Roger Beckie


Relevant Degree Programs

Affiliations to Research Centres, Institutes & Clusters


Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - April 2022)
A field investigation on the transport and fate of fugitive gas from energy wells in glaciogenic sediments (2021)

Recent petroleum resource development has generated concerns regarding the release of fugitive natural gas (comprised primarily of methane) from compromised wellbores into the shallow subsurface. Free-phase fugitive gas can move vertically to the ground surface, contributing to greenhouse gas emissions, or it can accumulate in confined spaces, creating explosive hazards. When free-phase gas dissolves into groundwater, it can be oxidized via microbially-mediated reactions that may deteriorate groundwater quality. To assess environmental risks posed by subsurface fugitive gas, it is critical to understand the transport and fate of fugitive gas in the shallow subsurface, particularly in petroleum resource development regions. The subject of this thesis is a controlled release field experiment to investigate the migration, dissolution behavior, and biogeochemical activity of fugitive gas as well as the hydrogeological controls on its migration in a shallow (
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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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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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A field investigation characterizing the hyporheic zone of a tidally-influenced river (2010)

This research investigated groundwater surface water interaction (GWSi) of the Fraser River, in the Lower Mainland of British Columbia. At the Braid Street site, GPR, seismic reflection surveys, bulk resistivity profiling, and sediment sampling were used to map the sediments of the riverbed and the area of contaminated groundwater discharge. Groundwater profiling revealed that three water types occur within the upper 2 m of the riverbed sediments a result of mixing of river water, contaminated (fresh) groundwater, and saline groundwater. The distribution of groundwater solutes indicate that during a single tidal cycle, river water penetrates the riverbed to a depth of approximately 15 cm but the long term effects of tidal pumping of river water into the riverbed is observed to a depth of approximately 1 meter below river bed (m.b.r.b.) Temperature measurements coupled with independent hydraulic head measurements within the riverbed confirmed that GWSi under tidal forcing produced a 1 m-deep hyporheic zone (HZ). Time-averaged riverbed temperature profiles displayed a distinct compressed convex-upward pattern, clear evidence of net groundwater discharge. However, the instantaneous time series data indicate that riverbed temperatures, to a depth of 1 m were affected by tidal-forcing. Heat transport modeling revealed that instantaneous velocities within the shallow sediments of the riverbed are rather high during either a flooding or ebbing tide. Further, the magnitude of the tidally-induced pressure gradient was found to be significantly greater than the pressure gradient attributable to flow across large river bottom bedforms, indicating that bedform-driven exchange is limited to within a few centimeters beneath the riverbed. At the Kidd2 site, where saline encroachment occurs up river, hyporheic and hypoaigic (recharge by saline water) processes occur. During low winter river flows saline encroachment up river occurs and hypoaigic exchange dominates. During low tide the saline water is pushed out of the river and hyporheic exchange dominates. Hyporheic exchange dominates exchange during freshet leading to deep freshening of the aquifer on the order of 2+ m.

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Particle and Streamline Numerical Methods for Conservative and Reactive Transport Simulations in Porous Media (2010)

Reactive transport modeling has become an important tool to study and understand the transport and fate of solutes in the subsurface. However, the accurate simulation of reactive transport represents a formidable challenge because of the characteristics of flow, transport and chemical reactions that govern the migration of solutes in geological formations.In particular, solute transport in natural porous media is advection-controlled and dispersion is higher in the direction of flow than in the transverse direction. Both characteristics create difficulties for traditional numerical schemes that result in numerical dispersion and/or spurious oscillations. While these errors can often be tolerated in conservative transport simulations, they can be amplified in presence of chemical reactions resulting in much larger errors or unstable solutions.In this thesis, new Lagrangian based methods to simulate conservative and reactive transport in porous media are investigated. First, the derivation of a new meshless approximation based on smoothed particle hydrodynamics (SPH) to simulate conservative multidimensional solute transport, including advection and anisotropic dispersion, is presented. Second, a hybrid scheme that combines some of the advantages of streamline-based simulations and meshless methods and that allows simulating longitudinal and transverse dispersion without requiring a background grid is also derived. The numerical properties of both methods are analyzed analytical and numerically. Furthermore, both formulations are compared with existing numerical techniques in a set of two- and three-dimensional benchmark problems.It is demonstrated that the proposed schemes provide accurate and efficient solutions of physical transport processes in heterogeneous porous media and overcome most of the issues in existing numerical formulations. The new methods have the potential to remove or minimize numerical dispersion and grid orientation effects and, in the case of the hybrid streamline method, also eliminate spurious oscillations even in presence of large longitudinal to transverse dispersivity ratios.Therefore, the results presented in this thesis confirm that the Lagrangian formulations of solute transport investigated here are viable and compelling alternatives to simulate reactive transport versus more standard numerical techniques.

