Hydrogeochemical models for the prediction of drainage quality from full-scale mine waste-rock piles are often parameterized using data from small-scale laboratory or field experiments of short ...duration. Yet, many model parameters and processes (e.g., sulfide-oxidation rates) vary strongly with the spatiotemporal dimensions of the experiment: the “upscaling” of prediction models remains a critical challenge for mine-waste management worldwide. Here, we investigate scale dependence in laboratory and field experiments that spanned orders-of-magnitude in size (i.e. 2 kg to 100,000 kg) at the Antamina mine in Peru. Normalized drainage mass loading rates systematically decreased with increasing scale, irrespective of waste-rock type. A process-based reactive-transport model was used to simulate observed rates and reproduce the geochemical composition of drainage across scales. Long-term trends in drainage quality could be quantitatively reproduced when the model was parameterized with mostly scale- and experiment-specific measured bulk properties or literature values, leaving geochemical rate coefficients the sole calibrated model parameters. Analysis of these fitted parameters revealed that the scale dependence of geochemical rates was largely explained by reactive mineral surface area. This work demonstrates that practical drainage quality predictions for full-scale waste-rock piles can be established from readily available bulk parameters determined at multiple scales.
•More than 15 years of multi-scale experimental hydrogeochemical field data is synthesized.•The scale dependence of weathering rates is investigated using numerical modeling.•The contributions of mineralogy, surface area, temperature on scale dependence are quantitatively resolved.•The predictive capability of a field-parameterized reactive-transport model is demonstrated.
Soil near waste rock often contains high concentrations of antimony (Sb), but the mechanisms that mobilize Sb in a soil closely impacted by the waste rock piles are not well understood. We ...investigated these mobility mechanisms in soils near historical waste rock at the world's largest Sb mine. The sequential extraction (BCR) of soil reveal that over 95 % Sb is present in the residual fraction. The leached Sb concentration is related to the surface protonation and deprotonation of soil minerals. SEM-EDS shows Sb in the soil is associated with Fe and Ca. Moreover, X-ray absorption spectroscopy (XAS) results show Sb is predominantly present as Sb(V) and is associated with Fe in the form of tripuhyite (FeSbO4) as well as edge- and corner-sharing complexes on ferrihydrite and goethite. Thus, Fe in soils is important in controlling the mobility of Sb via surface complexation and co-precipitation of Sb by Fe oxides. The initially surface-adsorbed Sb(V) or co-precipitation is likely to undergo a phase transformation as the Fe oxides age. In addition, Sb mobility may be controlled by small amounts of calcium antimonate. These results further the understanding of the effect of secondary minerals in soils on the fate of Sb from waste rock weathering and inform source treatment for Sb-contaminated soils.
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•Sequential extractions show the residual fraction contains the largest amount of Sb.•The leached Sb concentration is related to the surface protonation and deprotonation of soil minerals.•SEM-EDS results Sb in the soil is associated with Fe and Ca, which attenuate Sb mobility.•Sb is associated with Fe through tripuhyite, as well as through the inner-sphere complexes of ferrihydrite and goethite.•The initially surface-adsorbed Sb(V) is likely to undergo a transformation to incorporated Sb(V) as the Fe oxides age.
The weathering of mine waste rock can cause release of metal-laden and acidic drainage that requires long-term and costly environmental management. To identify and quantify the geochemical processes ...and physical transport mechanisms controlling drainage quality, we monitored the weathering of five large-scale (20,000 t) instrumented waste-rock piles of variable and mixed-composition at the Antamina mine, Peru, in a decade-long monitoring program. Fine-grained, sulfidic waste rock with low-carbonate content exhibited high sulfide oxidation rates (>1 g S kg−1 waste rock yr−1) and within 7 years produced acidic (pH < 3) drainage with high Cu and Zn concentrations in the g L−1 range. In contrast, drainage from coarse, carbonate-rich waste rock remained neutral for >10 years and had significantly lower metal loads. Efficient metal retention (>99%) caused by sorption and secondary mineral formation of e.g., gypsum, Fe-(oxy)hydroxides, and Cu/Zn-hydroxysulfates enforced strong (temporary) controls on drainage quality. Furthermore, reactive waste-rock fractions, as small as 10% of total mass, dominated the overall drainage chemistry from the waste-rock piles through internal mixing. This study demonstrates that a reliable prediction of the timing and quality of waste-rock drainage on practice-relevant spatiotemporal scales requires a quantitative understanding of the prevailing in-situ porewater conditions, secondary mineralogy, and spatial distribution of reactive waste-rock fractions in composite piles.
