Good Practice

A wide range of methods can be used to prevent, mitigate and/or remediate the adverse effects of surface mineral workings upon groundwater and surface water. These are described in detail in Chapter 7 of the Guide to Good Practice⁷⁸, and are listed alongside each of the identified potential effects in the Checklist which can be downloaded from the icon below.

However, final decisions regarding which issues need to be addressed for any given site, and how these should be dealt with, have to be made by MPA. Ideally, these decisions should be made in consultation with the mineral operator concerned, and with advice from appropriate statutory consultees, including the Environment Agency or SEPA (as appropriate).

Checklist (73K)
Checklist (111K)

Good Planning

One fundamental point which applies across the whole range of problems that can be encountered, is that adverse impacts can very often be avoided if the possibility of their occurrence is identified at the planning stage. By giving careful consideration to the location of proposed mineral workings at the earliest possible stage, it may be feasible to avoid important parts of an aquifer or to maintain a suitable distance from sensitive features such as existing or planned abstraction wells, surface watercourses or sensitive wetland areas.

Ideally, this should be done in a proactive fashion, at the time when preferred sites are being identified within the Minerals Local Plan. Options are far more limited once an application has been submitted, and a planning authority may find it difficult to refuse such an application on the grounds of hydrogeological impact, unless it has policies to support such an action within the Minerals Local Plan.

Where the location of a proposed new quarry cannot easily be altered, it should still be feasible to devise appropriate monitoring programmes, working methods and mitigation measures (where appropriate) before excavation begins, so that potential problems are prevented, as far as possible, rather than having to be cured. Part Three of the Guide sets out some of the ways in which this essential concept can be taken on board by MPAs as part of their routine forward planning and development control procedures.

Flexibility

Whatever concerns are identified and mitigation measures proposed, there is a need to retain flexibility, both within planning agreements and in operational practice. It is not possible to predict all of the effects of mineral extraction at the planning stage or prior to commencement of works. Although initial mitigation measures can be proposed where potential problems are identified early, understanding the hydrogeological system within a particular area is a continually developing process and regular review is essential to determine whether initial assumptions were correct. This review process may lead to an increase or a decrease in the monitoring programme and a change in what are considered to be the main concerns.

Timescale is as important as physical scale when considering the impact of potential effects. For example, larger effects may often be short term, ceasing when working is complete with water levels or other aspects of the natural environment returning rapidly to their natural condition. Mitigation of these short term effects is often feasible and can be tied in with other activities on the site.

In other cases, however, effects that may appear to be relatively minor may continue long after working is complete and may even be permanent. The full impact of such effects may be more difficult to predict and more expensive to deal with if ongoing mitigation is required long after the working life of the site. This means that the timescale as well as the size of an effect should be taken into account when considering the overall implications.

Ground Investigation

The good practice necessary to avoid problems arising from ground investigation are included in the checklist which can be downloaded, but are not considered here in any more detail.

Physical Presence

Understanding the effect of the physical presence of a mineral working on the hydrogeology of the area requires a detailed understanding of the processes already at work. A case study^cs39 gives a good example of how some simple investigations corrected a number of misapprehensions, to the satisfaction of all the professionals involved.

Physical disturbance of surface water features
If possible, locate the working so as to avoid disturbance.
If avoidance is not possible, then studies should be carried out to determine the importance of the features under threat, assess the baseline conditions which may need to be replicated, and determine the most appropriate techniques for reducing or compensating for the disturbance, e.g.

• creation of replacement surface water features which should have natural characteristics (Photos 2a)


• re-establishment of disturbed water-dependent species at suitable existing or replicated habitats


• re-establishment of disturbed species within the site on completion.

Modification of rainfall/run-off relationships
Site water which is discharged can be divided so that the appropriate amounts go into surface watercourses and groundwater recharge. Balancing ponds which allow water storage should be used to control increased surface run-off.

Alteration of the catchment size of headwater streams
Care should be taken to ensure that drainage ditches are connected to existing watercourses so that original catchment areas are more or less maintained. Where this is not possible, channels should be allowed to readjust their capacity by natural erosion and deposition, or if necessary, by engineering works. If this is not acceptable, then bank protection measures and/or flow balancing ponds may be required.

