Potential Effects

The hydrological and hydrogeological regime can be affected by the four distinct aspects of surface mineral extraction and associated activities shown in Table 3.

These four aspects are not mutually exclusive. Contamination, for example can arise either from operations within the quarry itself, or as a result of the modified groundwater flow paths induced by large-scale dewatering, which may bring polluted groundwater in from an external source. In general, however, the four headings provide a convenient way of considering the very wide range of individual effects that can, potentially, be induced by quarries on groundwater and surface water.

In addition to the four aspects listed above, there are also the impacts that can occur as part of the reclamation and restoration of the site. These will be listed in their own section.

Although the actual effects associated with any given quarry are often limited to a few key issues, there is a need for all potential issues to be given consideration at the initial planning stage and to be kept under review throughout the lifetime of the quarry.

While some very broad generalisations can be made regarding the likelihood of different issues being important in different geological and environmental settings, the specific issues which need to be addressed and the ways in which these are dealt with can only be adequately assessed on a site-specific basis.

In view of this, the original Guide to Good Practice⁷⁸ provides an explanation of each individual type of impact on groundwater and surface water that can occur in association with each different aspect of mineral extraction, reclamation and after-use, together with an explanation of the techniques which can be used to prevent these from occurring or to reduce their severity. The impacts are summarised here, while the mitigation techniques will be listed in the section on Good Practice.

Technically, this section does not lie within the compass of the web site, but is included for completeness. Most ground investigation work takes the form of boreholes, which obviously penetrate the ground and introduce a route of possible flow for the groundwater which is above and below the water table.

The physical presence of an excavation can have a number of effects on the water. Some of these are associated with particular types of excavation, while others are more general.

Excavations, however deep, that are located entirely within the unsaturated zone of an unconfined aquifer will remain virtually dry, except for rain falling within the site and minor seepages from perched groundwater resting on clay bands. However, excavations that extend down into the saturated zone will inevitably fill with groundwater up to the level of the surrounding water table, unless this is artificially lowered by dewatering, which of course is vital if excavation in the quarry is to carry on. The scale of the problem will depend on the type of groundwater flow, which will considered later.

Dewatering is most commonly carried out by intermittent pumping from a sump located in the deepest part of the quarry, to keep pace with the inflow of groundwater. Alternatively, dewatering can involve the use of drains, abstraction of groundwater from boreholes or wellpoints located around the perimeter of the site, from trenches or from shafts.

Whichever method is used, the effect is broadly the same, creating a "cone of depression" in the water table, centred on the excavation (Figures 6 - 8).



In most natural systems, where groundwater can flow more easily in some directions than others, the cone of depression will not be a symmetrical feature that is circular in plan. Rather, it will be irregular, extending much further in some directions than others. In such circumstances it can more usefully be regarded as a "zone of influence of dewatering" rather than a simple cone of depression.

Water table rebound can occur in areas where dewatering has been maintained over a very long period of time, and local development in the area has taken place with regard to the modified groundwater conditions. When mineral extraction eventually ceases in the area, and the water table is allowed to return to its natural level, new problems of flooding, contamination and land instability may be experienced, as a result of rising groundwater levels. Such problems are most commonly associated with large-scale deep-mining activity (as in the Durham and East Pennine coalfields, for example), but can also occur more locally as a consequence of surface mineral working.

The contamination of water is obviously one of the key concerns from an environmental viewpoint. The list below shows that this can occur in a variety of ways, each of which needs particular attention, if contamination is to be avoided.

In some cases, the impact of surface mineral workings on the water environment can be beneficial rather than detrimental. This is especially the case where good practice techniques are used in the reclamation of quarries and opencast sites, to improve the quality of groundwater and surface water, or to create new wetland environments and increase local biodiversity. It is emphasised that, although each of the effects listed above can take place, and many of them are documented in one or more previous case histories, it is by no means inevitable that they will occur. The likelihood of occurrence of any particular problem is entirely dependent upon site specific circumstances, including the local geological, hydrogeological, topographical and environmental conditions.