Water (Operational Phase) - Page 6.

 

Good Practice - continued.

Contamination

The most effective way of preventing the contamination of groundwater and surface water is to control the source of pollution itself. Failing that, pollution can be prevented by ensuring that contaminated water is not allowed to come into contact with groundwater or surface water courses until it has been suitably treated. This can be achieved by the careful design of surface drainage within the site (possible diverting existing surface water courses running through the site), and by the installation of appropriate water treatment works. These will be considered in each of the following subsections.

Contamination of surface and ground waters by suspended sediment.

Little can be done to prevent fine grained suspended sediment from being picked up and transported by site run-off. Concentrations are commonly highest in the early stages of quarry excavation when overburden is being transported. It is an offence to knowingly pollute any surface watercourse, so there must be adequate provision for the control of this suspended sediment. A limit on the discharge of suspended solids is commonly set by the Environment Agency in the order of 30 mg/l. Mitigation and preventative measures to reduce the suspended sediment are given in Table 7.

Table 7. Measures to reduce suspended sediment.

Design of site drainage to ensure that relatively clean water is kept away from potential sources of suspended sediment, (e.g. traffic areas, stock piles, spoil heaps, washing plant, etc.).

Recycling of process water to reduce volumes of dirty water requiring treatment.

Ensuring that all water is treated, as appropriate, before leaving site.

Use of settlement lagoons.

Use of organic flocculants in conjunction with settlement lagoons to deal with excessive or very fine suspended solids.

Use of vegetated channels and reed beds for filtering.

Use of in-line turbidity meters to automatically monitor suspended solids content of pumped water and manage treatment accordingly.

 

A good example of recycling is to keep the water required for wheel-washes in a closed system. Water can be kept in the washing, drainage and filtering process through a number of commercially available systems, or something that is custom designed as in Photograph 3.

Photograph 3. Closed water system for wheel-wash.

Settlement lagoons are the primary method and are usually very efficient. There are a range of different designs and in high flow, high solid situations, a series of two or three may be required. Photographs 4 - 7 give some good examples of the design of some settlement lagoons.

Photograph 4 shows well maintained primary lagoons. The floating pump in the foreground is taking clean water for dust suppression by water bowsers. Photograph 5 shows the T-piece linking 2 lagoons in a way which allows free flow of water below the surface from one to the other, without any surface scum or oil being transferred. Photographs 6 & 7 show how the final stage of treatment is by specially constructed polishing lagoons. These are of the rectangular, broad-crest weir type, where the water rises through the centre and flows out over the complete perimeter. This encourages very slow flow rates, which encourage further settling of sediment.

The lagoons in Photograph 6 are surrounded by a storm overflow drain, which during periods of extreme rainfall, when the lagoons may not be able to cope with the volume of water, prevent the overflow from spreading everywhere.

From a safety point of view, note also the good quality security fencing and lifebuoys positioned around the lagoons. There are also a number of warning signs posted.

Photograph 4. Settling lagoon.		Photograph 5. Settling lagoon with outflow taking water below surface level.

 

Photograph 6. Polishing lagoons.		Photograph 7. Polishing lagoons.

 

After treatment the water will then be discharged into the appropriate water course. Usually these will be via constructed channels, which if there is a significant fall which would encourage fast flow, should be designed to prevent scour. Photograph 8 shows a concrete channel which is suitable for use on virgin ground, while the rock gabians shown in Photograph 9 are more appropriate for use on backfill, where there may some settling.

Photograph 8. Channel leading to discharge point.		Photograph 9. Rock gabians to reduce scouring effect.

 

The use of flocculants is no longer encouraged by the Environment Agency because of the potential impact of the chemicals on water quality. The use of vegetated channels and reed beds are an alternative, and have been very effective in dealing with heavy metal contaminants absorbed into suspended sediment particles. They are often used in conjunction with chemical treatment such as the addition of lime to trigger the precipitation of heavy metals in the form of oxides. They can also be used prior to discharge, where fine clay or silt is a problem. An example of using gravel berms and planted reeds to remove colloidal chalk over a sustained period is given in the Case Studies section cs43.

Further detailed guidance on aspects of controlling the impact of quarry operations on surface water is available in a handbook produced as part of an ALSF project L0026.

Pollution from natural contaminants, including Acid Rock Drainage

As ever, prevention is better than a cure. One option is to reduce the amount of time that each part of the workings are de-watered, thereby reducing the amount of time that pyritic material is exposed to air for oxidation. This can be achieved by working in small cells. Alternatively, capping of temporary or permanent spoil heaps with rock, clay, soil or synthetic materials, can minimise the infiltration of rainwater.

Some specific treatment methods are listed in Table 8.

Table 8. Treatment of pollution from natural contaminants.