Removal of unsaturated zone: increased vulnerability to contamination.
Take steps to control all sources of pollution, so it is not there in the first place! (see section on contamination). Retain a sufficient thickness of unsaturated strata beneath the base of the excavation and the highest observed or predicted groundwater levels. Depth restrictions are used in some cases. Predicting groundwater levels can be very difficult and should be based on long term monitoring where possible.

Removal of unsaturated zone: changes in rates of groundwater response to rainfall
The same steps should be taken as above.

Removal of unsaturated zone: loss of temporary groundwater storage
If this is likely to be a major effect, then depth restrictions may be required as above. Compensation ponds have been used as a means of replacing the natural role of the unsaturated zone in sustaining summer flows. They can be filled during the winter and released during the summer, but their effectiveness is not fully proven.

Excavation below the water table: loss of groundwater resources.
If loss of storage is significant then the following options may need to be considered:

• limiting the depth of excavation;


• use of compensation ponds;


• backfilling with permeable fill;


• reclaim the site to open water without lining.

Excavation below the water table: re-adjustment of hydraulic gradients.
This can be limited by progressive working and rolling restoration. The long-axis of excavation can be aligned at right angles to the local groundwater gradient.

Excavation below the water table: evaporation losses from open water.
This is not usually a major problem. Minimising the area of open water at any one time and reducing the exposure to wind by planting trees or building embankments will help.

Removal of confining strata - floor heave and seepage.
A geotechnical assessment will be required to ensure an adequate thickness of rock is left to contain the aquifer pressure. If problems occur, then pressure could be released by pumping from boreholes.

De-Watering

General, including loss of ground water resources.
If de-watering cannot be avoided by site location, then a number of mitigation measures are possible. These all require good understanding of the groundwater and surface water systems, and are listed below.

• Limiting excavation so that sensitive areas are not affected.


• Recharging the abstracted water in to the ground, rather than discharging it away from the area. This can be difficult, as it is vital that the water being recharged is not contaminated with silt or micro-biological material.


• Carry out de-watering in small cells, so as to reduce the pumping rates and surface area of de-watering.


• Installation of a low-permeability cut-off barrier around the site or between excavation and sensitive area.

For shallow aquifers, such as sand and gravel deposits, cut off barriers may be constructed by digging a trench of adequate depth and length, and then filling it with material of low permeability such as clay, bentonite slurry or a suitable geomembrane. A case study^cs40 is a good example of the successful use of a low-permeability clay filled trench, together with close monitoring and a detailed action plan, to prevent the de-watering of a SSSI very close to a quarry. For deeper aquifers, cement grout can be injected into a series of specially drilled boreholes and forced by pressure into the surrounding rock to form a continuous grout curtain. This can be expensive and effect the groundwater on the "out-side" of the barrier.

In some situations where there is a delay between excavating an area and restoring it, it may be possible to reduce the duration of de-watering by creating bunds and 'flooding' those areas to allow the water table to recover^cs44.

Drying up of abstraction wells.
If the general measures outlined above are not sufficient to prevent the drying up of abstraction wells, then alternative supplies may need to be found, by constructing new wells or deepening existing ones. A case study^cs41 describes the successful deepening of a well used to supply water to a cottage.

Impacts on surface water features.
In addition to the general measures outlined above, the following measures may be employed.

Water being pumped out of the excavation can be discharged (after appropriate treatment) where it is required. The Mendips schemes, including Whatley Quarry and Torr Quarry, involve diverting some or all of the water pumped from the quarries into one or more streams at controlled rates of discharge, when the flows within an agreed 'control section' of one of the streams fall below a predetermined threshold. In effect, the dewatering process represents a local diversion of the natural flow of groundwater into surface watercourses, without significant loss of water resources, limited mainly to the immediate 'zone of influence' of water table lowering around the quarries. However, where the abstractions represent a significant proportion of the total flow of water through the aquifer, then downstream sections of the aquifer may be greatly derogated.

