Geochemical Transport Model Research Articles

Chemically reactive multicomponent transport simulation in soil and groundwater: 1. Model development and evaluation

The model, REACTRAN2D, is for simulating the transport of chemically reactive components in conjunction with energy transport in groundwater systems. It is coded on the basis of two well-known models, SUTRA and MINTEQA2. On the assumption that local equilibria exist, this coupled geochemical and heat transport model is solved by a parallel and sequential iteration approach. It has been compared with HYDROGEOCHEM and limited field data. Very close matches were achieved. This model can deal with chemically reactive transport involving solid phase, aqueous phase and gas phase, therefore, having a great potential to simulate the full range of geochemical problems covered by MINTEQA2.

On fluid flow and mineral alteration in fractured caprock of magmatic hydrothermal systems

Geochemical evolution in hydrothermal fractured rock systems occurs through a complex interplay of multiphase fluid and heat flow and chemical transport processes.Building on previous work, we present here simulations of reactive hydrothermal flow that include (1) detailed fracture‐matrix interaction for fluid, heat, and chemical constituents, (2) gas‐phase participation in multiphase fluid flow and geochemical reactions, (3) the kinetics of fluid‐rock chemical interaction, and (4) heat effects on thermophysical and chemical properties and processes. The present study uses, as an example, water and gas chemistry data as well as caprock mineral composition from the hydrothermal system in Long Valley Caldera (LVC), California. The flow system studied is intended to capture realistic features of fractured magmatic hydrothermal systems. The purpose of this “numerical experiment” is to gain useful insight into process mechanisms such as fracture‐matrix interaction, liquid‐gas phase partitioning, and conditions and parameters controlling water‐gas‐rock interactions in a hydrothermal setting. Simulation results indicate that almost all CO2 is transported through the fracture. Cooling and condensation results in an elevated CO2 partial pressure. The CO2 is the dominant gas‐phase constituent close to the land surface. Close to the heat source, dissolution dominates over precipitation. Away from the heat source precipitation dominates because chemical constituents, transported from the bottom, precipitate in a lower‐temperature environment. Mixing with cold meteoric water enhances mineral dissolution and precipitation effects. The rock alteration pattern is sensitive to reaction kinetics. The predicted alteration of primary rock minerals and the development of secondary mineral assemblages are generally consistent with field observations in the LVC. The observed sequence of argillic alteration in the LVC consists of an upper zone with smectite and kaolinite (in the lower‐temperature region), a lower illite zone, and an intermediate mixed illite and smectite zone. The sequence is reasonably well reproduced in the numerical simulation. In addition, calcite and chlorite precipitation in the hot region coincides with the observations. Using the reactive geochemical transport model, we have successfully simulated spatial distribution of the three argillic alteration zones.

Impact of Measurement Uncertainty in Chemical Quantities on Environmental Prognosis by Geochemical Transport Modelling*

In Germany, geochemical modelling takes a strong position in two aspects of broader public interest. The first aspect is the safety assessment of a nuclear waste repository, the second is remediation of uranium mining areas. In both aspects, the application of geochemical modelling is stipulated by authorities. This situation results from the possibility to model highly complex situations by computers. The increase in computing power experienced in recent times now offers techniques to assess the sensitivity of modelling results to uncertain input data both in the thermodynamic data base and the site-specific field data. Both aspects are investigated by using Monte Carlo methods in combination with non-parametric statistics. A probabilistic geochemical modelling of uranium mill tailings leaching is demonstrated by application of TReaC modelling code using a simplified site model.

