Waste Rock Piles Research Articles

Secondary arsenic minerals and arsenic mobility in a historical waste rock pile at Kaňk near Kutná Hora, Czech Republic

The arsenic mineralization in historical waste rock pile at Kaňk site near Kutna Hora developed over a period of about 500 years. The objective of this study was to determine principal secondary arsenic mineral phases and their environmental stability. The only common primary As-bearing mineral – arsenopyrite - occurs in the mineral assemblage of Kutna Hora base-metal deposit together with quartz, pyrite, sphalerite, and pyrrhotite. Most of arsenic is bound in supergene minerals (scorodite, jarosite-beudantite, bukovskýite, pitticite), which are relatively stable under oxidizing conditions prevailing in the pile. The Kaňk site is a type locality for bukovskýite, kaňkite, zýkaite, and parascorodite. In long-term perspective, the most stable minerals from viewpoint of As-binding appear to be scorodite and beudantite. A higher mobility was observed for As incorporated into jarosite and poorly crystalline to amorphous phases (FeIII -oxyhydroxides, pitticite). This study has not confirmed significant mobility of arsenic within the pile and water infiltrating in recharge periods of the year (late winter-early spring) should not mobilize arsenic at a significant rate. However, monitoring of the stability of secondary As-phases and dissolved arsenic in the environment around the pile is required to avoid future migration of arsenic out of the pile.

Enrichment of uranium, arsenic, molybdenum, and selenium in sediment cores from boreal lakes adjacent to northern Saskatchewan uranium mines

Assessment of flux rates and concentrations of potentially toxic elements in sediment cores from 5 lakes adjacent to the McClean Lake uranium mines in northern Saskatchewan (impact lakes) compared to results from a set of 5 upwind and distant lakes (reference lakes) clearly indicated enrichment of several elements in lakes close to the mining operations. The flux of uranium to the sediments of the 5 impact lakes was significantly higher post-1960 from the mean pre-1960 background conditions by 1.5 to 23 times. Flux rates of arsenic, molybdenum, and selenium also indicated significant enrichment from background conditions in the majority of the impact lakes. In contrast, large and significant increases in flux only occurred in 1 of the 5 reference lakes for arsenic. The highest levels of enrichment were found in the 3 lakes either in closest proximity to the tailings management facility, waste-rock piles, or downstream from the treated effluent management system. High flux rates and concentrations of elements such as arsenic and vanadium in both the reference lakes and in pre-mining samples of the impact lakes indicated that potentially toxic elements can occur naturally in this region. Sediment core analyses of elemental flux and concentration in both reference lakes and pre-1960 conditions in the impact lakes provided an estimate of natural background conditions. Our data suggest that the McClean Lake mine may be a source for the enrichments of toxic elements in the lakes in close proximity to the mining operations.

Mitigation of Acid Mine Drainage via a Revegetation Programme in a Closed Coal Mine in Southern New Zealand

Acid mine waters (pH 3–5) at the closed Wangaloa coal mine have resulted from surface runoff and groundwater seepage in contact with pyrite-bearing waste rock piles. The low nutrient content, physical factors, and elevated boron levels, all combined with the low pH (down to pH 2) of most waste rocks have limited early planned revegetation and natural plant colonisation. A renewed programme of site rehabilitation, started in 2000, focussed on establishment of near-complete ground cover and functioning ecosystems. Small patches (tens of m2) of low-pH, boron-rich, unvegetated substrates persist after more than 10 years of rehabilitation and introduction of a wide variety of species, but natural colonisation is slowly advancing on these remnants. The rehabilitation has coincided with and contributed to a rise in runoff water pH from ≈4.5 to ≈5.6. This decrease in severity of acid mine drainage (AMD) has apparently become sustainable without further intervention. Near-complete vegetative cover occurred by a combination of a major planting programme and natural colonisation from nearby islands of established native species. Both processes were accompanied by development of functioning ecosystems, and supported by increasing invertebrate diversity and abundance, which are ensuring the persistence of the ameliorative effects on AMD at the site. Vegetation established via natural processes can function ecologically at a higher level than comparable planted vegetation but may not lead to the desired plant cover on some substrates.

