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A strategic programme for NERC Lowland catchment research
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How water moves underground

Geological structure and history influence both water pathways and changes in water quality in a catchment. LOCAR has increased our understanding of these influences, offering opportunities to manage water resources better. Water managers need this information so that they can predict the movement of pollutants in the groundwater and can decide how much water to extract.

The concept of a stream catchment is apparently straightforward – it is the area of land whose water drains into the stream. From any given point on a stream, the divide between the land that slopes towards the stream and that sloping away from it can be easily picked out on a map. But for chalk streams, which depend upon groundwater flow, things aren't so simple. LOCAR discovered that in the River Pang catchment, much of the underground water moves directly to the adjacent River Thames and not to the Pang.  More about flow heterogeneity in the Chalk

The water quality in chalk-fed streams and rivers depends in part on where the water comes from. If it runs rapidly through cracks in the rock straight into streams it will carry chemicals and sediments from the surrounding fields. If it takes the slower path percolating through pores in the rock, the impact of the chemicals will be delayed so water quality will be better (but some of the pollutants will emerge into rivers much later). More about Chalk pollution

Tracers (harmless, inert chemicals used to label a small volume of water) were used by LOCAR scientists to show how fast the water moves through the rocks. In some cases, when tracers were injected in places where streams disappear into the ground, they reappeared in springs a day or two later. In other places the water did not emerge at all. High up in the Pang catchment on the chalk downs, any new water sinking into the ground pushes down the water that entered the catchment earlier. This ‘piston flow’ process means that a given litre of water entering the ground today might take many decades before actually emerging in a stream. More about the permeability of the Chalk

LOCAR has recognised four important types of groundwater movement:

  • shallow but fast flow within a few tens of metres of the water table;
  • deeper slower flow which transmits water to a main river like the Thames;
  • very rapid flow where water runs through major cracks which channel rainfall directly to springs that in dry weather provide the major part of the flow of the surface stream (such as the Blue Pool spring which sustains the Pang in dry summers);
  • river valley flow where the water moves through the sands and gravels beneath the valley floor and stream channel (under dry valley tributaries, for example).

Shallow flow generally contains water that fell as rain in the last few days or weeks whereas the deeper system may contain much older water that takes ten years or more to work its way through the rocks to the stream. These waters tend to mix in the valley bottoms where the rapidly flowing young water feeds the larger springs and upwellings and the older water emerges at the base of valley sides and in seepages along the channel.

In the valleys, surface streams often disappear into porous rock, but LOCAR work confirmed that beneath these dry valleys water is often moving through the gravels that partially fill the floor. These gravels formed during cold periods in the last two million years when ice covered areas further north in Britain. Today they are an area of concentrated, relatively rapid groundwater flow, influencing where water reaches the flowing rivers and also acting as a store of water beneath the rivers. It is helpful to think of the river not merely as the channel we can see, but as a large part of the valley floor underlain by gravels and sands all carrying water downstream. More about characterising groundwater flow in the Chalk