Ulrich Mayer


Research Classification

Research Interests

groundwater contamination
groundwater remediation
low-temperature geochemistry
mine waste management

Relevant Thesis-Based Degree Programs

Research Options

I am available and interested in collaborations (e.g. clusters, grants).
I am interested in and conduct interdisciplinary research.

Research Methodology

reactive transport modeling


Master's students
Doctoral students

Development of multicomponent reactive transport modeling tools for multiphase systems

Refinement of multicomponent reactive transport modeling approaches for mine waste management

I support public scholarship, e.g. through the Public Scholars Initiative, and am available to supervise students and Postdocs interested in collaborating with external partners as part of their research.
I am open to hosting Visiting International Research Students (non-degree, up to 12 months).
I am interested in supervising students to conduct interdisciplinary research.

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Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Reactive transport modelling of soil-plant interactions (2023)

The biogeochemical interactions between plants and soils in the critical zone (CZ) are well-known for diversity and complexity. These systems are thus difficult to characterize, despite their importance in ecology, agriculture, and role in carbon (C) and other nutrient cycles. Numerical modelling of reactive transport (RT), which includes both subsurface transport and geochemistry, has been suggested as a tool to quantitatively explore CZ systems, as RT models consider many interrelated and non-linear processes simultaneously. Here, we explore the degree of complexity (e.g. inclusion of individual processes and feedbacks) necessary to characterize soil-plant interactions in RT models through two distinct research problems at diverse physical and temporal scales of soil-plant systems, while simultaneously providing a quantitative assessment and interpretation of the controlling processes for each of the research problems. Research Problem 1 explores C sequestration through the oxalate-carbonate pathway of the Iroko tree (Milicia excelsa) in both a one-dimensional and two-dimensional context. We find the potential for the Iroko to sequester large quantities of C in the form of carbonate minerals is high, provided a continual source of calcium (Ca) from weathering of local feldspar, as well as from groundwater. While the one-dimensional study worked well for understanding C and Ca mass balances, two-dimensional analysis further illuminated processes both near to and far from the tree, particularly regarding subsurface gas concentrations and transport. In Research Problem 2, RT modeling was used to simulate and interpret a non-invasive imaging experiment of lab-grown maize (Zea mays). A series of RT models was developed, accounting for various degrees of complexity of the bulk soil, attempting to reproduce observed high-resolution soil moisture and O₂ concentration data. The simulation that best matched experimental data included spatial heterogeneity of porosity, permeability, and water retention parameters in the bulk soil, along with inclusion of heterogeneities unique to the system. Synthesis of the research problems indicates that RT modeling can be an effective tool to characterize diverse soil-plant interactions and highlights the benefit of strategic consideration of model parameterization and complexity, tailored to the research questions at hand.

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Reactive transport modelling of greenhouse gas cycling and emissions from macroporous agricultural soils (2022)

Agricultural lands are important anthropogenic sources of greenhouse gases (GHG) and play a vital role in affecting global climate change. GHGs (i.e., CO₂, N₂O and CH₄) are all produced (or consumed) as a result of microbial processes; however, the size of the concentrations and fluxes depends heavily on soil structure characteristics. Macroporous agricultural soils have a multitude of pore domains (e.g., macropores, soil matrix, soil aggregates) leading to varying degrees of fluid flow, solute and gas transport. Consequently, a spectrum of biogeochemical C-N processes may occur in macroporous soils that collectively, and coupled with flow and mass transport, determine GHG cycling. In this work, various modeling approaches were developed to assess the mechanisms of GHG production, consumption, transport and emissions, and interpret the simulation results in the context of observations at a field site in Ontario, Canada, featuring macroporous agricultural soil. The modeling approaches include a classical uniform reactive transport model, a dual-permeability reactive transport model with an emphasis on non-equilibrium gas transport and exchange, a discrete macropore model, and a more complex hybrid multi-domain model. Simulation results from all modeling approaches suggest that spatial distributions (e.g., hotspots) of GHG emissions can be attributed to differences in soil characteristics, soil nutrient supply, and organic C availability, whereas short- (hot moments) and long-term (seasonal) temporal variations are strongly affected by environmental factors including seasonal temperature and in-season acute precipitation events. In addition, results showed that all modeling approaches were successful in reproducing observed spatial and long-term temporal variations in pore gas GHG concentrations and fluxes; however, the hybrid multi-domain model clearly improved the simulation of hot moment N₂O emissions related to acute precipitation events compared to other approaches, which highlights the potential of improving the simulation of N₂O hot moments through accounting for three interconnected subdomains. The strengths and weaknesses of modeling approaches for simulating GHG cycling and emissions from macroporous agricultural soil were compared and evaluated. All modeling approaches showed promise for future studies with the aim to develop a more complete understanding of how complex soil structure affects the spatiotemporal variations of GHG cycling from macroporous agricultural soils.

