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Mine waste-rock piles can release low quality drainage that is harmful to the surrounding environment. Many studies have investigated recently placed waste rock, but fewer have examined geochemical processes within, and downgradient of, old waste rock, even though these processes may be expected to persist for many decades. The Ore Chimney property, in Ontario, Canada, was the site of gold exploration activities that produced a small waste-rock pile; it was abandoned in 1934. Elevated concentrations of Zn are restricted to within 50 m of the waste rock, and pH remains neutral across the site. Water and sediment analyses and geochemical modeling indicate that several processes are involved in pH buffering and contaminant control. Water samples taken at the edge of the waste rock were not acidic, indicating that mechanisms within the waste rock, including carbonate buffering and preferential oxidation of sphalerite over pyrite, are preventing acid mine drainage (AMD). Natural attenuation mechanisms are operating within wetlands at Ore Chimney with the most likely controls for Zn transport in ground and surface water being carbonate mineral precipitation, co-precipitation with Fe and Mn oxides and oxyhydroxide minerals and Al sulphate minerals, and adsorption onto calcite and organic matter. This investigation shows that, after long time frames, natural attenuation mechanisms may act to effectively immobilize metal contaminants.

One of the large-scale field waste rock experiments (test piles) conducted as part of the Diavik Waste Rock Project was deconstructed, providing a spatially located set of geochemical, mineralogical, and particle-size distribution samples. Geostatistical analyses were conducted for sulfur and carbon content and saturated hydraulic conductivity, which affect the geochemical evolution of waste rock, to investigate the spatial dependence of these parameters. Analyses included population statistics, experimental semi-variogram estimation, and theoretical semi-variogram fitting. Population statistics were calculated for additional data sets from samples collected during the construction of the test piles. The population statistical analyses indicated that log-normal distribution provided the best fit for all investigated data sets. Experimental semi-variograms were estimated for the spatially located data set (test pile deconstruction) using the classical estimator, and theoretical semi-variograms were fitted. This investigation showed that the spatial distribution of sulfur, carbon, and hydraulic conductivity within the core of the test-pile experiments can be approximated using a log-normal distribution with a mean and standard deviation calculated using the samples collected during construction of the piles, and that little to no spatial relationship was present for these parameters at the scale of sampling. That the saturated hydraulic conductivity of the matrix material can be represented by the same statistical distribution throughout the test pile is significant because water flow, as well as mineral surface area and reactivity are dominantly controlled within the matrix portion of the test pile. Reactive transport simulations are included to demonstrate the influence of the matrix material on effluent geochemistry.

In situ thermal recovery is utilized extensively for unconventional bitumen extraction in the Cold Lake-Beaver River (CLBR) basin in Alberta, Canada. Public health concerns have been raised over potable groundwater contamination and arsenic release adjacent to these operations within the CLBR basin, which have been linked to subsurface heating of aquifer sediments. Under localized heated conditions, As-bearing aquifer sediments have been shown to undergo water-rock interactions and release constituents at near neutral pH conditions; however, release mechanisms have yet to be conclusively reported. To investigate the hydrogeochemical processes of aquifer heating and solute transport in detail, this study applies a novel heated column design to mimic saturated aquifer materials in contact with a thermal recovery well while constraining flow and geochemical conditions. Two column experiment scenarios were considered using: 1) quartz [SiO2] sand with 0.6 wt% pyrite [FeS2]; and 2) aquifer sediments collected from the CLBR region. Heated temperature gradients between 50 °C and 90 °C were maintained within a 0.6 m section of the 3 m column with a flow rate of one pore volume per week. During heated low oxygen (

The Diavik Waste Rock Project, located in a region of continuous permafrost in northern Canada, includes complementary field and laboratory experiments with the purpose of investigating scale-up techniques for the assessment of the geochemical evolution of mine waste rock at a large scale. As part of the Diavik project, medium-scale field experiments (∼1.5 m high active zone lysimeters) were conducted to assess the long term geochemical evolution and drainage of a low-sulfide waste rock under a relatively simple (i.e. constrained by the container) flow regime while exposed to atmospheric conditions. A conceptual model, including the most significant processes controlling the sulfide-mineral oxidation and weathering of the associated host minerals as observed in a laboratory humidity cell experiment, was developed as part of a previous modelling study. The current study investigated the efficacy of scaling the calibrated humidity cell model to simulate the geochemical evolution of the active zone lysimeter experiments. The humidity cell model was used to simulate the geochemical evolution of low-sulfide waste rock with S content of 0.053 wt.% and 0.035 wt.% (primarily pyrrhotite) in the active zone lysimeter experiments using the reactive transport code MIN3P. Water flow through the lysimeters was simulated using temporally variable infiltration estimated from precipitation measurements made within 200 m of the lysimeters. Flow parameters and physical properties determined during previous studies at Diavik were incorporated into the simulations to reproduce the flow regime. The geochemical evolution of the waste-rock system was simulated by adjustment of the sulfide-mineral content to reflect the values measured at the lysimeters. The temperature dependence of the geochemical system was considered using temperature measurements taken daily, adjacent to the lysimeters, to correct weathering rates according to the Arrhenius equation. The lysimeter simulations indicated that a model developed from simulations of laboratory humidity cell experiments, incorporating detailed representations of temporally variable temperature and water infiltration, can be scaled to provide a reasonable assessment of geochemical evolution of the medium-scale field experiments.

The Diavik Waste Rock Project consists of laboratory and field experiments developed for the investigation and scale-up of the geochemical evolution of sulfidic mine wastes. As part of this project, humidity cell experiments were conducted to assess the long-term geochemical evolution of a low-sulfide waste rock. Reactive transport modelling was used to assess the significant geochemical processes controlling oxidation of sulfide minerals and their dependence on temperature and sulfide mineral content. The geochemical evolution of effluent from waste rock with a sulfide content of 0.16 wt.% and 0.02 wt.% in humidity cells was simulated with the reactive transport model MIN3P, based on a conceptual model that included constant water flow, sulfide mineral content, sulfide oxidation controlled by the availability of oxidants, and subsequent neutralization reactions with carbonate and aluminosilicate minerals. Concentrations of Ni, Co, Cu, Zn, and SO4 in the humidity cell effluent were simulated using the shrinking core model, which represented the control of oxidant diffusion to the unreacted particle surface in the sulfide oxidation process. The influence of temperature was accounted for using the Arrhenius relation and appropriate activation energy values. Comparison of the experiment results, consisting of waste rock differentiated by sulfide mineral content and temperature, indicated surface area and temperature play important roles in rates of sulfide oxidation and release of sulfate and metals. After the model was calibrated to fit the effluent data from the higher sulfide content cells, subsequent simulations were conducted by adjusting only measured parameters, including sulfide mineral content and surface area.