At Foster Yeoman's Torr Quarry ^cs32, a reservoir (Photo 2b) has been constructed which pumps clean water, taken from the quarry, to the local streams. Independent computer controlled pumps deliver water to two brooks as and when needed with their respective augmentation rates. The threshold level varies seasonally, to allow for the fact that flows are naturally lower during the summer. The reservoir is also developing as a wildlife habitat, with over 50 species of bird spotted including Little Egret, Green Sandpiper and a pair of Little Ringed Plovers with a reed-bed also being planned.



Stream augmentation is generally beneficial to down-stream dependants since it allows flow volumes and water quality to be maintained throughout the year, even during natural low-flow periods. However, by keeping flows unnaturally high at such times, this could lead to changes to natural ecosystems in some cases. Care is therefore needed to replicate the natural flow regime as far as possible, making provision for appropriate seasonal variations. Attention also needs to be given to water quality, since abstracted groundwater is unlikely to have the same chemical, biological and temperature characteristics as natural surface water.

Water table rebound
The prediction of the level that the water table will revert to after dewatering stops is difficult, and required extensive monitoring and modelling. If it is anticipated, then the following measures can be employed.

• The ground levels in disused quarries can be raised before redevelopment.


• Slope angles in cuttings and embankments can be reduced to allow for potential slope instability that could occur as groundwater levels rise.


• Retention of strips of unexcavated rock to help maintain stability of backfilled slopes.

Contamination caused or exacerbated by changes to groundwater flow paths.
Ideally the contamination itself should be dealt with, but if this is not possible then the measures listed above may be used, together with:

• use of low permeability cut-off barriers;


• control of groundwater flow paths through strategic groundwater pumping and/or recharge;


• sensitive sites may be protected from contamination by causing further, deliberate changes to local groundwater levels.

Saline intrusion caused or exacerbated by changes to groundwater flow paths.
In coastal areas, where this may be a problem, strategic groundwater pumping may create a hydraulic barrier.
A "stand-off" may be needed to prevent saline water being drawn in to abstraction boreholes.

Subsidence and settlement caused by falling groundwater levels and induced flows.
Identify vulnerable areas by means of geotechnical surveys and hydrogeological assessment. Areas at risk should be subject to the general mitigation methods outlined above, and detailed monitoring should be carried out.

Degradation of archaeological sites.
If possible, de-watering should be avoided using one of the methods outlined above. Guidance on managing archaeology in wetland environments have been published by English Heritage⁷⁹ and useful information is available⁸⁰ on preventative, in situ conservation techniques for archaeological sites, including those in wetland areas.

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 (Table 13). This can be achieved in a number of ways which will be considered in each of the following subsections.

Contamination of surface and groundwaters 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 30mg/l. Mitigation and preventative measures to reduce the suspended sediment include:

• 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 "process" water is to keep the water required for wheelwashes 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 Photo 3.



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 2 or 3 may be required. Photos 4 - 7 give some good examples of the design of some settlement lagoons.

Photo 4 shows well maintained primary lagoons. The floating pump in the foreground is taking clean water for dust suppression by water bowsers. Photo 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. Photos 6&7 shows 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 Photo 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. The drains are lined with Reno mattresses to prevent scouring.

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.

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. Photo 8 shows a concrete channel which is suitable for use on virgin ground, while the rock gabians shown in Photo 8a are more appropriate for use on backfill, where there may some settling.



The use of vegetated channels and reed beds 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.

Pollution from natural contaminants, including Acid Rock Drainage
As ever, prevention is better than a cure, so measures which might prevent contamination in the first place would include the following.

• Reducing 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.


• 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 14.

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 (Photos 9 & 10). This should be part of a larger treatment system.



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 outlines above. Some contaminants will need particular attention which is beyond the remit of this web site^{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. 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, there were over 5,000 pollution incidents where oil was involved, most from leaking tanks during storage or delivery.

Measures to minimise the risk of contamination from industrial fuels, oils and solvents include:

• 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 15 and all new oil stores should have complied since 1^st March 2002, those at "significant risk" had to comply from 1^st September 2003, with all remaining oil stores complying from 1^st September 2005.