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Fugitive gas migration from leaking oil and gas wells (2020)

Fugitive gas migration (GM) due to leaking oil and gas wells poses a major risk to the environment. The unintentional and uncontrolled release of methane (CH₄, the primary component of natural gas) from a compromised well can lead to aquifer contamination, explosive conditions, and greenhouse gas emissions to the atmosphere. Currently, there is a lack of knowledge on the occurrence, distribution, fate, and transport of fugitive gas. This has limited the development of effective and reliable monitoring techniques at oil and gas sites.This thesis presents results from: i) GM monitoring across well pads in Northeastern British Columbia (BC), and ii) two large-scale field experiments in Borden, Ontario and Northeastern, BC. Soil gas effluxes and stable carbon isotope ratios identified considerable variation in the spatiotemporal distribution of fugitive gas on well pads. The field experiments were designed to characterize the transport and fate of fugitive gas via controlled subsurface injections of natural gas into: i) the saturated zone of a shallow unconfined sand aquifer (Borden) and; ii) a thick vadose zone of glacio-lacustrine deposits (Northeastern, BC). Results from the experiment at Borden suggest that the distribution of fugitive gas is affected by the rate of natural gas leaking from a well and, characteristics of the subsurface lithology. Subtle heterogeneities led to extensive lateral GM and episodic effluxes to the atmosphere. A higher rate of gas leakage allowed greater lateral and vertical GM. Cessation of gas leakage was associated with enhanced CH₄ dissolution, greater oxidation, and lower effluxes. Geochemical impacts were caused by processes induced by natural gas leakage, and CH₄ oxidation. At the field site in Northeastern, BC, changes in barometric pressure directly controlled the magnitude and temporal distribution of fugitive gas from the thick vadose zone. Despite active gas release, increases in barometric pressure suppressed CH₄ effluxes. Decreases in barometric pressure allowed gas to break through low-permeability soils, even after injection stopped. The findings from these studies have significant implications to improve oil and gas well site monitoring to identify the occurrence and risks of GM.

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Reactive transport modeling of unsaturated hydrology and geochemistry of neutral and acid rock drainage in highly heterogeneous mine waste rock at the Antamina Mine, Peru (2019)

Drainage quality from variably-saturated mine waste-rock dumps is controlled by multiple processes that are effective at different scales. The objective of this research is to improve the conceptual understanding of coupled hydrological and geochemical processes in mine waste rock using reactive transport modeling.Multicomponent reactive-transport was modeled using the code MIN3P to investigate sulfide oxidation and acid buffering reactions constrained by two field-scale studies of fine-grained reactive intrusive material at the Antamina mine, Peru: 1) 1 m-high field barrel and 2) 10 m-high experimental pile. At the field-barrel scale, the uniform flow and solute transport model was able to capture long term concentration trends in the discharge. Sulfide mineral oxidation along with pH-buffering reactions, and Cu and Zn secondary mineral precipitation/dissolution were considered the main processes controlling metal concentrations. Results indicate seasonal fluctuations in dissolved concentrations controlled by precipitation/dissolution of secondary minerals in wet and dry cycles and a long-term trend towards more acidic drainage. At the pile scale, the uniform-flow and solute transport model was successful in matching the field-observed basal discharge and cumulative outflow. The results demonstrate the importance of preferential flow not only in rock-like, but also in soil-like waste-rock piles and indicate that calibrating an unsaturated flow model to observed outflow alone is insufficient to evaluate flow patterns and residence times in waste rock. Therefore, mobile-immobile, dual-porosity and dual-permeability approaches were implemented into the MIN3P code and the enhanced code was used to improve the simulation of tracer breakthrough at the pile scale, relative to the uniform flow and solute transport model. Although substantial improvements could be obtained by using the dual domain approaches, observed tracer peak concentration and tailing were not well captured, suggesting the presence of a third immobile region with a very slow release rate. Based on the geochemical system developed for the field barrel scale and the dual-domain model developed for the pile scale, the applicability of the enhanced code for modeling of multicomponent reactive transport in waste rock at the pile scale was demonstrated, and in the process, the distribution of reactivity in preferential flow and matrix regions was evaluated.

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Application of molybdenum (and zinc) stable isotopes to trace geochemical attenuation in mine waste (2018)

Mining activities generate tremendous quantities of waste rock and tailings that must be carefully managed to prevent contamination of water resources by metal-leaching. Proper environmental management of mine drainage requires a detailed understanding of the mechanisms that control the mobility of metals in mine waste. This thesis applied stable-isotope analyses of molybdenum (Mo) and zinc (Zn) to constrain the geochemical attenuation processes controlling transport of these metals in mine waste. The key outcomes of this work are: (1) the establishment of a robust protocol for determining high-precision Mo isotope ratios in mine-waste samples using double-spike multi-collector inductively coupled plasma mass-spectrometry; (2) the demonstration that mine drainage at field sites becomes enriched in heavy Mo isotopes because Mo attenuation preferentially removes light isotopes; the predominant Mo attenuation mechanisms considered being sorption onto (oxyhydr)oxides and precipitation of molybdate minerals; (3) the demonstration that mine drainage becomes depleted in heavy Zn isotopes under alkaline pH conditions because of preferential removal of heavy Zn isotopes during Zn adsorption and/or precipitation of secondary minerals; and (4) the determination of new Mo isotopic fractionation factors for the precipitation of powellite (CaMoO₄) and wulfenite (PbMoO₄)—important sinks of Mo in mine waste environments. Overall, this thesis demonstrates that metal stable isotope analyses are an informative new tool now available to trace the processes that control metal transport in the environment. Further improvements in the quantification of metal removal using stable isotopic analyses should become possible with ongoing research into the causes of metal stable isotope fractionation.

