The Chalk aquifer of the North Downs

The North Downs of south-east England are a prominent feature of the landscape forming the northern limb of the Wealden anticline and the southern margin of the London Basin. They extend from the Hog’s Back near Farnham in the west to the white cliffs of the Kent coast in the east. The Chalk dips in a broadly northerly or north-north-easterly direction eventually dipping below Palaeogene and superficial cover into the London Basin. The Chalk aquifer of the North Downs is one of the most intensively developed in the British Isles and comes under the jurisdiction of both the Thames and Southern Regions of the Environment Agency providing a resource for six water companies. Recharge is dominated by winter rainfall and occurs over the majority of the Chalk outcrop area. Low permeability cover tends to deflect rainfall as run-off to its edges which results in a high degree of solution activity in the Chalk due to the relative acidity of the soils associated with Palaeogene deposits and the clay-with-flints; this not only tends to enhance recharge but also results in minor (but significant) karstification of the underlying Chalk. Groundwater flows in a broadly northerly or northnorth- easterly direction and tends to develop permeability by solution. It would appear that current recharge could probably be dissipated within a relatively thin interval close to the water table. Variations of sea level, particularly during the Pleistocene, have provided a range of base levels for groundwater flow from the Chalk in this region. This has resulted in the development of several permeable horizons at levels associated with former water tables down to a depth of about –130 m OD. Groundwater flow will generally follow one of these permeable horizons down dip for a certain distance, but will then step up via fractures and faults to the nearest outlet where it can discharge to surface water, developing permeability en route. Springs and surface flows occur where the water table intersects the land surface. Consequently, the heads of Chalk streams tend to migrate up and down gradient seasonally as the water table rises and falls in response to recharge — the typical bourne behaviour recorded in Chalk catchments. Springs are also found on geological contacts, such as that between the Chalk and the Palaeogene. At the coast, seasonal fluctuations are less pronounced and springs are an important element of the marshland environment of this area. Where the aquifer is unconfined, the natural groundwater quality is good. However, the groundwater is vulnerable to pollution as shown by the high nitrate concentrations (mainly from diffuse agricultural sources) and the occasional detection of pesticides and organic solvents. Significant point source pollution occurred between 1907 and 1974 with the discharge of highly saline mine water from the Tilmanstone and Snowdon collieries. As the aquifer becomes confined below the Palaeogene cover it is better protected from pollution, but the generally more reducing conditions and longer transit times lead to deterioration in the natural quality. The heavy demands put on the aquifer for public supply can conflict with important environmental demands such as baseflow to rivers and spring flow to the north Kent marshes (much of which is protected by international conventions and/or European legislation). Perhaps the most widely publicised of such conflicts occurred in the Darent valley where groundwater abstraction was blamed for the drying up of the river in the early 1970s and again from 1989. A joint project by Thames Water and the National Rivers Authority (forerunner of the Environment Agency) resulted in the Darent Compensation Scheme which effectively maintains flow in the river. The North Downs has a long history of water supply development which has concentrated mainly on the Chalk aquifer. It is likely that the future impact of climate change processes will only exacerbate the existing conflict between supply and demand. The future for the Chalk aquifer of the North Downs will unfold within a framework of increasing national and EC legislation aimed at environmental protection and enhancement which will demand increasingly stringent control over water abstraction, use and reuse.