Water (Pre-operational Phase) - Page 4.

 

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, and these are described in detail in the Guide to Good Practice L0017. 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 the MPA concerned. Ideally, these decisions should be made in consultation with the mineral operator, and with advice from appropriate statutory consultees, including the Environment Agency or SEPA (as appropriate).

As well as considering mitigation measures associated with ground investigation, this section outlines some of the aspects of "good planning".

Ground Investigation

Two main types of impact need to be considered: those that may be caused simply by the hydrogeological pathways created by the existence of boreholes and trial pits; and those caused by in-situ testing within boreholes which involve the artificial addition or removal of water. As with all types of impact, avoidance should always be the first consideration. However, avoidance of impacts relating to ground investigations by means of careful planning and site selection cannot be relied upon and attention therefore needs to be focused on site-specific solutions involving the use of good practice investigative techniques.

Exploratory Boreholes and Trial Pits

By their very nature, all forms of intrusive ground investigation have the potential to create links or pathways between the surface and subsurface, and between discrete geological layers within the subsurface itself. Such pathways may allow water to flow vertically between the different layers – either downwards if surface water is allowed to drain into an open borehole or pit, or upwards if a borehole intersects groundwater under pressure within a confined aquifer. A borehole may even intersect two or more separate aquifer units with different hydrostatic pressures, allowing groundwater to flow between them, from higher to lower pressure.

The significance of these effects, if they were allowed to happen, relates mainly to the possibility of groundwater contamination, or at least changes in groundwater chemistry, as a result of mixing. Polluted or saline groundwater within one aquifer unit could, for example, contaminate relatively clean water in another.

The degree of significance will be influenced, to a large extent, on the receptors which depend on those resources, whether these be water abstraction wells, natural springs, ecosystems or aspects of the built or historic environment. Even subtle changes in water chemistry can be significant in certain situations, for example where they result in changes in acidity or 'redox' potential, or where they exceed the thresholds of tolerance for particularly sensitive species. Changes in pH can give rise to the precipitation of dissolved material on the one hand, or the dissolution of the host strata, on the other, depending on circumstances.

All of these potential impacts can be controlled by preventing the formation of unwanted hydrogeological pathways in the first place. This, in turn, can be achieved by undertaking thorough geological desk studies prior to the commencement of intrusive investigations (so as to understand the potential risks in advance), and then by adopting good practice ground investigation techniques including, where appropriate, those that are designed specifically for use in the investigation of contaminated land.

Water can, for example, be prevented from flowing between different aquifer units or between contaminated and 'clean' horizons by the use of 'double drilling' techniques; i.e. drilling an oversize borehole to the base of the first unit (or to the base of a known contaminated layer), backfilling with cement or bentonite grout, and then drilling a smaller diameter borehole through this to lower levels. Similarly, boreholes and trial pits should be backfilled if necessary with cement/bentonite grout to prevent the ingress of any surface contamination, whilst those which need to remain in use (e.g. for monitoring purposes) can be fitted with a secure and lockable cover to prevent contamination by accidental spillage or vandalism.

Further details of appropriate techniques are given in the main Guide to Good PracticeL0017a (from which Table 6 is taken and updated).

Table 6. Codes of practice and guidance for site investigations.

British Standards Institution (2010): British Standard 5930:1999+A2:2010: Code of Practice for Site Investigations

British Standards Institution (2001): British Standard 10175: Code of Practice for the Investigation of Potentially Contaminated Sites

CIRIA (1995): Remedial Treatment For Contaminated Land (Vol III - Site Investigation And Assessment) CIRIA Special Publication 103

Environment Agency (1999): Decommissioning Redundant Boreholes and Wells

Environment Agency (2001): Technical Aspects of Site Investigation in Relation to Land Contamination: (Vols 1 and 2)

Environment Agency (2003): Guidance on Monitoring of Landfill Leachate, Groundwater and Surface water.

