Comparison of unsaturated flow and solute transport through waste rock at two experimental scales using temporal moments and numerical modeling

Complex electrical measurements of waste rock during acid mine drainage generation and release: Kinetic column tests

Acid mine drainage (AMD) emanating from waste rock piles (WRPs) at mining sites is a global concern. Successful rehabilitation of these sites requires effective characterization and monitoring of the waste rock during AMD generation/release. Traditional approaches involve ex-situ analysis of waste rock and porewater samples collected via corings and monitoring wells; however, this is highly disruptive, costly, and provides sparsely distributed point information across enormous volumes typical of WRPs. Geoelectrical techniques are a promising approach for non-invasive continuous imaging; however, their application has been limited to 'one-off' imaging with few studies on monitoring waste rock evolution. The objective of this study is to assess the geoelectrical signatures of changing waste rock during AMD generation/release. Field waste rock samples were extracted from three mine WRPs and first characterized for mineralogy and acid generation potential. Kinetic tests were then performed on each sample using leaching columns and humidity cells, with simultaneous measurements of effluent quality and complex electrical conductivity (real and imaginary components measure conduction and polarization, respectively). Results show that real conductivity was highly sensitive to changes associated with AMD leachate quality (e.g., 28,800 to 68 mg/L acidity) and surface of the waste material. Imaginary conductivity measurements identified changes in the waste mineralogy over time, though these signatures were not very distinct, which is likely due to low sulfide contents and limited oxidation (e.g., 0.59 wt% sulfide and 33% air saturation). This study improves our understanding of geoelectrical signatures associated with real waste rock, demonstrating the potential application of the electrical resistivity tomography and induced polarization techniques for mine waste investigations.

Mechanistic insights into contaminant transport dynamics in the saturated porous system in the presence of low permeability region using numerical simulations and temporal moment analysis

The influence of low permeability porous media (LPPM) on contaminant transport dynamics in saturated porous systems was investigated using numerical simulations and temporal moments of contaminant concentrations. Two-dimensional flow and contaminant transport simulations were conducted, considering various parameters such as longitudinal dispersivity (ranging from 15 to 60 m), the ratio of transverse to longitudinal dispersivity (ranging from 0.05 to 0.2), retardation factor (ranging from 1 to 4), and hydraulic gradient (ranging from 0.005 to 0.02) for both homogeneous and heterogeneous porous systems. The findings revealed significant differences in the transport behavior of conservative and highly reactive contaminants between the porous systems without and with the LPPM region. The center of mass of contaminant plume and peak concentration zone were observed inside the LPPM region for the heterogeneous porous system, especially during the source off-loading period. Furthermore, asymmetric distributions of the zeroth temporal moment (ZTM), mean residence time (MRT), and variance of the breakthrough curve (BTC) were observed along the longitudinal distance within the LPPM region for heterogeneous porous system, highlighting the impact of heterogeneity on contaminant plume evolution dynamics. The moment analysis results provided insights into the influence of LPPM region on time-averaged contaminant transport dynamics in adjacent porous systems. These findings can help risk managers understand the complex fate and transport dynamics in heterogeneous porous systems. Future studies could explore the modelling of multispecies contaminants in heterogeneous saturated porous systems subjected to fluctuating water table.

Diavik Waste Rock Project: Simulation of the geochemical evolution of a large test pile using a scaled temperature and sulfide-content dependent reactive transport model

The Diavik Waste Rock Project (DWRP) project included four principal components focused on the development of techniques for assessing the environmental impacts of waste rock at mine sites. These components were small-volume laboratory experiments, intermediate- and large-volume field experiments, and assessment of the operational-scale waste-rock stockpiles, which facilitated characterization of waste-rock weathering at different scales. The heavily instrumented large-scale field experiments (test piles) were constructed to replicate, as closely as practicable, the temperature, water flow, and gas transport regimes of a waste-rock pile that is exposed to annual freezing and thawing cycles and to facilitate characterization of the long-term weathering of a low-sulfide waste rock. An integrated conceptual model of sulfide-bearing waste-rock weathering, developed at the small scale, was applied to assess the capacity of the conceptual model to capture the geochemical evolution of the waste rock at the large field-scale test-pile experiment. The integrated conceptual model was implemented using reactive transport code MIN3P, taking into account scale-dependent mechanisms. The test-pile mineralogy was similar to the small-scale laboratory experiments and included low-sulfide waste rock with an S content of 0.053 wt% (primarily pyrrhotite). The flow regime of the test pile was simulated using parameters measured as part of other DWRP investigations, including temporally variable infiltration estimates that represented the measured precipitation events at the site. The temporally and spatially variable temperature of the test pile was interpolated from values measured using instrumentation installed at the beginning of the experiment and was included in the simulation to refine the temperature dependence of the geochemical reactions. To allow continuous, multi-year simulation, freezing was also simulated to represent the conditions experienced at the test-pile experiment. Normalized root mean square error analysis of the large-scale field experiment simulation results indicated most parameters compare well to measured daily mass flux (i.e., the fraction of the range of annual values encompassed in the residual was less than 0.5 for SO₄, Fe, Ni, Si, Ca, K, Mg, Na, and pH and 1.0 or less for all parameters except Cu). The method of using an integrated conceptual model developed from the results of humidity cell experiments to implement a mechanistic approach for assessing the primary geochemical processes of waste-rock weathering on a large scale was shown to provide reasonable results; however, the results are specific to the study site and the approach requires application to various sites under different geological and climatological conditions to facilitate further refinement.