Chemical weathering of sedimentary rocks is of great importance in determining seepage water chemistry, carbon, iron, calcium and sulfur turnover, as well as mineral transformation. In this study, we used the numerical code MIN3P to investigate controls on seepage water chemistry during chemical weathering of marine mudrocks. In particular, we focused on the pyrite- and kerogen-bearing formation, Opalinus Clay (with outcrops in the area of the Swabian and Franconian Alb in Southern Germany), a typical fine-grained sedimentary mudrock that had been deposited during the Middle Jurassic in a shallow marine environment. In the geochemical model we considered four reactive minerals, i.e. , pyrite, kerogen, calcite and siderite (assuming silicate minerals to be stable), and ran model scenarios over a time period of 10 kyrs (since the last ice age). Our numerical results show that chemical weathering of Opalinus Clay is driven by oxygen ingress (which depends on effective gas diffusion, and thus on water saturation). Due to oxidation of pyrite and kerogen seepage water acidifies, which leads to dissolution of carbonate minerals, i.e. , calcite and siderite. As a consequence, porosity and groundwater alkalinity increase, and CO 2 is released into the atmosphere at early decades. Following the consumption of primary reactive minerals, iron oxides precipitate in the oxic zone. We compared our model results with field data of water saturation, porosity, and water chemistry. The overall reasonable fit between model results and field data demonstrates the applicability of the numerical code MIN3P to quantify chemical weathering of pyrite-bearing sedimentary mudrocks and to predict seepage water chemistry that is impacted by geochemical water-rock interactions.

Drill cuttings were collected at 1 m depths from an instrumented, low sulfur, experimental waste rock pile containing a 4C-pyrrhotite that had been exposed to the extreme freeze-thaw cycle of a tundra climate. Boreholes were drilled from the top to base in the center of the pile and near the core-batter transition. Waste rock samples were analyzed for carbon, sulfur, and metal concentrations; sulfur oxidation states; and variation in iron and nickel forms due to oxidative dissolution of pyrrhotite. Results from X-ray absorption spectroscopy and aqueous extraction experiments were used to relatively compare samples from various depths in the boreholes, which indicate sulfide weathering fronts that decrease in intensity from the top to core to base at the center of the pile and from the core-batter transition to the center of the pile. The tundra climate and waste pile configuration produce a permafrost base, a seasonally frozen core, and an atmospheric-like zone near the surface. The fluctuation of the freeze-thaw cycle caused the greatest sulfide weathering near the surface and lesser weathering in the core and base of the pile. Metal- and sulfur-rich leachate from the higher weathering zone likely is collecting on a variable and seasonal frozen surface beneath the surface layer that causes metals and S to precipitate and (or) sorb during a portion of the year. The accumulation of sulfur and metals with the flux of this frozen surface produces a nickel and possibly an iron and sulfur enrichment zone beneath the surface layer in the center of the pile. The weathering front from the top to core to base of the pile and from core-batter transition to the core corresponds to a previously formulated leachate model, but the enrichment zone below the surface zone is unique to this conceptual waste-rock weathering model for this tundra climate.

Field experiments previously conducted to assess organic carbon (OC) amendments for in situ biological treatment of tailings porewater at the Greens Creek Mine (Alaska, USA) showed sulfate reduction, metal-sulfide precipitation, and decreased fluxes of sulfate and dissolved metals. Here, we develop two reactive transport models using the reactive transport code MIN3P to simulate hydrogeochemical processes and δ 34S–SO4 isotope fractionation over four years in test cells containing unamended (control) and amended (5 vol % OC) tailings. These models successfully simulate observed data including pH, SO4, δ 34S–SO4, Ca, Fe, K, Mg, Mn, Si, and Zn. The models also indicate that dissolution of carbonate and, to a lesser extent, aluminosilicate minerals neutralize acidic porewater generated from sulfide mineral oxidation and sulfate reduction reactions. Application of a constant kinetic fractionation factor of 0.9820 to simulate measured δ 34S–SO4 trends confirms that sulfate removal principally results from microbially-mediated sulfate reduction in conjunction with OC oxidation and subsequent metal-sulfide precipitation. Gypsum precipitation/dissolution and thiosulfate disproportionation have negligible effects on modelled porewater δ 34S–SO4 signatures. Our simulations are consistent with the previous findings that metal-sulfide precipitation controls Fe and Zn attenuation in amended tailings and that coprecipitation reactions contribute to metal removal. Overall, these simulations demonstrate that coupled reactive transport modelling incorporating stable isotope fractionation can improve the understanding of hydrogeochemical and biogeochemical controls within in situ treatment systems, further illustrating the benefits and limitations of this technique for improving water quality of mine drainage.

Temperature changes can drive cycling of semi-volatile pollutants between different environmental compartments (e.g. atmosphere, soil, plants). To evaluate the impact of daily temperature changes on atmospheric concentration fluctuations we employed a physically based model coupling soil, plants and the atmosphere, which accounts for heat transport, effective gas diffusion, sorption and biodegradation in the soil as well as eddy diffusion and photochemical oxidation in the atmospheric boundary layer of varying heights. The model results suggest that temperature-driven re-volatilization and uptake in soils cannot fully explain significant diurnal concentration fluctuations of atmospheric pollutants as for example observed for polychlorinated biphenyls (PCBs). This holds even for relatively low water contents (high gas diffusivity) and high sorption capacity of the topsoil (high organic carbon content and high pollutant concentration in the topsoil). Observed concentration fluctuations, however, can be easily matched if a rapidly-exchanging environmental compartment, such as a plant layer, is introduced. At elevated temperatures, plants release organic pollutants, which are rapidly distributed in the atmosphere by eddy diffusion. For photosensitive compounds, e.g. some polycyclic aromatic hydrocarbons (PAHs), decreasing atmospheric concentrations would be expected during daytime for the bare soil scenario. This decline is buffered by a plant layer, which acts as a ground-level reservoir. The modeling results emphasize the importance of a rapidly-exchanging compartment above ground to explain short-term atmospheric concentration fluctuations.

