End of life: how are new technologies helping to shut down spent mines?

The closure of a mine site is an intensive process, requiring the removal of a wide range of dangerous equipment and waste in order to return it to a safe ecological area. But technology has come a long way over the last few years to ease the process, with new developments improving land management, sinkhole and groundwater remediation. Molly Lempriere takes a look.

The closure of a mine site is a highly complex process, involving the disposal of damaging waste and equipment, as well as the challenge of returning the land to ecological safety. In the past many sites have been either abandoned or ineffectively remediated due to a lack of understanding, tools and technology.

This is changing, however. As regulations have increased and tightened so technology has stepped up to help with everything from land management and sinkholes to soil reclamation and ground water. Technology has predominantly focused on the remediation of sites. Mining activity can create a large amount of contaminants within the soil and groundwater at the site and nearby. This includes toxic chemicals which are the result either of mining techniques, or chemical reactions at the site.

Alternatively some of the biggest advances in mine closures have been made in geomorphic software, technology which allows mine sites to be returned to a natural and cohesive form. Previous to geomorphic advances the main techniques were insufficient and often caused further damage such as tiered landscapes that were unstable, or valley fills which often lead to large-scale pollution of water.

Here are some of the most promising technologies and techniques helping companies around the world to shut down their spent mines efficiently and safely.

Mine waste solidification and stabilisation

Solidification and stabilisation technologies are used to trap and contain harmful substances such as those created by mining to prevent them leaching into groundwater, lakes and streams. Both work by binding contaminated soil with agents such as clay and concrete, trapping them so that they can no longer affect surrounding water.

While solidification simply traps the contaminants, stabilisation causes a chemical reaction which also neutralises them. Specialised machinery is able to blend and facilitate the binding process within the soil before being covered by a cap. This further protects the ground and surface water within a spent mine site, ensures contaminants are secured within a set area and cannot cause sinkholes or pollute local streams and ponds.

"Already in the US solidification and stabilisation has been used in over 250 Superfund projects."

Both solidification and stabilisation can be done in-situ at the mine site itself, and it is a quick and cheap mode of site remediation. Already in the US it has been used in over 250 Superfund projects, a programme run by the Environmental Protection Agency (EPA) to clean-up the hundreds of contaminated sites around the country, many of them mine sites.

Bioremediation of shutdown mining operations

Bioremediation has already proved incredibly successful, helping to clean up over 100 polluted mine sites around the US, and is the most popular choice of soil remediation technology. The process uses microbes which occur naturally within soil and groundwater that feed on pollutants. Bioremediation creates the correct conditions to stimulate the growth of these microbes, which then consume contaminants such as petroleum and solvents, converting them into small amounts of wastewater and CO2.

Where the right sorts of microbes are not present they can be added using bioaugmentation; elsewhere amendments can be added to soil or water to stimulate effective conditions. Bioremediation can often be conducted in-situ, and where it is not possible to create the right conditions underground using amendments, the process can be simulated within tanks onsite that create the precise conditions.

This is a popular technology as it can be done at the mine site itself and requires little labour, although it is often more time-consuming than alternatives. It also has the advantage of removing the contaminants as opposed to simply trapping them, which occurs with technologies such as solidification.

Using nanotechnology to clean-up sites

While still a young technology, the use of nanoparticles to clean-up damaged sites is becoming increasingly popular. In particular, nanotechnology is being using to neutralise groundwater, the contamination of which is a common problem at spent mines due to the large amount substances and minerals created or exposed by the mining process. The materials that make-up these nanoparticles can absorb and break down toxic materials, thereby neutralising a mine site’s groundwater.

"The materials that make up nanoparticles can absorb and break down toxic materials, thereby neutralising a mine site’s groundwater if applied."

Nanotechnology is the use of particles with dimensions approximately between one and 100 nanometres, which have a large surface area-to-volume ratio that ensures they are highly reactive. Due to the minute size of nanoparticles they can be quickly dispersed over a wide area as they easily pervade the soil, capable of moving through areas of land too dense for macroparticles, such as the microbes used in bioremediation. They can easily be modified in a variety of ways to suit particular contaminants, using catalysts, coatings and encasements.

A particular benefit on nanotechnology is its ability to be pumped directly into water, negating the necessity to pump up ground water to remediate it. The adaptability of the particles to specific contaminants ensures that this more direct usage does not have a knock-on effect to the remediated land and water.

Sinkhole prediction

One of the problems often associated with spent mine sites is sinkholes. Mining dramatically alters the water table, which can lead to the erosion of soluble rocks such as chalk and gypsum. As these erode they can no longer support the upper layers of rock and soil, which collapse, creating a sinkhole. Sinkholes can also be created when caverns and caves created by the mining itself, buried with the closure of the mine, fall in on themselves.

However, NASA believes it can use its radar technology to predict the occurrence of sinkholes early enough to implement various currently available technologies and techniques to stop the sinkhole before it begins. By mapping high-risk areas over a long period of time, the agency announced that it can measure small surface changes which indicate the underground shifts that pre-empt sinkholes.

The creation of sinkholes by spent mines pose one of the greatest dangers to both humans and the environment in the wake of a mine shutdown, therefore the ability to predict them could drastically increase the safety of old sites. The process would take years, and the technology is still relatively untested, but should it be successful, knowledge of a coming sinkhole would allow companies to prevent them using technologies such as solidification.

Geomorphic software

Probably the biggest change in mine site reclamation technology is the use of geomorphic software. This allows a full map of the site, both above and below ground, to be created. The map takes into account probable seismic activity, as well as water flow patterns to allow for cohesive land reformation choices.

This understanding of future morphing of the landscape can be used to design a landscape that will withstand such changes without detrimental shifts. Before the availability of such software, mine sites were often tiered following their closure to account for the deep holes created. However, as this did not take into account the land’s natural movement and processes they often collapsed or morphed, leaving a dangerous and unusable site.

"GeoFluvTM designed complex upland slopes that integrate with a naturally-designed stream drainage network."

There are several examples geomorphic software now available, which have already been tested on spent mine sites. For example the patented GeoFluvtm method, the core of Carlson Software’s Natural Regrade software has been used to remediate spent and active spent mines, across the United States, as well as Australia and Spain. The software designed complex upland slopes that integrate with a naturally-designed stream drainage network to make a functional reclamation landform that is in keeping with the natural terrain. The software also allows quick editing of material movement to allow design construction at a low cost. This enables the site to return to a far more natural state, capable of natural fluctuations without damage that requires maintenance and repair, while minimizing, and in many cases lowering, construction earth movement costs.