Gradual Acquisition of Professional Knowledge, Audit Quality and Audit Fees

The acquisition of professional knowledge is a core issue in the formation of auditor industry expertise; however, previous literature has neglected the time required for auditors to acquire professional knowledge. We examine the audit quality and fees of audit firms in different stages of an auditor acquiring professional knowledge and find that, in the initial stage of the process of knowledge acquisition, audit quality and audit fees decrease. However, in the long run, knowledge learning has a more obvious effect on the improvement of audit quality and audit fees. Specifically, knowledge learning has a positive effect on the development of audit firms.

Understanding the Effect of Stepwise Irrigation on Liquid Holdup and Hysteresis Behavior of Unsaturated Ore Heap

Liquid is a crucial medium to contain soluble oxygen, valuable metal ions, and bacteria in unsaturated heap leaching. Liquid retention behavior is the first critical issue to be considered to efficiently extract low-grade minerals or wastes. In this study, the residual liquid holdup of an unsaturated packed bed was quantitatively discussed by liquid holdup (θ), residual liquid holdup (θresidual), relative liquid holdup (θ′), and relative porosity (n*) using the designed measuring device. The detailed liquid holdup and the hysteresis behavior under stepwise irrigation are indicated and discussed herein. The results show that relative porosity of the packed bed was negatively related to particle size, and intra-particle porosity was more developed in the −4.0 + 2.0 mm packed bed. The higher liquid retention of the unsaturated packed bed could be obtained by using stepwise irrigation (incrementally improved from 0.001 to 0.1 mm/s) instead of uniform irrigation (0.1 mm/s). It could be explained in that some of the immobile liquid could not flow out of the unsaturated packed bed, and this historical irrigation could have accelerated formation of flow paths. The θ was sensitive to superficial flow rate (or irrigation rate) in that it obviously increased if a higher superficial flow rate (u) was introduced, however, the θresidual was commonly affected by n* and θ′. Moreover, the liquid hysteresis easily performed under stepwise irrigation condition, where θ and θresidual were larger at u of the decreasing flow rate stage (DFRS) instead of u of the increasing flow rate stage (IFRS). These findings effectively quantify the liquid retention and the hysteresis behavior of ore heap, and the stepwise irrigation provides potential possibility to adjust liquid retention conditions.

Reactive transport modelling to investigate multi-scale waste rock weathering processes

Prediction of drainage quantity and quality is critical to reduce the environmental risks associated with weathering mine waste rock. Reactive transport models can be effective tools to understand and disentangle the processes underlying waste-rock weathering and drainage, but their validity and applicability can be impaired by poor parametrization and the non-uniqueness conundrum. Here, a process-based multicomponent reactive transport model is presented to interpret and quantify the processes affecting drainage quantity and quality from 15 waste- rock experiments from the Antamina mine, Peru. The deployed uniform flow formulation and consistent set of geochemical rate equations could be calibrated almost exclusively with measured bulk waste-rock properties in experiments ranging from 2 kg to 6500 tons in size. The quantitative agreement between simulated dynamics and the observed drainage records, for systems with a variety of rock lithologies and over a wide range of pH, supports the proposed selection of processes. The controls of important physicochemical processes and feedbacks such as secondary mineral precipitation, surface passivation, oxygen limitations, were confirmed through sensitivity analyses. Our work shows that reactive transport models with a consistent formulation and evidence-based parametrization can be used to explain waste-rock drainage dynamics across laboratory to field scales.

Scale dependence of effective geochemical rates in weathering mine waste rock

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.