Continuous monitoring of a 15 m high heavily instrumented experimental waste rock pile (0.053 wt.% S) since 2006 at the Diavik diamond mine in northern Canada provided a unique opportunity to study the evolution of fresh run-of-mine waste rock as it evolved over annual freeze–thaw cycles. Samples were collected from soil water solution samplers to measure pore water properties, from twelve 4 to 16 m 2 basal collection lysimeters to measure basal leachate properties in the region underlying the crest of the pile (the core), and from basal drains to measure aggregate total pile leachate properties. By 2012, monitoring of pore water geochemistry within the core structure of the test pile revealed an apparent steady state with respect to weathering geochemistry, represented by (i) a flush of pre-existing blasting residuals and applied tracers, (ii) declining pH, (iii) a stepwise progression and subsequent equilibrium with acid-neutralizing phases (depletion of available carbonates; equilibrium with respect to aluminum hydroxide phases and subsequent iron (III) hydroxide phases), and (iv) concordant release of SO 4 , major cations (Ca, Mg, K, Na, Si), and trace metals (Al, Fe, Ni, Co, Cu, Zn). Distinct, high concentration ‘spring flushes’, characteristic of drainage in northern environments and primarily explained by a combination of fluid residence time and the build-up of oxidation products over the winter, were released from core drainage each season. Following the initial flush, the concentration of all dissolved constituents steadily declined, with distinct minimums prior to freeze-up. The opposite trend was observed in the cumulative pile drainage, in which early season leachate dominated by snowmelt and batter flow had low concentrations and late season leachate dominated by contributions from the core of the pile (indicated by season end merging of core and cumulative drainage geochemistry) had higher concentrations. Northern waste rock pile drainage geochemistry is strongly influenced by freeze–thaw cycling and varying core and batter subsystem contributions to total drainage. A comprehensive understanding of thermal cycling in waste rock piles is an important component of temporal predictions of drainage water composition based on up-scaling or reactive transport modeling.

Leachate from a humidity cell experiment provided a geochemical framework to evaluate the early evolution of weathering of the same waste rock in an Arctic environment. Comparison of laboratory and field results indicates the hydrogeochemical system within the higher sulfide (0.01–0.27 wt.% S) waste rock pile has attained a peak weathering state, indicated by a shift to acid neutralization by weathering products of Al-bearing minerals, stabilization of the pH near 4.5, mobility of metals from sulfide and Al-bearing minerals, and a highly correlated relation between SO 4 release and outflow from the waste rock pile. Further weathering of the waste rock should be driven by external environmental factors of temperature and precipitation/infiltration instead of internal factors of wetting and transient acid-neutralization processes. An evaluation of sulfide depletion indicates that 4% of the sulfide in the 4 . Weathering is strongly seasonal because of the Arctic climate, which produced a daily sulfide-depletion rate corresponding 0–0.02% and a peak annual depletion of 100 kg or 1.5% of the remaining sulfide content in the

The oxidation of sulfide minerals in waste rock has the potential to generate low-quality drainage that can present a significant challenge to mine owners, regulators, and other stakeholders. Challenges involved in managing waste rock include the large volume of waste rock produced and the difficulty in predicting the quality and quantity of leach water due to the chemical and physical heterogeneities in the waste rock, and the highly non-linear coupling of geochemical and physical processes. Many important studies have been conducted over the past decade, particularly at the field scale, that have investigated the geochemical, hydrological, microbiological, and gas and heat transport aspects of waste-rock. These studies show that although the parameters and processes that influence AMD generation and solute release are fundamentally similar between different waste-rock piles, major differences in the dominant mechanisms of water, gas and heat transport result from differences in the physical and mineralogical properties of the rock piles, and the climatic conditions, including the amount of precipitation and prevailing temperatures. Accurate prediction of the leach water quality from waste-rock requires a detailed characterization of the properties of the rock piles and the coupling of processes specific to the particular conditions. This paper provides a review of the physical and mineralogical characteristics of waste-rock piles, followed by a discussion of the principal processes related to sulfide oxidation and solute loading, and concluding with a discussion on acid mine drainage prediction and prevention techniques.

Anaerobic biodegradation of organic amendments and contaminants in aquifers can trigger secondary water quality impacts that impair groundwater resources. Reactive transport models help elucidate how diverse geochemical reactions control the spatiotemporal evolution of these impacts. Using extensive monitoring data from a crude oil spill site near Bemidji, Minnesota (USA), we implemented a comprehensive model that simulates secondary plumes of depleted dissolved O2 and elevated concentrations of Mn2+, Fe2+, CH4, and Ca2+ over a two-dimensional cross section for 30 years following the spill. The model produces observed changes by representing multiple oil constituents and coupled carbonate and hydroxide chemistry. The model includes reactions with carbonates and Fe and Mn mineral phases, outgassing of CH4 and CO2 gas phases, and sorption of Fe, Mn, and H+. Model results demonstrate that most of the carbon loss from the oil (70%) occurs through direct outgassing from the oil source zone, greatly limiting the amount of CH4 cycled down-gradient. The vast majority of reduced Fe is strongly attenuated on sediments, with most (91%) in the sorbed form in the model. Ferrous carbonates constitute a small fraction of the reduced Fe in simulations, but may be important for furthering the reduction of ferric oxides. The combined effect of concomitant redox reactions, sorption, and dissolved CO2 inputs from source-zone degradation successfully reproduced observed pH. The model demonstrates that secondary water quality impacts may depend strongly on organic carbon properties, and impacts may decrease due to sorption and direct outgassing from the source zone.

Chromium isotope analysis is rapidly becoming a valuable complementary tool for tracking Cr(VI) treatment in groundwater. Evaluation of various treatment materials has demonstrated that the degree of isotope fractionation is a function of the reaction mechanism, where reduction of Cr(VI) to Cr(III) induces the largest fractionation. However, it has also been observed that uniform flow conditions can contribute complexity to isotope measurements. Here, laboratory batch and column experiments were conducted to assess Cr isotope fractionation during Cr(VI) reduction by zerovalent iron under both static and saturated flow conditions. Isotope measurements were accompanied by traditional aqueous geochemical measurements (pH, Eh, concentrations) and solid-phase analysis by scanning electron microscopy and X-ray absorption spectroscopy. Increasing δ(53)Cr values were associated with decreasing Cr(VI) concentrations, which indicates reduction; solid-phase analysis showed an accumulation of Cr(III) on the iron. Reactive transport modeling implemented a dual mechanism approach to simulate the fractionation observed in the experiments. The faster heterogeneous reaction pathway was associated with minimal fractionation (ε=-0.2‰), while the slower homogeneous pathway exhibited a greater degree of fractionation (ε=-0.9‰ for the batch experiment, and ε=-1.5‰ for the column experiment).

A series of replicate flow-through cell experiments was conducted to characterize Cr isotope fractionation during Cr(VI) treatment by granular zero-valent iron (ZVI). Synthetic groundwater containing 50 mg L−1 Cr(VI) was pumped upward through a custom-made cell packed with ZVI under anaerobic conditions. The geochemical evolution of the system was monitored using pH and redox measurements, while aqueous effluent samples were retained for analysis of cations and Cr isotopes. Real-time, in situ X-ray absorption near edge structure (XANES) spectroscopy collected via a Kapton® window in the cell provided additional information on the speciation of the reaction products. Increases in δ53Cr values corresponding to decreases in Cr(VI) concentration suggested the occurrence of redox processes. Spectroscopic results correlated well with the isotope data, indicating reduction of Cr(VI) to Cr(III). The isotope data did not appear to follow a single trend. A two-stage system was proposed to explain the complex isotope trend, where the rapid Cr removal was associated with very little fractionation (e = −0.2‰), whereas slower removal was associated with a greater degree of fractionation (e = −1.2‰ to −1.5‰). Reactive transport modeling was used to quantify distinct isotope fractionation values (e), differentiated by a significant change in the Cr removal rate.