The zinc stable isotope signature of waste rock drainage in the Canadian permafrost region

Leachate from a well-instrumented experimental-scale waste-rock pile (test pile) at the Diavik Diamond mine, Northwest Territories, was monitored. The well-characterized waste rock consists of granite, pegmatitic granite and biotite schist with an average total sulfur and carbonate carbon concentration of 0.053 and 0.027wt.%, respectively. The leachate emerging from the southern basal drain of the waste rock pile has been monitored since 2007. The zinc stable isotope footprint was characterized alongside standard monitoring parameters during two field seasons, May to November 2011 and 2012. The pH ranged between 4.3 and 6.8 and carbonate alkalinity was low or undetectable (<35mgL−1 CaCO3). The pH was governed by the oxidation of sulfide minerals and the dissolution of primary carbonate minerals and secondary Al and Fe oxyhydroxysulfates and hydroxides. Dissolved Al and Fe concentrations averaged 6.78mgL−1 and 175μgL−1, respectively. The main processes controlling Zn concentrations in the range of 0.4 and 4.7mgL−1 (average=2.2mgL−1) were the oxidative dissolution of sphalerite (ZnS) and the attenuation by secondary Fe and Al hydroxides. Zinc isotopes were fractionated mass dependently. Zinc isotope ratios, ranging between −0.16 and +0.18‰ (average=+0.05‰, n=43) were consistent with values reported for sphalerite from other deposits. The deviations in isotope ratios (Δ=0.36‰) were significant in comparison to analytical uncertainties (0.06‰). Zinc isotope ratios and concentrations were largely uncorrelated, suggesting that the processes affecting Zn mobility had little or no impact on the Zn isotope signature. These data suggest that the Zn isotope ratios of the waste-rock leachate may be used as a fingerprint to track anthropogenic, mine-derived Zn sources in an environment under fluctuating pH, temperature and ionic strength.

Diversity and soil-tissue elemental relations of vascular plants of Callahan Mine, Brooksville, Maine, U.S.A

Metal-contaminated soils provide numerous stressors to plant life, resulting in unique plant communities worldwide. The current study focuses on the vascular plants of Callahan Mine in Brooksville, ME, USA, a Superfund site contaminated with Cu, Zn, Pb, and other pollutants. One hundred and fifty-five taxa belonging to 50 families were identified, with the Asteraceae (21%), Poaceae (11%), and Rosaceae (9%) as the most species-rich families. Ninety-six species encountered at the Mine were native to North America (62%), including 11 taxa (7%) with rarity status in at least one New England state. Fifty-one species were non-native (33%), including nine taxa (6%) considered invasive in at least one New England state. We characterized how the plant community changed across different habitats at the Mine, from disturbed and exposed (waste rock piles, tailings pond) to inundated and relatively undisturbed (wetland, shore), and documented concurrent shifts in the ionic content of the soils across the habitats. We found substantial differences in both the plant community and soil chemical features among habitats. Habitats separated out along a single axis of an ordination of the plant community, with wetland and shore habitats at one extreme and tailings pond and waste rock-pile habitats at the other. The first principal component axis of the 21 soil variables was significantly predicted by the ordination of the plant community, indicating a gradient of increasing organic matter, Fe, Mg, Mn, total N, Na, and K roughly parallel to the gradient of increasing wetland vegetation. None of the plant species tested accumulated substantial concentrations of metals in their leaf tissue except Salix bebbiana and Populus balsamifera, which accumulated 1070 ppm and 969 ppm Zn in dry leaf tissue, respectively—approximately one-third of the concentration considered as hyperaccumulation for Zn.

Assessment of Unsaturated Flow for a Large Coal Mine Waste Rock Pile

Some waste rock piles create some problems such as acid mine drainage (AMD), leaching of heavy metals, and slope stability concerns. These problems are related to the flow of water through the rock pile. Understanding the physical and hydrological properties of the waste rock piles is important for flow and solute transport modeling. To study physical properties and unsaturated flow in rock pile, some soil samples were collected from surface of the large coal waste rock pile and tested for some common geotechnical parameters in the laboratory. The results showed that materials are so heterogeneous and highly supporting segregation and natural gravity sorting of waste rock materials from top to toe. Numerical simulation was performed to investigate the unsaturated flow conditions in the pile. The results obtained showed that pile is unable to fully saturate and drain during the simulated period. It was also found that high evaporation and also coarse grain nature of waste rock materials results holding few amounts of water in the system.

