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Discussion |
1 Institute of Geological Sciences, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK (s.bottrell{at}see.leeds.ac.uk)
2 Limestone Research Group, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, UK
3 Limestone Research Group GEES (School of Geography, Earth & Environmental Sciences) University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
4 Worthington Groundwater, 55 Mayfair Avenue, Dundas, Ontario, Canada, L9H 3K9
5 Consultant Hydrogeologist, 12 Culcheth Hall Drive, Culcheth, Warrington WA3 4PS, UK (rick{at}brassingtonhydrogeology.co.uk)
S. Bottrell, D. Lowe, J. Gunn & S. Worthington write: We read with interest the hydrogeological analysis of the thermal waters rising from Dinantian limestones in Derbyshire presented by Brassington (2007) and note the similarities to a similar analysis by Gunn et al. (2006). Their analysis of deep thermal groundwater flows used similar geological reasoning and arrived at rather similar overall conclusions and a closely comparable conceptual model. The flow paths suggested in both works that are derived on a geological basis are confirmed by consideration of new data on sulphate stable isotopes (
34S and 18O) and Sr isotopes on thermal and non-thermal waters presented by Gunn et al. (2006), although there are also some differences, particularly with respect to the Buxton waters.
Chemical and isotopic compositions exhibited by the Buxton thermal waters differ markedly from those of the waters from other thermal risings and imply that a significant component of flow has interacted with non-limestone lithologies. The low Ca concentration and calcite saturation index of only 0.08 (Christopher 1981) are certainly atypical of limestone spring waters. Elevated radiogenic 87Sr content above those for all other thermal and non-thermal waters (together with, for example, high Mg and Mn concentrations as noted by Edmunds (1971)) indicate interaction of the deep Buxton flows with overlying sandstone aquifers at the limestone contact under the Goyt Syncline. Indeed, from the geochemical evidence, flow via sandstone aquifers in the Goyt Syncline could be a significant source of recharge for the Buxton thermal waters, although Brassington's hydrograph analysis clearly indicates that there is also a component of flow from the limestone. On the basis of their isotopic composition, the low concentrations of sulphate in the Buxton waters are consistent with derivation dominantly from pre-industrial rainfall (compare the pre-anthropogenic rainfall sulphate S isotope compositions derived from peat stratigraphy in north Derbyshire by Coulson et al. (2005)).
The group of thermal risings along the eastern margin of the Dinantian limestone outcrop (Matlock, Stoney Middleton, etc.) have very much higher sulphate concentrations and sulphate isotopic compositions identical to those of Dinantian evaporites, known to exist at c. 700 m depth, toward the base of the Dinantian sequence. This implies that the higher solubility of gypsum or anhydrite evaporites might have been critical to the early development of deep flow paths within this part of the aquifer (e.g. Worthington & Ford 1995). The Sr isotopic composition of these thermal waters shows no evidence for interaction with units other than Carboniferous limestones and evaporites. The depth of 700 m is consistent with the less elevated temperature of these risings compared with the Buxton waters. Shepley (2007, p. 130) also suggests that the thermal waters that enter Meerbrook Sough circulated to ‘depths greater than 600 m below surface’, which agrees well with the model of Gunn et al. (2006) in which Matlock-type waters are shown as migrating to c. 800 m depth.
Shepley (2007) modelled the discharge characteristics of one of the main thermal outputs from the Derbyshire White Peak Carboniferous limestone aquifer at Meerbrook Sough. The model was based on a 5% contribution of thermal water discharge to the Meerbrook Sough flow. High sulphate is characteristic of the Matlock thermal waters and the relatively high sulphate concentration of Meerbrook Sough compared with the ranges for thermal v. non-thermal springs (Table 1) might be taken to indicate a higher thermal water contribution than this (up to 30%). However, the sulphate isotopic data for Meerbrook Sough (Table 1) are rather variable and 34S depleted compared with Matlock thermal waters, indicating that a significant part of the elevated sulphate at Meerbrook Sough derives from other sources, probably oxidation of sulphide minerals in old mine workings and in the limestones and/or mineral veins in the rock volume dewatered by the adit (see Bottrell et al. 2000). In fact, the conclusion of Gunn et al. (2006) that c. 5% of the groundwater discharge from the whole White Peak limestone catchment is via deep thermal flows indicates that the analysis of flow by Shepley (2007) is for a groundwater catchment that is typical of this aquifer.
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F.C. Brassington writes: I am grateful to Professor Bottrell and his colleagues for their comments on my paper (Brassington 2007) and their discussion linking my conclusions with those derived from their study (Gunn et al. 2006). The aims of the two papers were slightly different, theirs being to examine deep-seated geotechnical processes in the aquifer using isotopic data, whereas mine was to derive a conceptual model that may assist understanding of why the thermal springs developed in the first place.
I note with interest the comments made on the possible influence on the Buxton system by groundwater derived from the Namurian aquifers within the Goyt Syncline. I agree that the Buxton waters differ from the other thermal springs, and the arguments for a Namurian component are attractive. However, I remain of the view that this thermal cell is essentially one within the Dinantian aquifer, as are the other postulated six cells that support the other thermal centres (see Brassington 2007, table 1). I also contend that the Buxton thermal flow cell follows a flow direction that is broadly described by my figure 8 (Brassington 2007), with possibly a minor component derived from groundwater in the Goyt Syncline. Such a flow direction with a downward limb towards the centre of the Derbyshire Dome structure provides an explanation for all the thermal springs being located close to the Dinantian–Namurian contact. Each thermal centre has different geochemical characteristics, which I have suggested reflect local differences in the geology. Such a conclusion does not preclude a contribution from the Goyt Syncline and it would be interesting to compare isotopic data from these Namurian aquifers with data from Buxton to shed more light on this aspect.
I share the pleasure that Professor Bottrell and his colleagues take in our independent studies having reached complementary conclusions and am pleased that we have been able to contribute to a better understanding of a complex, unusual and hitherto little studied groundwater system.
S. Bottrell1, D. Lowe2, J. Gunn3, S. Worthington4& F.C. Brassington5
1Institute of Geological Sciences, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK (e-mail: s.bottrell@see.leeds.ac.uk)
2Limestone Research Group, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, UK
3Limestone Research Group GEES (School of Geography, Earth & Environmental Sciences) University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
4Worthington Groundwater, 55 Mayfair Avenue, Dundas, Ontario, Canada, L9H 3K9
5Consultant Hydrogeologist, 12 Culcheth Hall Drive, Culcheth, Warrington WA3 4PS, UK (e-mail: rick@brassingtonhydrogeology.co.uk)
Received for publication 1 June 2007. Accepted for publication 5 October 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 springs. Quarterly Journal of Engineering Geology and Hydrogeology, 40 2007. 35–46.
Coulson, J., Bottrell, S.H., Lee, J., Recreating atmospheric sulphur deposition histories from peat stratigraphy: diagenetic conditions required for signal preservation and reconstruction of past sulphur deposition in the Derbyshire Peak District, UK. Chemical Geology, 218 2005. 223–248.[CrossRef][ISI][GeoRef]
Christopher, N.S.J. 1981. The karst hydrogeochemistry of the Carboniferous Limestone of North Derbyshire. PhD thesis, University of Leicester.
Edmunds, W.M. Hydrochemistry 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. Quarterly Journal of Engineering Geology and Hydrogeology, 40 2007. 123–135.
Worthington, S.R.H., Ford, D.C., High sulphate concentrations in limestone springs: an important factor in conduit initiation? Environmental Geology, 25 1995. 9–15.[CrossRef][GeoRef]
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