Secondary water quality impacts can result from a broad range of coupled reactions triggered by primary groundwater contaminants. Data from a crude-oil spill research site near Bemidji, MN provide an ideal test case for investigating the complex interactions controlling secondary impacts, including depleted dissolved oxygen and elevated organic carbon, inorganic carbon, CH4, Mn, Fe, and other dissolved ions. To better understand these secondary impacts, this study began with an extensive data compilation of various data types, comprising aqueous, sediment, gas, and oil phases, covering a 260m cross-sectional domain over 30years. Mass balance calculations are used to quantify pathways that control secondary components, by using the data to constrain the sources and sinks for the important redox processes. The results show that oil constituents other than BTEX (benzene, toluene, ethylbenzene, o-, m- and p-xylenes), including n-alkanes and other aromatic compounds, play significant roles in plume evolution and secondary water quality impacts. The analysis underscores previous results on the importance of non-aqueous phases. Over 99.9% of the Fe2+ plume is attenuated by immobilization on sediments as Fe(II) and 85–95% of the carbon biodegradation products are outgassed. Gaps identified in carbon and Fe mass balances and in pH buffering mechanisms are used to formulate a new conceptual model. This new model includes direct out-gassing of CH4 and CO2 from organic carbon biodegradation, dissolution of directly produced CO2, and sorption with H+ exchange to improve pH buffering. The identification of these mechanisms extends understanding of natural attenuation of potential secondary impacts at enhanced reductive dechlorination sites, particularly for reduced Fe plumes, produced CH4, and pH perturbations.

Formation of acid rock drainage is a temporal balance of acid-generating and alkalinity-generating reactions during mineral weathering. Accessory calcite and apatite are commonly present in granitoid rocks, and their weathering can be a source of net alkalinity. To evaluate the acid-consuming capability of the calcite and apatite group minerals in a granitoid waste rock, a humidity cell experiment was conducted with a mix of a high carbonate tonalite and a low carbonate pegmatite representative of the country rock surrounding a kimberlite pipe at the Diavik Diamond Mine in Canada. The alkalinity generated from the tonalite + pegmatite was evaluated through release of Ca and Sr, which indicated a primary alkalinity-generating period during the first 25 weeks of the 80-week experiment. This period represents decomposition of readily available carbonates (infillings) and phosphates (euhedral grains) that release acid-consuming CO3 and PO4 into solution. The total alkalinity—represented by the release of Ca—produced during the first 25 weeks was compared to the total acidity—represented by the release of SO4—produced during the first 25 weeks of a previous humidity cell experiment with an acid-generating (Type III) waste rock from the same site. The easily weathered carbonates and phosphates of the tonalite + pegmatite have the potential to produce sufficient net alkalinity to consume the net acid generated from the early weathering of sulfides in the Type III waste rock. Aqueous extractions performed on post-experiment samples of the tonalite + pegmatite indicate remaining carbonate and phosphate minerals that were not readily available at the experimental ≤6.3 mm size. The heterogeneity of carbonate content and availability in granitic country rock at the mine site are reflective of the variability of accessory calcite in granitoids and indicate a continued need for site-specific determination of alkalinity-generating reactions with the weathering of waste rock.

Diffusive groundwater pollution caused by agricultural and atmospheric inputs is a pressing issue in environmental management worldwide. Various researchers have studied nitrate contamination since the substantial increase of nitrogen fertilization in agriculture starting in the second half of the 20th century. This study addresses large scale reactive solute transport in typical landscapes and aquifers exemplified by geological analogues of southwestern Germany.. Fate of nitrate and other solutes (e.g. agricultural nitrate, ammonium, natural sulfate and dissolved organic carbon) was studied in a typical small river floodplain. Reactive transport model of Ammer river floodplain shows that agriculture nitrate is reduced rapidly in the Ammer floodplain sediments. However, there is a potential for geogenic production of ammonium in sediment layers high in organic carbon and peat, which might be a major source of nitrate in the drains. Part of the nitrate in drains and creeks in the Ammer valley thus could be of geogenic origin. Such findings are relevant for regional land and water quality management.

The interior thermal regime of a field-scale experimental waste rock pile in the Northwest Territories, Canada, was studied. Test pile construction was completed in the summer 2006, and temperature data was collected continuously since that time to February 2009. The temperature data indicates the test pile cooled over the study period, with an average heat energy release of −2.5 × 104 and −2.6 × 104 MJ in 2007 and 2008, respectively. The mean annual air temperature (MAAT) at the site was −8.9 °C during the period between 2006 and 2009, with a permafrost table at a depth of 4 m in bedrock away from the pile. Because of this cold environment, the upward movement rate of the 0 °C isotherm into the test pile at its base was approximately 1.5 m a−1 during 2007 and 2008. Thermistor strings installed immediately below the base of the test pile showed the test-pile basal temperatures remained near and below 0 °C during the study period. Furthermore, due to low rates of sulfide mineral oxidation, elevated temperatures in the interior of the test pile were not observed. The average air velocity in the pore space in July 2007 and 2008 was about one third of that during January of each year based on temperature distributions. Therefore, due to higher air velocity during the winter, it is expected that heat transfer is greater during winter.

Wind-induced gas transport in a test-scale unsaturated waste rock pile was investigated at the Diavik diamond mine, Northwest Territories, Canada. Differential gas pressures were measured in 2008 at 49 locations within a field-scale experimental waste rock pile (test pile) and at 14 locations on the surface of the test pile at 1-min intervals. Wind speed and direction were measured at 10-min intervals and decomposed into north, south, east, and west vectors. Correlations between wind vectors and pressure measurements indicate that the wind influences pressure fluctuations in the test pile. The strength of the correlation is roughly inversely proportional to the distance between the measurement ports and the atmospheric boundary. The relationship between the magnitude of the wind vector and pressure fluctuations on the surface of the test pile is non-linear. However, the relationship between internal and surface pressure measurements is linear, suggesting that gas flow within the test pile follows Darcy’s Law. Spectral analysis demonstrates that the dominant periods of the wind range from 1 to 14 d. A 1-D analytical solution to the flow equation is used to demonstrate that long periods have the most pronounced effect on transient gas flow within the test pile and that the penetration depth of the wind-induced gas pressure wave is a function of wind period and permeability of the test pile.

