Question 6 of 6: Dust and Water Issues

Question 6: Dust and Water Issues (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) Human health and environmental concerns from dust

Windblown dust from exposed waste dumps and tailings surfaces is dominated by respirable particulate matter (PM10 and PM2.5), which is small enough to penetrate deep into the lungs and can irritate the respiratory tract, aggravate asthma and chronic obstructive pulmonary disease, and, where the dust contains crystalline silica – common in hard-rock waste and tailings of almost any commodity – carries a long-term silicosis risk for workers and, at lower intensity, nearby residents. At metal mines specifically, dust particles can carry adsorbed or mineral-bound heavy metals (arsenic, lead, cadmium and others), adding an inhalation and ingestion (hand-to-mouth, garden produce, livestock forage) exposure pathway for toxic and, in some cases, carcinogenic elements that would not arise from an equivalent volume of inert non-metal dust. Environmentally, deposited dust can add metal or acid-generating sulphide loading to soils and surface water well beyond the mine's original footprint, smother adjacent vegetation and reduce photosynthesis, and expose wildlife through ingestion. Non-metal mining operations (aggregate, industrial mineral, potash, coal) still generate significant crystalline-silica exposure risk and can create their own distinct impacts – alkaline dust from lime/cement-associated operations altering downwind soil pH, or saline dust from potash affecting sensitive vegetation – even where heavy-metal toxicity is not a concern. Beyond direct health effects, dust is also a community nuisance (reduced visibility, soiling) and a road-safety hazard on haul roads.

b) Two dust control/suppression methods

Water spraying (fixed sprinklers or water trucks). Water is applied directly to exposed tailings surfaces, waste dump faces and haul roads to bind fine particles and add cohesion. It is low in capital cost, uses equipment and a water source usually already present on site, and can be deployed immediately on any newly exposed surface. Its main drawback is that the effect is temporary – wetting must be repeated frequently, especially in hot, dry or windy conditions – it consumes water that must be accounted for in the site water balance (see part c), and it is largely ineffective, or even counter-productive (ice formation), at sub-zero temperatures common on Canadian sites for much of the year.

Chemical dust suppressants/binders (e.g. calcium or magnesium chloride, lignosulfonates, polymer emulsions). These are sprayed onto haul roads or exposed surfaces to form a longer-lasting crust or binding matrix, typically remaining effective for weeks to months rather than hours. This substantially reduces both water consumption and the frequency/cost of reapplication, and on haul roads can double as a surface stabilizer that reduces maintenance. Drawbacks include a higher per-application cost than plain water, the need for careful application-rate control to avoid over-application, and, for chloride-based products, a potential for salt loading to nearby soils and surface water if runoff is not managed.

c) Water balance diagram

Inputs are measured or estimated as follows: precipitation directly onto the pond from an on-site rain gauge or weather station multiplied by the pond surface area; the tailings slurry inflow from a flow meter and density gauge on the tailings delivery pipeline; surface run-on from the delineated external catchment using a rainfall-runoff estimate (e.g. the rational method or a calibrated hydrologic model); and groundwater inflow, where the pond intercepts the water table, from a piezometer network combined with Darcy's law and an estimated hydraulic conductivity. Outputs are measured as evaporation from pan-evaporation data (with a pan coefficient) or a Penman-type estimate multiplied by pond surface area; reclaim/decant water directly from a flow meter on the return pipeline to the mill; seepage from toe-drain weirs and/or a piezometer network feeding a Darcy's-law calculation; and any regulator-authorized controlled discharge from a calibrated weir or flow meter at the discharge point, which is also the compliance point for the MDMER limits discussed in Question 2.

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

Collector ditch. A ditch excavated around, typically downstream of, the toe of the tailings embankment to intercept seepage water as it emerges near the surface and route it back to the process water system or to treatment. Design considerations include sizing the ditch depth and grade to the anticipated seepage flow and local water table, deciding whether to line it, and planning for periodic maintenance because iron/manganese-oxide precipitates from ARD-affected seepage commonly foul collector ditches. It is inexpensive, quick to build, and easy to inspect and extend as the facility grows, but it only captures seepage that has already reached the surface near the toe, provides no control of deeper seepage through the foundation, can lose effectiveness when frozen, and needs ongoing pumping and maintenance.

Cutoff trench. A trench excavated down to an impermeable or low-permeability foundation stratum beneath the embankment, then backfilled with compacted clay/till or a soil-bentonite mix, forming a positive barrier that cuts off a pervious foundation seepage path before it can develop. It requires a foundation investigation to confirm that a suitable impermeable layer exists at an achievable depth (without one, a cutoff trench is ineffective), compatible backfill material, and dewatering during excavation. Where feasible it is highly effective and becomes a permanent, low-maintenance barrier once complete, but it carries a high capital cost for deep foundations, is difficult or impossible to retrofit once the embankment already covers the area, and carries excavation and dewatering risk below the water table.

Slurry wall (soil-bentonite or cement-bentonite vertical cutoff). A continuous vertical barrier built by excavating a narrow trench under bentonite slurry (to keep it open and stable) and backfilling with a low-permeability soil-bentonite or cement-bentonite mix, which can be keyed into a deep impermeable layer or simply extended to a target design depth where no natural barrier exists. Design requires a mix design and backfill gradation achieving the target permeability (often 1 x 10^-7 to 10^-9 cm/s), specialized contractor equipment, and quality-control verification of continuity (coring or geophysical survey). It can reach significant depth, can be retrofitted around an existing facility and is very effective, but it is the most expensive of the three methods, needs specialized construction expertise, and its long-term integrity depends on the bentonite not desiccating or reacting chemically with unusually acidic or high-TDS seepage – a compatibility check worth flagging on any ARD-affected site.