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On the role of multicomponent diffusion and electrochemical migration for reactive transport in porous media (2017)

In multicomponent solutions, electrostatic coupling between charged species leads to a process called “electromigration”. Neglecting electromigration results in a charge imbalance and an incomplete and unrealistic description of mass transfer. Although not commonly considered in reactive transport codes, electromigration can strongly affect mass transport processes and can explain unexpected behaviors such as uphill diffusion or isotope fractionation. Including electrostatic coupling in reactive transport codes enables simulation of problems involving mass transport by advection and diffusion, electromigration and geochemical reactions, such as electrokinetic remediation and the geobattery concept associated with buried ore bodies. There are generally two methods for coupling charge and mass continuities. The first method is based on the null-current approach which assumes negligible electric current transmission. The second method considers explicit coupling of mass and electric fluxes. In this study both methods are investigated and their implications for reactive transport are examined.To this end, MIN3P, a fully coupled 3D reactive transport code, was extended by integrating the Nernst-Planck and Gauss-Ampère equations. The implementation of the Nernst-Planck equations was verified by inter-comparison with other existing reactive transport codes based on a set of benchmark problems. At the same time, these benchmark problems illustrate the effect of electric coupling during multicomponent diffusion and electrochemical migration.By explicit coupling of the Nernst-Planck and Gauss-Ampère equations, MIN3P was further enhanced to simulate electrokinetic remediation and the resulting code was tested for desalination problems. In addition, scenario and sensitivity analysis were used to investigate the potential for spontaneous exsolution of gases in response to gas generation at the electrodes of electrokinetic remediation systems.Finally, a process-based model linking surface-measureable self-potential signals to electrochemical transport and geochemical reactions associated with buried metallic bodies was developed. The enhanced code provides a reactive transport modeling framework for process-based forward modeling of self-potential signals and associated geochemical signatures of buried ore bodies and allows a quantitative investigation of the “geobattery concept”. The code was tested based on published data from a laboratory experiment involving a buried iron bar and used to evaluate the geobattery concept based on an illustrative example of a buried ore body.

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The Role of Hydrology, Geochemistry and Microbiology in Flow and Solute Transport through Highly Heterogeneous, Unsaturated Waste Rock at Various Test Scales (2015)

Drainage chemistries from unsaturated waste rock are affected by a number of hydrological, geochemical and microbiological processes. These processes are generally coupled and reliable prediction of solute loads is a difficult task for most mine sites, making it challenging to provide accurate estimates of future water treatment requirements. An assessment of preferential and matrix flow, geochemical controls and microbially-enhanced weathering is beneficial to provide an improved understanding of the dominant drainage controls. A multi-scale waste rock study was implemented at the Antamina mine (Peru) to assess drainage controls using 10-m high pile experiments, 1-m field barrels and 0.8-m laboratory columns. Observed drainage chemistries show a strong seasonal pattern in response to changes in infiltration rates, with increasing concentrations during the dry season and decreasing concentrations during the wet season. These seasonal fluctuations are less pronounced for finer-grained material, likely due to lower proportions of preferential flow, indicating that hydrological processes provide a key control on observed drainage chemistries. Microbiological analyses show iron-oxidizing neutrophiles are ubiquitous to all rock types, whereas proportions of acidophiles are strongly influenced by lithology. Drainage waters are more acidic and contain higher metal loads with increasing proportions of acidophilic microbes. Breakthrough curves of an externally-applied bromide tracer show that infiltration migrates mostly through matrix materials, with a minor proportion following preferential flow paths. Long tails indicate a portion of mass enters extremely slow matrix flow paths and/or immobile domains. Chloride, originating from blasting residues, was used as an internal tracer. Mean residence times from chloride breakthroughs are longer than bromide values for the same spatially-specific region, suggesting a slow release of chloride from low permeability matrix material. Flow and solute transport processes were successfully modeled using a dual-porosity mobile-immobile approach in HYDRUS1D. This research provides an improved understanding of the governing hydrologic and geochemical processes relevant to Antamina waste rock with implications for the large-scale waste rock dumps at site. Other aspects of this research, such as using blasting residues as an internally-applied tracer and using a dual-domain approach to model flow and solute transport, will be of value to other mines with similar conditions.

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Novel Approaches for Quantifying Source Zone Natural Attenuation of Fossil and Alternative Fuels (2014)

Natural attenuation represents an environmentally and economically sustainable means to manage contaminants in situ. To assess potential human and environmental risks associated with this strategy, there is a need to characterize the extent of contaminant containing source zones. Moreover, to obtain public and regulatory support, there is a need to demonstrate that mass losses are occurring. However, quantifying the extent and rate of natural attenuation under field conditions remain challenging. Here a novel approach for assessing and monitoring sites impacted by hydrocarbon and ethanol-blended fuels using surficial gas effluxes is developed. The approach is tested at several sites including those impacted by a crude oil pipeline rupture, ethanol-blended fuel train car derailments, and historic refinery operations. The approach was refined through inclusion of isotopic measurements, comparison to other approaches for monitoring microbial activity, and evaluation of seasonal dynamics and microbial communities. Process-based reactive transport modeling was used to integrate and interpret field data and develop and illustrate a more robust conceptual model of the processes occurring at the different field sites.Results demonstrate that surficial gas effluxes are able to both delineate contaminant containing source zones, and distinguish between the rates of natural soil respiration and contaminant mineralization. In scenarios where methane oxidation goes to completion, carbon dioxide fluxes are sufficient for evaluating natural attenuation rates; when methane escapes oxidation, measurements of methane fluxes are also needed. Results also demonstrated that measurement of radiocarbon is particularly useful for determining the contribution of contaminant degradation to the measured efflux. Comparison of seasonal dynamics showed that both biological and physical parameters must be considered when quantifying average annual contaminant degradation rates while comparison to other approaches for measuring microbial activity showed good correlations with gas effluxes. Comparison across the field sites investigated, showed degradation rates were relatively high at ethanol-blended fuel release sites. In-depth microbiological evaluation of microbial communities at one ethanol-blended fuel release site showed a substantial change in the microbial community associated with the release.Overall, the novel methods provide a useful approach for assessing the extent and rate of natural attenuation at hydrocarbon contaminated field sites.