The reactive and hydraulic efficacy of zero valent iron permeable reactive barriers (ZVI PRBs) is strongly affected by geochemical composition of the groundwater treated. An enhanced version of the geochemical simulation code MIN3P was applied to simulate dominating processes in chlorinated hydrocarbons (CHCs) treating ZVI PRBs including geochemical dependency of ZVI reactivity, gas phase formation and a basic formulation of degassing. Results of target oriented column experiments with distinct chemical conditions (carbonate, calcium, sulfate, CHCs) were simulated to parameterize the model. The simulations demonstrate the initial enhancement of anaerobic iron corrosion due to carbonate and long term inhibition by precipitates (chukanovite, siderite, iron sulfide). Calcium was shown to enhance long term corrosion due to competition for carbonate between siderite, chukanovite, and aragonite, with less inhibition of iron corrosion by the needle like aragonite crystals. Application of the parameterized model to a field site (Bernau, Germany) demonstrated that temporarily enhanced groundwater carbonate concentrations caused an increase in gas phase formation due to the acceleration of anaerobic iron corrosion.

Transport of reactive solutes in groundwater is affected by physical and chemical heterogeneity of the porous medium, leading to complex spatio-temporal patterns of concentrations and reaction rates. For certain cases of bioreactive transport, it could be shown that the concentrations of reactive constituents in multi-dimensional domains are approximately aligned with isochrones, that is, lines of identical travel time, provided that the chemical properties of the matrix are uniform. We extend this concept to combined physical and chemical heterogeneity by additionally considering the time that a water parcel has been exposed to reactive materials, the so-called exposure time. We simulate bioreactive transport in a one-dimensional domain as function of time and exposure time, rather than space. Subsequently, we map the concentrations to multi-dimensional heterogeneous domains by means of the mean exposure time at each location in the multi-dimensional domain. Differences in travel and exposure time at a given location are accounted for as time difference. This approximation simplifies reactive-transport simulations significantly under conditions of steady-state flow when reactions are restricted to specific locations. It is not expected to be exact in realistic applications because the underlying assumption, such as neglecting transverse mixing altogether, may not hold. We quantify the error introduced by the approximation for the hypothetical case of a two-dimensional, binary aquifer made of highly-permeable, non-reactive and low-permeable, reactive materials releasing dissolved organic matter acting as electron donor for aerobic respiration and denitrification. The kinetically controlled reactions are catalyzed by two non-competitive bacteria populations, enabling microbial growth. Even though the initial biomass concentrations were uniform, the interplay between transport, non-uniform electron-donor supply, and bio-reactions led to distinct spatial patterns of the two types of biomass at late times. Results obtained by mapping the exposure-time based results to the two-dimensional domain are compared with simulations based on the two-dimensional, spatially explicit advection-dispersion-reaction equation. Once quasi-steady state has been reached, we find a good agreement in terms of the chemical-compound concentrations between the two approaches inside the reactive zones, whereas the exposure-time based model is not able to capture reactions occurring in the zones with zero electron-donor release. We conclude that exposure-time models provide good approximations of nonlinear bio-reactive transport when transverse mixing is not the overall controlling process and all reactions are essentially restricted to distinct reactive zones.

In a methanogenic crude oil contaminated aquifer near Bemidji, Minnesota, the decrease in dissolved CH(4) concentrations along the groundwater flow path, along with the positive shift in δ(13) C(CH) (4) and negative shift in δ(13) C(DIC) , is indicative of microbially mediated CH(4) oxidation. Calculations of electron acceptor transport across the water table, through diffusion, recharge, and the entrapment and release of gas bubbles, suggest that these processes can account for at most 15% of the observed total reduced carbon oxidation, including CH(4) . In the anaerobic plume, the characteristic Fe(III)-reducing genus Geobacter was the most abundant of the microbial groups tested, and depletion of labile sediment iron is observed over time, confirming that reduced carbon oxidation coupled to iron reduction is an important process. Electron mass balance calculations suggest that organic carbon sources in the aquifer, BTEX and non-volatile dissolved organic carbon, are insufficient to account for the loss in sediment Fe(III), implying that CH(4) oxidation may also be related to Fe(III) reduction. The results support a hypothesis of Fe(III)-mediated CH(4) oxidation in the contaminated aquifer.

MIN3P, a multicomponent reactive transport model for variably saturated porous media, is used to simulate the outputs of column tests carried out using zero valent iron (ZVI) for nickel contaminated groundwater remediation. The objective of this study is to investigate the main chemical reactions involved in contaminant removal and the main causes of the reactivity decline of ZVI over time. According to the results of the model the major causes of ZVI reactivity loss is identified in the mineral precipitation of α-FeOOH on iron surface that probably caused ZVI passivation and led to a decline of the electron transfer rate. An existing empirical relationship between mineral precipitation and the reactivity loss of ZVI, included in the model, reproduced the changes in nickel removal observed during different laboratory column tests.

In granular iron permeable reactive barriers (PRBs), hydrogen gas formation, entrapment and release of gas bubbles, and secondary mineral precipitation have been known to affect the permeability and reactivity. The multicomponent reactive transport model MIN3P was enhanced to couple gas formation and release, secondary mineral precipitation, and the effects of these processes on hydraulic properties and iron reactivity. The enhanced model was applied to a granular iron column, which was studied for the treatment of trichloroethene (TCE) in the presence of dissolved CaCO(3). The simulation reasonably reproduced trends in gas formation, secondary mineral precipitation, permeability changes, and reactivity changes observed over time. The simulation showed that the accumulation of secondary minerals reduced the reactivity of the granular iron over time, which in turn decreased the rate of mineral accumulation, and also resulted in a gradual decrease in gas formation over time. This study provides a quantitative assessment of the evolving nature of geochemistry and permeability, resulting from coupled processes of gas formation and mineral precipitation, which leads to a better understanding of the processes controlling the granular iron reactivity, and represents an improved method for incorporating these factors into the design of granular iron PRBs.

High resolution direct-push profiling over short vertical distances was used to investigate CH 4 attenuation in a petroleum contaminated aquifer near Bemidji, Minnesota. The contaminant plume was delineated using dissolved gases, redox sensitive components, major ions, carbon isotope ratios in CH 4 and CO 2 , and the presence of methanotrophic bacteria. Sharp redox gradients were observed near the water table. Shifts in δ 13 C CH4 from an average of − 57.6‰ (± 1.7‰) in the methanogenic zone to − 39.6‰ (± 8.7‰) at 105 m downgradient, strongly suggest CH 4 attenuation through microbially mediated degradation. In the downgradient zone the aerobic/anaerobic transition is up to 0.5 m below the water table suggesting that transport of O 2 across the water table is leading to aerobic degradation of CH 4 at this interface. Dissolved N 2 concentrations that exceeded those expected for water in equilibrium with the atmosphere indicated bubble entrapment followed by preferential stripping of O 2 through aerobic degradation of CH 4 or other hydrocarbons. Multivariate and cluster analysis were used to distinguish between areas of significant bubble entrapment and areas where other processes such as the infiltration of O 2 rich recharge water were important O 2 transport mechanisms.