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Monitoring contamination in surface water and groundwater in a catchment with an unconfined porous aquifer overlying fractured bedrock (2009)

The factors influencing the design of an integrated surface water and groundwater contaminant monitoring network are examined for a system where a permeable unconfined aquifer overlies fractured bedrock in a headwater stream catchment. The unconfined porous aquifer is modeled as a homogeneous and isotropic porous medium with deterministic properties. The fractured bedrock is modeled as a lower permeability rock mass, but with a stochastic fracture network composed of three orthogonal fracture sets. The surface domain consists of a V-shaped overland flow zone and a linear stream channel with constant width. A continuous contaminant source zone is situated close to the land surface. The analysis is based on a steady state representation of the groundwater and surface water flow system and transport of a non-reactive solute in both the surface and subsurface domains. The probability of plume detection is defined in terms of the likelihood of detecting contamination in a performance monitoring network prior to its detection at a downstream compliance boundary. The ratio of the permeability of the unconfined porous aquifer to the bulk permeability of the underlying fractured bedrock is a key factor influencing detection probabilities. A higher permeability unconfined aquifer reduces detection probabilities in the stream, and increases detection probabilities in the subsurface. Fracture network connectivity and aperture size have a significant influence on detection probabilities in the subsurface, especially in the bedrock. Dilution of contamination in the headwater stream by inflowing non-contaminated water has a strong influence on detection probabilities. Due to the effect of dilution, sampling both stream water and streambed groundwater will enhance detection probabilities in the stream. The contaminant detection threshold is a key factor in influencing the stream water and groundwater monitoring. A lower detection threshold will increase detection probabilities in the surface water and groundwater systems, especially in the stream water. The results in this study demonstrates the importance of integrating stream water and groundwater samples in the design of a performance monitoring network and documents the factors that determine contaminant detection probabilities in the surface stream, the unconfined aquifer, and the underlying fractured bedrock.

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Master's Student Supervision (2010 - 2021)
Hydrocarbons in soil as pathfinders for base and precious metal exploration: an orientation study at the Mt. Washington epithermal Au-Ag-Cu prospect, British Columbia, Canada (2021)