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Unsaturated hydrology, evaporation, and geochemistry of neutral and acid rock drainage in highly heterogeneous mine waste rock at the Antamina Mine, Peru (2014)

Physical and geochemical heterogeneities in mine waste rock complicate the prediction and assessment of waste rock effluent water quantity and quality. The objective of this research is to provide a holistic conceptual understanding of the hydrological and geochemical processes that control effluent water quantity and quality, and the complex interactions among processes at the field scale. To this end, a prodigious dataset from three experimental waste-rock piles at the Antamina Cu-Zn-Mo skarn-deposit mine was compiled and analyzed. Analyses included solid-phase mineralogy and physical characteristics; effluent and pore-water hydrology and geochemistry; and an aqueous tracer study.The instrumented piles (36 m x 36 m x 10 m) are each composed of a single rock type and are exposed to almost identical atmospheric conditions, isolating the effect of rock type on hydrological regimes. Physical waste rock heterogeneities result in highly variable hydrology that is strongly dependent on material particle size distributions and especially the presence of large boulders. The hydrological regimes include wide ranges of velocities for matrix flow (
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Numerical modeling of density-driven chemical oxidation of chlorinated solvents (2009)

A popular method for the treatment of aquifers contaminated with chlorinated solvents is chemical oxidation using potassium permanganate (KMnO₄). Numerical modeling was employed to investigate permanganate-based remediation under free convection conditions, considering contaminant treatment, and geochemical reactions including the oxidation of naturally occurring organic matter, mineral dissolution and precipitation, and ion exchange reactions. The MIN3P multicomponent reactive transport code was enhanced to simulate the remediation technology. The modified code (MIN3PD) provides a direct coupling between density-dependent fluid flow, solute transport, contaminant treatment, and geochemical reactions. The code was utilized to identify the processes most important to remediation efficiency and the geochemical response to a KMnO₄ injection. The investigation was achieved through a sensitivity analysis, the development of a three-dimensional model of a field trial of TCE oxidation, and automated inverse modeling. Investigation results elucidate the important role of density-induced flow and transport on the distribution of the oxidant solution. The soil organic matter content, aquifer hydraulic conductivity and porosity, and the DNAPL dissolution kinetics comprise primary system attributes controlling the geochemical evolution and remediation efficiency. Detailed monitoring data collected during the field trial were used to evaluate the ability of the MIN3PD model to adequately simulate permanganate-based groundwater remediation. The calibrated model reproduced the transient distribution of aqueous species, and supported a quantitative evaluation of remediation efficiency. Automated inverse modeling was employed to systematically evaluate the quality of the model calibration, and identify optimal values of model input parameters. The investigation supported a quantitative evaluation of the conceptual model of the remediation technology, and capabilities and limitations of the numerical model.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Characterization of gas migration related to ethanol blended fuel spills and stray gas (2023)

Increased energy demand and technical advancements have led to unconventional resource development and alternative ethanol-blended fuel use. While these options are effective at contributing to meet energy demands, potential harmful effects due to release of constituents and degradation products into the environment must be considered to accurately assess risk and develop appropriate preventative, monitoring, and mitigation strategies. Gas migration and fate were studied in two related contexts: biodegradation of gasoline containing 20% ethanol (E20) and the release of stray gas into an aquifer. Secondary water quality impacts, vadose zone gas movement, and surficial release, were characterized, and effects of microbially-mediated reactions, porous media properties, and barometric pressure fluctuations were assessed. Two soil columns of differing permeability were used for each scenario. E20 fuel spill analysis included measurement of CO₂ and CH₄ surface efflux, soil gas concentrations (O₂, CH₄, CO₂, N₂, Ar, benzene, and toluene), volatile fatty acids, alkalinity and pH, cations, and aqueous benzene, toluene, and ethanol concentrations. Assessment of stray gas migration involved CO₂ and CH₄ surface efflux, soil gas concentrations (O₂, CH₄, CO₂, N₂, Ar), isotopic analysis (δ¹³C-CO₂, δ¹³C-CH₄, δ²H-CH₄), barometric pressure, hydraulic heads, and mass flow rates.Results reveal significant impacts on gas migration attributable to soil type in both cases. E20 fuel release resulted in greater aerobic oxidation of ethanol and the petroleum products in higher permeability soil. Microbial toxicity was not found to impede biodegradation in the buffered system; however, Mn²⁺ and Fe²⁺ metal release was observed. Two different flow experimental conditions were considered for the stray gas experiments: Stray gas release subject to constant flow rate or under constant pressure conditions. Stray gas migration was observed to be continuous under constant mass flow conditions, while dominantly discontinuous flow was seen under constant pressure conditions. Discontinuity of gas flow was found to be correlated with barometric pressure fluctuations, with more pronounced effect in finer grained soil. Under both release conditions, substantial methane (CH₄) oxidation was observed; however, the fraction of CH₄ oxidized declined for higher stray gas release rates. Irrespective of the nature of gas release, CH₄ and CO₂ were released across both column surfaces.

