Waste Rock Dump Research Articles

Diversity of cyanobacteria and algae in dependence to forest-forming tree species and properties rocks of dump

Restoration of ecosystems on the lands disturbed by the mining industry is very slow. Acceleration of these processes is the main purpose of revegetation measures. The purpose of the current research is to show changes of cyanobacteria and algae diversity and abundance depending on the composition of different reforested stands and technosols qualities of the waste rock dumps in West Donbas coal basin, Nikopol manganese ore basin and Kryvyi Rih iron ore basin (Ukraine) that were revegetated in 1966 and 1975. The main technosols physical and chemical qualities, diversity and abundance of cyanobacteria and algae are identified. It is discovered that abundance and diversity of cyanobacteria and algae depends on qualities of soils and forest-forming tree species. The increase in physical clay in soils (particles < 0.01 mm) has a positive effect on cyanobacteria diversity and development, the increase in humus effects positively species diversity of yellow-green and green algae. A higher pH intensifies development of cyanobacteria. Development of groups of cyanobacteria and algae shows dependence on the composition of forest-forming tree species and on their environment-forming impact. The environment-forming impact depends on properties of soils and decreases under the unfavourable conditions. Our study proved that the composition and quantity of cyanobacteria and algae have relation to forest-forming tree species and properties. The results can be used for the structural design of artificial edaphotopes, for a choice of forest-forming tree species that have a high capability to create forest ecosystems equivalent to natural ones.

Faro Waste Rock Project: Characterizing geochemical heterogeneity in sulfide- and carbonate-rich waste rock

Sustained oxidation of sulfide minerals in waste rock generates acid mine drainage for time frames of hundreds to thousands of years. Management to minimize long-term degradation of water supplies and ecosystem health requires a thorough understanding of geochemical processes occurring within the waste rock. A comprehensive field investigation was conducted on sulfide and carbonate-rich waste-rock dumps at the Faro Mine Complex, south-central Yukon Territory, Canada. This investigation included installation of three highly instrumented boreholes, in situ measurements of physical and geochemical parameters (e.g., water content, temperature, air permeability, and pore-gas O2 and CO2 concentration), and collection of pore-water, pore-gas, and solid-phase samples. Field and laboratory measurements indicate the waste rock is lithologically and mineralogically segregated, with sulfide-rich (>20 wt% S) waste rock dominating in the lower benches (20–50 mbgs) and carbonate-bearing waste rock dominating in the upper benches and other parts of the dumps. The observed segregation strongly influences spatial variability of geochemical parameters. Three distinct geochemical zones are observed within the waste-rock dumps, including a rapid O2-supply zone, with near-atmospheric concentrations of O2 (0–30 mbgs); a strong O2-depletion zone (30–50 mbgs), characterized by abundant sulfide minerals and significant O2 depletion to <5 vol%; and a thermally-induced O2-enriched zone (50 mbgs to the pre-mining surface), where thermally-driven gas transport results in replenishment of O2 concentrations (7–18 vol%). These observations indicate sulfide-mineral oxidation and pH neutralization via dissolution of carbonate and aluminosilicate minerals influence pore-water pH and concentrations of dissolved sulfate and metal(loid)s. High concentrations of dissolved metal(loid)s (e.g., up to Fe 36,900 mg L−1, Zn 14,200 mg L−1, Pb 6.16 mg L−1, Al 8,750 mg L−1, As 63.6 mg L−1, Mn 2,170 mg L−1, Cd 21.7 mg L−1, and Cr 1.53 mg L−1) and SO4 (up to 114,000 mg L−1) in pore water correlate with extreme values of pH (as low as 1.4). In the absence of an effective remediation strategy, the oxidation of still-abundant un-oxidized sulfide minerals will continue through vast portions of the waste-rock dumps.