A novel method for reducing acid mine drainage using green liquor dregs

When exposed to the weather, sulfidic minerals release sulfuric acid, metals and metalloids. This leachate can devastate nearby ecosystems for centuries. This article reports a novel barrier system based on green liquor dregs that reduces acid generation, is inexpensive, and is practical for implementation at operational mines. Two waste rock piles were constructed. One pile was left open to the atmosphere, and the other was sealed with green liquor dregs and partially capped with a polyethylene liner. This test was designed in collaboration with personnel from Boliden Mineral AB, a Swedish mining company in order to ensure that the results would be practical to implement at an operational mine. Leachate flow and chemistry were monitored in both piles over a period of 12 months. Effluent volume was 40 % lower in the test pile, while thirteen of the twenty elements assessed showed average concentrations that were 50 % lower in the test pile than in the control pile. For the most environmentally toxic elements As, Cd, Pb and Hg, reductions were even more significant, ranging between 67 and 87 %. These positive results are hypothesized to be primarily due to the chemical and physical characteristics of the green liquor dregs. The novelty of this result is that it is the first barrier system that fulfills the multiple needs of the Swedish mining industry: It is effective in reducing acid mine drainage while remaining both economical and practical. No other barrier system meets these requirements.

Radon as a natural tracer for gas transport within uranium waste rock piles.

Acid mine drainage (AMD) has been identified as the main cause for outflow of acid water and radioactive/non-radioactive contaminants. AMD encompasses pyrites oxidation when water and oxygen are available. AMD was identified in uranium waste rock piles (WRPs) of Indústrias Nucleares do Brasil-Caldas facility (Brazilian uranium mine), resulting in high costs for water treatment. AMD reduction is the main challenge, and scientific investigation has been conducted to understand oxygen and water transportation within WRPs, where 222Rn is used as natural tracer for oxygen transportation. The study consists of soil radon gas mapping in the top layer of WRP4 using active soil gas pumping, radon adsorption in active charcoal and 222Rn determination using high-resolution gamma-ray spectrometry. A sampling network of 71 points was built where samples were collected at a depth of 40 cm. Soil radon gas concentration ranged from 33.7 to 1484.2 kBq m(-3) with mean concentration of 320.7±263.3 kBq m(-3).

Numerical modelling of transfer processes in a waste rock pile undergoing the temporal evolution of its heterogeneous material properties

Waste rock piles producing acid mine drainage (AMD) are complex and heterogeneous unsaturated systems in which multiple coupled processes are involved: multiphase flow of fluids (liquid and gas), heat transfer and mass transport. Numerical modelling is required to consider together these processes. Such modelling was used to evaluate the effects of closing options for a waste rock pile, including resloping and the placement of a cover, on AMD production from the South Dump of the Doyon Mine (Quebec, Canada). The thermal data recorded within the pile during 15 years show a steady reduction of temperature over the last 10 years. The objective of this study was to numerically reproduce the temporal evolution of thermal conditions to answer two key questions: (1) Can the evolution of material properties over time explain observed changes in thermal conditions? (2) If waste rock properties have evolved, is it effective to cover the pile to mitigate AMD production? Modelling showed that reducing the permeability of materials in the model can reproduce observed temperature changes over time, which cannot be achieved by reducing the reactivity of materials. Results also show that the evolution of hydraulic properties of the waste rock pile can have a direct effect on the global oxidation rate because of their control over the supply of atmospheric air and oxygen in the pile. When the production of AMD is already in decline, simulations also show that a simple remodelling of the surface may be sufficient to significantly decrease the oxidation rate and provide immediate environmental gains, without a cover. However, there may be other benefits related to cover placement that have not been evaluated by our study, especially in relation to the management of water runoff and leachate.