The goal of this research is to advance hydrocarbon-pathfinder techniques as non-proprietary geochemical exploration tools for base- and precious-metal mineralization. Three objectives were set to achieve this goal: 1) identification of the physical, chemical, and biological links between buried economic mineralization and hydrocarbon concentrations and compositions at surface; 2) development of a robust, commercially viable and cost-effective analytical techniques for hydrocarbons applied to non-petroleum-based exploration; and 3) to optimize hydrocarbon sampling methodologies and strategies.To better understand hydrocarbon-sulphide interactions, hydrocarbon sampling methodologies were tested with a geochemical orientation survey over the Mt. Washington epithermal Au-Ag-Cu occurrence on Vancouver Island, British Columbia. A novel analytical technique, solid phase micro-extraction gas chromatography with flame ionization detection (SPME-GC-FID), was developed in conjunction with the commercial laboratory GeoFrontiers Corporation to determine hydrocarbon concentrations in soil. Soil gas samples, ground, and surface waters were also collected and analyzed for hydrocarbons.Surficial geology mapping shows that major and trace element distributions in soil are strongly controlled by the depositional facies of glacial cover. Epithermal pathfinder element concentrations in B-horizon soil and groundwater indicate that hydromorphic dispersion is also a significant factor in this study area. Key hydrocarbon pathfinder concentrations in soil and soil gas are highest proximal to sulphide mineralisation. Anomalous tridecane and hexadecane plume geometries in the surface environment are linked to epithermal mineralization in bedrock and commodity and pathfinder element anomalism in soil. Hydrocarbon anomalies in soil and soil gas at Mt. Washington are interpreted to be of biogenic origin. Hydrocarbons are naturally present in soils as products of biosynthesis by microbes, plants, fungi, yeast, and insects. Sulphide oxidising environments contain distinct microbial ecologies including primary producers such as Acidithiobacillus Ferrooxidans. which contribute to hydrocarbon cycling in soil. Thermogenic hydrocarbons interact with basin-hosted hydrothermal systems and may also contribute to anomalous concentrations of organic compounds associated with sulphide minerals.

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Regional groundwater conditions in northeast BC: results from a monitoring well network in an area of historical and ongoing unconventional natural gas development (2021)

Whether oil and gas development pervasively introduces natural gas into shallow groundwater systems remains a controversial issue. While dissolved thermogenic methane attributed to petroleum development activities has been documented at shallow depths in some locations, various studies have also shown that this methane is naturally occurring, even in regions with heavy development. Here, we assess natural gas in near-surface (28 mg/L) with isotopic signatures of a thermogenic coal bed methane (CBM) source. This finding suggests that high levels of naturally-occurring methane may be of concern in the region where highly transmissive aquifers intercept coal-bearing bedrock. Lastly, as a sub-study, the hydrogeology of a paleovalley system within the study area was conceptualized using data from newly installed MWs and available hydrogeological data for buried-valley aquifer systems in NEBC and the Western Canadian Sedimentary Basin (WCSB). Using this conceptual model, a regional-scale, steady-state, groundwater-flow model was constructed to assess recharge magnitude and mechanisms, and residence times to inform aquifer management. Among model results, the calibrated average aerial recharge rate was 16 mm/a, within the range of recharge estimates previously reported for NEBC. Our findings can support management of groundwater resources in similar hydrogeological settings common to NEBC. Supplementary materials available at:

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Using microbial fuel cells for the remediation of hydrocarbon-contaminated aquifers (2021)

Bio-electrochemical systems (BES) have been proposed as an emerging technology for enhancing groundwater remediation and are an interesting alternative for hydrocarbon-contaminated reducing aquifers where natural attenuation may be slow. BES take advantage of the ability of exoelectrogenic bacteria to transfer electrons from organic substrates to an extracellular electron acceptor, such as the anode of a microbial fuel cell (MFC). An electrical connection between the oxidizing and reducing compartments of the MFC allows reduction of oxygen at the cathode coupled to the oxidation of the reduced contaminant in the reducing compartment, accompanied by electricity production. Electricity production has been proposed as a proxy to monitor the progress of the remediation.The effects of additional electron donors, like ferrous iron, over the contaminant degradation efficiency and electricity production in BES have not been thoroughly studied. This research applied chemical, mineralogical, and microbiological analyses to study the degradation of naphthalene in a series of MFC experiments. The main objective was to test whether a reactor inoculated with native microorganisms from a local contaminated aquifer could successfully remediate naphthalene contamination in a reducing environment where iron was potentially an electron donor. An additional experiment was developed to address naphthalene sorption to electrodes and other reactor materials.The sorption experiment revealed that naphthalene dynamics in the MFC were significantly affected by sorption/desorption to reactor materials, so interpretation of MFC results required the consideration of naphthalene sorption and diffusion processes.The MFC experiments in this study did not find any advantage in providing an electrical connection between reducing and oxidizing zones of the bioreactors in terms of naphthalene degradation achieved in the system. However, the former did show the additional benefit of generating a small current. MFC experiments showed an increased electricity production when iron was available, however, the experiments with no iron achieved higher removal of naphthalene. The results from this study suggest that measuring electricity production is no substitute for direct measurement of contaminant biodegradation, since iron, sulfur, and naphthalene metabolites were involved in electricity production.