Subsoil Amendment With Poultry Manure as Topsoil Substitute for Promoting Successful Reclamation of Degraded Mine Sites in Ghana

Background: Despite the key role topsoil plays in reclamation, there are situations where topsoil is in deficit or unavailable, especially at degraded and abandoned mine sites in Ghana. The sites pose serious ecological and safety risks, underscoring the urgent need to finding alternative substrate for restoration. This study investigated the feasibility of using amended-subsoil as topsoil substitute for reclamation. The hypothesis was that amendment of stockpiled-subsoil with poultry manure positively influences tree growth and ground vegetation cover (GVC), which promote better soil stabilization at degraded mine sites. A graded waste-rock dump was covered with a 70 cm layer of the stockpiled subsoil at Newmont Ghana Gold Limited. Two experimental plots (24 × 15 m) were established with the treatments poultry manure (PLM 23 t ha-1) and control (no PLM), followed by seeding of Cowpea (Vigna unguiculata) and planting of potted-seedlings of five forest tree species. The Laser-point-quadrat method was used to estimate GVC, whereas erosion was visually observed. Diameter and height data of planted trees and surviving numbers were collected.Results: There was significant increase in tree growth and in GVC for the poultry manure treatment compared to the control. The manure provided sufficient nitrogen to overcome nitrogen deficiency and facilitated quicker and stronger vegetation growth that yielded superior soil stabilization.Conclusions: The findings demonstrate the potential of manure application in promoting successful restoration of the many degraded and abandoned mine sites in Ghana to productive uses.

Physicochemical assessment of a waste rock dump in the hidden valley gold mine: a case study for bioremediation in Papua New Guinea

Deforestation, environmental degradation, and pollution from the mining activities are much studied global environmental concerns. The effect of waste rock dumping from mining activity on the forest soil characters and suitability of waste rock dumps for bioremediation is least explored. Such information is necessary for making rehabilitation decisions. In this study, soil physicochemical characteristics and some index polluting elements were assessed in the waste rock dump stations (n = 16) and adjacent natural forest stations (n = 14) in a gold mine situated in the Morobe province of Papua New Guinea. Results indicated that the waste rock dump was poorer in organic C, but rich in available P, exchangeable Ca and Mg contents, and extractable Cu and Mn. Comparison of established diagnostic norms for potential sufficiency and toxicity of nutrient elements with those observed in the study indicated that organic carbon content, exchangeable K, Ca and Mg as well as, extractable Cu content was at an insufficient level in a majority of stations of waste rock dump for arable crops. Besides, the mean Cd content of waste rock dump was 15-times greater than that in forest stations while that of extractable Mn was 400-fold higher. Findings of the study warranted the use of adaptable and hardy forest species instead of arable crops for safe bioremediation of waste rock dump.

Macronutrients and heavy metals contents in the leaves of trees from the devastated lands at Kryvyi Rih District (Central Ukraine)

This research was performed at Petrovsky waste rock dump, the Central part of the Kryvyi Rih iron-ore &amp; metallurgical district (Dnipropetrovsk region, Ukraine). The macronutrients (K, Ca, Mg, P and S) and heavy metals (Fe, Mn, Zn, Cu, Pb and Cd) contents in the leaves of three species of the trees (Ash-leaved maple, Silver Birch and Black locust) that were collected on devastated lands were assessed. It was established that trees which grow on the Petrovsky dump take place under evident shortage of nutrients (especially K and P) and excess of metals (especially Fe, Mn and Zn). It is revealed that Ash-leaved maple and Black locust are more resistant to the geochemical conditions of devastated lands. In this regard, these species of trees can be recommended for forest restoration &amp; reforestation on devastated lands.

Ecological and geological determination of the initial pedogenesis on devastated lands in the Kryvyi Rih Iron Mining &amp; Metallurgical District (Ukraine)

