River Granta – Assessment of Risk of Deterioration under the Water Framework Directive

Case Study

Cambridge Water commissioned ESI (supported by APEM) to assess the risk of deterioration associated with a planned increase in abstraction at one of its groundwater sources in the catchment of the River Granta, near Cambridge.

Background

The River Granta has in the past been subject to very low flows in dry years. Investigations by Cambridge Water in 2005 indicated that abstractions from two of its Chalk groundwater sources were having a significant impact on baseflows in the river (albeit mitigated to some extent by an augmentation scheme within the catchment).

 

To resolve these problems the Environment Agency proposed adding a Hands Off Flow condition to the abstraction licences of these sources. This would put the Granta waterbody on a “Pathway to Good” under the Water Framework Directive (WFD), with a target of achieving a “supports Good” status by 2021.

 

This Water Supply Zone is largely self-contained, and few options exist for bringing in additional water from outside. As a result Cambridge Water proposed to use another borehole within the catchment but further from the river to supply water when the two sources are capped by the HOF conditions. However, as this borehole abstracts groundwater that would ultimately provide baseflow to the River Granta, the Environment Agency needed to see evidence that the use of this abstraction would not pose a risk of deterioration to the River Granta.

The Project

ESI, supported by aquatic ecology specialists APEM, were commissioned to carry out the investigations and risk assessment.

 

Initially ESI reviewed the results from the Environment Agency’s Cam-Bedford Ouse groundwater model to identify the likely effects of abstraction on flow. This was combined with the available aquatic ecology data to try to develop an understanding of the links between abstraction effects and ecological status. This work suggested that the main impacts on ecology in the catchment were related to water quality and sediment and that there was little relationship between abstraction related reduction in flows and ecological status.

 

As a result of this review Cambridge Water commissioned some detailed walkover surveys and monitoring (carried out by APEM) in the headwaters of the Granta. These would both provide additional information to support the conceptual model of this area as well as forming a baseline against which any risk of deterioration could be assessed. ESI carried out further model runs using the Cam-Bedford Ouse groundwater model to assess how the potential shortfall in supply in the zone could be made up from the alternative source without creating a risk of deterioration.

The Outcome

The results of these investigations were used to update Cambridge Water’s Options Appraisal for the water supply zone so that appropriate plans could be taken through to AMP7.