Ulrich Mayer

Professor

Research Interests

hydrogeology
low-temperature geochemistry
groundwater contamination
groundwater remediation
mine waste management

Relevant Degree Programs

 

Research Methodology

reactive transport modeling
soil gas efflux measurements
Gas Chromatography

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Development of multicomponent reactive transport modeling tools for multiphase systems

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
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 (2010 - 2018)
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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Unintentional contaminant transfer from groundwater to the vadose zone via gas 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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