In our time, a very urgent problem is the cessation of negative impacts on the environment and the return to the practical use of the territories of devastated lands. In this regard, it is important to find out the basic laws of primary soil formation in the area of these man-made neoplasms. The initial soil formation conditions were analyzed on 19 experimental sites which represent the main varieties of devastated land in the Kryvyi Rih Iron Mining and Metallurgical District (Central Ukraine): (i) waste rock dumps of old iron mines (old name “Forges”), (ii) tailing storage facility of underground iron mines, (iii) waste rock dumps of the Iron Ore Mining and Dressing Plant, (iv) waste rock dumps of the Granite Quarry Plant. It was established that on the devastated lands in Kryvyi Rih District, the initial soil formation occurs in very difficult conditions. Therefore, over 25- 100 years only very primitive soils were formed. The following features are inherent to them: (1) primitive soil profile (thickness 10-100 mm), (2) low levels of soil organic substance content (9.5-11.5 %), (3) alkaline indicators of the soil solution (pHH2O – 8.08-8.92, pHKCl – 7.42-8.23), (4) low levels of cation exchange capacity (6.34-8.47 mMol /100 g). By results of correlation calculations, among the factors of soil formation time (duration of soil formation) and input of plant ash elements’ fall are characterized by the maximum number of statistically significant correlation coefficients and their numerical values. In terms of chemical composition of the technosol, the values of organic matter content and exchangeable acidity (pHKCl) were the most predictable soil formation factors. Generally physical / chemical characteristics of geological rocks (as parent material) and time were the two most important factors in determining the initial pedogenesis on devastated lands in the Kryvyi Rih Iron Mining &amp; Metallurgical District (Ukraine).

Processing Hard Coal Mining Wastes of the Luhansk Region as Man-Made Metal Deposits

Coal mining waste rock dumps contain a lot of rare and precious microelements and are a single complex mechanism of the chemical and biochemical transformations of substances, and the key role in such transformations belongs to sulfuric acid formed as a result of the vital functions of thionic bacteria Th. Ferrooxidans. The aim of the present work was to develop an environmentally friendly biotechnology for processing coal mining waste rock dumps of the Luhansk Region as man-made metal deposits. The spectral analysis of the waste rocks of the Luhansk Region revealed high concentrations of precious, rare-earth, and trace elements. A high prevalence of gallium and germanium clarkes was found, and their actual concentration in the waste was close to the minimum industrial concentration. The results of chemical analysis of the waste rock for Al2O3 showed that its concentration was close to that in poor bauxites, a traditional raw material for alumina. A method and technology of the biochemical leaching of aluminum, gallium, and germanium, which make use of the natural processes of sulfuric acid formation in waste rock were proposed, and their efficiency was experimentally confirmed. The proposed biochemical technology for the processing of coal mining waste rocks can serve as the basic method for diminishing their environmental impact and rational use of natural resources.

Global-scale remote sensing of mine areas and analysis of factors explaining their extent

Mines are composed of features like open cut pits, water storage ponds, milling infrastructure, waste rock dumps, and tailings storage facilities that are often associated with impacts to surrounding areas. The size and location of mine features can be determined from satellite imagery, but to date a systematic analysis of these features across commodities and countries has not been conducted. We created detailed maps of 295 mines producing copper, gold, silver, platinum group elements, molybdenum, lead-zinc, nickel, uranium or diamonds, representing the dominant share of global production of these commodities. The mapping entailed the delineation and classification of 3,736 open pits, waste rock dumps, water ponds, tailings storage facilities, heap leach pads, milling infrastructure and other features, totalling ~3,633 km2. Collectively, our maps highlight that mine areas can be highly heterogeneous in composition and diverse in form, reflecting variations in underlying geology, commodities produced, topography and mining methods. Our study therefore emphasises that distinguishing between specific mine features in satellite imagery may foster more refined assessments of mine-related impacts. We also compiled detailed annual data on the operational characteristics of 129 mines to show via regression analysis that the sum area of a mine's features is mainly explained by its cumulative production volume (cross-validated R2 of 0.73). This suggests that the extent of future mine areas can be estimated with reasonable certainty based on expected total production volume. Our research may inform environmental impact assessments of new mining proposals, or provide land use data for life cycle analyses of mined products.