Transport of metals leaching from land-disposed oil field wastes

The potential for groundwater contamination by metals leaching from land-disposed oil field exploration and produc tion (E&P) wastes is an environmental concern. In this study, a geochemical transport model is adopted to consider multi- species metal migration. Objectives are to characterize the chemical composition of E&P wastes, to evaluate the mobility and pollution potential of metals in E&P wastes and to inves tigate the utility of simplified modelling approaches. A chem ical transport model coupling a hydrologic submodel is used for the analysis. The hydrologic transport submodel considers one-dimensional advective-dispersive transport of compo nents in the aqueous phase under steady unsaturated flow con ditions. The geochemical submodel considers speciation, pre cipitation-dissolution and ion exchange reactions assuming local equilibrium conditions prevail. Simulations are per formed to evaluate the movement of metals through the unsaturated zone for the base case corresponding to a typical waste pit. Sensitivity analyses are performed for selected input parameters that are expected to have significant effect on metal transport. Breakthrough curves (BTCs) are obtained for metal species As, Ba, Cd, Cr, Cu, Pb and Zn at the water table. Inspection of BTCs reveals multiple peaks and complex inter relationships among model parameters which cannot be reproduced using single species transport models with linear reaction terms. Precipitation plays a significant role in attenu ation of metal concentrations in E&P wastes. Calculated vadose zone attenuation ratios indexing the pollution poten tial of various metals span a wide range of values due to the dif ferences in reactivity of metals. Results for the relative vadose zone concentration with respect to maximum contaminant levels in drinking water suggest that, in general, chloride is likely to to be more critical than trace metals in controlling the groundwater quality. Cadmium and copper are the next most likely to lead to non-compliance. Under conditions where Cr(III) is the dominant species in waste, chromium attenuation is greatly increased due to precipitation.

A test of long-term, predictive, geochemical transport modeling at the Akrotiri archaeological site

A study of elemental transport at the Akrotiri archeological site on the island of Santorini, Greece, has been conducted to evaluate the use of natural analog data in support of long-term predictive modeling of the performance of a proposed geologic repository for nuclear waste at Yucca Mountain, Nevada. Akrotiri and Yucca Mountain have many analogous features including silicic volcanic rocks, relatively dry climates, and oxidizing, hydrologically unsaturated subsurface conditions. Transport of trace elements from artifacts buried in volcanic ash 3600 years ago at Akrotiri is analogous to transport of radioactive wastes in the proposed repository. Subtle evidence for a plume of Cu, Zn, and Pb has been detected by selective leaching of packed earth and bedrock samples collected immediately beneath the site where bronze and lead artifacts were excavated. The geologic setting of the artifacts and the hydraulic properties of the enclosing media were characterized. A numerical model of the type used in repository performance assessments was developed for elemental transport at the site. Site characterization data were used to build the model but no prior information on the nature of the contaminant plume was provided to the modelers. Some model results are qualitatively consistent with field data, including the small amount of material transported, limited amounts of sorbed material, and relatively elevated sorption on a packed earth layer, However, discrepancies result from incomplete representation of heterogeneity and complexity and poorly constrained model parameters. Identification of such system characteristics and model limitations in relevant systems is a major contribution that analog studies can contribute in support of repository modeling.

Simulation of reactive geochemical transport in groundwater using a semi-analytical screening model

A reactive geochemical transport model, based on a semi-analytical solution to the advective-dispersive transport equation in two dimensions, is developed as a screening tool for evaluating the impact of reactive contaminants on aquifer hydrogeochemistry. Because the model utilizes an analytical solution to the transport equation, it is less computationally intensive than models based on numerical transport schemes, is faster, and it is not subject to numerical dispersion effects. Although the assumptions used to construct the model preclude consideration of reactions between the aqueous and solid phases, thermodynamic mineral saturation indices are calculated to provide qualitative insight into such reactions. Test problems involving acid mine drainage and hydrocarbon biodegradation signatures illustrate the utility of the model in simulating essential hydrogeochemical phenomena.