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Investigating factors associated with fugitive gas migration in Northeast British Columbia, Canada (2020)

Oil and gas wells are engineered with barriers to prevent fluid movement along the wellbore. If the integrity of one or more of these barriers fails, it may result in subsurface leakage of natural gas outside the well casing, a process termed fugitive gas migration (GM). Knowledge of the occurrence and causes of GM is essential for effective management of the potential risks of GM. In BC, oil and gas producers are required to report well drilling, completion, production, and abandonment records for all oil and gas wells to the provincial regulator. This well data provides a unique opportunity to identify well characteristics associated with higher likelihoods for GM to develop. Here I employ multilevel logistic regression to understand the associations between various well attributes and reported occurrences of GM, found in 0.6% of the 25,000 oil and gas wells in BC. My results indicate that there is no meaningful association between the occurrence of GM and hydraulic fracturing or directional drilling. Overall there appears to be no engineering attribute in the study database that is conclusively associated with GM. The best predictors of GM are indicators of well integrity loss, such as surface casing vent flow, remedial treatments, and blowouts, and geographic location. I ascribe the spatial clustering of GM cases to the local geologic environment, and I speculate that there are links between particular intermediate gas-bearing formations and GM occurrence in the Fort Nelson Plains area. The results of this study suggest that oil and gas wells in high GM occurrence areas and those showing any attribute associated with integrity failure, such as surface casing vent flows, should be prioritized for monitoring to improve the detection of GM.

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Biogeochemical Characterization of the Hyporheic Zone below the Fraser River near Vancouver, British Columbia (2016)

The aim of this thesis is to characterize the biogeochemical conditions in the hyporheic zone of the Fraser River in order to help assess the fate of creosote-derived contaminants in groundwater. By investigating hyporheic biogeochemistry, a framework is established for further studies which could quantitatively assess the potential for monitored natural attenuation as a remedial option for groundwater contamination. At our site on the North Arm of the Fraser, groundwater flows horizontally towards the river at a velocity of 0.1 to 0.2 m/day. Transverse mixing between fresh groundwater and underlying saline groundwater is believed to be responsible for cation exchange reactions. From intertidal monitoring wells out to the hyporheic zone (HZ), Fe and Mn concentrations are found to increase despite dilution, and this is believed to be a result of either cation exchange, or oxidation of organic matter (OM), H₂S, or CH₄. As groundwater reaches the river, it turns upward and discharges into the HZ. In the river channel, fresh groundwater discharges to the HZ at distances between 70 and 85 m from the shoreline’s high-water mark (HWM), while the deeper, saline groundwater discharges at distances greater than 100 m. A custom-made cryogenic probe was designed to collect representative, high-quality sediment samples from the HZ. A vertical characterization of biogeochemistry is presented. Chloride is used as a conservative tracer to determine a dilution factor of 170 in the top 85 cm of sediment, but dilution is believed to persist beyond this depth. DNA sequencing results infer a large variety of geochemical processes. Since multiple known aerobic microorganisms are found to have highest proportional abundances in the top 30 cm of the HZ, this depth range is considered to represent the portion of the HZ which is continually exposed to significant amounts of oxygen from the river. Porewater geochemistry also supports this delineation, as Fe(II) is non-detectable above the depth of 30 cm. The estimation of residence time in the HZ, the delineation of the aerobic portion of the HZ, and the characterization of dilution rates provide valuable information on the biogeochemical and physical components of natural attenuation in the HZ.

