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Sediment analysis using wet extraction techniques and model simulations were carried out to investigate the role of ferric iron during biodegradation of dissolved petroleum hydrocarbon compounds at a field site in Perth, Western Australia. Sediment cores were analysed for iron and sulphur species. The total iron concentrations were found to be low while the fraction of Fe(II) was surprisingly high. Pyrite was detected in the contaminated zone, but no iron monosulphides were found. A reactive multi-component transport model, coupling advective- dispersive transport of organic compounds and inorganic aqueous components with a geochemical equilibrium model and a biodegradation module, was applied to simulate qualitatively the fluxes and reactions involved in the biodegradation of BTEX compounds. Both field investigations and modelling suggest that ferric iron minerals play no important role as electron accepters while sulphate provides the major part of the oxidation capacity.

The transport and biochemical transformations of the iodinated X-ray contrast medium (ICM) iomeprol were studied at the stream/groundwater interface. During a one-month field experiment piezometric pressure heads, temperatures, and concentrations of redox-sensitive species, iomeprol and 15 of its transformation products (TPs) were collected in stream- and groundwater. The data set was analyzed and transformation processes and rates identified by comparing conservative and reactive transport simulations. ICM and TP transformations were simulated as a cometabolic process during organic carbon degradation. Using iomeprol/TPs ratios as calibration constrain mitigated the uncertainties associated with the high variability of the ICM wastewater discharge into the investigated stream. The study provides evidence that biodegradation of ICM occurs at the field-scale also for predominantly denitrifying conditions. Under these anaerobically dominated field conditions shortest simulated half-life (21 days) was in the same range as previously reported laboratory-determined half-lives for aerobic conditions.

Calcite is an important, relatively soluble mineral phase that can affect uranium reactive transport in subsurface sediments. This study was conducted to investigate the distribution of calcite and its influence on uranium adsorption and reactive transport in the groundwater–river mixing zone of the Hanford 300A site, Washington State. Simulations using a two-dimensional (2D) reactive transport model under field-relevant hydrological and hydrogeochemical conditions revealed the development of a calcite reaction front through the mixing zone as a result of dynamic groundwater–river interactions. The calcite concentration distribution, in turn, affected the concentrations of aqueous carbonate and calcium, and pH through dissolution, as river waters intruded and receded from the site at different velocities in response to stage changes. The composition variations in groundwater subsequently influenced uranium mobility and discharge rates into the river in a complex fashion. The results implied that calcite distribution and concentration are important variables that need to be quantified for accurate reactive transport predictions of uranium, especially in dynamic groundwater–river mixing zones.

The Total Variation Diminishing (TVD) algorithm (Zheng & Wang 1999) was selected which is feasible to reduce the influence of numerical dispersion effectively. The general grid was 4 × 4 m. The grid refinement at the wells was optimized by adjusting to a tracer breakthrough (O2) which was evaluated during the testing phase of the pilot plant. We received a good agreement with1 INTRODUCTIONHigh Arsenic (As) concentrations up to 9 mg/L were found in the groundwater on a military site in Northern Germany. Laboratory experiments suggested that bivalent iron [Fe(II)] has the potential to immobilize As which is composed of inorganic as well as various organic species (Hollander et al. 2008; Kruger et al. 2008). Subsequently, a pilot plant was built at one of the hot-spots and within a two-year operational period, As concentration was reduced from 1.9 mg/L to less than 0.25 mg/L (Kruger et al. 2012).

A newly developed groundwater and electrokinetic (EK) flow and reactive transport numerical model was applied to simulate electrokinetic in situ chemical oxidation (EK-ISCO) remediation. Scenario simulations that considered the oxidation of a typical organic contaminant (tetrachloroethene) by permanganate were used to gain a better understanding of the key processes and parameters that control remediation efficiency. In a first step a sensitivity analysis was carried out to investigate a range of EK, hydraulic and engineering parameters on the performance of EK-ISCO. While all investigated parameters affected the remediation process to some extent, the duration and energy required for remediation were shown to be most dependent upon the applied voltage gradient, the natural oxidant demand and the concentration of the injected oxidant. Secondly, the efficacy of EK-induced oxidant transport was further examined for a heterogeneous aquifer system with random permeability fields. Oxidant migration under EK was slower in low-permeability media due to the increased oxidant consumption of competing reductants. Instead of injecting oxidant only at the cathode, locating injection wells between the electrodes greatly increased the contaminant degradation by decreasing the distance the amendment had to migrate before reaching the contaminant.

