Water supply and hot springs, Bristol and Gloucester region
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
Large supplies of water are derived from springs and underground sources in a wide variety of formations.
The older Palaeozoic rocks yield only very small amounts of water. Considerable amounts are obtained from the sandstone and the conglomerate at the top of the Old Red Sandstone on the eastern side of the Forest of Dean, but in the Bristol and Somerset coalfields large supplies are not normally present in these rocks.
The Carboniferous Limestone is one of the best sources of water in the region. Large public supplies are derived from perennial springs or underground streams and rivers issuing from the foot of the limestone hills. Such overflow springs are due to the confining of the limestone waters by impermeable Triassic mudstones banked against the limestone and its marginal fringe of Dolomitic Conglomerate. Most of Bristol’s water supply, as well as that of Burnham, Weston-super-Mare and many parts of north-west Somerset comes from this source.
Fissured ‘pennant sandstone’ and other thick sandstones occurring in the Coal Measures also yield much water. Many drowned and abandoned shafts and iron workings have been adapted for water supply, both in the Forest of Dean and in the Bristol Coalfield. Current working of the mines, the draining of the country by adits and pumping from old pits has greatly lowered the water table in these areas. This has led to the drying-up or excessive lowering of the levels of many of the older wells in the Coal Measures and the development of an extensive network of piped supplies fed by central pumping stations.
In the New Red Sandstone both the Dolomitic Conglomerate (mentioned above) and the Triassic sandstones are important aquifers.
The Triassic red mudstones are relatively waterless, or yield hard and saline waters, but sandstones interbedded with the marls yield appreciable supplies in north Somerset, Bristol and north Gloucestershire. Only minor quantities of water are obtained from the White Lias and Blue Lias Limestones, while in the overlying thick clays saline waters, such as those of Cheltenham Spa, are frequently encountered. In central Somerset and the Vale of Gloucester, many small but often unsatisfactory supplies are obtained from patches of head or gravel resting on Mesozoic clays.
Some water supplies are obtained from the Upper Lias sands, for example from the Midford Sands in the Bath district; but on the whole these rocks contain too much silt to yield large constant supplies. The overlying Inferior Oolite and Great Oolite limestones are important aquifers and provide supplies for south-east Somerset and much of the otherwise waterless uplands of the Cotswolds.
Bath–Bristol hot springs
The origin and composition of the Bath thermal waters have been the subject of much scientific curiosity. Notable associated discoveries include the extraction of helium from the exsolved gases by Sir James Dewar, the identification of their radioactivity due to radium by Strutt in 1904 and that due to radon by Munro in 1928. The King’s Spring, the principal spring at Bath, emerges through the Lower Lias within a reservoir of Roman construction. It has a temperature of 45°C, which has been constant to within very narrow limits since records began in 1754. Two smaller springs at Bath, the Cross Bath and the Old Royal (or Hetting) Springs, have similar chemical compositions to that of the King’s Spring but with temperatures of 41°C and 47°C respectively. Thermal water also occurs at Clifton and Hotwells in the Avon Gorge, Bristol. The Hotwells Spring, which has a temperature of 24°C, discharges from Carboniferous Limestone into river muds just above low tide level.
Systematic investigation of the hot springs has been in progress since 1977 and the results are described in a comprehensive account edited by G A Kellaway (1991). The total yield of the three principal springs at Bath is of the order of 275 000 gals/day (1250 m3/d) and they issue from open fissures in structually complex Lower Carboniferous rocks overlain unconformably by Triassic and Lower Jurassic strata. The floor of the valley at Bath is formed of Lower Lias clay largely obscured by river gravel and alluvium. The fissures postdate the Lower Jurassic rocks and predate the river deposits.
Regional sampling of representative groundwaters from local aquifers show that the Bath thermal water is of unique composition and that the Hotwells water represents a mixture of Bath-type thermal water and shallow Carboniferous Limestone water in the ratio of 1:2.3. The hydrogen and oxygen isotopic compositions of the thermal water demonstrate that it is of meteoric origin. Both the age of the thermal water and the maximum depth at which it is circulating are open to question. Recent estimates of age based on geochemical and radioactivity evidence vary from ‘a few hundreds or thousands of years’ to 6000–8000 years. The local geothermal gradient is only known within broad limits but it has been calculated, for instance, that within the depth range of the Carboniferous Limestone under the Somerset Coalfield (2700 to 4300 m) a maximum subsurface temperature of 80°C ± 16°C (silica geothermometry) could be attained. A map showing the pre-Roman geomorphology of Bath (Kellaway in Cunliffe and Davenport, 1985) indicates the position of streams draining the hot springs in Iron Age times. Archaeological evidence shows that Mesolithic man occupied, at least temporarily, the site of the hot springs, and the position of these remains supports the view that the gravel filling of the King’s Spring was already in place by the end of Devensian times. The date of the first emergence of the spring is therefore older than that attributed to the water now rising from them.
- KEllaway, G A. (editor).1991. Hot springs of Bath: investigations of the thermal waters of the Avon Valley. (Bath: Bath City Council.)
- Cunliffe, B, and Davenport, P. 1985. The temple of Sulis Minerva at Bath. Vol. 1 The site. Oxford University Committee for Archaeology, Monograph, No. 7, 4–8.