Fingerprinting mine water in the eastern sector of the South Wales Coalfield

Abstract

AbstractCoal has been mined in the South Wales Coalfield for centuries. Substantial dewatering operations have attended these activities, particularly with the advent of steam and electric pumps that facilitated deeper mining operations. However, underground coal mining in South Wales has been in decline since 1913. The closure of the last deep mine in the eastern part of the coalfield in 1992 entailed a cessation of pumping over a large geographical area, with a resultant recovery of the groundwater. A consequence of this recovery is that emissions of mine water are occurring at the surface.Surface water emissions of known provenance from the Upper Coal Measures were sampled in 2000 and characterized in terms of their major ion composition. Also, known emissions of mine water within the catchments of the Rivers Ebbw, Ebbw Fach, Rhymney and Taff Bargoed were sampled, and the major ion data plotted on trilinear Piper diagrams. The results indicate that two distinctive hydrochemical facies could be identified from these plots. By combining knowledge of the underground mine workings and lithology, and by comparison with the Upper Coal Measures ‘fingerprint’, it is possible to divide the mine water emissions into those sourced from the Upper and Middle and Lower Coal Measures, and at times to infer a degree of mixing between the two.The conclusion is that in the eastern valleys of the South Wales Coalfield, the Upper Coal Measures water has a calcium-sulphate facies and the Middle and Lower Coal Measures water has a sodium-sulphate to sodium-bicarbonate facies. This information can be added to the available knowledge to aid in assessing the nature and the progress of groundwater recovery within the coalfield.This paper refers to preliminary work that has been carried out in the South Wales Coalfield to attempt to ‘fingerprint’ mine water emissions to identify their underground source. This, in turn, provides valuable information on the hydrogeological regime and, coupled with the geological and mining setting, may further assist in identifying underground flow paths. This information helps to fill in some of the ‘knowledge gaps’ and provide a tool for use in the prediction of future surface mine water emissions and in the formulation of treatment-mitigation strategies. The current work uses observations and interpretations of the hydrochemistry of the coal Measures to develop a robust and simple to use method of characterizing and classifying mine water emissions within the South Wales Coalfield. It is anticipated that a similar interpretive process could be applied to mine water emissions in other coalfields, although it should be stressed that this cannot be considered as a ‘stand alone’ process and that the initial characterization of the mine water needs to be carried out with reference to the geological and mining setting for any particular area under consideration.