DUST CONTROL, RECLAMATION, AND SLOPES
Editorial focus by Harold HoughFeb/Mar 2010
Controlling dust around reclamation sites is different than controlling dust on roads. Here, the key is to first control the dust until a carpet of vegetation has grown enough to keep the dirt in one place. In many cases, a dust control product is added to the seed as liquid mulch. The chemical acts as a glue to keep the dust down, while protecting the seed until germination. However, in the long run, good dust control depends less on the dust control agent and more on the reclamation plan, the soil, the slope, and other products that aren’t often considered in terms of dust control.
One of the biggest problems is slopes. Steep slopes cause more erosion and hinder reclamation. This means that mining engineers and mine reclamation specialists may have different slope requirements. The reclamation experts prefer slight grades and larger footprints that are easier to reclaim, while operations managers want reclamation to take as small a space as possible in order to facilitate mining operations. As a result of this conflict, some reclamation specialists are forced to stack waste rock in piles with a 2.5 to 1 slope while tailings dams have slopes that range from 2.5:1 to 3:1. Although Nevada’s Carlin Trend it isn’t as rugged as many mining areas in the West, according to Nevada Mining Association, a major reclamation problem is reducing waste rock slopes to 3:1.
Then there is the problem of the mine’s location, soil content and its ability to support plant growth. Many mines in the west can be found at altitudes of over 5,000 feet, near the Continental Divide and having annual growing seasons of approximately 100 days. In addition, many soils are acidic, low in nutrients and organics, and final reclamation slopes are prone to erosion. This means long term dust control first requires treating the slope so it can be a better host to vegetation.
At some mines in Nevada, the current method for handling steep slopes is to contour rip and broadcast seed. Four tons of mulch is hydraulically sprayed on each acre. The success rate of a 250 acres area seeded two growing seasons ago is an average 30% cover, with up to 70% in some areas.
At the Pipeline operations in Nevada, reclamation consists of waste rock dump shaping. The reclamation engineers are trying to make the shaping more natural. Slopes range between 2.5:1 and 3:1. However, techniques are very conventional with terracing, broadcast seeding, harrowing, and using fences.
According to mine environmentalists, the region only gets an average of seven inches of moisture a year, which makes reclamation harder. This means three types of vegetation that do well there are Salt Bush, Shad Scale, and Sage Brush. Instead of saving topsoil, the mine just imports alluvium from its Pipeline operation.
Newmont’s method for handling steep slopes is contouring, applying minerals and nutrients for better plant growth, and applying seeds with hydro mulching. Costs vary widely, but can run into thousands per acre. The biggest reclamation problem according to Newmont reclamation experts is weather. The altitude is high and the climate is dry enough that the timing and occurrence of rain and snow are critical for successful reclamation.
Western gold mines aren’t the only ones with a steep slope problem. The Munger Canyon coal mine had 2 : 1 and greater slopes that needed to be reclaimed. Their solution was to roughen the slope with a track-hoe and then immediately hand seeding the slope with annual plant seeds. They later hand seeded with permanent perennial seedmix. This was followed by hand spreading and crimping noxious weed-free straw mulch. Small stacks of straw bales were scattered throughout the site by the contractor as grading operations were completed, since there was no vehicle access to most of the site following completion of grading. Overall, the project was a success. The graded slopes are stable and blend into the adjacent steep slopes. There has been minimal erosion to date and very little settling in the portal backfill. Last year, the mine won a reclamation award from the state of Colorado for their steep slope reclamation project.
There are also some products, which aren’t dust control agents, but help control dust on steep slopes by encouraging plant growth. Erosion control netting is a temporary measure to protect the soil surface. It is usually made of a synthetic material that is laid and anchored over straw or other mulch to hold the mulch in place and protect it from wind and water damage. It reduces soil erosion and provides a good environment for the seeds to germinate, and acts as an interim dust control method. The material is photodegradable or biodegradable, so it will eventually decompose and is not a threat to the environment. Another product that can help is Slope Interrupter Devices - tube-like devices composed of a mesh or netting enclosing a biodegradable plant fiber that are installed on the slope contour.
Plants that have gained a foothold on a steep slope can be reinforced with Turf Reinforcement Materials (TRM). These are three-dimensional fabrics of synthetic materials placed in drainage channels to help anchor the plants, which are necessary to slow water speed on slopes.
When thinking of dust control on slopes, it is necessary to think about more than a chemical that tacks the dirt down. Long term dust control requires soil treatment, plant selection, contouring, re-vegetation, and protecting that vegetation from wind, erosion, and the weather.