The fuel containers shown in Figures 11-13 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.



The large oil tank shown in Photo 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.



A detailed diagram of an effective oil trap is given in Figure 9. This will only be required if measures to prevent water becoming contaminated with oil have not succeeded.



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 (Photo 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 Photo 16 is not the outfall, but the point at which the pipe goes into the water, before terminating in the centre of the river.

Reclamation

Recent changes in environmental legislation and planning guidance means there is now a requirement to consider the whole life of a site, from excavation to reclamation. The effects of reclamation on surface and groundwater are therefore considered here.

Reclamation to open water
There is potential for contamination of both the open water body itself, and groundwater, which can be prevented by the following.

• If the open water is designated as a local nature reserve, it will usually receive a degree of management.
• Ban or restrict motorised sports.
• Restrict adjacent land uses to those with little or no pollution risk.
• Control or prevent run-off from adjacent land.
• Ensure backfill and landscaping material is free of contamination.
• Avoid stagnation by ensuring sufficient throughflow.

Restoring to open water can be part of creating a wetland habitat (Photo 17), or it can be for recreational purposes such as a watersports centre (Photo 18) or diving centre^cs46. In line with the guidelines above, the only powered craft allowed on the lake is the rescue boat, and an electric powered launch.



Additional measures to prevent contamination of the groundwater would be to line the excavation with low permeability material. Care must also be taken where open water reclamation schemes are sited in floodplains.

Backfilling with overburden or imported fill: effects on site drainage and surface run-off
This will depend on the nature of the backfill material, but suitable topography should be created to provide efficient and controlled run-off. Land drains can be installed to compensate for low permeability material.

Backfilling with overburden or imported fill: effects on groundwater levels and flow paths.
Low permeability fills can distort patterns of groundwater flow, so there should be good monitoring and modelling to predict whether the following measures might be needed. If the hydrogeology is clearly understood then it is often possible to take very simple measures to ensure that there are no negative impacts^cs45.
Installation of channels of higher permeability material, and shaping the up-slope end of the quarry, to facilitate the flow of groundwater around the backfilled excavation.
Installation of land drains, discharging into surface watercourses, and vertical gravel drains to deal with problems of excess water and elevated ground water levels.

Backfilling with overburden or imported fill: effects on water quality
Measures to limit the effects on water quality of backfilling include the following.

• Wherever possible, use only inert backfill and landscaping materials.
• Restrict the period of time for which potentially contaminating material is exposed to oxidation.
• Use low-premeability liners, grout curtains or cut-off barriers to isolate contaminated material.
• Restrict use of contaminated backfill to areas above the predicted rebound groundwater levels.

Reclamation to Engineered Landfill
Landfill and waste management sites within disused quarries represent a major potential threat to the water environment unless they are designed and operated strictly in accordance with current best practice. Detailed guidance on such practice is given in the former DoE's Waste Management Paper 26B, "Landfill Design, Construction and Operational Practice". A simple list of measures is given in Table 16

Low level restoration On-going pumping is usually required in this situation, although the pumping requirements can be reduced by partial exclusion of groundwater by means of bunds or liners.

The requirement for long-term pumping to keep the site drained would normally be covered through planning obligations (Section 106). Consideration should be given to the potential effects of stopping the pumping and also to the effects of long-term pumping on other water users or wetland areas which

After Use

Agricultural after-use
Agricultural practices which could be harmful to groundwater and/or surface water, such as the intensive use of nitrate fertilisers, cannot easily be controlled through the planning system, once the land has been reclaimed for agricultural purposes. However, to minimise the risks, the following measures may be considered as part of the reclamation design.

• Control and minimise the use of fertilisers as much as possible.
• Retain an adequate thickness of unsaturated material beneath the excavation to help attenuate contaminants.
• Backfill with inert, preferably low-permeability material after excavation, to restore or increase the thickness of the unsaturated zone.
• Avoid prolonged delays in the re-use of stripped topsoils which contain chemical residues.