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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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Iron and Manganese Reduction Driven by Organic Matter and Mixing of Fresh and Saline Groundwater in the Fraser River Delta Aquifer, Vancouver, Canada (2015)

We present results of field investigations of the biogeochemistry of an aquifer a few km from the ocean adjacent to the Fraser River in Vancouver, Canada. At the site, a wedge of relatively dense saline ocean water enters the aquifer in the hyporheic zone at the river bottom, migrates away from the river along the base of the aquifer to a maximum distance of approximately 500m inland, where it overturns and mixes with fresh groundwater. The mixed saline - fresh water then flows back under a regional freshwater gradient and eventually discharges to the river at the top of the saline wedge. Pore waters show iron concentrations peak at over 300 mg/L (5.4 mM) and manganese at 7 mg/L (0.13 mM) at the upper mixing zone - the interface between terrestrial recharge and top of the overturned saline groundwater. The reducible concentrations on the sediment are approximately 784-2,576 mg/kg (14-46 mM/kg) iron and 110-330 mg/kg (2-6mM/kg) manganese. The dominant process is the reductive dissolution of iron and manganese oxide minerals via organic matter oxidation, although acid-volatile sulfide and methane measurements show that both sulfate reduction and methanogenesis are also occurring. Dissolved organic matter (DOM) concentrations ranged between 5 and 30 mg/L. Excitation – emission fluorescence spectroscopy is used to help identify the distinct sources of DOM, which include terrestrial from fresh recharge, detrital from sediments and from inflowing ocean water. One-dimensional kinetic reactive-transport modeling that includes primary mineral redox reactions and secondary mineral precipitation was used to: i) interpret the role of mixing of fresh and saline water, ii) to constrain reduction-rate parameters and metabolic activity levels from field data, including the oxidation rate of organic matter by iron and manganese oxides, probably accompanied with sulfate reduction and methanogenesis; iii) to understand how other secondary minerals further control aqueous ferrous iron and manganese concentrations through mineral precipitation/dissolution processes; v) to gain insight into the long-term evolution of the geochemistry at the site.

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A field cell and humidity cell study of metal attenuation in neutral rock drainage from the Antamina mine, Peru (2012)

This work focuses on the attenuation of Mo and Zn in neutral pH drainage from waste rock at the Antamina mine in Peru. The study was designed to test the hypothesis that Mo or Zn containing leachate from one waste rock type can be attenuated when allowed to contact a different waste rock type. Mixed material stacked field cells and humidity cells connected in series, where leachate from a Mo or Zn - releasing waste rock type flowed through a second waste rock material type, were used to test this hypothesis. Both of these were new methods, which had not before been reported in the peer reviewed scientific literature related to the study of waste rock geochemistry. Results from both the humidity cells (laboratory conditions) and field cells (field conditions) showed the same general attenuation patterns. When drainage from Mo-releasing waste rock flowed through Pb-rich black marble waste rock, Mo was removed from solution. Mo attenuation was not observed when the order of the waste rock materials was reversed such that drainage from Pb-rich material flowed through Mo-releasing intrusive rock. As, also released from the same Mo-releasing intrusive rock, showed the same attenuation pattern as Mo. Geochemical modeling suggested that wulfenite precipitation was responsible for the observed attenuation of Mo. Zn was removed from leachate both by contact with Mo-releasing intrusive rock and by contact with calcite-rich grey hornfels material. Like Zn, Cd was removed from solution by contact with calcite-rich grey hornfels. Scanning Electron Microscopy (SEM) suggested that Zn may have been incorporated into the crystal structure of phyllosillicate clay minerals; however, further work is needed to confirm this mechanism. Insufficient data were available to develop a hypothesis as to the specific attenuation mechanisms responsible for removing Cd and As from solution.In addition to shedding light on the geochemical processes controlling Mo and Zn in neutral mine drainage, this research also demonstrated the effectiveness of stacked field cells and humidity cells connected in series for the study of metal attenuation by waste rock mixing.  