Coupled intra-grain diffusional mass-transfer and non-linear surface complexation processes play an important role for the transport behaviour of U(VI) in contaminated aquifers. Two alternative model approaches for simulating these coupled processes have been analysed and compared: (i) the physical non-equilibrium approach that explicitly accounts for aqueous speciation and instantaneous surface complexation reactions in the intra-grain regions and approximates the diffusive mass exchange between the immobile intra-grain pore water and the advective pore water as multi-rate 1st-order mass transfer and (ii) the chemical non-equilibrium approach that approximates the diffusion-limited intra-grain surface complexation reactions by a set of multiple 1st-order surface complexation reaction kinetics, thereby eliminating the explicit treatment of aqueous speciation in the intra grain pore water. Model comparison has been carried out for column and field scale scenarios, representing the highly transient hydrological and geochemical conditions in the U(VI)-contaminated aquifer at the Hanford 300A site, Washington, USA. It was found that the response of apparent U(VI) adsorption/desorption kinetic behaviour to hydrogeochemically induced changes in U(VI) sorption strength is more pronounced in the physical than in the chemical non-equilibrium model. The magnitude of the differences in model behaviour depends particularly on the degree of disequilibrium between the advective and immobile phasemore » U(VI) concentrations. While a clear difference in U(VI) transport behaviour between the two models was noticeable for the column-scale scenarios, only minor differences were found for the Hanford 300A field scale scenarios, where the model-generated disequilibrium conditions were less pronounced as a result of high frequent groundwater flow reversals.« less

[1] Reactive transport modeling is a critical element in assessing the potential of natural attenuation of groundwater pollutants. In the present study, we developed a comprehensive quantitative model that incorporates the key processes affecting the long-term fate of complex organic compound mixtures released from coal tar–type dense nonaqueous phase liquid sources. The model framework addresses the simulation of the long-term dynamics of source zone depletion, the fate of the released compounds during reactive transport in the groundwater, the evolution of the aquifer's biogeochemical response, in particular its redox conditions, and the redox-dependent carbon isotope fractionation of selected organic compounds. The modeling framework was applied for the interpretation of observed biogeochemical and isotopic data from a well-characterized coal tar–contaminated site in northern Germany. The simulations highlight the diversity of fates of the individual compounds, which result from their widely varying physicochemical characteristics, and also how complex interactions develop over the lifetime of the contamination. The highly transient release of contaminants from the coal tar as pool and as heterogeneously distributed blobs in the source zone triggers continuously changing biogeochemical conditions and isotope signatures. The modeling results illustrate how difficult and uncertain the assessment of contaminant fate can be if the collected data cover only a small time window relative to the transport time scale. This emphasizes the need for a holistic understanding of the governing processes that control the effectiveness of monitored natural attenuation before it is implemented as a passive remediation strategy at nonaqueous phase liquid–contaminated sites.