Biochar application to hardrock mine tailings: Soil quality, microbial activity, and toxic element sorption

Waste rock piles from historic mining activities remain unvegetated as a result of metal toxicity and high acidity. Biochar has been proposed as a low-cost remediation strategy to increase soil pH and reduce leaching of toxic elements, and improve plant establishment. In this laboratory column study, biochar made from beetle-killed pine wood was assessed for utility as a soil amendment by mixing soil material from two mine sites collected near Silverton, Colorado, USA with four application rates of biochar (0%, 10%, 20%, 30% vol:vol). Columns were leached seven times over 65days and leachate pH and concentration of toxic elements and base cations were measured at each leaching. Nutrient availability and soil physical and biological parameters were determined following the incubation period. We investigated the hypotheses that biochar incorporation into acidic mine materials will (1) reduce toxic element concentrations in leaching solution, (2) improve soil parameters (i.e. increase nutrient and water holding capacity and pH, and decrease compaction), and (3) increase microbial populations and activity. Biochar directly increased soil pH (from 3.33 to 3.63 and from 4.07 to 4.77 in the two materials) and organic matter content, and decreased bulk density and extractable salt content in both mine materials, and increased nitrate availability in one material. No changes in microbial population or activity were detected in either mine material upon biochar application. In leachate solution, biochar increased base cations from both materials and reduced the concentrations of Al, Cd, Cu, Pb, and Zn in leachate solution from one material. However, in the material with greater toxic element content, biochar did not reduce concentrations of any measured dissolved toxic elements in leachate and resulted in a potentially detrimental release of Cd and Zn into solution at concentrations above that of the pure mine material. The length of time of effectiveness and specific sorption by biochar is variable by element and the toxic element concentration and acidity of the initial mine material.

Mobility of Po and U-isotopes under acid mine drainage conditions: an experimental approach with samples from Río Tinto area (SW Spain)

Under acid mine drainage (AMD) conditions, the solubilities and mobilities of many elements are vastly different from conditions prevailing in most natural waters. Studies are underway in the Río Tinto area (Iberian Pyrite Belt), in order to understand the behavior and mobility of long-lived U-series radionuclides under AMD conditions. A set of leaching experiments utilizing typical country rocks from the Tinto River basin, waste rock pile composite materials, iron-rich riverbed sediments and gossan (weathered naturally rock) were performed towards this purpose. Initial leaching experiments using distilled water kept in contact with solid material for 300, 100, 50 and 1 h resulted in very low concentrations of U with 234U/238U activity ratios close to equilibrium and activity concentrations of 210Po < 0.03 mBq/g. Leaching experiments performed with sulfuric acid media (0.1 and 0.01 M), and contact times between the solid and solution for 24 h were conducted to quantify the amount of U-isotopes and 210Po leached, and the radioactive disequilibria generated between the radionuclides in the leachate. These experiments show that Po mobility in acidic conditions (pH around 1–2) is very low, with 210Po activity in the leachate to be 6% in average for the solid sample. By contrast, mobility of U-isotopes is higher than that of Po, around 1.2%.

Effect of Dense Material Layers on Unsaturated Water Flow Inside a Large Waste Rock Pile: A Numerical Investigation

The construction method used to build waste rock piles influences their internal structure. Commonly used methods typically lead to the creation of compacted material layers within otherwise loose, coarse-grained waste rock. These dense layers, which typically have a finer grain size, affect the movement and distribution of water inside the pile. Long-term numerical simulations of unsaturated flow in a large pile were conducted to investigate the effect of such layers. The simulations led to various observations that provide a better understanding of the hydrogeological behaviour of the modeled pile (based on an actual case). The results show how water distribution and seepage within the pile are influenced by the presence of these layers. Other factors, including the magnitude of precipitation (or recharge) and pile size, were also investigated. This article presents the main results of the simulations, with some comments on their practical implications for pile design.

Lab to field scale effects on contaminated neutral drainage prediction from the Tio mine waste rocks

