Evaluating the potential impact of opencast coal mining on water quality (Groundwater Regulations 1998) An assessment framework for Scotland

Assessing the impact of opencast coal mining on water quality Executive Summary The European Directive on Groundwater (80/68/EEC) was fully transposed into Scottish law by the introduction of the Groundwater Regulations 1998. These regulations forbid the introduction of certain substances (denoted as "List I substances") into groundwater, and also place limitations on the extent to which other substances ("List II substances") may be permitted to enter groundwater. Scottish opencast mining, which constitutes an 'activity' under the Groundwater Regulations, poses little risk of introducing List I substances into groundwater, but it has substantial potential to lead to the migration of several 'List II' substances. A code of practice recently published by the Scottish Executive (2003) provides sound advice on the control of pollution from non-mineral pollution sources during opencasting operations. It also identifies the need for the "assessment of the risk of release of List I and List II substances from storage in stockpiles or backfilling, prior to the activity being undertaken". The present document has been prepared to satisfy the need for an assessment framework for List I and II pollutants released by weathering of soils and rocks disturbed by opencast coal mining. Although this assessment framework assumes the user is familiar with opencast coal mining as it is currently practised in Scotland, a brief overview of opencast coal mining techniques is provided in Annex I in order to ensure the document is accessible to non-specialists. The development of the assessment framework has been founded upon a comprehensive state-of-the-art review of the key issues relating to the potential release of List I and II substances from soils and rocks disturbed by opencasting. This review is presented in full in Annex II. The principal topics which it addresses are: • The geochemical processes responsible for pollutant release and attenuation in coal-bearing strata affected by opencast mining, including: the oxidation of coal-bearing strata (including pyrite weathering); the role of the two most abundant iron-rich carbonate minerals (siderite and ankerite); neutralisation reactions involving other minerals (including aluminosilicates); and identification of possible sources of List I and List II substances in Scottish opencast drainage. • Mining hydrogeological controls on pollutant loadings, including: the hydrogeology of unworked coal-bearing strata in Scotland; the hydrogeological behaviour of opencast backfill; and relationships between opencast sites and surrounding groundwater systems. • Methods for characterising the pollutant release potential of opencast backfill, including consideration of "static tests" (acid-base accounting), "dynamic …