Two-dimensional Reactive Transport Model Research Articles

Two-dimensional reactive transport model as a new approach for identifying the origins and contribution of arsenic in a soil and water system

Distribution of arsenic (As) in the environment can occur through various paths, depending on the interrelationship between soil and surface water, and groundwater. This study aimed to identify the origins and pathways of As among various potential origins in soil and water systems using two-dimensional reactive transport models (2D RTMs) and to evaluate the contribution of each origin to the geochemical distribution of As in the study area, which included a reservoir, two streams, and two abandoned mines. The 2D RTMs were prepared by combining hydraulic and geochemical reactive models. The MODFLOW-2005 package was used to simulate the water flow in the study area. The geochemical reaction and distribution of As were simulated using Geochemist's Workbench. The concentration of As in the reservoir was ∼50 μg/L, while that in the soils collected around the mine X and Y was 15.0–1853.2 mg/kg (median: 126.1 mg/kg) and 3.6–1629.2 mg/kg (median: 60.9 mg/kg), respectively. Hydraulic and geochemical input data were derived from field surveys and laboratory experiments. Based on the geochemical and hydrogeological input data, 2D RTMs determined that 32.0 kg of As accumulated into the reservoir over 18 years through surface water, which is from mines X (2.7 kg) and Y (29.3 kg). Hence, mine Y would be the primary origin of As to the reservoir. Based on 2D RTM, 2.4 kg of As entered the reservoir via groundwater (DO <0.1 mg/L). Also, precipitation of sulfide minerals (e.g., orpiment; As2S3) influences As dispersion and contribution under subsurface conditions. These modeling results suggest that 2D RTMs could be a new approach for identifying and assessing the contribution of potential As origins in soil and water system.

Simulating meteoric and mixing zone carbonate diagenesis with a two-dimensional reactive transport model

Meteoric and mixing-zone diagenesis can dramatically alter the geochemical signatures of shallow marine carbonates. Most preserved pre-Cretaceous carbonates were deposited in shallow marine environments and thus may have been susceptible to meteoric and mixing-zone diagenesis. However, a quantitative understanding of how the geochemical composition of carbonates changes during diagenesis still requires further development. Here, we present a new two-dimensional (2D) reactive transport model coupled with a 2D coastal hydrology model to simulate carbonate diagenesis and provide insights into its impact on the isotopic and elemental compositions of carbonates in the geological record. Using this model, we have simulated the stratigraphic trends and relationship between isotopic records (for example, δ13C and δ18O values) observed in modern (Recent-Miocene) sections where the impact of meteoric diagenesis has been clearly characterized. Our model can also reproduce anomalous Neoproterozoic carbonate geochemical profiles where the effects of meteoric diagenesis have been debated. Further, our model indicates that linear carbonate C-O isotope co-variations can either be generated in the mixing zone between freshwater and marine pore waters, or in the freshwater phreatic zone with a downward decrease in recrystallization (with no net carbonate dissolution or precipitation) rate. In addition, numerous processes were observed to decouple δ18Ocarb values from other isotopic and elemental signatures during carbonate diagenesis, indicating that a lack of linear correlation between δ18Ocarb values and other geochemical variables does not necessarily suggest limited meteoric alteration. Sensitivity analyses show that the steady-state timescale is controlled by compositional differences between fluid endmembers, the calcite-water element distribution coefficient, the recrystallization rate, porosity, and the groundwater discharge rate. Given that reactive transport models have proven to be powerful theoretical tools in many disciplines of Earth sciences, our hope is that this model will promote a more quantitative understanding of meteoric and mixing zone diagenesis of marine carbonates.

Reactive Transport Simulation of Fracture Channelization and Transmissivity Evolution.

Underground fractures serve as flow conduits, and they may produce unwanted migration of water and other fluids in the subsurface. An example is the migration and leakage of greenhouse gases in the context of geologic carbon sequestration. This study has generated new understanding about how acids erode carbonate fracture surfaces and the positive feedback between reaction and flow. A two-dimensional reactive transport model was developed and used to investigate the extent to which geochemical factors influence fracture permeability and transmissivity evolution in carbonate rocks. The only mineral modeled as reactive is calcite, a fast-reacting mineral that is abundant in subsurface formations. The X-ray computed tomography dataset from a previous experimental study of fractured cores exposed to carbonic acid served as a testbed to benchmark the model simulation results. The model was able to capture not only erosion of fracture surfaces but also the specific phenomenon of channelization, which produces accelerating transmissivity increase. Results corroborated experimental findings that higher reactivity of the influent solution leads to strong channelization without substantial mineral dissolution. Simulations using mineral maps of calcite in a specimen of Amherstburg limestone demonstrated that mineral heterogeneity can either facilitate or suppress the development of flow channels depending on the spatial patterns of reactive mineral. In these cases, fracture transmissivity may increase rapidly, increase slowly, or stay constant, and for all these possibilities, the calcite mineral continues to dissolve. Collectively, these results illustrate that fluid chemistry and mineral spatial patterns need to be considered in predictions of reaction-induced fracture alteration and risks of fluid migration.