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Processes governing carbon dioxide exchange between the atmosphere and hydromagnesite-magnesite playas in Atlin, British Columbia (2022)

The hydromagnesite-magnesite playas in Atlin, BC provide a unique opportunity for studying the carbonate-bicarbonate system and carbonate mineral stability at the Earth’s surface. Based on analysis of pore water samples and mineralogical data, Power et al. (2014) concluded that the playas degas CO₂ from Mg-HCO₃-rich groundwater and in-situ carbonate mineral precipitation, but CO₂ emissions were not quantified directly. In this thesis, eddy covariance (EC) and dynamic closed chamber (DCC) systems were co-located to directly quantify rates and characterize processes governing the CO₂ flux across the playa-atmosphere interface. Data were collected continuously over 27 days in 2020 and 14 days in 2021. The results from the DCC method show distinct diurnal oscillations of CO₂ fluxes, with average daytime fluxes of +0.15±0.34 μmol mˉ² sˉ¹ (2020) and +0.15±0.19 μmol mˉ² sˉ¹ (2021) and nighttime fluxes of -0.24±0.31 μmol mˉ² sˉ¹ (2020) and +0.04±0.18 μmol mˉ² sˉ¹ (2021) (positive upward and negative downward). Fluxes measured via the DCC method indicate minimal net exchange of carbon across the playa-atmosphere interface during the monitoring period. These observations imply that DCC-measured CO₂ fluxes are governed predominantly by changes in CO₂ solubility in alkaline porewater related to diurnal temperature fluctuations and variations in CO₂ concentrations in ambient air above the ground surface. However, EC measurements show a continuous positive flux averaging +1.38±0.62 μmol mˉ² sˉ¹ (2020) and +1.07±0.43 μmol mˉ² sˉ¹ (2021). The net CO₂ flux measured by EC was attributed to a source undetected by the DCCs with possible contributions from soil respiration at the playa margins or surrounding forests, or directly released via preferential pathways from a deeper source within the playa. The use of two complementary flux measurement methods revealed that CO₂ fluxes vary as a function of scale and location at this site. These findings provide insights on CO₂ flux dynamics in sparsely vegetated arid and semi-arid regions and the application of these methods for monitoring and verification of ex-situ carbon mineralization at sites with enhanced mineral weathering. The complex interactions between minerals, fluids and the atmosphere make Atlin an ideal site to test complementary methods for measuring CO₂ fluxes.

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Simulating discontinuous gas flow and methane reactive transport in the saturated zone (2022)

The release of natural gas to shallow groundwater systems from energy wells suffering integrity issues (termed gas migration, GM) can lead to adverse impacts on groundwater quality, safety concerns associated with explosion risks, and the release of greenhouse gases to the atmosphere. The physiochemical processes occurring during GM are difficult to characterize and model. However, modeling is important to develop strategies for detecting and monitoring GM and formulate conceptual models. In this study: (i) A previously developed numerical model based on macroscopic invasion percolation (macro-IP) and multicomponent mass transfer was enhanced to simulate gas releases in the shallow subsurface. Model simulations were compared to previously conducted bench-scale gas injection experiments and results show that gas flow is highly sensitive to the entry pressure field distribution assigned within the model domain and the critical gas saturation used to model gas-water flow based on macro-IP. However, mass transfer and the resulting domain-scale dissolved-gas transport was relatively insensitive to these parameters. (ii) Hypothetical natural gas releases were simulated in a two-dimensional shallow confined aquifer to identify and evaluate GM indicators. Free-phase gas movement was modeled using macroscopic invasion percolation. The resulting free-phase gas distribution was then inputted to the multicomponent reactive transport model MIN3P. A variety of scenarios were tested to understand the impact on dissolved gas concentrations downgradient of the release. The scenarios included the presence of multiple sources of free-phase gas and different biogeochemical conditions in the aquifer. Simulations show that exsolution of dissolved background gases during simultaneous dissolution of free-phase hydrocarbons cause delayed and variable breakthrough of dissolved gas concentrations at monitoring points across the domain. The results suggest that monitoring of background dissolved gases with total dissolved gas pressure can help enhance the monitoring network at sites impacted by GM. It is clear from the modeling results that multicomponent mass transfer and transport need to be considered when interpreting GM indicators. The findings from this study will aid in taking necessary steps towards up-scaling and implementing numerical models at larger scales to simulate GM scenarios.

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Reactive transport modeling of waste rock weathering in permafrost environments (2021)

Developing an understanding of the thermo-hydrological-chemical (THC) behavior of waste rock piles (WRPs) at mine sites in cold-region climates is important for anticipating contaminated drainage. In cold-region climates, freeze-thaw cycles and the possible development of permafrost within WRPs add to the complexity of the coupled processes occurring in WRPs but also provide opportunity for reclamation strategies, in particular through the placement of thermal covers to isolate the waste rock from weathering. Reactive transport modeling (RTM) has proven a versatile tool that can help characterize the coupled processes within mine waste. In this thesis, RTM code MIN3P-HPC has been enhanced to account for the effects of seasonal freeze-thaw cycles on the weathering behavior of WRPs with specific focus on the development of permafrost and drainage quality. The code was used to perform a sensitivity analysis for a hypothetical full-scale sulfide-bearing WRP to assess the influence of two key factors: climate warming and sulfide reactivity. Simulation results indicate that for some WRPs hosting permafrost under present-day conditions, a marginally warmer climate has the potential to lead to self-heating of the pile and substantially increase the mass loadings. Moreover, even if permafrost within a WRP persists in a long term, the results illustrate that a warming climate can increase mass loadings significantly due to an increased active layer thickness and a longer duration of thawed conditions. Subsequently, the code was used to evaluate the effectiveness of thermal covers at mine sites in cold-region climates, constrained by observational laboratory and field data from the Meadowbank mine, located in the Kivialliq region. Favorable agreement of simulated and measured thermal data provides confidence in the utility of the numerical model to simulate coupled processes within a covered pile subjected to freeze-thaw cycles. The model was expanded to interpret the long-term functionality and effectiveness of the cover as a function of cover thickness. The results of this research demonstrate the capabilities of MIN3P-HPC for simulating problems in permafrost environments affected by seasonal freeze-thaw cycles and to evaluate weathering of WRPs in such environments. This modeling tool has potential to assist with the design of reclamation strategies.

