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Mine water discharges - an environmental threat?

Introduction

The issue of Mine Water discharge is something that must be dealt with after any mine workings have closed down. Mining activity can 'extensively [modify] the natural groundwater flow regime' (Burke & Younger 2000, P.149) and so once mining activity has stopped, the resulting groundwater rebound is one of the major causes of this discharge. As the water table begins to rise back to its original position (it has to be lowered during mining to prevent flooding the mine) it is widely accepted that 'rebound commonly results in a marked deterioration in the quality of mine waters' (Younger, Banwart & Hedin 2002, P.230). This deterioration can take the form of Acid Mine Drainage (AMD), high Fe concentrations, high concentrations of metals other than Fe, High sulphate levels and "Yellow Boy". These contaminants all pose a great rise to the surrounding surface waters and local environment if left untreated and so the process of remediation in either an active or a passive form is essential to mitigate their effects.

This report looks at the problems caused by mine water drainage and its impacts, as well as at notable cases of AMD such as at the Wheal Jane tin mine, and at the Witbank Coalfield, South Africa. It also examines both active and passive methods of remediation in order to determine how great of an environmental threat mine water drainage really are.

Problems

The most common issue associated with mine water drainage is that of Acid Mine Drainage or AMD. AMD occurs when pyrite (FeS2) comes into contact with water and air, and the resulting chemical reactions can form both sulphuric acid and iron hydroxide. AMD is specifically water that has become polluted from contact with coal or mining activity. AMD has four main characteristics although the discharge does not have to have all of the following caricerteriscts to be classified as AMD.

"High acidity ... High metal concentrations -- iron is most common, High sulphate levels, Excessive suspended solids, which result in siltation that smothers insects" (SCRIP 2004)

The high acidity of AMD poses one of the greatest threats as when the mine water meets a stream or other water resource it can very effectively lower its pH. This change in pH levels can results in the precipitation of iron hydroxide from solution, which will then coat the base of stream forming the rust coloured "yellow boy". Once a stream base has become coated with this "yellow boy" the AMD is no longer just impacting on the water quality. This covering of iron hydroxide essentially smothers all insects living at the stream base because it prevents them from taking the oxygen from the water, '[once] the aquatic insects die, the fish have little or no food'. (SCRIP 2004) This shows how AMD can a detrimental impact on the local eco-system, not just on local water quality.

There are many notable cases where Acid Mine Drainage has been discharged on a large scale, one example being at the former Wheal Jane tin mine. Although the principal product of the mine was Cassiterite, 'older interconnecting workings also produced pyrite and arsenopyrite' (Pirrie, Power, Rollinson et al n.d). Once the mine was closed, the pumping that controlled water levels within the mine was stopped and on 17th November 1991 the mine water 'discharged into the River Carnon via an adit draining the mine' (Pirrie, Power, Rollinson et al n.d). The water that was discharged was characteristic of AMD, with an acidic pH of around 2.9 and containing a high concentration of metals. The water was treated with Lime to raise its pH, however during maintenance, 50,000m3 of of AMD were released into the Carnon River. The release of this much AMD (which contained not only Fe hydroxides, but Cadmium too) meant that there was great public pressure to treat the immediate impact of the AMD discharge on the river water, and for more long term solution to the issue of mine water discharge from Wheal Jane.

Another instance of AMD occurred at the Witbank Coalfield, South Africa. Here, mine water is discharging from several abandoned mines into the Blesbokspruit, which in turns feeds into the Olifants River. The result of this is that 'the water in the Blesbokspruit has a low pH and a high total dissolved solids' (Bell, Hälbich & Bullock 2002, P. 265). This causes problems further downstream within the rivers catchment area as it flows through the Kruger National Park which causes problems 'from the point of view of tourism and nature conservation' (Bell, Hälbich & Bullock 2002, P. 266) as well as from an agricultural point of view as much of the area is intensively farmed. The heavy metals found within stream waters have a detrimental effect on water quality and can leach from the water into the soil, impacting on the farming of the local area. These heavy metals are also found in algal mats at seepage points. The levels of heavy metals found within the waters are significantly higher than the safe levels determined by South African standards and so remediation is necessary to improve water quality for downstream users, and to ensure the area is still able to be farmed.

Further problems of mine water pollution are found within the surface-spoil heaps left behind. Perched groundwater, found within these spoil heaps, can cause problems as 'they often contain significant quantities of sulphide minerals, particularly pyrite' (Gandy & Younger 2003, P. 207) and so the flow of water through them results in the contamination of 'local water courses and groundwater' (Gandy & Younger 2003, P.207). Once the water has been contaminated with pyrite, the pH is lowered and under oxidising conditions iron hydroxides and ox hydroxides can precipitate out. This forms the familiar 'yellow boy' which coats stream beds and prevents insects and plants from taking oxygen out of the water.