Saline solutions are the most commonly used hydrological tracers, because they can be easily and economically monitored by in situ instrumentation such as electrical conductivity (EC) loggers in wells or by geoelectrical measurements. Unfortunately, these low-cost techniques only provide information on the total concentration of ions in solution, i.e.; they cannot resolve the ionic composition of the aqueous solution. This limitation can introduce a bias in the estimation of aquifer parameters where sorption phenomena between saline tracers and sediments become relevant. In general, only selected anions such as Cl- and Br- are recognised to be transported unretarded and they are referred to as conservative tracers or mobile anions. However, cations within the saline tracer may interact with the soil matrix through a range of processes such as ion exchange, surface complexation and via physical mass-transfer phenomena. Heterogeneous reactions with minerals or mineral surfaces may not be negligible where aquifers are composed of fine alluvial sediments. The focus of the present study was to examine and to quantify the bias between the aquifer parameters estimated during model-based interpretation of experimental data of EC measurements of saline tracer relative to the aquifer parameters found by specific measurements (i.e. via ionic chromatography, IC) of truly conservative species. To accomplish this, column displacement experiments with alluvial aquifer materials collected from the Po lowlands (Italy) were performed under water saturated conditions. The behaviour of six selected, commonly used saline tracers (i.e.; LiCl, KCl, and NaCl; LiBr, KBr, and NaBr) was studied and the data analysed by inverse modelling. The results demonstrate that the use of EC as a tracer can lead to an erroneous parameterisation of the investigated porous media, if the reactions between solute and matrix are neglected. In general, errors were significant except for KCl and KBr, which is due to the weak interaction between dissolved K + and the sediment material. The study shows that laboratory scale pre-investigations can help with tracer selection and to optimise the concentration range targeted for in situ multilevel monitoring by unspecific geoelectrical instrumentation. © 2011 Elsevier B.V.

[1] This study explores field-scale modeling of U(VI) reactive transport through incorporation of laboratory and field data. A field-scale reactive transport model was developed on the basis of laboratory-characterized U(VI) surface complexation reactions (SCRs) and multirate mass transfer processes, as well as field-measured hydrogeochemical conditions at the U.S. Department of Energy, Hanford 300 Area (300 A), Washington. The model was used to assess the importance of multirate mass transfer processes on U(VI) reactive transport and to evaluate the effect of variable geochemical conditions caused by dynamic river water-groundwater interactions on U(VI) plume migration. Model simulations revealed complex spatiotemporal relationships between groundwater composition and U(VI) speciation, adsorption, and plume migration. In general, river water intrusion enhances uranium adsorption and lowers aqueous uranium concentration because river water dilution increases pH and decreases aqueous bicarbonate concentration, leading to overall enhanced U(VI) surface complexation. Strong U(VI) retardation was computed for the field-measured hydrogeochemical conditions, suggesting a slow dissipation of the U(VI) plume, a phenomenon consistent with field observations. The simulations also showed that SCR-retarded U(VI) migration becomes more dynamic and synchronous with the groundwater flow field when multirate mass transfer processes are involved. Breakthrough curves at selected locations and the temporal changes in the calculated mass during the 20 year simulation period indicated that uranium adsorption/desorption never attained steady state because of the dynamic flow field and groundwater composition variations caused by river water intrusion. Thus, the multirate SCR model appears to be a crucial consideration for future reactive transport simulations of uranium contaminants at the Hanford 300 A site and elsewhere under similar hydrogeochemical conditions.

International audience; A model-based analysis of concentration and isotope data was carried out to assess natural attenuation of chlorinated ethenes in an aerobic fractured bedrock aquifer. Tetrachloroethene (PCE) concentrations decreased downgradient of the source, but constant δ13C signatures indicated the absence of PCE degradation. In contrast, geochemical and isotopic data demonstrated degradation of trichloroethene (TCE) and cis- 1,2-dichloroethene (DCE) under the prevailing oxic conditions. Numerical modeling was employed to simulate isotopic enrichment of chlorinated ethenes and to evaluate alternative degradation pathway scenarios. Existing field information on groundwater flow, solute transport, geochemistry, and δ13C signatures of the chlorinated ethenes was integrated via reactive transport simulations. The results provided strong evidence for the occurrence of aerobicTCEandDCEdegradation. The chlorinated ethene concentrations together with stable carbon isotope data allowed us to reliably constrain the assessment of the extent of biodegradation at the site and plume simulations quantitatively linked aerobic biodegradation with isotope signatures in the field. Our investigation provides the first quantitative assessment of aerobic biodegradation of chlorinated ethenes in a fractured rock aquifer based on compound specific stable isotope measurements and reactive transport modeling.

