‘ Uisge Mèinne ’: mine water hydrogeology in the Celtic lands, from Kernow (Cornwall, UK) to Ceap Breattain (Cape Breton, Canada)

Abstract

Abstract Mining has historically made a major contribution to the economies of all the contemporary Celtic lands. At the start of the third Millennium, the bulk of mining activity has ceased, and problems associated with hydrogeological changes in abandoned mines are now common to all these lands. In global terms an unusually high proportion of mining in the Celtic lands has been by underground methods, a fact which is reflected in the range of hydrogeological problems encountered in these countries. Recent experiences in these countries offer insights which should be useful elsewhere when currently-active deep mines are eventually abandoned. Particular lessons are drawn from case studies and analyses of previously unpublished data from Alba (Scotland), Ceap Breattain (Cape Breton, Canada), Cymru (Wales) and Kernow (Cornwall). These lessons are: (1) The importance of recognizing the predominance of mined features in the post-closure hydrogeology of abandoned mines. (2) Dynamic temporal changes in hydrogeological behaviour arise from collapse of mined voids, caused by fluvial erosion by rapidly-flowing mine waters and/or by pneumatic fracturing by mine gases compressed in pockets during mine water rebound; these changes can have significant implications for human safety and environmental protection. (3) Net-acidic mine waters are generally restricted to situations in which high-sulphur strata are present in (i) recently-flooded deep-mine workings (ii) shallow partially flooded mine workings and (iii) perched groundwater systems in spoil heaps and opencast backfill. (4) Net-alkaline mine waters are associated with (i) low-sulphur strata in any hydrogeological setting and (ii) high-sulphur strata at depth in long-flooded workings. In practice, this means that the net-alkaline mine waters are far more abundant than the net-acidic. (5) The presence of limestone in a mined sequence is not on its own a guarantee that mine waters will be net-alkaline; the patterns of groundwater flow (which determine the transport of limestone dissolution products through the mined system) must also be favourable. (6) Mine water often becomes hydrochemically stratified during rebound. However, when discharge from a mined system commences, this stratification can break down, resulting in discharges considerably poorer in quality than would have been inferred by sampling the uppermost waters alone.