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Investigating The Impact Of A Groundwater Abstraction On The Flows And Ecology Of A River And Special Area Of Conservation

Congress: 2015
Author(s): Victoria Price, Celia Figueira, Jane Dottridge
Mott MacDonald1

Keyword(s): Sub-theme 9: Water allocation among competing uses and users,
AbstractIntroduction

The European Union's Habitats Directive (Council Directive 92/43/EEC) requires member states to: designate Special Areas of Conservation (SACs); maintain or restore protected habitats; and ensure conservation measures are in place in SACs. Under the Regulations, water companies as competent authorities, are required to demonstrate that their abstractions do not have an adverse effect on the ecology in designated sites.

Anglian Water has both surface water and groundwater abstractions at a site within the River Wensum Special Area of Conservation (SAC). Pits formed from old sand and gravel workings are used for settlement and storage before water is sent for treatment and then into supply.

The Habitats Directive Review of Consents procedure concluded that the River Wensum SAC was in an unfavourable condition for four reasons: low flows, high levels of phosphates, siltation, and the presence of mill structures. The Environment Agency identified the surface water abstraction as a cause of low flows, with potential links between the groundwater abstraction, the aquifer system, the settlement pits, and the river. Anglian Water's solution to address low flows was agreed with the Environment Agency and Natural England.

Anglian Water has carried out investigations to define the impact of the abstraction on the flows and ecology of the SAC from both surface water and groundwater abstractions. In 2013, a solution to the surface water abstraction was agreed followed by an investigation considering the impact of the groundwater abstraction. The groundwater study comprised three components: a 16 week pumping test of the two boreholes, a comprehensive ecological study, and the development of a groundwater model calibrated with the pumping test data and used to simulate potential impacts of abstraction on the environment.

Method

Detailed hydrological, hydrogeological and ecological data were collected over the period of the 16 week pumping test from July to October 2013. These data not only informed the understanding of the hydraulic connection between the river, the pits and the aquifer system but were also used to refine a local groundwater model.

A sub-model was extracted from the Environment Agency's regional North East Anglian Chalk groundwater model to represent the groundwater and surface water catchments of the boreholes of interest. Data from the pumping test was used to refine the model, and in particular to more accurately represent the interaction between surface water in the pits and groundwater in the superficial deposits (river terrace gravels and alluvium) and the Chalk. Three scenarios were run to assess the impact of the groundwater abstractions on the flows in the River Wensum SAC.

Refinements made to the local sub-model were subsequently incorporated into the regional groundwater model, requiring collaboration between Anglian Water, the Environment Agency, AMEC and Mott MacDonald. The refinements to the regional model included the addition of the pits, which have local importance for storage and recharge, and improved connections between the Chalk aquifer and the overlying superficial deposits aquifer.

In addition to the groundwater and surface water monitoring, ecological surveys were undertaken in 2013 in order to establish baseline conditions before, during and after the pumping test employed by Anglian Water between July and October 2013. Both macrophyte and macroinvertebrate surveys were undertaken at six sites along the River Wensum.

Results and discussion

During the pumping test, groundwater levels decreased in both the Chalk and superficial deposits aquifers. There was also a sharp decline in the water levels in the pits. Flow in the river followed the expected decline as part of a normal summer recession; the overall trend of the flow gauging indicated that the river continued to gain past the site. However, there was a slight reduction in flow and change of the flow accretion profile near the site which may have been caused by the groundwater abstraction.

Modelled flows in the River Wensum for three abstraction scenarios were compared with the simulated baseline conditions during a period of near average summer flows (July and August 2013) to understand the impact of the abstraction on the river flows. The maximum modelled change in the flow in the River Wensum SAC resulted from a scenario which simulates the use of the groundwater abstractions under the terms of the current groundwater licence. Flow in the river was simulated to reduce by less than 5% in the SAC; this reduction is not considered to have an adverse impact on the ecology of the river.

Temperature data recorded in the pits and observation boreholes indicated that surface water in the pits moved into the superficial deposits aquifer and then the Chalk aquifer at a location between two of the largest pits.

The ecological surveys undertaken in 2013 and historical ecological data collected from previous studies support the conclusion that the ecological communities present in the study area are not suffering from low flow conditions. Overall the Ecological Status is classified as 'good' or 'high' in all survey months at all sites, with no distinguishable trend between sites up and downstream of the abstraction point.

Conclusion

From the modelling and ecological survey results, it is concluded that, for near average summer flow conditions:

The current operational use of the abstraction boreholes has no significant adverse impact on the flow or ecology in the River Wensum SAC; and
The ecological communities present in the stretch of the River Wensum immediately up and downstream of the abstraction point show no history of being impacted by low flow conditions.

A multi-disciplinary approach was successfully used to address the requirements of the Habitats Directive to show that the impact of the abstraction does not have an adverse impact on the ecology of the SAC. Drawing information from different sources provided a holistic view of the problem and potential solutions.

The Council of the European Communities. (1992, May 21). Habitats Directive. Council directive 92/43/EEC on the conservation of natural habitats and of wild fauna and flora.

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