To recap, springs issue from the base of a permeable water bearing rock (the aquifer) where it overlies a less permeable rock which has allowed groundwater to accumulate on top of it. Its surface is referred to as a water table. Springs form where a slope intersects the water table allowing it to flow freely out of the aquifer. The local examples are the springs which issue from the Chalk resting on the Gault clay along the south facing slope of the North Downs.
The term “water table” may be misleading as it implies a level surface which suggests that springs should erupt at a uniform elevation along a slope. This is not always the case, and on the larger scale the surface of water table in chalk is convex, falling towards the valley, and on a smaller scale it is modified by variations in the permeability of the chalk.
The chalk contains joints and bedding planes through which water flows preferentially and in particular, the West Melbury Marly Chalk Formation, at the base of the Grey Chalk, contains alternate permeable and less permeable layers which influence the location of springs. Where impermeable layers exist above the main water table, they can support local perched water tables (i.e. above the main water table).
A characteristic of Chalk, which also affects the flow of springs, is that it is soft and soluble. Once a spring starts to flow, the groundwater flow towards the spring enlarges the flow path by erosion and dissolution which increases the flow and further enlarges the flow path. So, they are self-perpetuating and can become permanent features. (This characteristic is used by the operators of water extraction boreholes in chalk. As a well is “developed” the water yield increases).
In the urban environment some spring water may find its way into the permeable backfill to services trenches in the road, or into the granular sub-base of roads. These then effectively act as “French Drains”- a primitive type of land drain comprising a gravel filled trench which is used to collect and divert groundwater flow. Where this happens, the water can erupt through weak points in the road construction as “road springs”. Occasionally, in times of high groundwater levels, you see water issuing from manhole covers. This may be evidence of ground water having penetrated through discontinuities in the pipework, and which exceeds the discharge capacity of the downstream pipework causing the excess to erupt around manhole covers.
Locally, these road springs are particularly obvious in winter when the water table is high and where roads cross over the Gault – Chalk boundary. Good examples can be seen on the lower part of Beachborough and near the bottom end of Sandling Road. When the water table is sufficiently high, it causes springs to issue from perched water tables higher up the slope in the permeable parts of the West Melbury Marly Chalk Formation. Good examples of these can be seen near Broadstreet on the south side of Lyminge and on the north side of Etchinghill. The geological maps show linear deposits of Head on both these locations, which can be indicators of ancient water courses.
The occurrence of alternating permeable and impermeable beds, characteristic of the lower part of the Grey Chalk, provides the conditions necessary for the development of artesian pressure. Ground water supplied from a source at a higher elevation is under pressure if it is trapped below an impermeable layer. Where this water reaches the surface through discontinuities in the capping layer, it issues under pressure sufficient to produce a noticeably rapid flow (sometimes sufficient to produce fountains). It is likely that some of the road springs are under a certain amount of artesian pressure, suggested by their high flow rate.
A way to keep an eye on the height of the water table, if you are interested in such things, is to observe the progression upslope of the highest spring eruption as winter progresses and then their regression in spring. (This method suggests that the water table during the winter of 2024 was particularly high).
Andrew Coleman
Rev. 21/04/2024
Geomusings 