Assessment of the effects of vegetational cover on the long-term stabilityof a waste rock dump

Abstract Waste rock dump reclamation includes re-grading the banks to flatten the overall slope followed by re-vegetation. These measures are required to reduce surface erosion, provide physical stability, and meet future use goals. In some cases, the presence of trees can cause localized shear zones in a waste rock dump by the load of the tree mass and the action of wind, the presence of roots in the soil and the death of some species. In geotechnical assessment, however, the influence of trees on slope stability of waste rock dumps is only qualitative. This article assesses the effects of trees on slope stability of a reclaimed waste rock dump of an iron ore mine. Historic data of construction, laboratory tests, and stability analysis were evaluated. Slope stability assessments were performed after an extensive geotechnical and forest campaign, considering two scenarios - the final waste rock dump without the vegetational cover and the final waste rock with vegetational cover. Tree vegetation in slopes produces both positive and negative effects on slope stability. For scenario 1, the lowest factor of safety occurs for a potential failure surface near the bottom of the waste dump, passing by a layer 6 m depth from the surface. In scenario 2 the factor of safety is increased by 10%. For deeper failure surface, the smallest factor of safety occurs 30 m deep. However, very tall trees at the top of the waste dump are subject to wind action, which has a negative impact on slope stability.

Atmospheric control of radon emissions from a waste rock dump

At the former Schlema-Alberoda mining site, located in the south-east of Germany, waste rock dump 38neu was built along a natural slope on the edge of a valley. The dump is 750 m wide, with a maximum thickness of 30 m and a total height of 100 m. Dump remediation involved surface re-sloping and placement of a 1 m thick low-permeability soil cover. A characterisation aimed to verify the soil cover efficiency: its air permeability, radon concentrations under the cover and radon fluxes, as well as differential air pressures across the soil cover. In summer, relatively high radon concentrations and fluxes were found to occur across the cover in the lower slope of the dump. Differential pressures indicate that gas flow is upwards in the dump below the 9·5°C mean atmospheric temperature and downwards otherwise. A numerical model was developed to explain the relation between dump airflow and atmospheric conditions. The results indicate that airflow is controlled by dump gas buoyancy relative to atmospheric air. Preferential gas entry and exit occur across the dump’s lower slope, where differential pressures are highest, explaining the higher radon fluxes observed near the base of the dump under high atmospheric temperatures.

Conceptual hydrogeological and numerical groundwater flow modelling around the Moab Khutsong deep gold mine, South Africa.

Prediction of response to fissure water pumping from a shaft on the surrounding areas is conducted with a numerical model, which was refined using environmental stable isotope (ESI), Tritium and hydrochemical datasets. The numerical model domain covers the local carbonate rock units mainly because it is known to be the main source of groundwater.Based on the hydrochemical dataset of the fissure water pumped at the shaft, strong signature of Na–Cl type water originating from the seepage of a waste rock dump as well as polluted water from mine offices, loading/unloading and parking lot areas, is evident. The environmental isotope data demonstrated that fissure water intercepted at the shaft has strong footprint of surface water and polluted groundwater from nearby areas. The 30-year numerical model prediction indicates that pumping at a rate of 2500 m3/day or more would cause the radius of influence to expand to areas where the central tailings storage facility is located and could possibly draw highly acidic water. Interception of acid rock drainage could cause sever corrosion of pump infrastructure, enhance dolomite dissolution and local ground subsidence as well as possibly compromise the structural integrity of the central tailings dam.It is recommended that the current groundwater monitoring network be revised for taking proactive action. The study highlights the relevance of integrating various sets of data to improve understanding of the conceptual flow before any numerical modelling is attempted.

Fungal richness in the extreme environments of the Libiola mine (eastern Liguria, Italy): correlations among microfungi, lithology, mineralogy, and contaminants

A small sulphide-rich waste-rock dump from the derelict Libiola mine (eastern Liguria, Italy) was studied for evaluating the fungal richness as well as the correlation among microfungi and lithology, mineralogy, and contaminants. The waste-rock dump is characterized by severe environmental conditions in particular for the (1) high metal and sulphur contents, (2) presence of reactive sulphides (mainly pyrite) triggering acid mine drainage processes, (3) acid-to-strong acid pH values, and (4) high cementation grade of the sediments evolving up to hardpans. Despite these critical and dangerous environmental conditions, a total of 603 CFUs belonging to 30 filamentous fungal species were isolated. Correlations analyses underlined that serpentinite-rich samples resulted particularly poor of fungal richness, with respect to sulphide-mineralized basalts. Only some Penicillium species, in fact, resulted adapted to survive in serpentinitic samples. Finally, among the isolated species, Cladosporium cladosporioides, C. iridis, C. tenuissimum, Metarhizium anisopliae, Penicillium brevicompactum, P. expansum, and Phanerochaete sordida appeared particularly well adapted to the extreme edaphic conditions of the Libiola mine. These results allowed the selection of particularly adapted and tolerant fungal strains usable in biotechnological processes for bioremediation.

