Groundwater may be obtained from almost any stratum in the sedimentary succession in the British Isles, as well as from igneous and metamorphic rocks. In many, such as clays and shales, volcanics and metamorphics, the permeable zone may well be limited to the depth to which weathering may reach, and this is likely to be no more than some 50 metres beneath the ground surface. In those strata which are not generally recognised to be aquifers, well-yields tend to be small (of the order of only a few cubic metres per day), uncertain as a continuous source (tending to fail in prolonged droughts), with an indifferent water quality, and with the sources vulnerable to pollution.
Of those featured in the Generalised list of aquifers in the UK, the Chalk, the Upper Greensand, the Lincolnshire Limestone and the Permo-Triassic sandstones are the most important from the viewpoint of public supply. From aquifers such as these, yields of 3,000 to 4,500 cubic metres per day are not unusual. For the next category, including the Lower Greensand and the Magnesian Limestone, yields to individual wells of 1,500 to 3,000 cubic metres per day can generally be expected. In the other aquifers, while occasional sources sufficient for large supplies may be developed, they tend to be important only locally. Throughout Wales, Scotland and Northern Ireland, aquifers are less extensively developed and tend to be only of relatively local importance.
The groundwater resources of an aquifer are naturally replenished from rainfall. During the summer months, when the potential evapotranspiration is high and soil moisture deficits are appreciable, little infiltration takes place. Water levels in the aquifer fall as storage is depleted by flow to rivers and springs and by pumped abstractions. The normal recharge of an aquifer takes place during the winter months when the potential evapotranspiration is low and soil moisture deficits are negligible; groundwater levels rise in response to this recharge.
Only the largest artificial reservoirs in the United Kingdom have sufficient capacity to support demands through the driest summers, assuming that they were full at the start of the summer, without some continuous contributions from river intakes. Prolonged dry spells lead in many rivers to reduced flow, particularly where the natural groundwater contribution (baseflow) is limited. Consequently, while surface water droughts may be in part due to the failure of runoff from winter rainfall to fill the reservoirs, they are more frequently caused by a decrease in the summer flows of streams and rivers. Surface water droughts do, however, lead to increased consumption of water (where available). By way of contrast, a groundwater drought is caused by a lack of winter rainfall. Potentially, the most serious droughts occur when, as in 1975/76, a dry summer succeeds a notably dry winter, or as in 1988-92 in eastern England, recharge is significantly below average over a series of successive winters.
The Observation Well Network
Groundwater level observation wells (in this context, a well includes both shafts - constructed by hand digging - and boreholes - constructed by machinery) are generally used for one of two purposes; to monitor levels regionally and thus to estimate groundwater resource fluctuations, or to monitor the effects locally of groundwater abstractions.
The observation well network was reviewed in 1981 by the British Geological Survey (then the Institute of Geological Sciences) with the aim of selecting 200 to 300 sites from the existing national archive, to be used for periodical assessments of the national groundwater situation. The selection was based upon the hydrogeological units identified in an investigation of the groundwater resources of the United Kingdom REF1; one site was chosen for each aquifer present within each unit. For Scotland and Northern Ireland, this was not possible due to the very limited number of potential observation wells available. In England and Wales, the total number finally selected was 175 REF2. Minor changes to the national network have been made in recent years.
Details of the wells currently in this national network are given in the Register of Selected Groundwater Observation Wells.
Measurement and Recording of Groundwater Levels
The majority of observation wells are still measured manually either weekly or monthly. The usual instrument is an electric probe suspended upon a graduated cable or tape, contact being made by the water to complete a circuit which gives either an audible or visual signal at the surface. Measurements are normally made to the nearest 10 millimetres, although instruments may be accurate to 1 millimetre. Some observation wells are equipped with continuous water level recorders. These recorders measure level either by a float or with a pressure transducer. Data are recorded either on paper charts, punched tapes (now rarely used) or solid state loggers. At a small but increasing number of observation boreholes provision is made for the routine transmission - usually by telephone line - of groundwater levels to local, or regional centres.
REF1. Monkhouse, R.A., and Richards, H.J. 1982. Groundwater resources of the United Kingdom. Commission of the European Communities, published Th. Schaeffer Druckerei GmbH, Hannover, 252 pages.
REF2. Monkhouse, R.A., and Murti, P.K. 1981. The rationalisation of groundwater observation well networks in England and Wales. Institute of Geological Sciences, Unpublished Report No. WD/81/1, 18 pages.