Severe petroleum hydrocarbon contamination (styrene and the BTEX compounds: benzene, toluene, ethylbenzene and the isomers of xylene) from leaking sewers was detected in a Quaternary aquifer below a chemical plant in the Padana Plain, Italy. From 1994, active pump and treat remediation has been employed. The site is bordered by canals which, in combination with variable pumping rates and groundwater flow directions, control groundwater levels. In this study we sought to determine the fate of styrene at the site within a mixed styrene/BTEX plume where the hydraulic boundaries induced strong seasonal variations in flows. In order to determine the fate of styrene, detailed field investigations provided intensive depth profile information. This information was then incorporated into a staged flow and reactive transport modelling. Three sets of measurements were obtained from sampling multilevel samplers (MLSs) under different hydraulic conditions at the site. These included measurements of BTEX, styrene, all major ions, pH and redox potential. A three-dimensional transient flow model was developed and calibrated to simulate an unconfined sandy aquifer with a variable flow field. Subsequently a reactive, multi-component transport model was employed to simulate the fate of dissolved BTEX and styrene along a selected flow line at the site. Each petroleum hydrocarbon compound was transported as independent species. Different, kinetically controlled degradation rates and a toxicity effect were simulated to explain the observed, selective degradation of pollutants in groundwater. Calibration of the model was accomplished by comparison with the three different sets of measurements obtained from the MLS devices. The results from various scenarios show that the detailed simulation of geochemical changes can be very useful to improve the site's conceptual model. © 2008 Elsevier B.V. All rights reserved.

A biogeochemical transport modeling study was carried out to analyze large-scale laboratory column experiments in which ethanol was used as an electron donor to create favorable conditions for the immobilization of selected trace metals (Zn and Cu) in groundwater. Microbial activity was explicitly simulated to capture the dynamic changes of the redox zonation within the column (i) in the early phase of the experiment (microbial lag) and (ii) in response to a significant decrease in the pH of the feed solution introduced after 188 days. The simulated redox dynamics agreed well with the observations after the pH-dependency of microbial growth was incorporated into the microbial model. The study showed that residual minerals may have buffered the pH for a period after the pH of the feed solution was decreased. Where the buffering capacity was exhausted,the pH decreased, leading to a successive downstream movement of the redox boundaries. The simulations reproduced the Zn immobilization within the sulfate-reducing zone as well as its partial remobilization after this zone moved further downstream. The immobilization of Cu within the denitrifying zone could also be well explained by incorporating malachite (Cu 2 (OH) 2 CO 3 ) precipitation in the simulations.

The Rayleigh equation is commonly applied to evaluate the extent of degradation at contaminated sites for which compound-specific isotope analysis (CSIA) data are available. However, it was shown recently that (i) the Rayleigh equation systematically underestimates the extent of biodegradation in physically heterogeneous systems, while (ii) it overestimates biodegradation if sorption-based carbon isotope fractionation is relevant. This paper further explores these two isotope effects not captured by the Rayleigh equation by means of a numerical modeling approach. The reactive multicomponent transport simulations show that the systematic underestimation is considerably larger for fringe-controlled and Monod-type degradation reactions than for previously assumed redox-insensitivefirst-orderdegradation kinetics, while forthe nonsteady state front portion of plumes, the Rayleigh equation may falsely indicate the occurrence of and/or overestimate biodegradation. The latter anomaly results from carbon isotope fractionation during sorption. It occurs for both supply-controlled degradation at the plume fringe and slow, reaction-controlled degradation inside the plume core. The numerical model approach enables a more accurate interpretation of CSIA data and thereby improves the quantification of biodegradation processes. © 2008 American Chemical Society.

[1] A numerical modeling approach was used to investigate the relevancy of the feedback mechanisms between geochemical reactions and variable density flow during free convection. The problem was studied by reformulating the classic Elder problem as a reactive multicomponent transport problem. It was found that for the studied system, the importance of this feedback decreased with increasing density contrast between intruding salinized water and the ambient fresh water body. For the smaller density contrasts, the flow patterns that develop during intrusion in the reactive simulations are characterized by different convection cells, higher rates of plume descent, and an increase in the total mass of solutes by up to 22% at the end of the simulation compared to the nonreactive simulations. For field situations, the results suggest that it may be necessary to consider the feedback between reactions and variable density flow where subtle density variations drive flow such as in contaminant plumes. For seawater intrusion problems, however, the feedback mechanism appears to be negligible for most cases.