Uranium is a pollutant of concern to both human and ecosystem health. Uranium's redox state often dictates its partitioning between the aqueous- and solid-phases, and thus controls its dissolved concentration and, coupled with groundwater flow, its migration within the environment. In anaerobic environments, the more oxidized and mobile form of uranium (UO2 2+ and associated species) may be reduced, directly or indirectly, by microorganisms to U(IV) with subsequent precipitation of UO2. However, various factors within soils and sediments may limit biological reduction of U(VI), inclusive of alterations in U(VI) speciation and competitive electron acceptors. Here we elucidate the impact of U(VI) speciation on the extent and rate of reduction with specific emphasis on speciation changes induced by dissolved Ca, and we examine the impact of Fe(III) (hydr)oxides (ferrihydrite, goethite and hematite) varying in free energies of formation on U reduction. The amount of uranium removed from solution during 100 h ...

Uranium is a pollutant of concern to both human and ecosystem health. Uranium's redox state often dictates whether it will reside in the aqueous or solid phase and thus plays an integral role in the mobility of uranium within the environment. In anaerobic environments, the more oxidized and mobile form of uranium (UO2(2+) and associated species) may be reduced, directly or indirectly, by microorganisms to U(IV) with subsequent precipitation of UO. However, various factors within soils and sediments, such as U(VI) speciation and the presence of competitive electron acceptors, may limit biological reduction of U(VI). Here we examine simultaneous dissimilatory reduction of Fe(III) and U(VI) in batch systems containing dissolved uranyl acetate and ferrihydrite-coated sand. Varying amounts of calcium were added to induce changes in aqueous U(VI) speciation. The amount of uranium removed from solution during 100 h of incubation with S. putrefaciens was 77% in absence of Ca or ferrihydrite, but only 24% (with ferrihydrite) and 14% (without ferrihydrite) were removed for systems with 0.8 mM Ca. Dissimilatory reduction of Fe(III) and U(VI) proceed through different enzyme pathways within one type of organism. We quantified the rate coefficients for simultaneous dissimilatory reduction of Fe(III) and U(VI) in systems varying in Ca concecentration (0-0.8 mM). The mathematical construct, implemented with the reactive transport code MIN3P, reveals predominant factors controlling rates and extent of uranium reduction in complex geochemical systems.

Contaminant attenuation processes in the vadose zone of a crude oil spill site near Bemidji, MN have been simulated with a reactive transport model that includes multicomponent gas transport, solute transport, and the most relevant biogeochemical reactions. Dissolution and volatilization of oil components, their aerobic and anaerobic degradation coupled with sequential electron acceptor consumption, ingress of atmospheric O(2), and the release of CH(4) and CO(2) from the smear zone generated by the floating oil were considered. The focus of the simulations was to assess the dynamics between biodegradation and gas transport processes in the vadose zone, to evaluate the rates and contributions of different electron accepting processes towards vadose zone natural attenuation, and to provide an estimate of the historical mass loss. Concentration distributions of reactive (O(2), CH(4), and CO(2)) and non-reactive (Ar and N(2)) gases served as key constraints for the model calibration. Simulation results confirm that as of 2007, the main degradation pathway can be attributed to methanogenic degradation of organic compounds in the smear zone and the vadose zone resulting in a contaminant plume dominated by high CH(4) concentrations. In accordance with field observations, zones of volatilization and CH(4) generation are correlated to slightly elevated total gas pressures and low partial pressures of N(2) and Ar, while zones of aerobic CH(4) oxidation are characterized by slightly reduced gas pressures and elevated concentrations of N(2) and Ar. Diffusion is the most significant transport mechanism for gases in the vadose zone; however, the simulations also indicate that, despite very small pressure gradients, advection contributes up to 15% towards the net flux of CH(4), and to a more limited extent to O(2) ingress. Model calibration strongly suggests that transfer of biogenically generated gases from the smear zone provides a major control on vadose zone gas distributions and vadose zone carbon balance. Overall, the model was successful in capturing the complex interactions between biogeochemical reactions and multicomponent gas transport processes. However, despite employing a process-based modeling approach, honoring observed parameter ranges, and generally obtaining good agreement between field observations and model simulations, accurate quantification of natural attenuation rates remains difficult. The modeling results are affected by uncertainties regarding gas phase saturations, tortuosities, and the magnitude of CH(4) and CO(2) flux from the smear zone. These findings highlight the need to better delineate gas fluxes at the model boundaries, which will help constrain contaminant degradation rates, and ultimately source zone longevity.

An automated data logging system designed to measure gas pressures within a 15-m-high waste rock test pile was installed at a diamond mine site in the Northwest Territories, Canada. Data collected from 12 Aug. 2007 to 15 Oct. 2007 shows distinct gas pressure gradients within the waste rock pile. The magnitude of the gradients within the pile shows a clear response to wind speed external to the pile. The direction of the gradients shows a response to the wind direction. The results demonstrate the ability to measure wind-induced gas pressure gradients within a waste rock pile or other similar porous structures. The general pattern of the observed gradients is inconsistent with the results of numerical modeling assuming homogeneous permeability within the pile. This inconsistency suggests that heterogeneity within the pile and an irregular landscape surrounding the pile affect the way in which the wind flows around and air flows through the rock pile. Calculations of O 2 fluxes using the observed gradients show that wind-induced air flow through the pile has the potential to be a significant mechanism of O 2 transport, similar in magnitude to other mechanism such as diffusion and convection. These results suggest that wind-driven advection may be an important process in waste rock piles where the supply of oxygen limits the overall rate of sulfide oxidation.

The strongly reducing nature of permeable reactive barrier (PRB) treatment materials can lead to gas production, potentially resulting in the formation of gas bubbles and ebullition. Degassing in organic C based PRB systems due to the production of gases (primarily CO2 and CH4) is investigated using the depletion of naturally occurring non-reactive gases Ar and N2, to identify, confirm, and quantify chemical and physical processes. Sampling and analysis of dissolved gases were performed at the Nickel Rim Mine Organic Carbon PRB, which was designed for the treatment of groundwater contaminated by low quality mine drainage characterized by slightly acidic pH, and elevated Fe(II) and SO4 concentrations. A simple 4-gas degassing model was used to analyze the dissolved gas data, and the results indicate that SO4 reduction is by far the dominant process of organic C consumption within the barrier. The data provided additional information to delineate rates of microbially mediated SO4 reduction and confirm the presence of slow and fast flow zones within the barrier. Degassing was incorporated into multicomponent reactive transport simulations for the barrier and the simulations were successful in reproducing observed dissolved gas trends.