The quality of mine drainage water is often predicted with laboratory prediction tests. However, laboratory and field results sometimes show significant differences in release rates or contaminant concentrations in drainage waters. Understanding the scale influence on the different factors that affect the geochemical behavior of mine wastes is necessary in order to extrapolate results from the laboratory to the field scale. The main objectives of the present study are to compare CND prediction results obtained at the laboratory (with columns, humidity cells, and weathering cells) and field scales (field test pads, waste rock pile), and to identify the main scale factors involved. This work focuses on CND-generating waste rocks from the Tio mine (Rio Tinto), in Québec, Canada, where Ni concentrations in the drainage waters of some of the waste rock piles sometimes exceed the regulated limits. Results obtained in the present study suggest that the liquid to solid ratio differences between the scales have a significant impact on the geochemical responses of the waste rocks. The results also demonstrate that the humidity cell and weathering cell tests exaggerate the dissolution of neutralizing minerals versus the sulfide oxidation rate for the waste rocks studied. This phenomenon most probably occurs because of the high liquid to solid ratios of these laboratory tests with regard to field conditions. On the other hand, the column test seems to accelerate the release rates versus the field, while the ratios of neutralization to oxidation rates relatively remain close to that in the field.

Identification of acid rock drainage sources through mesotextural classification at abandoned mines of Croydon, Australia: Implications for the rehabilitation of waste rock repositories

Developing effective strategies to manage acid rock drainage (ARD) from historic and abandoned mine sites is a significant rehabilitation challenge. In Australia, there are more than 50,000 recorded abandoned mine sites, many of which have associated ARD and water quality issues. Traditional rehabilitation strategies focus on utilising a blanket approach to management. However, if sources of ARD were instead thoroughly characterised, cost-effective management strategies based on mineralogy could be formulated, potentially enhancing site rehabilitation and ensuring longer-term success.A mesotextural method was developed to domain waste rocks into groups based on their mineralogical, textural and chemical similarities, using routine geological tools and field-based analytical instrumentation. This was tested at the abandoned mining operations at Croydon, North Queensland, from which uncapped sulphidic waste rock piles were sampled. Surface water and sediment samples collected from creeks up to 10km downstream of the site showed elevated concentrations of As, Cd, Cu, Ni, Pb, S and Zn relative to local background levels, indicating the necessity for effective rehabilitation strategies to be implemented at these sites. Ten mesotextural waste rock groups (A to J) were identified in the piles across both mine sites and comprise of hydrothermally altered rhyolites, and massive sulphides. Three major sulphide-bearing groups were identified (G, H and J). Mineralogical and geochemical data indicated that group J (quartz–pyrite) was acid forming, with pyrite containing significant concentrations of As, Pb, Zn and Cu. Pyrite was in early weathering stages with some hydrous ferric oxides observed on grain rims and fractures. Group H (arsenopyrite–quartz–pyrite) was also acid forming; with scorodite extensively precipitated in fractures and rims, likely retarding arsenopyrite oxidation. Significant quantities of Zn and Cd were leached from Group G (quartz–sphalerite–galena) in first flush experiments, and were also measured downstream of the Glencoe site (at which the majority of group G material was identified). Microtextural analyses showed galena had partial weathering to anglesite, suggesting a potential Pb source. High concentrations of Fe and Cd (8.5wt.% and 0.19wt.% respectively) were measured in sphalerite, which likely encouraged oxidation, and subsequent release of Zn. Considering the diversity of the sulphide mineralogy and the associated weathering pathways, a rehabilitation strategy which focuses on segregating waste on the basis of mesotextural classification should be considered.

Spatial Variability of Metal Bioaccumulation in Estuarine Killifish (Fundulus heteroclitus) at the Callahan Mine Superfund Site, Brooksville, ME

The former Callahan Mine Site in Brooksville, ME, is an open-pit, hardrock mine site in an intertidal system, thus providing a unique opportunity to evaluate how metal-enriched sediments and overlying water impact estuarine food webs. Copper, zinc, cadmium, and lead concentrations in sediment, whole water, and Atlantic killifish (Fundulus heteroclitus) were evaluated at sites in Goose Pond (GP; Callahan Mine Site) and at reference sites. The metal concentrations of sediment, water, and fish were spatially distinct and significantly greater at the mine site than in the reference estuary. Sediment concentrations were particularly elevated and were above probable effects levels for all four metals adjacent to the tailings pile. Even in this well-mixed system, water metal concentrations were significantly elevated adjacent to the tailings pile, and concentrations of Cu and Zn were above ambient water-quality criteria for chronic marine exposure. Neither organic matter in the sediment nor salinity or pH of the water explained the metal concentrations. Adjacent to the tailings pile, killifish metal body burdens were elevated and were significantly related to both sediment and aqueous concentrations. In conclusion, (1) the contaminated sediment and seepage from the tailings impoundment and waste rock pile no. 3 create a continual flux of metals into the water column, (2) the metals are bioavailable and bioconcentrating as evident in the killifish tissue concentrations, and (3) Callahan Mine is directly affecting metal bioaccumulation in fauna residing in the GP estuary and, potentially, in Penobscot Bay by the way of “trophic nekton relay.”