Reactive multicomponent transport modeling was used to investigate and quantify the factors that affect redox zonation and the fate of the pharmaceutical residue phenazone during artificial recharge of groundwater at an infiltration site in Berlin, Germany. The calibrated model and the corresponding sensitivity analysis demonstrated thattemporal and spatial redox zonation at the study site was driven by seasonally changing, temperature-dependent organic matter degradation rates. Breakthrough of phenazone at monitoring wells occurred primarily during the warmer summer months, when anaerobic conditions developed. Assuming a redox-sensitive phenazone degradation behavior the model results provided an excellent agreement between simulated and measured phenazone concentrations. Therefore, the fate of phenazone was shown to be indirectly controlled by the infiltration water temperature through its effect on the aquifer's redox conditions. Other factors such as variable residence times appeared to be of less importance.

Data obtained from a field study of an aquifer contaminated by landfill leachate and related laboratory experiments suggest that natural attenuation of phenoxy acid herbicides such as mecoprop (MCPP) occurs in the transition zone between the anaerobic plume core and the overlying aerobic water body. The location of this transition zone is assumed to be primarily controlled by vertical transverse dispersion processes occurring downstream of the pollution source. A reactive transport modeling study was carried out to evaluate this conceptual model. The transport was simulated for a two-dimensional vertical cross section to quantify the combined physical, geochemical, and microbial processes that affect the fate of the phenoxy acid herbicides. The simulations, showing removal of phenoxy acids, an increase of phenoxy acid degraders in the fringe zone, and a dependency of the results on vertical transverse dispersivity, are compatible with the hypothesis of fringe-controlled aerobic biodegradation of the phenoxy acids.

Artificial groundwater recharge by percolation through the unsaturated zone is an important technique to enhance the water quality for drinking water supplies. Purification of the infiltration water results mainly from microbially mediated redox-reactions that involve the degradation of a wide range of organic substances. The aim of this study was to identify the spatial and temporal distribution of the redox zones that develop beneath an artificial recharge pond, a system that is characterised by regular hydraulic changes between the saturated and unsaturated condition within every operational cycle. The most significant hydraulic changes result from the formation and removal of a clogging layer at the pond’s bottom. Geochemical analyses of suction cup water as well as oxygen and hydraulic measurements showed that generally nitrate and manganese reducing conditions dominated below the pond as long as water saturated conditions prevailed. Iron and sulphate reduction occurred only in patchily distributed zones directly below the clogging layer and resulted from chemical and physical heterogeneity. When the sediment below the clogging layer became unsaturated, atmospheric oxygen penetrated from the pond fringes into this region, allowing re-oxidation of previously formed sulphide minerals.

Riverbank filtration is a low cost treatment technology which is effective in removing various chemical, and biological contaminants from the surface water. In the United States, utilities that employ horizontal collector wells, have reported clogging of the riverbed in vicinity of the wells, particularly around the laterals that go toward the river. In this paper, we show the impact of clogging and associated reduction in leakage on flow and transport simulations.

Over a period of several decades multiple leaks of large volumes from storage facilities located near Hnevice (Czech Republic) have caused the underlying Quaternary aquifer to be severely contaminated with nonaqueous phase liquid (NAPL) petroleum hydrocarbons. Beginning in the late 1980's the NAPL plume started to shrink as a consequence of NAPL dissolution exceeding replenishment and due to active remediation. The subsurface was classified geochemically into four different zones, (i) a contaminant-free zone never occupied by NAPL or dissolved contaminants, (ii) a re-oxidation zone formerly occupied by NAPL, (iii) a zone currently occupied by NAPL, and (iv) a lower fringe zone between the overlying NAPL and the deeper underlying contaminant-free zone. The study investigated the spatial and temporal variability of the redox zonation at the Hnevice site and quantified the influence of iron-cycling on the overall electron balance. As a first step inverse geochemical modelling was carried out to identify possible reaction models and mass transfer processes. In a subsequent step, two-dimensional (forward) multi-component reactive transport modelling was performed to evaluate and quantify the major processes that control the geochemical evolution at the site. The study explains the observed enrichment of the lower fringe zone with ferrihydrite as a result of the re-oxidation of ferrous iron. It suggests that once the NAPL zone started to shrink the dissolution of previously formed siderite and FeS by oxygen and nitrate consumed a significant part of the oxidation capacity for a considerable time period and therefore limited the penetration of electron acceptors into the NAPL contaminated zone.