The addition of mine waste rock to topsoil improves microsite potential and seedling emergence from broadcast seeds in an arid environment

This study investigated the effect of method of blending and spreading topsoil-waste rock (as dictated by waste dump position), rock addition, topsoil source and seed burial on seedling emergence from seeds broadcast onto mine restoration sites. Seed of 10 species were surface sown onto a waste rock dump plateau with six cover treatments spread by loose tipping; three topsoil source treatments (sand dunes, sandplains and stony hills) x two waste rock treatments (topsoil mixed and without waste rock). Emergence was greater on the waste dump plateau than slopes, where very few seedlings emerged. On the waste dump plateau, the addition of rock to topsoil increased seedling emergence >2.5-fold. Soil surface temperatures were cooler and water content at a depth of 2 cm was higher in topsoil mixed with rock than without rock. Higher seedling emergence with the addition of rock to topsoil was due to the creation of microsites where sub-soil moisture was retained for longer durations. Lack of emergence on waste dump slopes was attributed to relatively low surface roughness that reflected operational issues associated with different methods of spreading and mixing topsoil with rock on plateau versus on slopes.

Biogeochemical Characterization of Metal Behavior from Novel Mussel Shell Bioreactor Sludge Residues

Acid mine drainage (AMD) remediation commonly produces byproducts which must be stored or utilized to reduce the risk of further contamination. A mussel shell bioreactor has been implemented at a coal mine in New Zealand, which is an effective remediation option, although an accumulated sludge layer decreased efficiency which was then removed and requires storage. To understand associated risks related to storage or use of the AMD sludge material, a laboratory mesocosm study investigated the physio-chemical and biological influence in two conditions: anoxic storage (burial deep within a waste rock dump) or exposure to oxic environments (use of sludge on the surface of the mine). Solid phase characterization by Scanning Electron Microscopy (SEM) and selective extraction was completed to compare two environmental conditions (oxic and anoxic) under biologically active and abiotic systems (achieved by gamma irradiation). Changes in microbial community structure were monitored using 16s rDNA amplification and next-generation sequencing. The results indicate that microbes in an oxic environment increase the formation of oxyhydroxides and acidic conditions increase metal mobility. In an oxic and circumneutral environment, the AMD sludge may be repurposed to act as an oxygen barrier for mine tailings or soil amendment. Anoxic conditions would likely promote the biomineralization of sulfide minerals in the AMD sludge by sulfate reducing bacteria (SRB), which were abundant in the system. The anoxic conditions reduced the risk of trace metals (Zn) associated with oxides, but increased Fe associated with organic material. In summary, fewer risks are associated with anoxic burial but repurposing in an oxic condition may be appropriate under favorable conditions.

Hydrochemistry of Surface and Groundwater in the Vicinity of a Mine Waste Rock Dump: Assessing Impact of Acid Rock Drainage (ARD)