In many natural and contaminated aquifers, geochemical processes result in the production or consumption of dissolved gases. In cases where methanogenesis or denitrification occurs, the production of gases may result in the formation and growth of gas bubbles below the water table. Near the water table, entrapment of atmospheric gases during water table rise may provide a significant source of O(2) to waters otherwise depleted in O(2). Furthermore, the presence of bubbles will affect the hydraulic conductivity of an aquifer, resulting in changes to the groundwater flow regime. The interactions between physical transport, biogeochemical processes, and gas bubble formation, entrapment and release is complex and requires suitable analysis tools. The objective of the present work is the development of a numerical model capable of quantitatively assessing these processes. The multicomponent reactive transport code MIN3P has been enhanced to simulate bubble growth and contraction due to in-situ gas production or consumption, bubble entrapment due to water table rise and subsequent re-equilibration of the bubble with ambient groundwater, and permeability changes due to trapped gas phase saturation. The resulting formulation allows for the investigation of complex geochemical systems where microbially mediated redox reactions both produce and consume gases as well as affect solution chemistry, alkalinity, and pH. The enhanced model has been used to simulate processes in a petroleum hydrocarbon contaminated aquifer where methanogenesis is an important redox process. The simulations are constrained by data from a crude oil spill site near Bemidji, MN. Our results suggest that permeability reduction in the methanogenic zone due to in-situ formation of gas bubbles, and dissolution of entrapped atmospheric bubbles near the water table, both work to attenuate the dissolved gas plume emanating from the source zone. Furthermore, the simulations demonstrate that under the given conditions more than 50% of all produced CH(4) partitions to the gas phase or is aerobically oxidised near the water table, suggesting that these processes should be accounted for when assessing the rate and extent of methanogenic degradation of hydrocarbons.

A quantitative density model for alkali germanate and alkali germano-phosphate glasses has been developed based on Raman spectra and density data. The model demonstrates that for lighter/smaller cations such as lithium and sodium, the transition from 4- to 3-membered GeO4 rings is the prominent structural change responsible for changes in the density trends with increasing alkali oxide concentrations. For the heavier/larger cations such as potassium and rubidium, the 4- to 3-membered ring transition remains important but other factors must be considered. These include increases in density due to additional mass of the alkali oxides and decreases in density due to depolymerization of the glass network. The model shows that these combined factors can account for the ‘germanate anomaly’ noted in these glasses.

A series of alkali germanophosphate glasses ((R2O)x(GeO2:P2O5)1−x where R=Na, K and Rb) with variable GeO2:P2O5 ratios (8:1, 6:1, 4:1 and 2:1) have been investigated using Raman spectroscopy. The glass network may be treated as being made up of separate germanate and phosphate components. Addition of alkali cations indicates that the alkalis preferentially modify the phosphate part of the network. Network depolymerization occurs by formation of Q2 and Q1 PO4 tetrahedra. At high Ge:P ratios the glasses exhibit a density anomaly. The anomaly is attributed to the formation of small three-membered GeO4 rings. However, alkali cation size and mass are contributing factors to the shape of the density anomaly in the K2O and Rb2O containing glasses. Depolymerization of the 2:1 alkali germanophosphate glasses is predominantly by formation of Q2 PO4 tetrahedra. At low Ge:P ratios (2:1) density trends are linear. No ring transition is observed in these glasses. Furthermore, the presence of [5]Ge cannot be ruled out, but if present, does not play a major role in generating a ‘germanate anomaly’.

Chromium isotope fractionation is indicative of mass-transfer processes, such as reduction of Cr(VI) to Cr(III) during groundwater remediation. Laboratory experiments comparing batch and column treatment of Cr(VI) using organic carbon suggest that the associated isotope fractionation may be influenced by solute-transport mechanisms. These batch and column experiments were simulated using the reactive transport model MIN3P to further evaluate the effects of Cr reduction and transport on isotope fractionation under saturated flow conditions. Simulation of the batch experiment provided a good fit to the experimental data, where a fractionation factor (α₅₃) of 0.9965 was attributed to a single, dominant Cr(VI) removal mechanism. Calibration of the column simulations to the experimental results suggested the presence of a second, more rapid Cr(VI) removal mechanism with α₅₃ = 0.9992. Results from this study demonstrate that the interpretation of Cr isotope fractionation during reduction can be complex, particularly where multiple removal mechanisms are evident. Reactive transport modeling of Cr isotope fractionation can provide a quantitative assessment of the contaminant removal mechanisms, thus improving the application of Cr isotope measurements as a tool to track Cr(VI) migration and attenuation in groundwater.

MIN3P was developed as a general purpose multicomponent reactive transport code for variably saturated media. The basic version of the code includes Richard’s equation for the solution of variably-saturated flow, and solves mass balance equations for advective-diffusive solute transport and diffusive gas transport. Biogeochemical reactions are described by a partial equilibrium approach, using equilibrium-based law-of-mass-action relationships for fast reactions, and a generalized kinetic framework for reactions that are relatively slow in comparison to the transport time scale. MIN3P has been used to support multiple field and laboratory investigations involving the fate of inorganic and organic substances and has served as a platform for additional code development: MIN3P-Bubble, an enhanced version to simulate gas generation and exsolution in the saturated zone, as well as gas entrapment and release due to water table fluctuations; MIN3P-Dusty, a version of the code that includes gas advection and multicomponent gas diffusion based on the Dusty Gas Model (DGM); and MIN3P-Soil, a version that includes plant-soil interactions. The capabilities of the basic code and the follow-up developments are demonstrated by simulating the oxidation of pyrite in mine waste, associated metal release, and subsequent attenuation processes; the interactions between the formation of “excess air” and biogeochemical reactions in the vadose zone and below the water table; the evolution of vadose zone gas composition and transport processes at a petroleum hydrocarbon spill site undergoing natural attenuation; and the effect of plant-soil interactions on mineral weathering and secondary mineral formation in soils and surficial sediments.

Isotope ratio measurements provide a tool for indicating the relative significance of biogeochemical reactions and for constraining estimates of the extent and rate of reactions in passive treatment systems. In this paper, the reactive transport model MIN3P is used to evaluate sulfur isotope fractionation in column experiments designed to simulate treatment of contaminated water by microbially mediated sulfate reduction occurring within organic carbon-based and iron and carbon-based permeable reactive barriers. A mass dependent fractionation model was used to determine reaction rates for 32S and 34S compounds during reduction, precipitation, and dissolution reactions and to track isotope-dependent mass transfer during SO4 removal. The δ34S values obtained from the MIN3P model were similar to those obtained from the Rayleigh equation, indicating that there was not a significant difference between the conceptual models. Differences between the MIN3P derived α value and the Rayleigh equation derived value were attributed to minor changes in the dissolution and precipitation rate of gypsum and mathematical differences in the fitting models. The results indicated that the prediction of δ34S was fairly insensitive to differences in the fractionation factor at the concentration ranges measured in the current study. However, more significant differences would be expected at low sulfate conditions.