The simulation of geochemical reactions in the Heng-Chun limestone formation, Taiwan

The potential for dissolution of the limestone formations in coastal freshwater-saltwater mixing zones can be analyzed by a geochemical transport model. Geochemical mixing theory suggests that mixing of sea water and calcite-saturated fresh groundwater can result in a solution that is undersaturated with respect to calcite and can lead to significant dissolution of carbonate rocks. The purpose of this study is to investigate the possible mechanisms of the limestone dissolution in Heng-Chun in southwestern Taiwan. The geochemical model EQ3NR/6 and the geochemical transport model HYDROGEOCHEM are used to simulate a series of chemical reactions and solute transport. The input data for the simulation of groundwater quality and hydrogeological parameters were obtained from 24 wells around the Heng-Chun coastal area with reasonably simplified boundary conditions. Results of the simulation suggest that the dissolution process is not controlled by chemical reactions but strongly depends on the groundwater flow.

Contaminant Transport at a Waste Residue Deposit: 2. Geochemical Transport Modeling

Contaminant transport in an aquifer at an incinerator waste residue deposit in Denmark is simulated. A two‐dimensional, geochemical transport code is developed for this purpose and tested by comparison to results from another code. The code is applied to a column experiment and to the field site. The ongoing geochemical processes and any unknown geochemical parameters are obtained through simulation of the column experiment that used soil and leachate from the field site. The application of the code to the field site, which has been monitored for more than 15 years, use this geochemical information along with the flow and nonreactive transport parameters obtained by the inverse modeling procedure described in the first paper [Sonnenborg et al., this issue] of this two‐paper series. The simulation results of the site model are compared with several measured component breakthroughs at monitoring wells. Contamination was first controlled by transport, and later by transport and ion exchange. In both the column and field site simulations the code is used to identify the controlling transport processes, physical or geochemical (ion exchange and mineral precipitation), and to estimate the involved parameters (primarily ion exchange selectivity coefficients).

Cation and proton exchange, pH variations, and carbonate reactions in a freshening aquifer

Freshening of aquifers is accompanied by sequential elution of the saltwater (seawater) cations from the sediment's exchange complex. The resulting Chromatographic patterns are modeled with a one‐dimensional geochemical transport model that can handle the complex interplay of transport and mineral and ion exchange equilibria. The transport part is based on the mixing cell approach, with different time steps for advective and diffusive transport used when required by small grid size. The chemical reactions are calculated explicitly after each time step with the geochemical model PHREEQE. Ion exchange is included in the form of association half reactions, which allows simulation of the dynamic nature of the exchange process. The variation in the constant for proton association is obviated with an activity coefficient for H‐X that is derived from the constant capacitance model. All coefficients for the exchange model are obtained by fitting to literature data to be able to perform the modeling as realistically as possible. The code is applied to a laboratory column experiment, and subsequently used to demonstrate Chromatographic development of solute profiles in a freshening aquifer. Sequential peaks of Mg2+, K+, and Na+ along a flow path in the Aquia aquifer in Maryland are modeled, and the results confirm that the variation of water qualities in this aquifer has basically a Chromatographic origin. Proton exchange acts here as a source of acid in NaHCO3 water in which calcite dissolves. This explains the Na+ to HCO3− ratio and high δ13C observed in these waters.

Modeling reactive transport of organic compounds in groundwater using a partial redox disequilibrium approach