Contaminated groundwater can be treated by chemical or biological methods, usually after it has been brought to the surface.

"Air sparging" may be used in-situ, which injects air beneath the groundwater table, oxygenating it, and helping the breakdown of contaminants.

The addition of lime will raise the pH of acidic water, causing heavy metals such as iron to precipitate out as hydroxides

Aeration of contaminated water before discharge.

Use of reed beds to absorb metals such as zinc, copper and iron.

Replanting to stabilize the surface of a spoil heap and to control pollutants within contaminated wetlands. Tree planting can be used to limit the infiltration of polluting salts, but care should be taken in floodplains that the trees do not disturb the normal river flow.

 

One method of removing high concentrations of iron from contaminated water is to aerate it. A fully lined lagoon is constructed and a system of pipes laid, through which air can be pumped (Photographs 10 & 11). This should be part of a larger treatment system.

Photograph 10. Lined lagoon ready for filling with iron contaminated water.		Photograph 11. Filled lagoon with are aerating contaminated water.

Working previously contaminated land.

As long as adequate mitigation measures are in place, mineral extraction can be considered an effective opportunity to remediate contaminated land. This is particularly true of opencast coal extraction. Mitigation and pollution control methods will be similar to those outlined above. Some contaminants will need special attention 81, 82.

Contamination from fuels, oils and solvents.

The impact of contamination from fuels and oils can be very severe, and so every effort should be made to prevent such material contaminating the water (see Table 9). In England, the Control of Pollution (Oil Storage) (England) Regulations came into effect in 2001, with advice and enforcement coming from the Environment Agency. In 2002, it has been reported that there were over 5,000 pollution incidents where oil was involved, most from leaking tanks during storage or delivery.

Table 9. Prevention of contamination from fuels, etc.

Use of safer alternatives to solvents wherever possible.

Use of electric pumps, rather than diesel pumps for de-watering.

Use of bunded or double skinned fuel tanks.

Use of designated fuelling points.

Use of chemical mats to absorb or neutralise accidental spillages in high risk areas.

Use of oil traps to prevent accidental spillage being discharged with site run-off.

On-site provision of flocculants to deal with spills quickly if they occur.

 

Any industrial or commercial site storing more than 200 litres above ground, must have a secondary containment facility, such as a bund or a drip tray, to prevent oil escaping. The requirements are listed in Table 10.

Table 10. Complying with the Control of Pollution Regulations, 2001.

Tanks, drums or other containers must be strong enough to hold the oil without leaking or bursting.

If possible, the oil container must be positioned away any vehicle traffic to avoid damage from collision.

A bund or drip tray must be positioned away from the container or its ancillary pipe work and equipment.

The bund must be sufficient to contain 110% of the maximum contents of the oil container.

Where more than one container is stored, the bund should be capable of storing 110% of the largest tank or 25% of the total storage capacity, whichever is the greater.

The bund base and the walls must be impermeable to water and oil and checked regularly for leaks.

Any valve, filter, sight gauge, vent pipe or other ancillary equipment must be kept within the bund when not in use.

No drainage valve may be fitted to the bund for the purpose of draining out rainwater.

Aboveground pipe work should be properly supported.

Underground pipe work should be protected from physical damage and have adequate leakage detection. If mechanical joints must be used, they should be readily accessible for inspection.

A number of other detailed requirements are included in the regulations, such as the positioning of sight gauges, fill points and other ancillary equipment.

 

The fuel containers shown in Photographs 12 & 13 are all designed to prevent any oil leakage from the outlet, and are known as bunded bowsers. Care must also be taken at the point where the plant is being filled by using splash trays or similar.

Photograph 12. Small oil bowser.		Photograph 13. Mobile oil bowser.water.

 

The large oil tank shown in Photograph 14 has a wall (or bund) built around it, not only to prevent leakage from the outlet, but also to contain any major failure of the oil tank itself.

Photograph 14. Bunded oil tank.

Contamination from industrial processes within the site.

This will be addressed in a similar way to those above. Additional measures could be to use a mobile electric-powered crusher and conveyor belt system, thus reducing the risk of fuel spills from dumper trucks. Covered conveyors, crushers and storage piles, together with safe storage of tyres, belts and drums etc., will minimise any contamination potential.

Discharging

The discharging of any treated water is important. It is likely to be into a local watercourse and should be done in such a way so as not to upset the flow characteristics of the existing watercourse too much (Photograph 15). Care should be taken to ensure there is no excessive scouring, and that the water mixes effectively. Some operators pipe the discharge water to the middle of a watercourse, to the point of maximum flow, so that it disperses quickly. The structure in Photograph 16 is not the outfall, but the point at which the pipe goes into the water, before terminating in the centre of the river.

Photograph 15. Water discharging into stream.		Photograph 16. Discharged water piped to middle of river.

 

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