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Applications of reactive transport modeling for assessing the effects of heterogeneities and internal structure on mass loadings from waste rock piles (2020)

Design strategies for waste rock piles (WRPs) to mitigate or reduce the potential for poor drainage quality which have associated environmental and economic liabilities are an important consideration for mining operations. Due to their size and complexity, conceptualizing internal processes and forecasting drainage quality from WRPs has proved challenging. Reactive transport modeling (RTM) is a tool that can help improve understanding of complex interactions of physical and geochemical processes within WRPs and compare the effects of pile design on drainage water quality. In this thesis, RTM is used to investigate the effect of physical and chemical heterogeneities, pile construction methods and scale on mass loadings in drainage from full-scale WRPs through a series of stylistic models. RTM is also applied to a laboratory-scale model to simulate an engineered cover system as part of the long-term design plan for the Tio Mine, Quebec, Canada. Results from the stylistic models indicate that heterogeneity and structural features in WRPs can cause significant variations in effluent quality, when compared to a homogeneous case. Simulated peak loadings are reduced due to the presence of heterogeneities and internal structure, at the same time prolonging the release of poor-quality drainage. The results also indicate that pile construction methods (e.g. end- and push-dumping) affect drainage release in different ways; however, these differences vanish for larger multi-lift piles. Simulations suggest that multiple lifts and the presence of buried traffic surface homogenize mass loadings, independent of construction methods. Simulations also indicate that peak mass loadings from waste rock piles are reduced by a factor of 2-3 when heterogeneities and structure are considered, which has implications for the design of water treatment systems. These results were put to practice in simulations of a laboratory-scale model, where heterogeneities proved essential for capturing effluent quality from different material zones, and for conceptualizing internal flow paths occurring within the dry cover system in place. In addition, results from this research demonstrate the capabilities of the recently developed unstructured grid code MIN3P-HPC for capturing complex shapes and geometries, which may prove useful for future investigations of capillary barrier effects and other design strategies in WRPs.

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Characterization of gas migration and surface emissions through a controlled release experiment at the Hudson's Hope field research station, BC, Canada (2020)

The expansion of unconventional oil and gas development has led to growing concern regarding the environmental impacts of gas migration (GM), which occurs at some wells. GM is the transport of natural gas outside of the well casing, which can lead to mobile fugitive gas (FG) that penetrates into neighbouring geological formations and can impact aquifers. FG that migrates to the surface contributes to greenhouse gas emissions that are difficult to quantify. This study is part of a research program that aims to increase knowledge of GM and FG through a controlled natural gas release experiment. The experiment was conducted near Hudson’s Hope in north-eastern British Columbia, Canada, a region of active unconventional natural gas development. The experimental site is underlain by heterogeneous quaternary deposits with a confining clay layer overlaying a sand aquifer. 100 m³ of a synthetic natural gas mixture was injected at the base of the aquifer at a constant rate for 66 days.This thesis focuses on monitoring of GM in the unsaturated zone and the quantification of surface effluxes. To this end, twelve long-term chambers were used to measure CO₂ and CH₄ effluxes, providing high resolution time-series data. Survey chamber measurements at 105 locations allowed for spatially distributed measurements at lower frequency. In addition, soil gas samples were collected from 22 soil gas sampling ports. The results illustrate that the injected gas migrated upgradient against the direction of groundwater flow and broke through at the surface six weeks after the injection started. Once the gas was detected, elevated CH4 fluxes were continuously detected at the surface in a constrained geographical region and only began to decrease one-week post-injection. Soil gas composition and isotopic data further support that the gas migrated through the soil towards the ground surface and that hydrocarbons were microbially oxidized. The free phase gas plume was only able to reach the surface due to the presence of a preferential pathway in the confining layer. Soil gas compositional data indicates that towards the end of the injection and post-injection, the free-phase gas plume began to explore alternative preferential pathways.

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Experimental investigation on the fate of ethanol-blended fuels in the subsurface (2017)

Currently, the North American market for motor fuels is distributing E10 (gasoline with an ethanol content up to 10%). Recent legislature is promoting a range of higher ethanol content fuels to be introduced into the market. The result is an increased risk of accidental release of these higher ethanol content fuels. A comprehensive understanding of the fate of ethanol-blended fuels, including interactions between its constituents (e.g. ethanol, benzene and toluene), degradation products (e.g. VFAs, CO₂, CH₄) and potential secondary water quality impacts (e.g. Mn²⁺,Fe²⁺), is lacking. Eight large columns were constructed to evaluate the impacts of fuel blends of varying ethanol contents on biodegradation, and to determine the effect of soil type on gas generation and migration. In each scenario, approximately 2L of fuel was injected into the lower quarter of the column, approximately 30cm above the water table. The saturated zone was analyzed for dissolved Mn and Fe, as well as EtOH and VFAs. Vadose zone analysis focused on measuring surficial CO₂ and CH₄ fluxes, in-situ soil gas concentrations of CO₂, CH₄, O₂, N₂, and Ar, as well as benzene and toluene. Isotopic analysis of vadose zone CO₂ was also completed.The results confirm that ¹³C isotopic analysis is well suited for identifying the predominant microbial substrate undergoing biodegradation. Fuel blends with higher ethanol content showed more elevated levels of dissolved Mn and Fe, demonstrating that metal mobilization occurs more readily in spills with higher ethanol content. Additionally, fuels with a higher ethanol content exhibited signs of elevated microbial stress through the increased production of butyric acids. Benzene and toluene measured near surface, and the surficial effluxes of CO₂ or CH₄ did not indicate a significant SVI risk in any of the scenarios tested, under the conditions studied.