The chemical transformation of organic contaminants in natural groundwater systems is clearly dependent upon local geochemistry which determines the thermodynamically favorable degradation reactions and the nature of local microbial populations. Conversely, groundwater geochemistry may be impacted significantly in terms of pH and redox couple speciation by the chemical transformation of sufficient quantities of organic compounds. Therefore an understanding of the coupling between degradation reactions, local geochemistry, and chemical transport is essential in predicting the chemical evolution of contaminated aquifers. Equilibrium‐based reactive chemical transport models are usually not utilized for problems involving the transport of degradable organic compounds due to slow reaction kinetics and the persistence of intermediate degradation products. In this study we propose a reactive geochemical transport model which considers these types of degradation reactions. An expert system approach is used to postulate a set of sequential, first‐order degradation reactions for the organic compounds based upon thermodynamic considerations and user‐defined rules. Redox disequilibrium provides the driving force for the abiotic or microbially mediated transformation of the organic compounds as well as the associated response of groundwater geochemistry. Coupling between local inorganic geochemistry and reacting organic compounds is achieved by assuring conservation of operational valence and mass balance. The composite geochemical model is in turn coupled with an integral finite difference transport algorithm using a two‐step sequential solution approach. The transport equation is solved separately for each inorganic aqueous species, complex, and dissolved organic species, allowing a high degree of flexibility in problem definition. We apply the model to an illustrative example problem concerning the introduction of aromatic hydrocarbons and chlorinated ethenes into an initially oxidizing aquifer. Modeling results agree well qualitatively with field and laboratory observations reported in the literature in terms of degradation patterns and the effects on groundwater geochemistry.

A Chromatographic Model for Water Quality Variations in the Aquia Aquifer (Maryland, Usa)

The Aquia aqt~fer Flushing of marine sediments with fresh water will induce a chromatographic pattern of the cations which are displaced from the exchange complex. Upstream from the salt/fresh water boundary, a sequence of Na +, K +, Mg 2+ and lastly Ca 2+ dominated water is expected (Ca 2+ is the displacing cation; HCO~ is the major anion). This sequence is present in the Aquia aquifer. Field data from this aquifer have been modeled with a 1D geochemical transport model to satisfactory agreement. The upper 30 miles of the aquifer must have been flushed about 8 times under the model conditions, which means that the now observed chromatographic pattern can have been established in 100 ka. Introduction Along towlines in the Aquia aquifer, ChapeUe and Knobel (1983) observed zonal bands with changes in the concentrations of the major cations that have been attributed to Ca 2+ for Na + exchange and dissolution of Mg-calcites. The pattern is akin to a chromatographic sequence, where excess cations from the seawater exchange complex are flushed in order of increasing selectivity: first Na + , then K +, and lastly Mg 2+ . The geochemical transport model PHREEQM (Appelo and Postma, 1993) was adapted to the hydraulic conditions in the Aquia, and run to see whether the sequence of water qualities could result from chromatographic transport. Chromatographic modeling PHREEQM is a 1D transport model that uses PHREEQE (Parkhurst et al., 1980) to calculate the chemical reactions among water and sediment. It has been developed to model salt/fresh water displacements in Dutch aquifers, and includes cation exchange in the full dynamic sense. A form of the constant capacitance model was used for proton exchange. Exchange parameters are taken from literature. The Aquia aquifer extends over 90 km from the outcrop near Washington D.C. to Chesapeake Bay in the East, where it is bounded by a facies change. The aquifer sediment has 10 to 35% glauconite and is sealed on top and bottom by clay layers. These properties make the Aquia probably the most ideal analogue of a laboratory chomatographic column that can be found on earth (fig. 1). Rec~K~ a r u Chesapeake Bay

The Biogeochemical Transport Code Drink: a Mechanistic Description.

AbstractDrinkis a 2D research code, which simulates the long term evolution of shallow, trenched, LLW disposal sites with significant groundwater flow. It employs a finite difference solver and is built around a geochemical transport model into which various functional units are interfaced. These units describe sorption, corrosion, microbiology, radionuclide decay, colloids, mineral precipitation and dissolution and gaseous release. Each of these units is described here, along with a 1 D simulation of uranium migration.

