Groundwater in Carboniferous carbonates : Field excursion to the Derbyshire "White Peak" District 26th June 2015

Carbonate rocks of Carboniferous (largely Viséan) age crop out widely in England and Wales but many of the outcrops are scattered and broken (Figure 1). The Peak District is one of six contiguous areas, the others being the Yorkshire Dales, the Northern Pennines, the Mendip Hills, South Wales (including the Forest of Dean) and North Wales (Gunn et al., 1998). These areas have commonly been regarded by hydrogeologists as having karstic drainage, whereas geologically more recent carbonates such as those deposited in Jurassic and Cretaceous times are commonly regarded as not being karst or, at best, 'weakly karstic'. In particular, there has been a tendency to equate karst drainage with surface landforms and to use 'caves as a measure of karst'. Hence, those carbonates with visually impressive surface karst landforms (dolines, blind and dry valleys) where there are well-developed and extensive cave systems are commonly assumed to have karstic hydrogeology and those without these landforms are assumed to be 'non-karstic'. One weakness of this simplistic approach is that caves in carbonates are simply conduits that have been enlarged by dissolution to a point where humans can explore them. Water flowing though a conduit that is, say, 0.2m in diameter and hence un-enterable will not behave any differently if after some distance the conduit attains a diameter of 0.4m and can be visited by humans. There is also an increased awareness that the marked spatial heterogeneity in carbonates at small scale (commonly expressed in terms of dual or tertiary porosity and permeability) is also present at the field scale. Hence, even the most karstic of regions, where groundwater moves through conduits and caves at velocities in the hundreds of metres per hour, is likely to also have areas where there are few conduits and where groundwater velocities may be as little as a few metres per day. The Derbyshire "White Peak" district provides an excellent location in which to discuss these concepts and their importance to management or water and mineral resources.