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A strategic programme for NERC Lowland catchment research
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Connections between rivers and groundwater

The Water Framework Directive requires that all bodies of water shall be of 'good status' by 2015. To achieve this we must fully understand how rivers and aquifers function and, more specifically, how changes to one of them affect the other.

A track on the Chalk downlands

The ecological health of a river and adjoining areas depends on the regular supply of appropriate amounts of water and nutrients under specific conditions (such as temperature and oxygen levels). Interfering with these conditions - by abstracting water (from wells or from the river), changing the quality of water entering the river or changing recharge to the groundwater - can have devastating impacts on the ecology in and along the river.  More about ecology & surface water & groundwater exchanges

The headwaters of rivers in permeable catchments are particularly vulnerable to exploitation of water resources because they have a high, natural variability in their flow. This variability is caused by seasonal fluctuations in groundwater levels. The use of water resources in these vulnerable areas must therefore be carefully managed to reduce the potential risk of both summer drought and winter flooding.

LOCAR scientists have been looking at the similarities and differences in the way aquifers and rivers interact in different catchments. This has allowed them to work out what is influencing the delivery of water to a river and how that water (and the associated chemicals) continues its journey to the river mouth. Relevant factors might include where the groundwater comes from locally, the type of rocks, the soils, the form and use of the land, and the plants, animals, insects and microorganisms present.

About 75 LOCAR researchers have been involved in intensive fieldwork programmes for over three years to analyse changes in river flow and water quality along the lengths of the five rivers in the three LOCAR catchments. Broader scale studies have involved examining some 350km of river, with highly detailed investigations over 45km of channel.

Isotopes (the heavy and light variants of a given element) of naturally occurring chemical elements have been shown to be particularly useful in understanding these environments - water that has travelled through different geologies or has been in the ground for different lengths of time emerges into the river with a particular isotope 'fingerprint'. Scientists then have a measure of how water, which initially fell on the catchment as rain, enters the river days or years later. Other tracers - deliberately injected or, like CFCs and radon, already present - were also used to trace routes and age water.

These data provide new evidence of how dry valleys can act as subterranean pathways allowing water to move into and out of the river. Some stretches of a river may lose water to the groundwater, while in other places the groundwater may re-emerge to add water to the river. LOCAR discovered that this natural exchange between aquifer and river is extremely variable within a catchment and can also change over time. For example, under low river flows fine sediments may be deposited on the river bed which can lead to a kind of hydraulic sealing of the river. Then, during periods of higher flow this 'lining' of the river may be stripped away, allowing connection again with the aquifer.

These findings are important for managing the quality of the river. If pollutants are to be eliminated we need to know where they come from and where and how they enter the river. LOCAR's work provides the understanding and detailed theories that help managers to know what to look for and where they might find it. It has also produced knowledge that will form the basis of new models required for estimating the impact of changing climate, land-management practice and water use on the ecological environment in and close to the river.