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Natural attenuation of biodiesel in a sandtank experiment (2017)

The use of alternative fuels, including biodiesel, has increased steadily in the last two decades, increasing the risk of accidental spills. However, a comprehensive understanding of the fate of biodiesel in the subsurface is currently lacking. A large sandtank experiment was conducted over 18 months to evaluate the spatial and temporal evolution of biodiesel biodegradation with a focus on vadose zone impacts. 80 L of biodiesel was applied to the center of the sandtank. Monitoring and analysis focused on two zones: the saturated zone including the capillary fringe, and the unsaturated zone. Measured parameters included surficial CO2 and CH4 effluxes, gas concentrations and their isotopic composition in the vadose zone, moisture contents and temperature. In the saturated zone, groundwater chemistry was characterized based on dissolved cation and anion concentrations, volatile fatty acids (VFAs), pH and alkalinity. The experimental results displayed a rapid response to the biodiesel release, revealed by increases of surficial CO2 effluxes and CO2-concentrations in the vadose zone, while O2 concentrations remained close to atmospheric levels. In the saturated zone, elevated VFA concentrations were observed together with pronounced increases in cation concentrations, specifically Ca, Mg, Fe and Mn, indicating the rapid development of anaerobic conditions. The generation of acidity associated with aerobic and anaerobic degradation reactions led to a decline in pH, locally to values below 5, likely inhibiting the progress of biodegradation. The onset of CH4 generation was delayed and coincided with reaching maximum VFA concentrations in the saturated zone. CH4 effluxes at the ground surface were limited; however, stable isotope analysis indicated that CH4 oxidation in the vadose zone was weak, likely due to low-pH conditions. Increases in dissolved concentrations of Fe and Mn were attributed to the reductive dissolution of Mn- and Fe-oxides, with possible contributions from the dissolution of Fe- and Mn-bearing dolomite.Carbon balance estimates showed that the biodiesel was recalcitrant to degradation, and at 590 days less than 5% of the biodiesel had been transformed to VFAs, CO2 and CH4. The average biodiesel degradation rate derived from the carbon balance is 1.3 x 10-8 mol L-1 H2O s-1, comparable to literature values.

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Investigating Metal Attenuation Processes in Mixed Sulfide Carbonate Bearing Waste Rock (2016)

The objective of this study was to identify the trace metal/secondary mineral phase associations in a heterogeneous waste rock dump that contains carbonate bearing lithologies and a mix of metal sulfides. The identification of attenuation processes can be used to better predict the drainage chemistry from waste rock at this site and/or other sites with similar waste rock. This study also provides the opportunity to investigate metal attenuation at the largest scale of complexity and compare these observations to those made from the smaller scale tests conducted for this site and is useful for understanding scalability of the smaller scale tests. This study shows that in carbonate bearing waste rock the predominant processes that attenuate copper (Cu) and zinc (Zn) are precipitation of hydroxycarbonate and hydroxysulfate phases and sorption onto iron oxides. Arsenic (As) and molybdenum (Mo) are associated with iron oxides, although for Mo this association was observed in only a few samples. Lead (Pb) was observed in association with iron oxides. Wulfenite observed in a few samples provides an additional attenuation process for Mo and Pb. The stability of the phases and potential for remobilization of these metals can also be suggested from this study. The hydroxycarbonate/hydroxysulfate phases are the least stable phases identified and can dissolve at pH
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Unintentional contaminant transfer from groundwater to the vadose zone via exsolution and ebullition during remediation of volatile organic compounds (2016)

Historical heavy use of chlorinated solvents in conjunction with improper disposal practices and accidental releases has resulted in widespread contamination of soils and groundwater in North America. As a result, remediation of chlorinated solvents is required at many sites. For treatment of source zone contamination, common remediation strategies include in situ chemical oxidation (ISCO) using potassium or sodium permanganate, and the enhancement of biodegradation by primary substrate addition. It is well known that these remediation methods tend to generate gas (carbon dioxide (CO₂) in the case of ISCO using permanganate, CO₂ and methane (CH₄) in the case of bioremediation). It is hypothesized that the generation of gas in the presence of volatile organic compounds (VOCs), including chlorinated solvents, may lead to stripping of the contaminants from the source zone due to gas exsolution and ebullition. This process may lead to ‘compartment transfer’, whereby contaminants are transported away from the saturated zone into the vadose zone, with possible implications for soil vapour intrusion. Two sites in the U.S. undergoing enhanced bioremediation have exhibited behavior suggestive of contaminant transfer into the vadose zone via gas generated during remediation. These sites provided the impetus for a more in-depth investigation into this process. To this extent, benchtop column experiments were conducted to observe the effect of gas generation during remediation of the common chlorinated solvent trichloroethylene (TCE/C₂Cl₃H). Two common in situ treatment strategies were simulated for source-zone subsurface contamination of TCE, including ISCO and enhanced bioremediation. Results confirm that these aggressive remediation methods can lead to gas production and induce vertical transport of contaminants away from the treatment zone, following the formation of a discontinuous gas phase (bubbles). The generation of gas and the potential for unintentional contaminant stripping and transport should be taken into consideration when treating VOCs to avoid release into the atmosphere or into underground structures via soil vapour intrusion. This study also suggests that the suitability of gas-generating remediation techniques in proximity to buildings and in populated areas should be evaluated with care.