Model-based optimization of field-scale electrokinetic treatment of dredged sediments

We developed a methodology for the cost optimization of electrokinetic treatment porous media contaminated by toxic metals. A two-dimensional reactive-transport model was implemented to simulate the transport of chemical species by diffusion, electromigration and electroosmosis, coupled with a geochemical model which calculates precipitation and dissolution of species, adsorption and desorption reactions, and aqueous speciation. The model was applied to the case study of an electrokinetic remediation prototype plant built in Livorno (Italy), treating 150m3 of dredged sediments contaminated by toxic metals. The plant consisted of an ex-situ treatment basin equipped with electrodic wells arranged on a rectangular grid, connected to an electrolyte management system for catholyte and anolyte pH control. We validated the model by comparing the simulated electric field with the measured electric potential and the simulated pH profiles with the pH values of field samples. Good agreement was achieved between the modelled and measured data. On the basis of the validated model, we performed a parametric study to evaluate the influence of electrode distance and sediment buffering capacity on treatment costs and calculated the overall cost as a function of these two parameters. The results and costs were evaluated in terms of Pb removal, which was taken as the representative toxic metal. The results revealed the existence of distinct minima, representing the best set of parameters which optimized the overall treatment costs. We believe that the methodology and results obtained can be employed as a valuable tool to support the evaluation and design of electrokinetic remediation systems.

Modeling biogeochemical processes and isotope fractionation of enhanced in situ biodenitrification in a fractured aquifer

Enhanced in situ biodenitrification (EIB) is a feasible technology to clean nitrate-polluted groundwater and reach drinking water standards. Aimed at enabling a better monitoring and management of the technology at the field scale, we developed a two-dimensional reactive transport model (RTM) of a cross section (26.5×4m) of a fractured aquifer composed of marls involving both biogeochemical processes and associated isotope fractionation. The RTM was based on the upscaling of a previously developed batch-scale model and on a flow model that was constructed and calibrated on in situ pumping and tracer tests. The RTM was validated using the experimental data provided by Vidal-Gavilan et al. (2013). The model considers several processes including (i) exogenous and endogenous microbial nitrate and sulfate respiration coupled to ethanol oxidation and linked to microbial growth and decay, and (ii) geochemical interactions (dissolution/precipitation of calcite), and (iii) isotopic fractionation of the reaction network (15N–NO3, 18O–NO3, 13C–DIC, 13C–ethanol, 13C–biomass, and 13C–calcite). Most of the calibrated microbiological parameter values at field scale did not change more than one order of magnitude from those obtained at batch scale, which indicates that parameters determined at the batch scale can be used as initial estimates to reproduce field observations provided that groundwater flow is well known. In contrast, the calcite precipitation rate constant increased significantly (fifty times) with respect to batch scale. The incorporation of isotope fractionation into the model allowed to confirm the overall consistency of the model and to test the practical usefulness of assessing the efficiency of EIB through the Rayleigh equation approach. The large underestimation of the Rayleigh equation of the extent of EIB (from 10 to 50%) was caused by the high value of hydrodynamic dispersion observed in this fractured aquifer together with the high reaction rates.

Two-dimensional reactive transport modeling of the alteration of a fractured limestone by hyperalkaline solutions at Maqarin (Jordan)

Two-dimensional reactive transport modeling of the Maqarin Eastern Springs site, a natural analogue for the alteration of a fractured limestone by high-pH Portland cement waters, has been performed using the CrunchFlow code. These 2D calculations included transport by advection–dispersion–diffusion along a single fracture and diffusion in the wall rock. Solute transport was coupled to mineral dissolution and precipitation. A limited sensitivity analysis evaluated the effect of different values of primary mineral surface areas, flow velocity and sulfate concentration of the inflowing high-pH solution.Major secondary minerals include ettringite–thaumasite, C–S–H/C–A–S–H and calcite. C–S–H/C–A–S–H precipitation is controlled by the dissolution of primary silicates. Ettringite precipitation is controlled by diffusion of sulfate and aluminum from the wall rock to the fracture, with aluminum provided by the dissolution of albite. Calcite precipitation is controlled by diffusion of carbonate from the wall rock. Extents of porosity sealing along the fracture and in the fracture-wall rock interface depend on assumptions regarding flow velocity and composition of the high-pH solution. The multiple episodes of fracture sealing and reactivation evidenced in the fracture infills were not included in the simulations. Results can qualitatively reproduce the reported decrease in porosity in the fractures and in the wall rock next to the fractures. Instances of porosity increase next to fractures caused by carbonate dissolution were not reproduced by the calculations.