Studies and modelling may need to be carried out to assess the potential impact of schemes on the water environment, and appropriate measures taken to avoid or compensate for these.

Forestry and habitat regeneration
Mitigation work, where necessary, will depend on the specific nature of the problem. Drainage schemes on sloping sites, for example, will need to be designed in such a way that they do not exacerbate flooding and erosion. On floodplain sites, the establishment of a dense scrub or woodland could hinder the flow of floodwaters and may not, therefore, be appropriate.

In one situation^cs47 where restoration to agriculture may have resulted in a local aquifer being affected by nitrates, the site was restored as an ecologically important heathland project.

Industrial, commercial and residential after-use
The main risk from these forms of after-use is pollution from industrial processes, waste and contaminated run-off. While pollution from these sources is controlled, to some extent, by existing regulations and good practice techniques, the level of potential risk can be greater in reclaimed mineral workings, because of the increased vulnerability of underlying aquifers. Appropriate mitigation measures for industrial developments in disused quarries include the following.

• Restrict industrial after-uses to those with little or no risk of pollution.
• Limitation and control of potential pollutants as much as possible.
• Retention of an adequate thickness of unsaturated material above the water table to help the attenuation of contaminants.
• Where this is not possible, backfill with inert materials to re-establish an adequate thickness of unsaturated material before redevelopment.
• Use of low-permeability capping material to restrict the infiltration of contaminants.
• Control of run-off from the site to limit impact on surface water. Treat before discharge if appropriate.

Where reclaimed sites are to be used for housing, attention would need to be given to the control of surface run-off that may be contaminated with vehicle fuels, oils and de-icing agents, particularly from busier roads and car parking areas. Special attention may also need to be given to the prevention of accidental leakage from sewers and domestic waste connections.

Assessment and Monitoring

An essential aspect of avoiding or minimising adverse effects is the use of appropriate and effective techniques to assess what may happen, and to monitor what actually happens. This applies both to the potential impacts caused by surface mineral workings, and to the effectiveness and possible side-effects of any mitigation works that are used.

A range of different assessment and monitoring procedures may be required, ranging from initial desk studies and baseline monitoring to the assessment of potential impacts, reporting and regular review. Each of the steps involved has its own specific purpose, often building on the results of a previous stage. It is worth bearing in mind however, that invasive monitoring procedures can themselves carry potential impacts. If a single monitoring borehole is used to monitor water water levels and quality in two different horizons, there is always the possibility that a pathway could be created which enables the waters to mix, as it can be difficult to install and maintain effective seals. Two of the case studies ^{cs37, cs38} describe successful solutions where two boreholes were constructed for each monitoring station, one to sample only the lower aquifer and the other to sample the upper one.

A common factor in most applications for surface mineral working is the need for adequate monitoring of all potential impacts on groundwater and surface water, and of consequential impacts on the environment in general. Such requirements, which may be set out in conditions attached to planning permissions or in Section 106 Planning Obligations, can have long-term man-power and financial implications for mineral operators. A clear test must, therefore, be applied by the MPA as to the necessity for such works. Liaison between the MPA, the Environment Agency and the mineral operator is essential to identify the specific purpose of each monitoring requirement, and to ensure that the monitoring is adequate but not excessive.

It is also important to identify, in advance, how the results of each monitoring exercise might lead to other actions (e.g. cessation of dewatering or implementation of specific mitigation measures if groundwater levels fall below an agreed threshold). Planners and other decision-makers are increasingly being faced with the need to make decisions on the basis of groundwater or surface water modelling results, especially for larger sites. Whilst planners need not be fully familiar with the detailed technical issues associated with modelling, it is essential that they at least have a basic understanding of the capabilities and limitations of these techniques, as reviewed in the main Guide to Good Practice⁷⁸.

Of paramount importance is the need to recognise that computer models, however good, are only simplified approximations of reality, and (at best) are only as good as the data fed into them. It is, therefore, essential that independent opinions from experienced hydrogeologists and other specialists, as appropriate, should always be sought before conclusions from modelling exercises are accepted. It is equally important that modelling should not automatically be requested as an aid to decision-making: it will not always be appropriate.