 

Borehole Tests

Three main types of in-situ hydrological borehole tests may be carried out and these are pump tests, rising head tests and falling head tests. Only the first is of real potential significance, as the head tests use very small amounts of water.

Reliance should be placed on careful monitoring of drawdown as a pump test is carried out, and on the immediate reduction or cessation of pumping if unacceptable drawdown is detected. Additional impacts may be associated with the discharge of water arising from a pump test, whether this be to a temporary recharge trench and/or to a nearby surface water feature. Impacts potentially include localised flooding, scouring, changes in the rate of flow, and contamination (including changes in water chemistry and/or suspended sediment). In view of the short duration of most pump tests, very few if any of these potential impacts are likely to pose a major threat, but the same elements of good practice should generally be followed, with monitoring to detect any adverse effects and with provision for the rate of pumping and/or the location of discharge to be changed if unacceptable impacts are detected.

Significantly higher levels of risk to the water environment can occur where investigations are carried out on sites that are already contaminated through previous land use (for example, industrial activities). In such cases it may be necessary for the abstracted water to be removed from the site by tanker for proper disposal at a suitably licensed site.

Good planning

The Guide to Good Practice L0017 illustrates a wide range of potential impacts on both groundwater and surface water. It also demonstrates, however, that opportunities exist to prevent or reduce these effects through careful assessment, planning and monitoring, as well as through the implementation of appropriate mitigation or remedial measures discussed in the Water – Operations section.

The guidance places emphasis on the need for open dialogue and forward planning as a means of avoiding potential impacts through careful site location, and on the implementation of studies and mitigation measures that are appropriate to each specific case, i.e. justified by the level of potential risk involved and fit for the intended purpose.

A number of general principles can be extracted from the guidance which are widely applicable to many different types of surface mineral extraction and which represent the essentials of best practice. These principles are given in Table 7.

Table 7. Essentials of best practice (derived from the Guide to Good Practice L0017).

Prevention	Prevention of adverse effects (through early considerations of site location, and/or through the implementation of best working practices), is more effective than trying to reduce or compensate for a problem, once it has occurred.
Increased awareness	Best practice is encouraged and supported by increased awareness of potential issues and available solutions by both planning officers and applicants.
Open dialogue between all interested parties	Best practice is enhanced by open dialogue between all interested parties throughout all stages of plan-making and before, during and after the formal determination of individual applications.
Pre-application discussions	There is particular merit in pre-application discussions which enable all potential issues to be identified, and often resolved, before the formal submission of a planning application.
Mineral operators	Mineral operators, because of their knowledge and experience of site conditions and practical engineering methods, are often best placed to suggest appropriate mitigation measures to deal with specific issues, once these have been identified.
Environment Agency / SEPA	The Environment Agency / SEPA, in their role as statutory consultees, are often best-placed to verify both the extent of potential issues relating to the water environment which need to be addressed, and the efficacy of proposed mitigation measures. Also, because of their expertise in hydrogeology and hydrology, these organisations are generally best placed to advise on the scope of groundwater and surface water monitoring requirements, and to review the results obtained.
Procedural frameworks	Clearly defined procedural frameworks for both forward planning and development control can help to ensure that adequate consultation takes place with key interest groups and that essential information is not overlooked.
Expert advice	The benefit of expert advice provided by consultees to mineral planning authorities can often be enhanced by asking specific questions or by highlighting specific issues, rather than simply asking for comments on an application.
Precautionary principle	The precautionary principle is of fundamental importance in assessing planning conditions: unless a particular issue can be positively ruled out through pre-application discussions, it is prudent to ensure that an adequate programme of monitoring is implemented to assess whether or not an effect takes place, and that, if problems do arise, provision is made for working practices to be modified or suspended until such time as the problem is resolved.
Commitment from all	A commitment is needed, from all parties involved in the control of surface mineral workings, to work together in the interest of achieving an acceptable balance between economic and environmental priorities.

 

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