[1] Managed aquifer recharge is an increasingly popular technique to secure and enhance water supplies. Among a range of recharging techniques, single-well aquifer storage and recovery (ASR) is becoming a common option to either augment drinking water supplies or facilitate reuse of reclaimed water. For the present study a conceptual biogeochemical model for reclaimed water ASR was developed and incorporated into an existing reactive multicomponent transport model. The conceptual and numerical model for carbon cycling includes various forms of organic and inorganic carbon and several reactive processes that transfer carbon within and across different phases. The major geochemical processes considered in the model were microbially mediated redox reactions, driven by the mineralization of organic carbon, mineral dissolution/precipitation, and ion exchange. The numerical model was tested and applied for the analysis of observed data collected during an ASR field experiment at Bolivar, South Australia. The model simulation of this experiment provides a consistent interpretation of the observed hydrochemical changes. The results suggest that during the storage phase, dynamic changes in bacterial mass have a significant influence on the local geochemistry in the vicinity of the injection/extraction well. Farther away from the injection/extraction well, breakthrough of cations is shown to be strongly affected by exchange reactions and, in the case of calcium, by calcite dissolution.

Numerical experiments of non-reactive and reactive transport were carried out to quantify the influence of a seasonally varying, transient flow field on transport and natural attenuation at a hydrocarbon-contaminated field site. Different numerical schemes for solving advective transport were compared to assess their capability to model low transversal dispersivities in transient flow fields. For the field site, it is shown that vertical plume spreading is largely inhibited, particularly if sorption is taken into account. For the reactive simulations, a biodegradation reaction module for the geochemical transport model PHT3D was developed. Results of the reactive transport simulations show that under the site-specific conditions the temporal variations in groundwater flow do, to a modest extent, affect average biodegradation rates and average total (dissolved) contaminant mass in the aquifer. The model simulations demonstrate that the seasonal variability in groundwater flow only results in significantly enhanced biodegradation rates when a differential sorption of electron donor (toluene) and electron acceptor (sulfate) is assumed.

Former mining areas generally suffer numerous environmental problems such as groundwater contamination with heavy metals and acidification. A reactive hybrid transport model has been developed to allow the quantification of uranium transport in flooded underground mines. This hybrid model specifically takes into account two different transport time scales within an underground mine, which result from the existence of a network of highly conductive shafts, drifts or ventilation raises within the considerably less permeable ore material in the underground. This paper introduces the model concept and addresses potential model applications.

Reference ECOL-ARTICLE-2002-001doi:10.1016/S0378-4754(02)00051-4 URL: http://www.elsevier.com/wps/find/journaldescription.cws_home/505615/description#description Record created on 2005-12-09, modified on 2016-08-08

Atrazine degradation rates are known to vary in dependence of a range of environmental factors. In the present study we used statistical techniques to identify the major dependencies and to quantify these relationships. It was found that atrazine degradation rates can be related to depth, organic carbon content, and redox potential. Furthermore, the effects of previous treatment, carbon amendment, and saturation were quantified. The statistical models are based upon a vast number of observations, and therefore are expected to be more reliable than existing atrazine degradation models. The two major factors in atrazine degradation were identified to be the availability of substrate and the oxidation state of the soil. The here found model for redox dependent degradation of atrazine can now be incorporated into rate expressions that are used in biogeochemical transport models where the redox-state is routinely computed.

