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Discussion |
12 Culcheth Hall Drive, Culcheth, Warrington WA3 4PS, UK (rick{at}brassingtonhydrogeology.co.uk)
F.C. Brassington writes: Shepley is to be congratulated on his detailed description of the Meerbrook Sough and analysis of the groundwater discharge from this abandoned mine drainage system (Shepley 2007). He has made a useful addition to the small but growing body of work on the hydrogeology of both the Derbyshire Carboniferous Limestone aquifer and the associated thermal groundwaters. I am particularly interested in his conclusion that the groundwater discharging through the Meerbrook Sough is predominantly from diffuse flow through the primary fracture network, particularly as this conclusion supports important aspects of my proposed conceptual model for the Derbyshire thermal springs (Brassington 2007).
Shepley's review of the Meerbrook Sough primarily considers the flow regime rather than the water temperature or chemistry. Edmunds (1971) gave the temperature of the sough discharge as 15.3 °C compared with that of the closest thermal springs at Matlock Bath, which is typically some 19.7 °C. He uses the water temperature to conclude that up to half the discharge from the sough is from thermal origins, although from consideration of the temperatures of the local groundwater this estimate appears conservative. The temperature measurements by Edmunds (1971) of other groundwater discharges indicate a range from less than 6 °C to over 9 °C for the non-thermal waters. Assuming the non-thermal groundwater entering the sough has a temperature at the top of this range, the thermal component would be about 60%, and taking the lower end of the range the thermal contribution would be about 67% of the total flow.
More detailed monitoring of water flowing from the sough (Shepley, pers. comm.) shows that the temperature has a seasonal variation of some 2–3 °C, with the value quoted by Edmunds (1971) being close to the average. The temperature also has long-term fluctuations, indicating that the proportion of thermal water changes under different hydrogeological conditions. Gunn et al. (2006) measured the temperature of the sough discharge as 17 °C, also indicating that the thermal component may be a larger proportion at times.
On the basis of the discussion above and the flow range quoted by Shepley (2007) of 50–70 Ml day–1, it can be concluded that the thermal discharge is more than 30 Ml day–1. The significance of this single discharge of thermal waters can be readily seen by a comparison with the flows of the thermal springs recorded by Edmunds (1971) (see Table 1). The largest flows are around 1 Ml day–1 and the total discharge from all the thermal springs is around 4 Ml day–1. On that basis, the Meerbrook Sough could be regarded as being the equivalent of a major karst conduit (cave system) that dominates the local drainage of groundwater flowing through the primary non-karst fractures, and is on a scale that is not yet known in the natural Derbyshire thermal flows.
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The entry of meteoric water into the system allowed karst processes to start forming the conduit and cave systems that exist today. Part of my hypothesis is that at some time the developing conduits encountered the primary fractures carrying the thermal flow, thereby providing a lower resistance flow path to the surface and, over time, producing the modern thermal springs, with the temperature of each thermal spring (see Table 1) depending on the depth at which the conduit flow commences.
The Meerbrook Sough appears to have performed the role of developing conduits by intercepting thermal waters in the southeastern corner of the Carboniferous Limestone outcrop some 4.5 km south of the Matlock Bath cluster of springs in an area where no thermal springs have been recorded. Where thermal flows have not been intercepted by conduits it can be expected that the thermal water discharges at the surface as seeps that are too small to be noticed, although this may also be the cause of the small number of temperature measurements made by Edmunds (1971) that are over 9 °C but where thermal springs were not considered to be present.
F.C. Brassington
12 Culcheth Hall Drive, Culcheth, Warrington WA3 4PS, UK (e-mail: rick@brassingtonhydrogeology.co.uk)
Received for publication 11 September 2007. Accepted for publication 1 November 2007.
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References |
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 TOP References |
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Brassington, F.C.A proposed conceptual model for the genesis of the Derbyshire thermal waters. Quarterly Journal of Engineering Geology and Hydrogeology, 40 2007. 35–46.
Edmunds, W.M. Hydrogeochemistry of groundwaters in the Derbyshire Dome with special reference to trace constituents. Report of the Institute of Geological Sciences, 71/7 1971.
Gunn, J., Bottrell, S.H., Lowe, D.J., Worthington, S.R.H., Deep groundwater flow and geochemical processes in limestone aquifers: evidence from thermal waters in Derbyshire, England, UK. Hydrogeology Journal, 14 2006. 868–881.[GeoRef]
Shepley, M.G.Analysis of flows from a large Carboniferous Limestone drainage adit, Derbyshire England by M. G. Shepley. Quarterly Journal of Engineering Geology and Hydrogeology, 40 2007. 123–135.
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