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•Decade-long monitoring of waste-rock weathering under natural conditions.•Primary weathering rates vary with waste-rock composition and particle size.•Metal sorption and secondary mineral formation affect drainage chemistry.•Reactive waste rock dominates the overall drainage quality from composite piles.•Demonstration of partial decoupling of primary mineralogy and drainage quality.
Subsurface natural gas release from leaking oil and gas wells is a major environmental concern. Gas migration can cause aquifer contamination, explosive conditions in soil gas, and greenhouse gas ...emissions. Gas migration is controlled by complex interacting processes, thus constraining the distribution and magnitude of "fugitive gas" emissions remains a challenge. We simulated wellbore leakage in the vadose zone through a controlled release experiment and demonstrate that fugitive gas emissions can be directly influenced by barometric pressure changes. Decreases in barometric-pressure led to surface gas breakthroughs (>20-fold increase in <24 hours), even in the presence of low-permeability surficial soils. Current monitoring strategies do not consider the effect of barometric pressure changes on gas migration and may not provide adequate estimates of fugitive gas emissions. Frequent or continuous monitoring is needed to accurately detect and quantify fugitive gas emissions at oil and gas sites with a deep water table.
Bacteria can adversely affect the quality of drainage released from mine waste by catalyzing the oxidation of sulfide minerals and thereby accelerating the release of acidity and metals. However, the ...microbiological and geochemical controls on drainage quality from unsaturated and geochemically heterogeneous waste rock remain poorly understood. Here, we identified coexisting neutrophilic and acidophilic bacteria in different types of waste rock, indicating that robust endemic consortia are sustained within pore-scale microenvironments. Subsequently, natural weathering was simulated in laboratory column experiments with waste rock that contained either in-situ microbial consortia or suppressed populations with up to 1000 times smaller abundance and reduced phenotypic diversity after heating and drying. Drainage from waste rock with in-situ populations was up to two pH units lower and contained up to 16 times more sulfate and heavy metals compared to drainage from waste rock bearing treated populations, indicating significantly higher sulfide-oxidation rates. The drainage chemistry was further affected by sorption and formation of secondary-mineral phases (e.g., gypsum and hydroxy-carbonates). This study provides direct evidence for the existence of diverse microbial communities in waste rock and their important catalytic role on weathering rates, and illustrates the mutual controls of microbiology and geochemistry on waste-rock drainage quality.
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•Integrated approach to study microbial and geochemical controls on drainage quality.•Characterization of microbial abundance and community structures in waste rock.•Waste-rock weathering under simulated field conditions using column experiments•Demonstration of microbial catalysis of waste-rock weathering•Discrete secondary minerals and adsorption affect waste-rock drainage quality.
Fugitive gas migration (GM) from compromised oil and gas wells remains a global concern. To understand environmental impacts from GM there is a need to characterize the transport and fate of fugitive ...gas in the vadose zone. We simulated subsurface wellbore leakage by injecting natural gas into thick unsaturated glacio‐lacustrine deposits in a region of petroleum development in Western Canada. Methane and carbon dioxide effluxes were monitored and soil‐gas samples were collected for molecular and stable carbon isotope analyses. A conceptual model was developed to demonstrate the physical and biogeochemical processes that control the spatial‐temporal variability of GM. Methane oxidation partially attenuated natural gas; however, gas transport and fate were strongly influenced by variations in grain‐size distribution and barometric pressure, resulting in episodic effluxes and lateral gas transport. To accurately detect, quantify and assess GM at oil and gas sites, adequate site characterization and continuous, spatially dense monitoring are necessary.
Plain Language Summary
Natural gas leaking from imperfectly sealed oil and gas wells can lead to explosive conditions in soil gas and methane emissions to the atmosphere. To understand the impacts and improve monitoring practices of fugitive gas migration, there is a need to characterize processes that control gas transport and fate in the unsaturated zone. We simulated subsurface wellbore leakage by injecting natural gas into thick unsaturated deposits in a region of petroleum development in Western Canada. The response was monitored by measuring methane and carbon dioxide concentrations and emissions on the ground surface and in soil gas in the unsaturated zone. Our results show that numerous compounding processes influence methane concentrations in the unsaturated zone and emissions to the atmosphere. While microbially mediated reactions consume methane in the unsaturated zone, variations in soil grain sizes and changes in barometric pressure strongly influence gas transport, which can lead to high episodic emissions in unpredictable locations. To accurately detect, quantify and assess fugitive gas migration at oil and gas well sites, adequate site characterization and continuous, spatially dense monitoring are necessary.