Field demonstration of CO2 leakage detection in potable aquifers with a pulselike CO2-release test.

This study presents two field pulselike CO2-release tests to demonstrate CO2 leakage detection in a shallow aquifer by monitoring groundwater pH, alkalinity, and dissolved inorganic carbon (DIC) using the periodic groundwater sampling method and a fiber-optic CO2 sensor for real-time in situ monitoring of dissolved CO2 in groundwater. Measurements of groundwater pH, alkalinity, DIC, and dissolved CO2 clearly deviated from their background values, showing responses to CO2 leakage. Dissolved CO2 observed in the tests was highly sensitive in comparison to groundwater pH, DIC, and alkalinity. Comparison of the pulselike CO2-release tests to other field tests suggests that pulselike CO2-release tests can provide reliable assessment of geochemical parameters indicative of CO2 leakage. Measurements by the fiber-optic CO2 sensor, showing obvious leakage signals, demonstrated the potential of real-time in situ monitoring of dissolved CO2 for leakage detection at a geologic carbon sequestration (GCS) site. Results of a two-dimensional reactive transport model reproduced the geochemical measurements and confirmed that the decrease in groundwater pH and the increases in DIC and dissolved CO2 observed in the pulselike CO2-release tests were caused by dissolution of CO2 whereas alkalinity was likely affected by carbonate dissolution.

Combined source apportionment and degradation quantification of organic pollutants with CSIA: 2. Model validation and application.

Compound-specific stable isotope analysis (CSIA) has proven a useful tool for the quantification of the extent of degradation (QED), and for source identification and source apportionment (SA) in contaminated environmental systems. However, the simultaneous occurrence of degradation processes and mixing of emission sources complicates the use of CSIA in combined SA and QED. In a companion study, we developed a mathematical model that allows for combined SA and QED of organic pollutants (and inorganic compounds such as nitrate) in a scenario of two emission sources and degradation via one reaction pathway. This work presents a validation of the model against virtual data from a two-dimensional reactive transport model. The model calculations for SA and QED were in good agreement with the simulation results, which suggests the correctness of the model assumptions. However, the application of the model to field data of benzene contamination was challenged by large uncertainties in CSIA data and the unknown interplay between competing degradation pathways. Nonetheless, the use of the model allowed for the identification of a prevailing contribution of one emission source and revealed a low overall extent of degradation at the field site. This indicates that the model can, for example, facilitate the characterization of air pollution or aquifer contamination with organic pollutants.

Spatial zonation limits magnesite dissolution in porous media

We investigate how mineral spatial distribution in porous media affects their dissolution rates. Specifically, we measure the dissolution rate of magnesite interspersed in different patterns in packed columns of quartz sand where the magnesite concentration (v/v) was held constant. The largest difference was observed between a “Mixed column” containing uniformly distributed magnesite and a “One-zone column” containing magnesite packed into one cylindrical center zone aligned parallel to the main flow of acidic inlet fluid (flow-parallel One-zone column). The columns were flushed with acid water at a pH of 4.0 at flow velocities of 3.6 or 0.36m/d. Breakthrough data show that the rate of magnesite dissolution is 1.6–2 times slower in the One-zone column compared to the Mixed column. This extent of rate limitation is much larger than what was observed in our previous work (14%) for a similar One-zone column where the magnesite was packed in a layer aligned perpendicular to flow (flow-transverse One-zone column). Two-dimensional reactive transport modeling with CrunchFlow revealed that ion activity product (IAP) and local dissolution rates at the grid block scale (0.1cm) vary by orders of magnitude. Much of the central magnesite zone in the One-zone flow-parallel column is characterized by close or equal to equilibrium conditions with IAP/Keq>0.1. Two important surface areas are defined to understand the observed rates: the effective surface area (Ae) reflects the magnesite that effectively dissolves under far from equilibrium conditions (IAP/Keq<0.1), while the interface surface area (AI) reflects the effective magnesite surface that lies along the quartz–magnesite interface. Modeling results reveal that the transverse dispersivity at the interface of the quartz and magnesite zones controls mass transport and therefore the values of Ae and AI. Under the conditions examined in this work, the value of Ae varies from 2% to 67% of the total magnesite BET surface area. Column-scale bulk rates RMgCO3,B (in units of mol/s) vary linearly with Ae and AI. Using Ae to normalize rates, we calculate a rate constant (10−9.56mol/m2/s) that is very close to the value of 10−10.0mol/m2/s under well-mixed conditions at the grid block scale. This implies that the laboratory-field rate discrepancy can potentially be caused by differences in the effective surface area. If we know the effective surface area of dissolution, we will be able to use the rate constant measured in laboratory systems to calculate field rates for some systems. In this work, approximately 60–70% of the Ae is at the magnesite–quartz interface. This implies that in some field systems where the detailed information that we have for our columns is not available, the effective mineral surface area may be approximated by the area of grains residing at the interface of reactive mineral zones. Although it has long been known that spatial heterogeneities play a significant role in determining physical processes such as flow and solute transport, our data are the first that systematically and experimentally quantifies the importance of mineral spatial distribution (chemical heterogeneity) on dissolution.