[1] The collection of representative samples of dissolved volatile compounds requires particular care to ensure that degassing does not occur during sampling, particularly for sites where the water table is deep. Furthermore, the investigation of physical and chemical processes involving volatile components can be enhanced by collecting vadose zone gas samples above the water table and collecting water samples very close to the water table. Here we describe the design and operation of a direct push profiler that allows the collection of gas and water samples from one hole. The profiler is capable of collecting water samples close to the water table and preserves the integrity of the samples collected from below deep water tables with respect to volatile compounds. The versatility of the profiler is due to the incorporation of a small-diameter bladder pump into the tip of a Waterloo profiler. Laboratory testing shows that samples collected from 10 cm below the water table and at 9 m depth below ground surface show no signs of degassing. Field measurements made at a petroleum spill site near Bemidji, Minnesota, provide a comparison of profiler results to samples collected from an existing well cluster, demonstrating the ability of the profiler to obtain representative samples under field conditions. Furthermore, the field tests demonstrate the utility of the direct push profiler in terms of obtaining high-resolution, high-quality data through the vadose zone and into the upper few meters of the aquifer.

Ebullition of gas bubbles through saturated sediments can enhance the migration of gases through the subsurface, affect the rate of biogeochemical processes, and potentially enhance the emission of important greenhouse gases to the atmosphere. To better understand the parameters controlling ebullition, methanogenic conditions were produced in a column experiment and ebullition through the column was monitored and quantified through dissolved gas analysis and reactive transport modeling. Dissolved gas analysis showed rapid transport of CH4 vertically through the column at rates several times faster than the bromide tracer and the more soluble gas CO2, indicating that ebullition was the main transport mechanism for CH4. An empirically derived formulation describing ebullition was integrated into the reactive transport code MIN3P allowing this process to be investigated on the REV scale in a complex geochemical framework. The simulations provided insights into the parameters controlling ebullition and show that, over the duration of the experiment, 36% of the CH4 and 19% of the CO2 produced were transported to the top of the column through ebullition.

[1] At many sites contaminated with petroleum hydrocarbons, methanogenesis is a significant degradation pathway. Techniques to estimate CH4 production, consumption, and transport processes are needed to understand the geochemical system, provide a complete carbon mass balance, and quantify the hydrocarbon degradation rate. Dissolved and vapor-phase gas data collected at a petroleum hydrocarbon contaminated site near Bemidji, Minnesota, demonstrate that naturally occurring nonreactive or relatively inert gases such as Ar and N2 can be effectively used to better understand and quantify physical and chemical processes related to methanogenic activity in the subsurface. In the vadose zone, regions of Ar and N2 depletion and enrichment are indicative of methanogenic and methanotrophic zones, and concentration gradients between the regions suggest that reaction-induced advection can be an important gas transport process. In the saturated zone, dissolved Ar and N2 concentrations are used to quantify degassing driven by methanogenesis and also suggest that attenuation of methane along the flow path, into the downgradient aquifer, is largely controlled by physical processes. Slight but discernable preferential depletion of N2 over Ar, in both the saturated and unsaturated zones near the free-phase oil, suggests reactivity of N2 and is consistent with other evidence indicating that nitrogen fixation by microbial activity is taking place at this site.

Reactive transport modeling was used to evaluate the performance of two similar column experiments. The experiments were designed to simulate the treatment of acid mine drainage through microbially mediated sulfate reduction and subsequent sulfide mineral precipitation by means of an organic carbon permeable reactive barrier. Principal reactions considered in the simulations include microbially mediated reduction of sulfate by organic matter, mineral dissolution/precipitation reactions, and aqueous complexation/hydrolysis reactions. Simulations of column 1, which contained composted leaf mulch, wood chips, sawdust, and sewage sludge as an organic carbon source, accurately predicted sulfate concentrations in the column effluent throughout the duration of the experiment using a single fixed rate constant for sulfate reduction of 6.9 x 10(-9) mol L(-1) s(-1). Using the same reduction rate for column 2, which contained only composted leaf mulch and sawdust as an organic carbon source, sulfate concentrations at the column outlet were overpredicted at late times, suggesting that sulfate reduction rates increased over the duration of the column experiment and that microbial growth kinetics may have played an important role. These modeling results suggest that the reactivity of the organic carbon treatment material with respect to sulfate reduction does not significantly decrease over the duration of the 14-month experiments. The ability of the columns to remove ferrous iron appears to be strongly influenced by the precipitation of siderite, which is enhanced by the dissolution of calcite. The simulations indicate that while calcite was available in the column, up to 0.02 mol L(-1) of ferrous iron was removed from solution as siderite and mackinawite. Later in the experiments after approximately 300 d, when calcite was depleted from the columns, mackinawite became the predominant iron sink. The ability of the column to remove ferrous iron as mackinawite was estimated to be approximately 0.005 mol L(-1) for column 1. As the precipitation of mackinawite is sulfide limited at later times, the amount of iron removed will ultimately depend on the reactivity of the organic mixture and the amount of sulfate reduced.

Soil–atmosphere exchange is important for the environmental fate and atmospheric transport of many semi-volatile organic compounds (SVOCs). This study focuses on modeling the vapor phase exchange of semi-volatile hydrophobic organic pollutants between soil and the atmosphere using the multicomponent reactive transport code MIN3P. MIN3P is typically applied to simulate aqueous and vapor phase transport and reaction processes in the subsurface. We extended the code to also include an atmospheric boundary layer where eddy diffusion takes place. The relevant processes and parameters affecting soil–atmosphere exchange were investigated in several 1-D model scenarios and at various time scales (from years to centuries). Phenanthrene was chosen as a model compound, but results apply for other hydrophobic organic compounds as well. Gaseous phenanthrene was assumed to be constantly supplied to the system during a pollution period and a subsequent regulation period (with a 50% decline in the emission rate). Our results indicate that long-term soil–atmosphere exchange of phenanthrene is controlled by the soil compartment — re-volatilization thus depends on soil properties. A sensitivity analysis showed that accumulation and transport in soils in the short term is dominated by diffusion, whereas in the long term groundwater recharge and biodegradation become relevant. As expected, sorption causes retardation and slows down transport and biodegradation. If atmospheric concentration is reduced (e.g. after environmental regulations), re-volatilization from soil to the atmosphere occurs only for a relatively short time period. Therefore, the model results demonstrate that soils generally are sinks for atmospheric pollutants. The atmospheric boundary layer is only relevant for time scales of less than one month. The extended MIN3P code can also be applied to simulate fluctuating concentrations in the atmosphere, for instance due to temperature changes in the topsoil.