Key Points
Fugitive gas migrated to the ground surface despite a deep vadose zone with low‐permeability silts and clays
A preferential pathway and intervals of decreased barometric pressure resulted in a hotspot with high episodic methane effluxes
Low‐permeability sediments and intervals of increased barometric pressure favored lateral gas migration and progressive methane oxidation
In mining environmental applications, it is important to assess water quality from waste rock piles (WRPs) and estimate the likelihood of acid rock drainage (ARD) over time. The mineralogical ...heterogeneity of WRPs is a source of uncertainty in this assessment, undermining the reliability of traditional bulk indicators used in the industry. We focused in this work on the bulk neutralizing potential ratio (NPR), which is defined as the ratio of the content of non-acid-generating minerals (typically reactive carbonates such as calcite) to the content of potentially acid-generating minerals (typically sulfides such as pyrite). We used a streamtube-based Monte-Carlo method to show why and to what extent bulk NPR can be a poor indicator of ARD occurrence. We simulated ensembles of WRPs identical in their geometry and bulk NPR, which only differed in their initial distribution of the acid generating and acid neutralizing minerals that control NPR. All models simulated the same principal acid-producing, acid-neutralizing and secondary mineral forming processes. We show that small differences in the distribution of local NPR values or the number of flow paths that generate acidity strongly influence drainage pH. The results indicate that the likelihood of ARD (epitomized by the probability of occurrence of pH<4 in a mixing boundary) within the first 100years can be as high as 75% for a NPR=2 and 40% for NPR=4. The latter is traditionally considered as a “universally safe” threshold to ensure non-acidic waters in practical applications. Our results suggest that new methods that explicitly account for mineralogical heterogeneity must be sought when computing effective (upscaled) NPR values at the scale of the piles.
•Predicting the risk of water acidification from mineralogically heterogeneous waste rock piles is uncertain.•Monte-Carlo analysis to test several WRPs scenarios via multicomponent reactive transport modeling•Guideline-recommended indicators (e.g. NPR=4) may not ensure effective pH buffering over time.
Molybdenum contamination is a concern in mining regions worldwide. Better understanding of processes controlling Mo mobility in mine wastes is critical for assessing potential impacts and developing ...water-quality management strategies associated with this element. Here, we used Mo stable isotope (δ98/95Mo) analyses to investigate geochemical controls on Mo mobility within a tailings management facility (TMF) featuring oxic and anoxic environments. These isotopic analyses were integrated with X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and aqueous chemical data. Dissolved Mo concentrations were inversely correlated with δ98/95Mo values such that enrichment of heavy Mo isotopes in solution reflected attenuation processes. Inner-sphere complexation of Mo(VI) with ferrihydrite was the primary driver of Mo removal and was accompanied by a ca. 1‰ isotope fractionation. Limited Mo attenuation and isotope fractionation were observed in Fe(II)- and Mo-rich anoxic TMF seepage, while attenuation and isotope fractionation were greatest during discharge and oxidation of this seepage after discharge into a pond where Fe-(oxyhydr)oxide precipitation promoted Mo sorption. Overall, this study highlights the role of sorption onto Fe-(oxyhydr)oxides in attenuating Mo in oxic environments, a process which can be traced by Mo isotope analyses.
Ancient river channels or subglacial drainage networks infilled with younger sediments can include significant deposits of highly permeable sands and gravels. Despite being hidden at surface, such ...systems are ubiquitous globally, can form highly productive groundwater reservoirs and have significant influence on regional hydrogeology, contaminant transport and local water resources. Consequently, the hydraulic characteristics of such buried-valley or “paleovalley” aquifers have been the subject of increasing study. In this study, the hydrogeology of the Sunset Paleovalley in Northeast British Columbia (NEBC, Canada) was conceptualized using data from newly installed, scientifically designed monitoring wells and available hydrogeological data for buried-valley aquifer systems in NEBC and the Western Canadian Sedimentary Basin. Using this conceptual model, a regional-scale, steady-state, groundwater-flow model was constructed to assess recharge magnitude and mechanisms, fluxes and residence times to inform aquifer management. The calibrated average aerial recharge rate was 16 mm/year, within the range of recharge estimates previously reported for NEBC (0.5–78 mm/year). The average residence times for buried valley sand/gravel and weathered bedrock aquifers were estimated at 3,200 and 2,900 years, respectively, and are indicative of a slowly flushed system, consistent with the 1,300 mg/L average total dissolved solids groundwater chemistry. The current groundwater extraction rates are a small fraction of the simulated groundwater discharge to the Kiskatinaw River. The findings can support management of groundwater resources in similar hydrogeological settings common to NEBC.
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