A Numerical Approach to Simulate the Biogeochemical Involvement in a Coastal Reduced Groundwater Environment

Bacteria-mediated reduction processes play a decisive role in the water quality alterations in the subsurface environment of coastal aquifers. Availability of organic carbon as the electron donor with other electron acceptors such as NO3−, MnO2, Fe(OH)3 and SO42− induces different bacteria to activate under aerobic and anaerobic conditions. A two-dimensional reactive transport model has been developed to simulate the bacteria-mediated reduction reactions in a coastal aquifer. The model explains the utilisation of O2, NO3−, MnO2, Fe(OH)3 and SO42− as electron acceptors for the oxidation of organic carbon in the aquifer under aerobic and anaerobic conditions. The conceptual model consists of three different phases named as bio phase, mobile phase and matrix phase. Model parameters are adopted from literature on bacteria-mediated multi-component modelling and bioremediation processes. Monod kinetic equation is assumed to formulate the bacterial growth. The model explains the behaviours of aerobic and anaerobic bacteria under the availability of organic carbon. Two scenarios are tested and numerical results are discussed. The present numerical study highlights the possibility of the simulation of the formation of reduced environments in coastal aquifers which has not received much attention of groundwater modelling community yet.

Two-dimensional reactive transport modeling of CO2 injection in a saline aquifer at the Sleipner site, North Sea

This paper presents a 2D reactive transport model of long-term geological storage of carbon dioxide. A data set from the Utsira formation in Sleipner (North Sea) is utilized for geochemical simulation, while the aquifer is approximated as a 2D cylindrically symmetric system. Using the reactive transport code TOUGHREACT, a 25 year injection scenario followed by a 10,000 year storage period are simulated. Supercritical CO2 migration, dissolution of the CO2 in the brine, and geochemical reactions with the host rock are considered in the model. Two mineralogical assemblages are considered in the Utsira formation, a sand formation that is highly permeable and a shale formation representing four semi-permeable layers in the system that reduce the upward migration of the supercritical CO2. The impacts of mineral dissolution and precipitation on porosity are calculated. Furthermore, the 2D cylindrical geometry of the mesh allows simulating both the upward migration of the supercritical gas bubble as well as the downward migration of the brine containing dissolved CO2. A mass balance of the CO2 stored in, respectively, the supercritical phase, dissolved in the aqueous phase, and sequestered in solid mineral phases (carbonate precipitation) is calculated over time. Simulations with lower residual gas saturation and with different mesh refinement are also performed to test the sensitivity on mass balance estimates.

Geochemical model of the granite–bentonite–groundwater interaction at Äspö HRL (LOT experiment)

The bentonite buffer material has become an integral component of many HLNW repository designs since its introduction by SKB in the KBS-3 concept. The bentonite buffer provides mechanical stability, hydrological isolation and chemical buffering and radionuclide retardation. The three functions are interconnected and, consequently, the geochemical stability of the bentonite buffer is essential for the performance of the HLNW repository. In this context, it is of paramount importance to understand how bentonite responds to some geochemical processes as a consequence of the geochemical variability of the repository system. For this reason, SKB is performing a field test of these geochemical interactions at the underground laboratory in Äspö, Sweden—the so-called LOT experiment. The aim of this experiment is to understand how bentonite reacts to changes produced by a thermal gradient and/or to the addition of some substances at different bentonite blocks, such as NaCl, KCl, gypsum, calcite and cement, among others. In order to predict the potential outcome of these experiments, we have modelled some of the special cases of the LOT experiment. The calculations have been performed using a two-dimensional reactive-transport model with the code PHAST. The results indicate that cation exchanger processes are essential for the chemical performance of the bentonite; additionally, calcite buffers pH in almost all cases, except where cement is present. In this case, equilibrium with portlandite imposes hyperalkaline conditions, and the precipitation of sulphides would lead to reducing conditions. Calcite dissolution–precipitation is governed by the calcium in solution, which is mainly controlled by the cation exchange in the bentonite. The effect of increased salinity in the system by adding KCl or NaCl to the bentonite results in an acidification of the solution. This is due to the release of Ca from the exchanger and the subsequent precipitation of calcite. Overall, the results of the calculations indicate that quite extensive changes will be apparent in the LOT experiments after the 5-year contact time. The outcome of the postexperimental observations will verify or falsify the results from the modelling calculations.