The Lambert W is a transcendental function defined by solutions of the equation W exp(W) = x. For real values of the argument, x, the W-function has two branches, W-0 (the principal branch) and W-1 (the negative branch). A survey of the literature reveals that, in the case of the principal branch (W-0), the vast majority of W-function applications use, at any given time, only a portion of the branch viz. the parts defined by the ranges -1 less than or equal to W-0 less than or equal to 0 and 0 less than or equal to W-0. Approximations are presented for each portion of W-0, and for W-1. It is shown that the present approximations are very accurate with relative errors down to around 0.02% or smaller. The approximations can be used directly, or as starting values for iterative improvement schemes.

Subsurface contamination by organic chemicals is a pervasive environmental problem, susceptible to remediation by natural or enhanced attenuation approaches or more highly engineered methods such as pump-and-treat, amongst others. Such remediation approaches, along with risk assessment or the pressing need to address complex scientific questions, have driven the development of integrated modelling tools that incorporate physical, biological and geochemical processes. We provide a comprehensive modelling framework, including geochemical reactions and interphase mass transfer processes such as sorption/desorption, non-aqueous phase liquid dissolution and mineral precipitatation/dissolution, all of which can be in equilibrium or kinetically controlled. This framework is used to simulate microbially mediated transformation/degradation processes and the attendant microbial population growth and decay. Solution algorithms, particularly the split-operator (SO) approach, are described, along with a brief résumé of numerical solution methods. Some of the available numerical models are described, mainly those constructed using available flow, transport and geochemical reaction packages. The general modelling framework is illustrated by pertinent examples, showing the degradation of dissolved organics by microbial activity limited by the availability of nutrients or electron acceptors (i.e., changing redox states), as well as concomitant secondary reactions. Two field-scale modelling examples are discussed, the Vejen landfill (Denmark) and an example where metal contamination is remediated by redox changes wrought by injection of a dissolved organic compound. A summary is provided of current and likely future challenges to modelling of oxidisable organics in the subsurface.

PHT3D, a three-dimensional biogeochemical tool for natural and enhanced remediation of organic compounds in saturated porous media, is presented. The model is based on four modules: a three-dimensional transport model accounting for advective-dispersive transport and three modules accounting for (bio)chemical reactions. These comprise a NAPL (non aqueous phase liquids) dissolution module, a geochemical equilibrium package and a biodegradation module. The modules are coupled via an operator-splitting method. Model capabilities are demonstrated by way of an illustrative simulation for a dissolved hydrocarbon plume undergoing natural biogeochemical attenuation.

A one-dimensional (1-D) multi-component model accounting for hydrological transport inorganic equilibrium chemistry and microbial activity during kinetically controlled biodegradation in groundwater of compounds such as benzene, toluene, ethylbenzene and xylenes (BTEX) is presented. The problem is solved numerically using an operator-splitting method to couple advective–dispersive transport of organic and inorganic solutes with a geochemical equilibrium package PHREEQC and a biodegradation module. The transport equations for inorganic solutes are solved for total aqueous component concentrations. Changes in such concentrations due to precipitation/dissolution of minerals and chemical speciation are accounted for within PHREEQC. For chemical elements occurring in multiple valence states, separate components are defined and transported. The biodegradation module simulates the sequential or parallel activity of multiple bacterial groups attached to soils and their biochemical effects. The model has been evaluated by comparison with an existing model simulation of a 1-D inorganic redox problem. An application of the model is shown for a synthetic case where BTEX compounds are degraded by sequential reduction of aqueous electron acceptors.

Numerical models of reactive solute transport in groundwater are often solved using an approximate two-step approach that separates the transport and reaction processes. Even though the transport and reaction steps are split, using a novel formulation it is possible to pose the reaction step in general form as a system of ordinary differential equations (ODEs) such that the original, fully coupled model is solved exactly. The reaction step in the ‘standard’ two-step method is shown to be a special case of this general form. In this paper, these two approaches are compared in terms of accuracy and efficiency. The general and the standard ODEs representing the reaction steps are solved using the public domain ODE solver, LSODI. The ODEs representing the standard reaction step are also solved by the fourth- order Runge–Kutta (RK) method. Both the RK method and LSODI are capable of solving the system of ODEs in the standard two-step method. The RK method is found to be the most efficient even though it requires comparatively smaller time steps to yield accurate solutions. The LSODI solution of the general ODEs representing the reaction step was found to be extremely time consuming without any significant gain in accuracy.