Mine Waste Rock: Insights for Sustainable Hydrogeochemical Management

Mismanagement of mine waste rock can mobilize acidity, metal (loid)s, and other contaminants, and thereby negatively affect downstream environments. Hence, strategic long-term planning is required to prevent and mitigate deleterious environmental impacts. Technical frameworks to support waste-rock management have existed for decades and typically combine static and kinetic testing, field-scale experiments, and sometimes reactive-transport models. Yet, the design and implementation of robust long-term solutions remains challenging to date, due to site-specificity in the generated waste rock and local weathering conditions, physicochemical heterogeneity in large-scale systems, and the intricate coupling between chemical kinetics and mass- and heat-transfer processes. This work reviews recent advances in our understanding of the hydrogeochemical behavior of mine waste rock, including improved laboratory testing procedures, innovative analytical techniques, multi-scale field investigations, and reactive-transport modeling. Remaining knowledge-gaps pertaining to the processes involved in mine waste weathering and their parameterization are identified. Practical and sustainable waste-rock management decisions can to a large extent be informed by evidence-based simplification of complex waste-rock systems and through targeted quantification of a limited number of physicochemical parameters. Future research on the key (bio)geochemical processes and transport dynamics in waste-rock piles is essential to further optimize management and minimize potential negative environmental impacts.

Long-term monitoring of waste-rock weathering at the Antamina mine, Peru

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.

Microbial and geochemical controls on waste rock weathering and drainage quality

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.

Evaluation of single- and dual-porosity models for reproducing the release of external and internal tracers from heterogeneous waste-rock piles

Accurate predictions of solute release from waste-rock piles (WRPs) are paramount for decision making in mining-related environmental processes. Tracers provide information that can be used to estimate effective transport parameters and understand mechanisms controlling the hydraulic and geochemical behavior of WRPs. It is shown that internal tracers (i.e. initially present) together with external (i.e. applied) tracers provide complementary and quantitative information to identify transport mechanisms. The analysis focuses on two experimental WRPs, Piles 4 and Pile 5 at the Antamina Mine site (Peru), where both an internal chloride tracer and externally applied bromide tracer were monitored in discharge over three years. The results suggest that external tracers provide insight into transport associated with relatively fast flow regions that are activated during higher-rate recharge events. In contrast, internal tracers provide insight into mechanisms controlling solutes release from lower-permeability zones within the piles. Rate-limited diffusive processes, which can be mimicked by nonlocal mass-transfer models, affect both internal and external tracers. The sensitivity of the mass-transfer parameters to heterogeneity is higher for external tracers than for internal tracers, as indicated by the different mean residence times characterizing the flow paths associated with each tracer. The joint use of internal and external tracers provides a more comprehensive understanding of the transport mechanisms in WRPs. In particular, the tracer tests support the notion that a multi-porosity conceptualization of WRPs is more adequate for capturing key mechanisms than a dual-porosity conceptualization.

Stochastic multicomponent reactive transport analysis of low quality drainage release from waste rock piles: Controls of the spatial distribution of acid generating and neutralizing minerals

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.

Molybdenum and zinc stable isotope variation in mining waste rock drainage and waste rock at the Antamina mine, Peru

The stable isotope composition of molybdenum (Mo) and zinc (Zn) in mine wastes at the Antamina Copper-Zn-Mo mine, Peru, was characterized to investigate whether isotopic variation of these elements indicated metal attenuation processes in mine drainage. Waste rock and ore minerals were analyzed to identify the isotopic composition of Mo and Zn sources, namely molybdenites (MoS2) and sphalerites (ZnS). Molybdenum and Zn stable isotope ratios are reported relative to the NIST-SRM-3134 and PCIGR-1 Zn standards, respectively. δ(98)Mo among molybdenites ranged from -0.6 to +0.6‰ (n=9) while sphalerites showed no δ(66)Zn variations (0.11±0.01‰, 2 SD, n=5). Mine drainage samples from field waste rock weathering experiments were also analyzed to examine the extent of isotopic variability in the dissolved phase. Variations spanned 2.2‰ in δ(98)Mo (-0.1 to +2.1‰) and 0.7‰ in δ(66)Zn (-0.4 to +0.3‰) in mine drainage over a wide pH range (pH2.2-8.6). Lighter δ(66)Zn signatures were observed in alkaline pH conditions, which was consistent with Zn adsorption and/or hydrozincite (Zn5(OH)6(CO3)2) formation. However, in acidic mine drainage Zn isotopic compositions reflected the value of sphalerites. In addition, molybdenum isotope compositions in mine drainage were shifted towards heavier values (0.89±1.25‰, 2 SD, n=16), with some overlap, in comparison to molybdenites and waste rock (0.13±0.82‰, 2 SD, n=9). The cause of heavy Mo isotopic signatures in mine drainage was more difficult to resolve due to isotopic heterogeneity among ore minerals and a variety of possible overlapping processes including dissolution, adsorption and secondary mineral precipitation. This study shows that variation in metal isotope ratios are promising indicators of metal attenuation. Future characterization of isotopic fractionation associated to key environmental reactions will improve the power of Mo and Zn isotope ratios to track the fate of these elements in mine drainage.