Geothermal prospects in the UK are represented by low enthalpy resources in deep sedimentary basins and 'Hot Dry Rock' (HDR) resources in radiothermal granites, and possibly in deep basement rocks where they are overlain by thick low conductivity sediments. The low enthalpy resources are in Permo-Triassic sandstones at temperatures of more than 40°C. Four deep exploration wells have been drilled to investigate the potential of these sedimentary aquifers. The main HDR resource potential is associated with major granite batholiths in southwest and northern England where temperatures are predicted to be 200°C at about 5.4 and 6 km respectively. The HDR potential is being investigated by the Camborne School of Mines at their test site in Cornwall where three boreholes have been drilled to depths of between 2 and 2.5 km. The Hot Dry Rock Accessible Resource Base at temperatures of more than 100°C and depths of less than 7 km is 36 X 10 21 joules (equivalent to 130 × 10 4 million tons of coal). The low enthalpy Geothermal Resource of the Permo-Triassic sandstones at temperatures of more than 40°C is 200 × 10 l8 joules (equivalent to about 8000 million tons of coal). If only a small fraction of these resources could be developed, it would be significant in terms of the UK’s energy balance.

Isotope techniques have been applied to a study of groundwater movement in the Chalk of the London Basin. The 'age' of the water as determined by •4C measurements increases toward the central confined part of the basin where it exceeds 25,000 years and therefore originated during the Pleistocene. The age distribution supports previous interpretations of the permeability pattern, partly based on hydrochemistry. Measurements of tritium and the Stable isotope ratios of carbon, oxygen, and deuterium were also made. The •aC?2C ratio varied from -13%o at outcrop to values less negative than .-19oo in the central part of the basin, which is approaching the value of +2.35%o for the aquifer matrix. The •80 and deuterium ratios indicate that the Pleistocene waters were recharged at a mean air temperature less than 1 øC cooler than that of present day recharge, probably because recharge in the Pleistocene was limited to summer periods because of frozen ground in the winter. INTRODUCTION The London Basin is a synclinal structure in Cretaceous and Eocene strata in southeast England (Figure 1). The principal geological formations and their general thicknesses are [Water R'esources Board, 1972]' for the Eocene, London clay, 60-160 m, and Lower London Tertiaries, 12-40 m; and for the Cretaceøus, Chalk, 180-250 m, Upper Greensand, 6 m, and Gault, 50 m. The London clay is a relatively impermeable, overConsolidated clay, but the Lower London Tertiaries are represented by a series of sands, loams, and clays. In the central and eastern parts of the basin, the lower part of the Lower London Tertiaries is arenaceous. This facies, which has been referred to as the Basal Sands [Water Resources Board, 1972], attains a thickness of more than 30 m east of London and is commonly in hydraulic continuity with the Chalk. The Chalk is a soft, fine-grained limestone. Groundwater flow in the aquifer is predominantly through fissures, which are mainly in the upper 60 m and tend to be well developed along valleys, particularly in the unconfined areas but sometimes also in the confined area. The Chalk is underlain by the Upper Greensand and the Gault; the latter, being an argillaceous deposit, forms the base of the aquifer system. Much of the London Basin is drained by the River Thames. Groundwater flows from the outcrops on the northern and southern flanks of the basin through the confined part of the aquifer to natural discharge areas in the Thames valley. Since the eighteenth century, groundwater in the Chalk and the Lower London Tertiary sands has been developed for water supply. Groundwater levels have fallen in the center of the basin by as much as 70 m, and an increasing proportion of the natural discharge has been intercepted by pumped wells which are now the main outlet from the confined area, natural discharge no longer taking place from the central and eastern parts of the basin. The groundwater in the Chalk and Lower London Tertiary sands at outcrop is of the calcium bicarbonate type. Generally, this changes into a sodium bicarbonate type...

Following the increase in oil prices in the mid‐1970s, Britain assessed its geothermal resources. Low‐temperature, hot‐water resources, in the range 40°C to 100°C, occur in Permo‐Triassic sandstones in several deep sedimentary basins. In total these resources are estimated at 69.1 × 10 18 joules (J) (2576 million tonnes coal equivalent). Resources also occur in Upper Palaeozoic aquifers but, as the permeability of these aquifers depends upon fissures, exploitation is difficult. The only surface manifestations of hot water at depth are the warm springs at Bath and Bristol and in the Peak District and Taff Valley, which issue from the Carboniferous Limestone. The potential of radiothermal granites for Hot Dry Rock (HDR) development has also been investigated, particularly in the Carnmenellis granite in Cornwall. Three boreholes drilled in the granite to depths of over 2 km have been connected by developing natural fractures. Water circulation between the boreholes and through the fractured rock has been successful.

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Summary The U.K. contains seven groundwater provinces. Regional groundwater flow systems occur in these provinces above an essentially impermeable basement of varying age. The pattern of flow in the U.K. is strongly influenced by the distribution of the Permo-Triassic sandstones. Intermediate flow systems are encouraged by the scarp and vale topography that has developed as a consequence of the alternating sequence of aquifers and aquicludes in the Mesozoic. Regional changes in groundwater chemistry reveal the direction of flow paths, and isotopic ratios and inert gas contents give an indication of the residence time of water in the flow systems. The composition of the groundwater in deep systems has been modified by shale-membrane filtration. Density settling may be a feature in thick relatively homogeneous aquifers.