Acid Rock Drainage (ARD) is a well-known problem related to the mining industry due to its hazardous environmental effects. Metal-rich drainage and acid effluent transmitted from mine waste dumps compromise environmental quality of groundwater and surface water systems destroying aquatic life and increasing human health risks. This study was aimed at assessing the acid and metal drainage potential from the Subriso East Rock Dump (SERD) located in the Wassa East district of Ghana on ground and surface water quality in the catchment using a system of monitoring boreholes, reference boreholes and river samples. Water samples were collected from deep and shallow monitoring boreholes and surface water within the immediate environs of the SERD from August 2012 to February 2013 for laboratory and statistical analysis. Parameters analyzed include sulphate, alkalinity, Arsenic (As), Manganese (Mn), Iron (Fe), Zinc (Zn), Copper (Cu), Cadmium (Cd), Mercury (Hg), Aluminum (Al), Silver (Ag) and lead (Pb) and their concentrations compared with Ghana Standards Authority (GSA) GSB (2009) and WHO (2017) standards. Results indicate that surface and groundwater were not impacted by the SERD possibly because there was no generation of acid or metal-loaded effluent from the SERD into the environment. Physicochemical variables between monitoring boreholes did not differ significantly from conditions in the reference boreholes. Similarly, comparison of upstream and downstream river conditions did not yield any statistical significance (p > 0.05). Mn and Fe concentrations were above the WHO (2017)/GSB (2009) standards. Heavy metal concentrations in surface and groundwater were below detection limits except manganese and iron whose concentrations exceeded the recommended guidelines. No significant environmental impacts exist that could be attributed to the waste rock dump and may be as a result of engineering designs and mechanisms which prevent acid generated water from reaching the external environment. Furthermore, the geology of the study area potentially could be slightly inert having the potential to generate ARD under appropriate conditions. Again, the young age of the waste rock dump is a factor that may contribute to ARD generation under appropriate condition. Routine monitoring of groundwater and surface water sources is required to determine future acid generation of the SERD and its environmental impacts. The results of this study will assist decision makers and environmental managers to plan effectively to mitigate future impacts as mining waste rock dumps are known to increase in acid generation potential with age.

Geomorphic design and modelling at catchment scale for best mine rehabilitation – The Drayton mine example (New South Wales, Australia)

Computer modelling and design tools can assist in environmental management. In particular, post-mining landscapes with large volumes of materials require shaping for optimal erosional stability and ecological and visual integration into the surrounding undisturbed landscape. This paper evaluates the complementary capabilities of landscape evolution modelling (SIBERIA) and geomorphic design software (Natural Regrade with GeoFluv). An existing 11.5-ha waste rock dump (Hunter Valley, New South Wales, Australia) served as the study site. The SIBERIA modelling demonstrated that geomorphic design reduced erosion by half that of conventional designs while being able to store an extra 7% of mine waste volume. Additionally, the spatial pattern of gullying was able to be predicted by modelling, which allowed management in subsequent geomorphic design, and successively more stable patterns. In conclusion, the joint use of the Natural Regrade with GeoFluv geomorphic design software with the SIBERIA landscape evolution model showed complementary capabilities for enhancing mine rehabilitation.

A comparison of measured and predicted diffusion coefficients applied to sand and silt sized acid mine drainage materials

Determining the rate at which Acid Mine Drainage (AMD) sulfide oxidation occurs in mining waste products is a central requirement for safe and sustainable long term design of storages, including tails storage facilities, ore stockpiles and waste rock dumps. Inappropriate design can result in AMD acidification, mobilisation of heavy metals and pollution of ground and surface waters. The use of soil gas (oxygen) transport modelling to model AMD based sulfide oxidation and potential acidity loads is widespread, but diffusion coefficients used for modelling are based on existing diffusion coefficient models derived for natural and agricultural soils. Mining wastes are often well sorted due to mineral processing, and differences in soil structure and porosity can impact on diffusion coefficient behaviour compared to natural soils. This study compares a variety of approaches to estimate the diffusion coefficient and compare these results to measured values for comparison and analysis. A diffusion column apparatus is used to compare diffusion coefficient models from the literature with test results from the laboratory in several mining derived AMD materials. The results of the comparison indicate that laboratory testing of diffusion provides more accurate estimation of soil gas diffusion coefficients in mine materials. The use of diffusion coefficient safety factor approaches are explored as possible alternatives, if measurement of mine material matrix soil gas diffusion coefficient is not possible.

Magnetometric resistivity: a new approach and its application to the detection of preferential flow paths in mine waste rock dumps