Methods of Remediation

Once mine water discharge has contaminated water supplies, there are many ways in which the water can be treated to remove any contaminants. These can be divided into active or passive treatments. Active remediation involves an 'ongoing [input] of artificial energy and/or (bio) chemical reagents' (Younger, Banwart & Hedin 2002, P.271) in order to treat contaminated water supplies. In comparison, passive remediation is based on the idea that over time and as water flows in 'streams, rivers, wetlands and lakes' (Younger, Banwart & Hedin 2002, P. 311) its quality will improve naturally.

Passive Remediation

Passive remediation is essentially the natural improvement of mine water discharge over time and in 2000 was 'the technology of choice for the long-term remediation of such discharges' (Younger 2000, P.84). Remediation methods include Reed-beds, SAPS (Successive Alkalinity Producing Systems, sometimes reffered to as RAPS), Subsurface reactive barriers and Roughing filters.

Reed-beds

These beds are a type of constructed wetland and serve two main purposes, filtration and settlement. Influent water is channelled into the aerobic reed bed and as it flows, 'particles of Ferric Hydroxide become caught and remain within the natural filter whilst the rest of the mixture progresses' (The Coal Authority 2009). Reed-beds are ideal to treat net-alkali, ferruginous water as in a good reed-bed system the 'reedbeds are 'of a sufficient size that all of the particles are removed before the water re-enters the watercourse' (The Coal Authority 2009). Once the Ferric Hydroxide has been caught and retained by the reeds, it then settles to the base of the reed-bed. (NIRBC n.d) suggest that a properly maintened reed-bed should last for approximately 15 years.

Active Remediation

Active remediation covers the majority of methods used to treat mine water discharges; examples include membrane processes, Sorption and ion exchange and pH modification.

Membrane Processes

A membrane processes is the removal of contaminants from water by pushing the water (under high pressure) through a membrane. The 'clean' water passes through the membrane while the solutes are left on the other side in a form of 'highly concentrated liquor' (Younger, Banwart & Hedin 2002, P.300) The pore size of this membrane is dictated by what the user wants to remove from the water, and in the treatment of mine water discharge it is commonly solutes than need to be removed. This requires a membrane with a pore size of < 0.001 µm and comes under the title "Reverse osmosis". For the past 30 years efforts have been made to make reverse osmosis a viable option for the large scale treatment of mine water discharge, however the 'cost of the membranes and the energy required to obtain the high operating pressures' (Younger, Banwart & Hedin 2002, P.300) have proved difficult to overcome. Recently Keyplan have developed

"A three-stage membrane process where multiple stages of ultrafiltration and reverse osmosis membrane systems operate in series, with interstage precipitation of low solubility salts" (Jacqueline Holman 2009)

The plant treats high volumes of AMD waters effectively (99% recovery), meaning that reverse osmosis has been made more effective and cheaper as 'expensive evaporators and crystallisers' (Jacqueline Holman 2009) are no longer needed to further treat water once it has passed through the membranes.

pH Modification

Another method of treating mine waters, specifically AMD, is through pH modification. The basis of this method is that by raising the pH of the water, 'iron, and then other metals [will] precipitate out of solution' (House of Commons Library 2009, P.34). This is done by liming the water and is a straightforward procedure, as well as being easy to modify based on the concentrations of the metals in the water. Limitations of liming can include its high cost to maintain due to scaling and the chemically unstable sludge that it produces. Also, in raising the pH of the water to remove metals such as manganese, there is the risk that this may cause 'remobilisation of other metal hydroxides (e.g. aluminium)' (House of Commons Library 2009, P.34).

Sorption and ion exchange

The purpose of remediation of mine water through sorption and ion exchange is to use the money made from the sale of the base metals extracted (such as Lead and Zinc) to offset, or even exceed the cost of the treatment. The most effective materials for sorption are expensive which causes a problem as the value of the base metals extracted via this process is not enough to sell to a smelting company for a large enough profit to offset the cost of the materials used to extract the metals. Although in lab-based studies it has been proven that this method of removing metals from water is effective on a small scale, the difficulty is in 'scaling-up the lab-based reactors to cope with large mine water flows' (Younger, Banwart & Hedin 2002, P.299). Further issue is found in that as time goes on the concentrations of these base metals would gradually decline 'due to the gradual improvement of minewater quality that often occurs post-closure' (House of Commons Library 2009, P.35)

Resource Recovery

In Pennsylvania, USA, a scheme called Resource Recovery is being used to offset the costs incurred by trying to clean up the local water supplies. They are not only selling metals extracted from AMD waters, but they propose to use AMD 'as the source of water for a pump storage electric generation facility and as coolant for a co-generation plant' (SCRIP 2004). The result of this is that these remediation processes not only clean up contaminated water supplies which are damaging to farming, wildlife and human health (if drinking water sources also become contaminated), but income can be generated from their by-products meaning they could become self-funding.

Discussion + Conclusions

References

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