8. Groundwater Storage in the Chalk in SE Britain
TJ Marsh

Annual average groundwater level in the Chalk at Washpit Farm(Norfolk), Stonor Park (Oxfordshire) and Little Bucket Farm (Kent). Levels are metres above mean sea level (at Newlyn, Cornwall; the Ordnance Datum) so that high values indicate that the groundwater level is close to the surface, low values that it is deep below the surface.

Specific Indicator
Groundwater levels in about 170 representative wells and boreholes (measured every 1-4 weeks mainly by the Environment Agency and Scottish Environment Protection Agency) are maintained in the National Groundwater Level Archive at the British Geological Survey, Wallingford. Annual average groundwater levels in three boreholes are chosen as specific indicators. They are all in Chalk, have long term records and are little affected by water abstraction. They are at Washpit Farm(Norfolk), Stonor Park (Oxfordshire) and Little Bucket Farm (Kent). Levels are in metres above mean sea level (at Newlyn, Cornwall; the Ordnance Datum) so that high values indicate that the groundwater level is relatively close to the surface, low values that the level is at greater depth.
[Source: British Geological Survey, Wallingford, OX10 8BB]

Relevance
The Chalk aquifer is the principal source of water in much of the English lowlands. Nationally, groundwater provides over 30% of the public water supply and, in permeable catchments, helps to sustain flows in rivers during the summer and autumn. Groundwater levels in the Chalk are particularly important in sustaining wetlands and headwater streams in the English lowlands. Through time, average groundwater levels provide an overall assessment of groundwater resources.

Sensitivity to climatic and other factors
The Chalk aquifer is particularly sensitive to climate change because it occurs in the driest part of the UK where, on average, annual rainfall exceeds evaporation by only a narrow margin. The vulnerability of groundwaters to reduced recharge rates was evident during the droughts of the 1990s when the headwaters of many Chalk streams were dry for weeks or months. Groundwater levels are especially sensitive to changes in rainfall over the winter, when levels are normally replenished.

Average groundwater levels also respond to abstraction rates and land use changes. Wells and boreholes have long been used for water supply purposes and, as a result, local or regional drawdown has occurred. In the Chalk below London (where it occurs below thick impermeable clay layers and so responds to only very long term climatic variability) heavy abstraction caused groundwater levels to fall by almost 70 metres in the 120 years up to the 1960s. Subsequently, much reduced abstraction rates have produced a partial but brisk recovery.

Change over time
Where unaffected by abstractions, groundwater levels in Chalk aquifers tend to maintain a stable long term average, but with peak levels in spring and low levels late in the year. Evidence for long term stability is provided by data for wells and boreholes which have records extending back to the nineteenth century, such as the Chilgrove House borehole, in the South Downs (commissioned in 1836). However, the absence of a long term trend does not mean that there are no significant and protracted perturbations about the mean.

If the UK climate becomes drier and warmer in SE Britain, average groundwater levels in the Chalk may be expected to decline in response to decreased average recharge. However, a tendency towards a more continental climate - with a greater proportion of the rainfall in the winter months - may maintain a relatively stable average level but with a greater seasonal range.