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Hydrogeochemical Site Characterization and Groundwater Flow Modeling of the Arsenic-Contaminated Gotra Aquifer, West Bengal, India (2011)

Groundwater geochemistry and flow have been studied in Gotra, West Bengal, India, where geogenic arsenic contaminates groundwater at levels above World Health Organization limits. The village is situated upon the natural levee of an abandoned channel, which terminates a fluvio-deltaic depositional sequence. The formerly prograding meander bend deposited point-bar sands during the Holocene that now comprise the 30m-thick shallow aquifer, while incising deeper Pleistocene sands and a shallow floodplain sequence. Hand-pumped tubewells are completed in point-bar sands, whereas irrigation wells are generally screened within Pleistocene materials. Hydraulic conductivity of the point-bar aquifer is approximately 5x10⁻⁴ m/s. A leaky-confining layer of levee/crevasse splay deposits overlies this aquifer beneath crop fields. Near the village, the shallow aquifer is confined by channel-fill silts that have an estimated conductivity of 1x10⁻⁷ m/s. The paleohorizon separating Holocene and Pleistocene sediments is marked locally by fine-grained material comprised of organic material and a hard clay. Groundwater elevations vary 7m over the course of a given year due to monsoonal climate and groundwater extraction. Convergent flow towards shallow pumping wells is a salient feature during irrigation season, and persistent localized downward gradients suggest that meteoric recharge and pumping are the predominant groundwater sources and sinks. A northeast trending geochemical gradient in the point-bar aquifer suggests that local flows transporting arsenic are directed away from the channel-fill silt. Concentration gradients of terminal electron acceptors and redox reaction products in the shallow aqueous profile also coincide with a flowpath originating from this unit. A numerical model of groundwater flow was developed based on a conceptual model derived from field observations to investigate controls of arsenic transport and to provide a context in which to interpret geochemical data. Short- (7-day) and long- (3-year) term transient simulations were implemented to simulate groundwater flowpaths, water balance, and average residence times. Modeling results support field observations that contamination is related to the channel-fill unit deposit, and suggest that the water balance has been significantly altered compared to pre-irrigation conditions. Results also suggest contaminant flushing will not occur over a human timescale.

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Microbiology and geochemistry of neutral pH waste rock from the Antamina mine, Peru (2010)

The microbial populations of waste rock piles and field cells producing neutral pH drainage at the Antamina mine were characterized to better understand processes which affect current and future drainage quality. Naturally weathered waste rock samples were collected and examined using a variety of high resolution imaging, geochemical, mineralogical, and microbiological techniques. Despite the relatively young age of the waste rock piles (1.5 years), populations dominated by neutrophilic sulfur oxidizers as large as 10⁸ bacteria per gram were found. An exponential relationship was found between the size of microbial communities and the contemporary sulfate loadings. These results indicate that the microbial populations rapidly grow to reach a mass which is proportional to the rate of substrate release, and then decrease as the host rocks reactivity diminishes. One sample from a field cell producing pH 6.2 drainage had a mixed population of neutrophiles and acidophiles capable of both S⁰ and Fe²⁺ oxidation. A mini-column study was conducted to determine the catalytic effect of microbiology in various rock types. No catalysis was identified in the sulfur concentrations of most mini-column series, with the exception of the one mini-column series constructed of material containing acidophilic S⁰ and Fe²⁺ oxidizing bacteria, which demonstrated strong microbial catalysis. It was also determined that concentrations of Mo as low as 10mg/l were toxic to these acidophilic bacteria, and dissolved Mo may inhibit the establishment of these geochemically important bacteria. A massive sulfide from this material was thoroughly examined using high resolution imaging techniques. Biofilms of bacteria were found upon and within a porous schwertmannite precipitate. The mixed population of neutrophilic and acidophilic bacteria and circumneutral drainage pH implies that the bacteria are living in acidic microenvironments surrounding sulfide minerals in which ferric iron leaching can take place. The large microbial populations and the close correlation between geochemistry and biology described in this study, emphasize the importance of biological processes in determining current and future drainage quality emanating from the mine waste.

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