Feature

Rock of Ages

Hampshire’s Hidden Aquifer

Long before people roamed the earth, the space that is now Hampshire lay beneath a shallow sea. The warm water teemed with coccolithophores - tiny algae which thrived in the plentiful sunlight. As eons passed, their exoskeletons settled on the sea floor and were pressed into a layer of chalk hundreds of metres high.

In time the sea receded, exposing the chalk to ice, wind, and rain. The elements wore away its soft surface, shaping it into rolling downs and winding valleys. In some places the rock was blanketed with material from retreating glaciers, while elsewhere it lay just below the soil. Through a collision of natural forces, wildlife gained a mighty ally.

'Gin-clear’ chalk stream water © Linda Pitkin/2020VISION

Buried treasure

True to its aquatic origins, chalk has an almost magical relationship with water. In our county, so much rain disappears into the ground that one might picture a vast lake deep below. The truth is rather more bizarre: the chalk is riddled with tiny holes, enabling it to soak up water like a giant sponge. This rock reservoir, known as an aquifer, has a truly staggering liquid capacity.

When the rock is fully saturated, its cargo makes a bid for freedom. It bursts onto the surface as springs - the sources of our chalk streams. In higher parts of the landscape, where the aquifer rarely fills completely, intermittent streams called 'bournes' can dry out into grassy hollows for months at a time. Those that appear only in the wettest seasons earn the title 'winterbournes'.

While this process is straightforward, it is anything but speedy. A stream's resurgence can occur months after the wet weather that prompted it, and water can take decades to fully traverse the aquifer. The timescale is predictably unpredictable: the only certainty is that what goes in will, eventually, come out again.

Wonder water

When water does re-emerge from the aquifer, it has been fundamentally changed by its subterranean journey. It leaves the rock filtered of impurities and slightly alkaline; since rain is naturally acidic, it dissolves a little of the chalk during its passage. This suits species like the white-clawed crayfish, which uses calcium to harden its shell, and water crowfoot, which is hungry for bicarbonate ions.

Passing through miles of rock lowers the water’s temperature, causing it to arrive at about 11 °C all year round. With this coldness comes abundant oxygen - a valuable quality for fish like brown trout and freshwater eels, which can suffocate in overly warm water. Aquatic plants are just as appreciative, and their increased growth lays the foundation for a vibrant ecosystem.

Sediment, which in excess can menace a whole host of species, is also found in lower-than-average quantities. In other landscapes much of the sediment in streams is washed in by heavy rain, but chalk is so absorbent that relatively little rain stays on the surface. This helps to keep the gravelly stream beds clean and the water itself famously 'gin-clear'.

White-clawed crayfish © Linda Pitkin-2020VISION

Water wisdom

The water in our aquifer is a precious resource that is shared by Hampshire’s wildlife and its human population. We draw directly from the aquifer to supply our homes and businesses, but if our demand is too high then the streams can run low.

Find out how you can help through the Save Every Drop project: www.hiwwt.org.uk/saveeverydrop