Geochemistry, water balance, and stable isotopes of a “clean” pit lake at an abandoned tungsten mine, Montana, USA

The Calvert Mine is a small tungsten-rich (scheelite) skarn deposit in a remote, mountainous region of southwest Montana, USA. The open-pit mine closed in the 1970s and subsequently flooded to form a pit lake that is roughly conical in shape, 30m deep and 120m in diameter, with no surface inlet or outlet. The lake is holomictic with a groundwater flow-through hydrology and an estimated residence time of 2.5–5y. Water isotopes show that the lake is at an approximate steady state with respect to water balance and has experienced 30% evaporation. The lake has a near-neutral pH, exceptional clarity, and extremely low concentrations of nutrients, sulfate, and most metals, including tungsten. Manganese concentrations are slightly elevated and increase with depth towards the sediment–water interface. Despite seasonally anoxic conditions in the deep water, dissolved Fe concentrations are orders of magnitude lower than Mn, suggesting that insufficient organic carbon is present in the sediment of this oligotrophic lake to drive bacterial Fe reduction. Based on stable isotope fingerprinting, diffuse seepage that enters a nearby headwater stream at the base of a large waste-rock pile can be directly linked to the partially evaporated pit lake. However, this seepage has neutral pH and low metal concentrations, and poses no threats to the environment. Stable isotopes of dissolved inorganic carbon (DIC) and dissolved oxygen (DO) are used to track the relative importance of photosynthesis and respiration with depth. In summer, a zone of high productivity exists near the base of the chemocline, releasing DO that is isotopically light. Respiration exceeds photosynthesis below the Secchi depth, which causes DO concentrations to approach zero towards the bottom of the lake. In winter, thick ice and snow cover prohibits photosynthesis. However, because of the low nutrient content, most of the volume of the lake remains oxic through the winter. Overall, the Calvert Lake is a good example of a pit lake formed from metal mining that has excellent water quality, which is a result of its favorable geology (paucity of sulfide minerals) and hydrology (flow-through lake with short residence time in a temperate climate).

The Diavik Waste Rock Project: Particle size distribution and sulfur characteristics of low-sulfide waste rock

Three large-scale instrumented waste rock piles were constructed at the Diavik Diamond Mine in the Northwest Territories, Canada. These experimental waste rock piles (test piles) are 15m high and are part of an integrated field and laboratory research program to characterize and compare low-sulfide waste rock and drainage at various scales. During test pile construction, samples of the <50mm fraction of waste rock were collected from two types of waste rock that are segregated during mining operations based on S content. The samples were analyzed for S content and particle size distribution. One test pile contained waste rock with an average of 0.035wt.% S in the <50mm fraction, within the operational S target of <0.04wt.% S for the lower S waste rock type. The second test pile contained waste rock with an average of 0.053wt.% S in the <50mm fraction, lower than the operational S target of >0.08wt.% S for the higher S waste rock type. The third test pile has a low permeability till layer and a low sulfide waste rock thermal layer covering a core of waste rock with average 0.082wt.% S in the <50mm fraction, which is within the operational S target of >0.08wt.% S for the higher S waste rock. Particle size distributions for the lower and higher S waste rock are similar, but the higher S waste rock has a higher proportion of fine-grained particles. Sulfur determinations for discrete particle sizes of the <50mm fraction illustrate higher S concentrations in smaller particles for both the lower S waste rock and the higher S waste rock. Similarly, S concentrations calculated for the >10m scale, from composite blast hole cuttings, are lower than those calculated for the <50mm scale. Acid–base accounting using standard methods and site-specific mineralogical information was used to calculate the ratio of neutralization potential to acid generating potential. A comparison of calculation approaches to pH and alkalinity data from humidity cell and test pile effluent suggest that ratios are very sensitive to the calculation method. The preferred calculation method was selected by comparing calculation results to pH and alkalinity data from humidity cell effluent collected over 95weeks and test pile effluent collected over five field seasons. The preferred acid–base accounting values were obtained by calculating the average neutralization potential divided by the average acid potential of a sample set. This approach indicates that waste rock with >0.05wt.% S is of uncertain acid-generating potential and effluent data indicate this waste rock generates acidic effluent; whereas lower S waste rock does not produce acidic effluent, consistent with the acid–base accounting predictions.

