Question 5 of 6: Dust, Water and Effluent

Question 5: Dust, Water and Effluent (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Health and environmental concerns of dust from waste dumps and tailings impoundments

Fugitive dust from exposed waste rock dumps and dry tailings beaches is a concern common to essentially every mine regardless of commodity, because the dominant gangue mineral in almost all hard-rock ore and waste is crystalline silica (quartz); chronic inhalation of respirable crystalline silica causes silicosis, a progressive and irreversible lung fibrosis, and elevates lung cancer, COPD and tuberculosis-susceptibility risk for workers and, at lower chronic exposure, nearby communities. On top of this baseline silica concern, METAL mining dust carries additional metal-specific toxicity: waste rock and tailings from a sulphide ore body can bear elevated arsenic, lead, cadmium or other metals that are inhaled directly or deposited onto soil, snowpack, surface water and vegetation, where they bioaccumulate and can enter the food chain via livestock, wild game or fish; a tailings beach may also carry residual process reagents (cyanide degradation products, flotation collectors/frothers) that become airborne as the beach dries. NON-METAL mining (industrial minerals, aggregate, coal, asbestos-hosted deposits) carries its own distinct dust hazards independent of metal toxicity: asbestiform mineral fibres cause asbestosis and mesothelioma, and coal dust causes coal workers' pneumoconiosis, both governed by exposure limits entirely separate from the silica/metal concerns above. Beyond direct human health effects, windblown dust from both dump and tailings surfaces reduces regional air quality and visibility, is a persistent nuisance and property-value concern for nearby communities, and can smother downwind vegetation, affecting wildlife habitat and Indigenous traditional land use – concerns that regulators weigh alongside the direct occupational-health picture when setting a mine's air-quality permit conditions.

b) Dust control / suppression methods for waste dumps and tailings impoundments

Chemical dust palliative / surface crusting agent. A hygroscopic salt (magnesium or calcium chloride), a bitumen emulsion, or a polymer/lignosulfonate binder is sprayed onto an inactive dump bench or a dried tailings beach to bind surface fines into a crust. It suppresses dust for weeks at a time rather than the hours a water application lasts, which markedly cuts water-truck fleet size and reduces freeze risk in a Canadian winter, and it is relatively low-cost per hectare treated compared to an engineered cover. Its drawbacks are that chloride salts are corrosive to equipment and can themselves leach to groundwater or surface water (requiring their own monitoring), the crust degrades with continued wind/traffic exposure and periodic re-grading, so reapplication is required on a recurring schedule, and it is only a surface treatment – it does nothing for a surface still receiving fresh material.

Progressive vegetative or physical cover. As soon as a dump bench or tailings beach segment is no longer receiving fresh material, it is capped with topsoil/growth medium and revegetated, or covered with a coarse gravel/rock layer. This is a durable, low-maintenance, largely one-time-cost solution once established, and it does double duty as progressive reclamation while also reducing surface-water infiltration (an ARD co-benefit, Question 2). Its main drawback is that it cannot be applied to an actively-growing surface still receiving fresh waste rock or tailings, vegetation establishment on nutrient-poor tailings can be slow and may require soil amendment or irrigation, and relative to the chemical palliative its up-front capital cost per hectare is higher even though its long-run cost is usually lower.

c) Water balance diagram for a surface impoundment tailings system

Each term in the diagram is measured or estimated by a different method. Direct precipitation on the pond and beach is estimated from an on-site or regional rain gauge multiplied by the wetted surface area. The mill process water / tailings slurry discharge, usually the dominant inflow, is measured directly with a flow meter on the tailings delivery line. Upstream catchment surface runoff is estimated with a rainfall–runoff model (e.g. the Rational Method or a curve-number method) applied to the contributing catchment area and its land cover. Groundwater inflow, where the impoundment is hydraulically connected to a permeable aquifer, is estimated from piezometer gradients and a site hydrogeological model, and is usually a minor term. On the outflow side, evaporation is estimated from pan-evaporation data (with a pan coefficient) or a Penman-type equation applied to the pond surface area; reclaim/decant water pumped back to the mill – typically the single largest managed outflow, since most operations recycle the bulk of their process water – is measured directly by a flow meter on the reclaim line; seepage through or beneath the embankment is measured from underdrain/toe-ditch collection flow, or calculated from Darcy's law (Question 1d) given the phreatic surface position and material permeability; and controlled discharge at the spillway or MDMER compliance point (Question 1c) is measured with a calibrated weir or flow meter and sampled under the mine's Environmental Effects Monitoring program.

d) Seepage control techniques for tailings embankments (all three discussed; any two satisfy the mark scheme)

Collector ditches. An open perimeter ditch is excavated around the toe or downstream of the embankment to intercept seepage as it emerges at or near the ground surface, channelling it to a sump or collection pond for treatment, monitoring or recycle back to the mill. It is the simplest and lowest-cost of the three methods, is easy to inspect visually and to maintain (regrading, desilting), and provides an immediate, direct measurement point for the seepage water balance term (Question 5c). Its main drawback is that it only captures seepage that has ALREADY emerged near the surface – it does nothing to intercept deeper seepage through a pervious foundation layer, and it does not by itself lower the phreatic surface within the embankment the way an internal drain does.

Cutoff trench. Where a shallow pervious foundation layer overlies a relatively impermeable layer, a trench is excavated through the pervious material down to the impermeable layer beneath the embankment footprint (typically at or near the upstream toe) and backfilled with compacted low-permeability material (clay or a bentonite-amended fill), forming a physical barrier that blocks underseepage through the foundation. It is highly effective where the impermeable layer is shallow enough to key into economically, and it addresses the seepage PATH directly rather than just collecting the symptom. Its main drawback is cost and constructability: it becomes impractical once the impermeable layer lies more than a few metres to perhaps ten metres deep, requires dewatering of the open excavation during construction, and is not compatible with a foundation with no identifiable impermeable layer to key into.

Slurry wall (soil-bentonite or cement-bentonite cutoff wall). A narrow, deep trench is excavated under a bentonite slurry (which hydrostatically supports the trench walls so they do not collapse) and progressively backfilled with a low-permeability soil-bentonite or cement-bentonite mix, forming a continuous vertical barrier that can be installed to tens of metres depth without full open excavation or extensive dewatering. It is the method of choice where the impermeable layer is too deep for an economical open cutoff trench, and a properly constructed wall achieves a very low hydraulic conductivity (often $10^{-8}$ to $10^{-9}$ m/s) over its full depth. Its drawbacks are the highest unit cost of the three methods and the need for specialized slurry-wall construction equipment and continuous quality assurance (verticality, panel-joint continuity) to ensure there is no gap in the wall through which seepage could bypass it.