A field experiment on cation exchange-affected multicomponent solute transport in a sandy aquifer

A field experiment was performed in an aquifer in order to study multicomponent cation-exchange processes under natural flow conditions. The aquifer is a glacial outwash plain with sandy aquifer material having a cation-exchange capacity (CEC) of ∼ 1.0 meg/100 g. A continuous injection of groundwater spiked with sodium and potassium as chlorides was accomplished over 37 days to resemble leachate contamination from landfills. The plume was monitored by sampling in a dense spatial network (length 100 m, width 20 m) over a period of 2.5 years in order to obtain breakthrough curves and spatial contour maps of the chemical compounds. Na and especially K showed a substantial retardation caused by cation-exchange processes despite the low CEC of the aquifer material. The average velocity of K + was only 10% of the velocity of chloride (0.7 m day −1). The relative migration velocity of Na + was not a constant in the plume, but apparently influenced by dilution. Ca 2+ and Mg 2+ were expelled from the cation-exchange sites of the aquifer material and subsequently transported with the same velocity as chloride. The breakthrough curves of the various compounds showed multiple peaks and low concentration zones. It was concluded by calculations with PHREEQE that changes in calcite equilibrium may occur in the lower part of the aquifer, while complexation processes seem to be of no importance. Cation exchange is then the most important process in this field experiment, and further evaluation of the data by a geochemical transport model including cation exchange is recommended.

Model simulations of a field experiment on cation exchange-affected multicomponent solute transport in a sandy aquifer

A large-scale and long-term field experiment on cation exchange in a sandy aquifer has been modelled by a three-dimensional geochemical transport model. The geochemical model includes cation-exchange processes using a Gaines-Thomas expression, the closed carbonate system and the effects of ionic strength. Information on geology, hydrogeology and the transient conservative solute transport behaviour was obtained from a dispersion study in the same aquifer. The geochemical input parameters were carefully examined. CEC and selectivity coefficients were determined on the actual aquifer material by batch experiments and by the composition of the cations on the exchange complex. Potassium showed a non-ideal exchange behaviour with KCa selectivity coefficients indicating dependency on equivalent fraction and K + concentration in the aqueous phase. The model simulations over a distance of 35 m and a period of 250 days described accurately the observed attenuation of Na and the expelled amounts of Ca and Mg. Also, model predictions of plateau zones, formed by interaction with the background groundwater, in general agreed satisfactorily with the observations. Transport of K was simulated over a period of 800 days due to a substantially attenuation in the aquifer. The observed and the predicted breakthrough curves showed a reasonable accordance taking the duration of the experiment into account. However, some discrepancies were observed probably caused by the revealed non-ideal exchange behaviour of K +.

A geochemical transport model for redox‐controlled movement of mineral fronts in groundwater flow systems: A case of nitrate removal by oxidation of pyrite

A one‐dimensional prototype geochemical transport model was developed in order to handle simultaneous precipitation‐dissolution and oxidation‐reduction reactions governed by chemical equilibria. Total aqueous component concentrations are the primary dependent variables, and a sequential iterative approach is used for the calculation. The model was verified by analytical and numerical comparisons and is able to simulate sharp mineral fronts. At a site in Denmark, denitrification has been observed by oxidation of pyrite. Simulation of nitrate movement at this site showed a redox front movement rate of 0.58 m yr−1, which agreed with calculations of others. It appears that the sequential iterative approach is the most practical for extension to multidimensional simulation and for handling large numbers of components and reactions. However, slow convergence may limit the size of redox systems that can be handled.

Geochemical calculations and observations on salt water intrusions. II. Validation of a geochemical model with laboratory experiments

Column experiments with aquifer sediments have been performed. A description is given of procedures and equipment which can maintain the anaerobic status of the sediments from sampling in the field to subsequent experimentation in the laboratory. From two sediments native pore solutions were displaced with SrCl 2 solutions; another was percolated alternately with groundwater and once diluted seawater. The chromatographic pattern which develops when exchangeable cations are eluted, was modelled with the earlier presented geochemical (multicomponent) transport model. Proper description of the SrCl 2-elution required reformulation of the cation exchange reaction for one sediment; the other sediment showed proton buffering and calcite dissolution with slow kinetics. These elutions could be modelled with constant exchange coefficients, but during fresh/saltwater displacements the selectivity for Ca/Mg exchange was variable.