A proposal for a modified in situ leaching method for extracting gold from oxidized gold ores using a non-cyanide lixiviant is described. A non-cyanide lixiviant is suggested because of the obvious concerns posed by injecting cyanide-bearing solutions into the subsurface. Oxidized gold ores were chosen as a focus because earlier research on the use of sodium thiosulfate as a lixiviant under anaerobic conditions indicated that the presence of pyrite led to rapid thiosulfate breakdown. A reconnaissance research program involving ore characterization and hydrometallurgical test work on samples from four Australian ore deposits and preliminary reactive transport modeling studies was carried out. This work showed that lixiviant-oxidant combinations of sodium thiosulfate and ferric EDTA and iodide and iodine are both capable of extracting high percentages of accessible gold from the selected samples in bottle roll tests under anaerobic conditions. The ore characterization and reactive transport studies suggested that both physical and chemical methods of permeability enhancement may be required to lift bulk permeability and the availability of gold for dissolution to sufficiently high levels to obtain adequate gold recoveries. Assuming that such methods prove to be both necessary and economically viable, the mining method would no longer be regarded as simple in situ leaching. Therefore, the term “in-place leaching” has been adopted for the proposed gold extraction system.

Arid and semi-arid climates are mainly characterised as those areas where precipitation is less (and often considerably less) than potential evapotranspiration. These climate regions are ideal environments for salt to accumulate in natural soil and groundwater settings since evaporation and transpiration essentially remove freshwater from the system, leaving residual salts behind. Similarly, the characteristically low precipitation rates reduce the potential for salt to be diluted by rainfall. Thus arid and semi-arid regions make ideal ‘salt concentrator’ hydrologic environments. Indeed, salt flats, playas, sabkhas and saline lakes, for example, are ubiquitous features of arid and semi-arid regions throughout the world (Yechieli and Wood,2002). In such settings, variable density flow phenomena are expected to be important, especially where hypersaline brines overlie less dense groundwater at depth. In contrast, seawater intrusion in coastal aquifers is a global phenomenon that is not constrained to only arid and semi-arid regions of the globe and is inherently a variable density flow problem by its very nature. These two examples make it clear that variable density flow problems occur in, but importantly extend beyond, arid and semi-arid regions of the globe. The intention of this chapter is therefore not to limit ourselves to modelling arid zone hydrological systems, but rather to present a more general treatment of variable density groundwater flow and solute transport phenomena and modelling. The concepts presented in this chapter are therefore not climatologically constrained to arid or semi-arid zones of the world, although they do apply equally there.

PHWAT is a new model that couples a geochemical reaction model (PHREEQC-2) with a density-dependent groundwater flow and solute transport model (SEAWAT) using the split- operator approach. PHWAT was developed to simulate multi- component reactive transport in variable density groundwater flow. Fluid density in PHWAT depends not on only the concentration of a single species as in SEAWAT, but also the concentrations of other dissolved chemicals that can be subject to reactive processes. Simulation results of PHWAT and PHREEQC-2 were compared in their predictions of effluent concentration from a column experiment. Both models produced identical results, showing that PHWAT has correctly coupled the sub- packages. PHWAT was then applied to the simulation of a tank experiment in which seawater intrusion was accompanied by cation exchange. The density dependence of the intrusion and the snow- plough effect in the breakthrough curves were reflected in the model simulations, which were in good agreement with the measured breakthrough data. Comparison simulations that, in turn, excluded density effects and reactions allowed us to quantify the marked effect of ignoring these processes. Next, we explored numerical issues involved in the practical application of PHWAT using the example of a dense plume flowing into a tank containing fresh water. It was shown that PHWAT could model physically unstable flow and that numerical instabilities were suppressed. Physical instability developed in the model in accordance with the increase of the modified Rayleigh number for density-dependent flow, in agreement with previous research.