batch reactor system to study equilibrium-reactive transport problems

It is difficult to design column experiments to study transport processes involving slow geochemical reactions that require long residence times to reach equilibrium. We propose a sequential equilibration reactor (SER) setup to study such equilibrium geochemical reactive transport problems. The proposed system consists of sequentially operated batch reactors that directly mimic typical one-dimensional grids used in numerical reactive transport models. The SER experimental setup has the characteristics of batch experiments and provides complete control over the reaction time; in addition, the setup also includes certain simple transport features. We conducted several single-reactor and multiple-reactor SER experiments to investigate arsenic adsorption and transport on iron-oxide coated sand, at different pH, solid–solution ratio, and initial arsenic concentration conditions. The data generated from the experiments are compared against predictions from a geochemical transport code (PHREEQCI) that used previously developed surface complexation model parameters to describe the reaction system. The model predictions matched the SER experimental data well. The proposed SER system provides a flexible alternative to column experiments and allows better control over system parameters such as pH, reaction time, and solid–solution ratio.

Estimating spatially-variable first-order rate constants in groundwater reactive transport systems

Numerical reactive transport models are often used as tools to assess aquifers contaminated with reactive groundwater solutes as well as investigating mitigation scenarios. The ability to accurately simulate the fate and transport of solutes, however, is often impeded by a lack of information regarding the parameters that define chemical reactions. In this study, we employ a steady-state Ensemble Kalman Filter (EnKF), a data assimilation algorithm, to provide improved estimates of a spatially-variable first-order rate constant λ through assimilation of solute concentration measurement data into reactive transport simulation results. The methodology is applied in a steady-state, synthetic aquifer system in which a contaminant is leached to the saturated zone and undergoes first-order decay. Multiple sources of uncertainty are investigated, including hydraulic conductivity of the aquifer and the statistical parameters that define the spatial structure of the parameter field. For the latter scenario, an iterative method is employed to identify the statistical mean of λ of the reference system. Results from all simulations show that the filter scheme is successful in conditioning the λ ensemble to the reference λ field. Sensitivity analyses demonstrate that the estimation of the λ values is dependent on the number of concentration measurements assimilated, the locations from which the measurement data are collected, the error assigned to the measurement values, and the correlation length of the λ fields.

A Method for Estimating In Situ Reaction Rates from Push-Pull Experiments for Arbitrary Solute Background Concentrations

This paper presents a general method and the mathematical equations for estimating in situ reaction rates from the tracer and reactive solute breakthrough curves obtained from single-well push-pull tests (PPTs). The in situ zeroth-order reaction rates can be obtained through the linear regression of the net mass transfer of reactive solutes versus time. The method was first tested for scenarios of various concentrations of reactive constituents and conservative tracer in background and injection solution using a numerical reactive transport model and was then applied to a set of field biostimulation data from PPTs performed at the U.S. Department of Energy9s Natural and Accelerated Bioremediation Research Program9s Field Research Center (Oak Ridge, TN). The results show that the method is general and can be applied to each of the scenarios as long as the concentrations in background water are known. While the method presents practitioners with a simplified and economic tool for a first approximation analysis of in situ reaction rates from PPT data, the derived reaction orders and rates are apparent and bulk properties by nature, masking the complexity of competing reactions regarding the reactive solute of interest.

Dolomitization, anhydrite cementation, and porosity evolution in a reflux system: Insights from reactive transport models

Significant volumes of world hydrocarbon reserves occur in dolostones, and the majority of these reservoirs are interpreted to be of reflux origin. We used a two-dimensional numerical reactive transport model to investigate systematically the temporal and spatial distribution of replacement dolomitization, dolomite cementation, anhydrite cementation, and porosity evolution in a reflux system. We tested the sensitivity of reflux dolomitization to brine concentration (mesohaline to near-halite-saturated brines), near-surface temperature, flow rate, porosity-permeability feedback relationships, reactive surface area, and the effect of initial porosity-permeability heterogeneity. Simulations support the contention that hypersaline reflux is capable of extensive pervasive dolomitization, and that mesohaline reflux dolomitization is viable. Reflux generated a tabular dolomite body that is thickest close to the brine source and thins basinward. Incorporation of initial porosity and permeability heterogeneity resulted in a dolomite body with pronounced lateral fingers. Replacement dolomitization increased porosity (up to 8%), but postreplacement reflux resulted in the precipitation of minor dolomite cements (overdolomitization). Anhydrite cements that were spatially and temporally associated with replacement dolomitization caused a significant porosity reduction of up to 25%. Simulated rates of replacement dolomitization by reflux are fast, up to three orders of magnitude faster than seawater dolomitization. The rate of dolomitization and anhydrite mineralization proved to be critically sensitive to the flow rate and the brine chemistry. Temperature and reactive surface area were important controls on the rate of dolomitization, whereas the feedback relationship between porosity and permeability was a relatively moderate control. Our reactive transport models predict the general spatial and temporal trends in dolomite, porosity, and anhydrite observed in some major dolomite reservoirs.

Numerical Model for Biological Oxidation and Migration of Methane in Soils

Concern over the potentially negative impacts of climate change has resulted in a search for techniques to reduce anthropogenic emissions of methane, a primary greenhouse gas. Microbial oxidation of methane in soils may serve as an inexpensive technique for reducing CH4 emissions from sources such as landfills and heavy oil wells. To gain a better quantitative understanding of the biological and physical processes limiting CH4 oxidation in soils and biofilters, a numerical reactive-transport model was developed. The model inputs include CH4 source strength, soil bulk density, moisture content, and biological kinetic parameters. The outputs consist of gas concentration profiles, CH4 oxidation rates, and surface flux rates. A series of soil column and batch incubation experiments were performed on a variety of soil types to calibrate and verify the model. The model was also verified by reproducing experimental results found in the literature.