The Diavik Waste Rock Project: Measurement of the thermal regime of a waste-rock test pile in a permafrost environment

The interior thermal regime of a field-scale experimental waste rock pile in the Northwest Territories, Canada, was studied. Test pile construction was completed in the summer 2006, and temperature data was collected continuously since that time to February 2009. The temperature data indicates the test pile cooled over the study period, with an average heat energy release of −2.5×104 and −2.6×104MJ in 2007 and 2008, respectively. The mean annual air temperature (MAAT) at the site was −8.9°C during the period between 2006 and 2009, with a permafrost table at a depth of 4m in bedrock away from the pile. Because of this cold environment, the upward movement rate of the 0°C isotherm into the test pile at its base was approximately 1.5ma−1 during 2007 and 2008. Thermistor strings installed immediately below the base of the test pile showed the test-pile basal temperatures remained near and below 0°C during the study period. Furthermore, due to low rates of sulfide mineral oxidation, elevated temperatures in the interior of the test pile were not observed. The average air velocity in the pore space in July 2007 and 2008 was about one third of that during January of each year based on temperature distributions. Therefore, due to higher air velocity during the winter, it is expected that heat transfer is greater during winter.

Numerical modeling of contaminated neutral drainage from a waste-rock field test cell

Contaminated drainage related to the leaching of soluble metals under near-neutral conditions, known as contaminated neutral drainage (CND), may arise when metal species are soluble at neutral pH. Such a phenomenon has been sporadically observed in effluent fromthe Tiomine waste-rock pile in Quebec, Canada, particularly from older sections of the pile, where Ni concentrations are increasing with time. It has been postulated that Ni is retained within the fresh waste rock as sorbed species, but as the rock ages, sorption sites become saturated and more Ni is released to the effluent. A field test program was initiated to evaluate the geochemical behavior of the waste rock. This paper presents a numerical analysis of CND generation from waste-rock field test cells including water flow and multi-component transport with geochemical reactions (e.g. sulfide oxidation, pH neutralization, and Ni sorption), using the code MIN3P. The model was able to represent the delay before Ni is seen in the effluent, as caused by sorption of Ni onto Fe-oxide particles. Once the sorption sites are saturated, the model allows Ni release into the effluent by millerite dissolution, expressed by the shrinking core model. A sensitivity analysis indicated that sorption parameters significantly affected the simulated results, so their selection should be based on sound independent field or experimental data.

Solid-Phase Distribution and Leaching Behaviour of Nickel and Uranium in a Uranium Waste-Rock Piles

The potential risk of surface and groundwater contamination by the heavy metals and radionuclides leached from uranium waste-rock piles (UWRP) is a major environmental concern in the uranium-mining district of Northern Saskatchewan, Canada. The main objective of this study was to evaluate the nickel and uranium leaching behaviour in the UWRP lithological materials. In addition to the chemical characterization, these selected UWRP geomedia samples were also subjected to the sequential extraction procedure, availability test to quantify leaching potential and cumulative leaching test (CLT). Sequential extractions results demonstrated substantial observed differences in the Ni and U distribution patterns among various operationally defined geochemical fractions. A large fraction of total Ni concentration was associated with non-labile residual fraction while U was mainly present in the labile fractions. The observed labile Ni and U concentrations also remained relatively high in the gneissic basement materials and underlying organic-rich lake sediment. In case of basement materials, both Ni and U concentrations in solution with the first CLT fraction exceeds their maximum permissible levels in both surface and groundwater. Leaching test results confirmed that Ni and U leachability depends on their total content distribution in various solid phase fractions, and several geochemical processes are controlling the solubility of Ni and U geochemical phases in the UWRP. Our experimental data suggest the potential for a long-term risk to surface and groundwater contamination from these UWRP.