The injection of a low frequency electrical current in the ground between two electrodes A and B generates a magnetic field that can be measured at the ground surface with sensitive magnetic sensors. The map of the magnetic field, measured at the frequency of the injected current, can be used to determine the paths of the current through the ground. When the current is channelled along preferential conductive paths, the MagnetoMetric Resistivity (MMR) method can be used to detect these paths. Conductive current paths can be associated with preferential flow paths of groundwater when the two electrodes A and B are in the direction of the flow and when the flow path is highly electrically conductive with respect to the background. We first review the background equations for the magnetic field in MMR. Then, we provide the kernel of the problem using Biot and Savart law to connect the components of the observed magnetic field to the current density distribution. We also develop a simple approach to invert the magnetic field in terms of electrical current paths. To illustrate how the method works, we develop five synthetic models to test the sensitivity of the method to the properties of the conductive targets channelling the electrical current. The targets are characterized by different shapes, sizes, depths, and conductivity contrasts with the background. Then, we proceed with a case study for which the MMR method is used to identify and map preferential groundwater flow paths bypassing a mine waste rock dump drainage collection trench into the tailings pond. In this case, the conductivity of the flow paths is much stronger than the background conductivity due to the high mineralization of the ground water along these paths. The method underlines the 3-D architecture of these flow paths.

Hydrogeology of a montane headwater groundwater system downgradient of a coal-mine waste rock dump: Elk Valley, British Columbia, Canada

Steelmaking-coal waste rock placed in mountain catchments in the Elk Valley, British Columbia, Canada, drain constituents of interest (CIs) to surface water downgradient of the waste rock dumps. The role of groundwater in transporting CIs in the headwaters of mountain catchments is not well understood. This study characterizes the physical hydrogeology of a portion of a 10-km2 headwater catchment (West Line Creek) downgradient of a 2.7-km2 waste rock dump placed over a natural headwater valley-bottom groundwater system. The study site was instrumented with 13 monitoring wells. Drill core samples were collected to determine subsurface lithology and geotechnical properties. The groundwater system was characterized using field testing and water-level monitoring. The valley-bottom sediments were composed of unconsolidated glacial and meltwater successions (<64 m thick) deposited as a series of cut and fill structures overlying shale bedrock. An unconfined basal alluvial aquifer located above fractured bedrock was identified as the primary conduit for groundwater flow toward Line Creek (650 m from the toe of the dump). Discharge through the basal alluvial aquifer was estimated using the geometric mean hydraulic conductivity (±1 standard deviation). These calculations suggest groundwater discharge could account for approximately 15% (ranging from 2 to 60%) of the total water discharged from the watershed. The residence time from the base of the waste rock dump to Line Creek was estimated at <3 years. The groundwater system was defined as a snowmelt (i.e., nival) regime dominated by direct recharge (percolation of precipitation) across the catchment.

Biotic and Abiotic Sequestration of Selenium in Anoxic Coal Waste Rock

Mobile selenium oxyanions ( $${\text{S}}{{\text{e}}^{{\text{VI}}}}{\text{O}}_{4}^{{2 - }}$$ and $${\text{S}}{{\text{e}}^{{\text{IV}}}}{\text{O}}_{3}^{{2 - }}$$ ) can be sequestered by biotic or abiotic reduction to non-mobile species or by adsorption to mineral surfaces. Microbial analyses and geochemical batch testing with samples collected from a coal waste rock dump in the Elk Valley, British Columbia, Canada were conducted to assess whether Se can be sequestered in anoxic, waste rock by these mechanisms. Bacteria that reduce Se(IV) and Se(VI) to Se(0) were isolated from the waste rock. Isolates that reduce Se(IV) to Se(0) were present in a water sample collected from an underlying rock drain. Three isolates were affiliated with Pseudomonas and Arthrobacter. One isolate was a putatively novel species. The production of Se(0) was confirmed by X-ray absorption near edge spectroscopy of a red precipitate isolated from a broth media containing rock-drain water. No adsorption or reduction of Se(VI) was observed in anoxic, abiotic (sterile) batch tests conducted with waste rock and a 1.0 mg/L Se(VI) solution, whereas Se(IV) was adsorbed by the waste rock and subsequently reduced to Se(0) in abiotic batch tests with a 0.7 mg/L Se(IV) solution. In non-sterile batch tests using waste rock and rock-drain water (0.39 mg/L Se(VI)), Se(VI) was biologically reduced to Se(IV), which was subsequently removed from solution by a combination of bioreduction, adsorption, and possibly abiotic reduction. This study suggests that, under anoxic conditions, Se sequestration in waste rock may occur via biotic reduction of Se(VI) to Se(IV) followed by adsorption of Se(IV) and abiotic and biotic reduction of Se(IV) to Se(0).