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Integrating environmental tracers and groundwater flow modeling to investigate groundwater sustainability, Gibsons, BC (2013)

Environmental tracers can provide information on groundwater age, recharge conditions and flow processes. This information is useful for evaluating groundwater sustainability and vulnerability by identifying groundwater provenance and information for water budgets. Gibsons, British Columbia is a growing coastal community relying on groundwater to supply drinking water to two thirds of its 4,300 residents. The Town of Gibsons is proud of its untreated groundwater resource and proactive about keeping it protected and sustainable for future generations. Samples of noble gases, tritium, and stable isotopes of oxygen and hydrogen were collected from the aquifer. Tracer results improved the site conceptual model by identifying a previously unknown contribution of mountain block recharge (MBR) and by providing recharge elevation estimates using noble gas thermometry. The updated conceptual model including the mountain block was integrated into a regional three-dimensional numerical groundwater flow model calibrated to both hydraulic heads and to recharge elevation, a non-traditional approach to model calibration. This is the first study to use recharge elevation as a calibration target, which proved to be imperative for constraining bedrock geometry and minimizing model non-uniqueness. Tracer and modeling results indicate that groundwater in the Gibsons aquifer contains a mixture of approximately 45% MBR and 55% bench recharge. The MBR component is pre-modern (> 50 years) groundwater that recharged at elevation and cold temperatures (~5°C) and has evolved hydrogeochemistry and high concentrations of excess air (EA; >0.005 ccSTP/g) and ⁴Heterr (>10-⁹ ccSTP/g). Bench recharge is modern (
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Numerical modeling techniques for assessing three-dimensional diffusion processes in heterogeneous rock samples on the sub-mm scale (2011)

The burial of high level nuclear wastes in geologic repositories requires careful consideration of the long-term hydrogeological and geochemical stability of the receiving formations. Care must be taken due to the high environmental sensitivity of the waste material and its potential long-term effect on groundwater. Studies into the host rocks' natural ability to minimize contaminant migration are a matter of high priority in the planning for long-term storage of high level radioactive waste. Focusing on porous sedimentary rock, this study aims to examine numerical techniques used to analyze and interpret experimental data that characterize the distribution of porosity in geologic samples at the µm to mm scale. Because repositories are hosted in natural fine-grained rock formations, transport in the vicinity of these repositories is diffusion-controlled and believed to be affected substantially by heterogeneities at these scales. Data available for the analyses consist of non-destructive, high-resolution measurements of porosity obtained using new developments in X-ray imaging. Advances in computing technology make it possible to numerically analyze the expected patterns of sub-mm-scale diffusive transport for these large experimental data sets. The modeling analyses examine 3D diffusive transport in heterogeneous rock samples and evaluate the effect of data resolution and image processing techniques on the connectivity of the transport pathways. The simulation results provide insight into small-scale diffusive transport of solutes, and guide the needs for dataset resolution and handling for these types of investigations. With increased availability of experimental results, further modeling studies could be conducted. These studies would aim at developing a link between simulation results and observed data to further develop the transport theory for contaminant migration on this scale.

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Using atmospheric noble gases and sulfur hexafluoride as indicators for transport and reaction processes in hydrocarbon contaminated sediments (2010)

Naturally occurring contaminant attenuation processes are investigated in a petroleum-hydrocarbon contaminated shallow aquifer, near Bemidji, MN. At this site, the biodegradation of hydrocarbons operates mostly under methanogenic conditions and generates CO₂ and CH₄. The main objectives of this study are to determine whether the full suite of noble gases, including He, Ne, Ar, Kr, and Xe, can be used to further delineate the fate of contaminants in the saturated and vadose zones and to identify mass transfer processes between these two compartments. Noble gases are sampled in the field and analyzed by way of an extraction line and mass spectrometry. In the vadose zone, gas consumption and production will induce pressure gradients, causing advective gas transport, which can be identified through concentration gradients of the inert gases. Noble gas data collected at the Bemidji site confirms the occurrence of advective gas transport, providing verification for previous field investigations and modeling that focused on Ar and N₂ as gas tracers. In addition, the present study reveals that heavier noble gases provide the strongest signal for identifying reaction-induced gas advection in the vadose zone, as a result of their lower diffusion coefficients. The biogenic addition of gas to the saturated zone promotes gas exsolution and bubble formation, which can be marked in the source zone by the depletion of dissolved noble gas concentrations in relation to atmospheric values. Modeling results support the hypothesis that ebullition, the buoyancy-driven upward migration of gas bubbles, is taking place locally in the source zone. The flux of gas across the water table, as a result of ebullition, is estimated at 0.177Lm-²day-¹. Ebullition is further investigated under laboratory closed system conditions. Results indicate that both atmospherically derived Ar and injections of SF₆ can be used as tracers for ebullition. However, the partitioning of gas tracers into free-phase hydrocarbons limits the applicability of gas tracer injections. An oil-gas partitioning experiment is carried out to assess the feasibility of SF₆ as a tracer in hydrocarbon contaminated settings. The results suggest that partitioning of SF₆ into oil is extensive, with a dimensionless oil-gas partitioning coefficient of 0.73.

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