Highways England seems confident that its planned tunnel at Stonehenge will not have too great an impact on local groundwater flows and the historical treasures that depend on them but its optimism will need to be tested before the scheme gets the formal go-ahead.
In February, Highways England launched a new consultation on its plans for the A303 between Amesbury and Berwick Down, including a tunnel passing close to Stonehenge that will be at least 1.8 miles (2.9 kilometres) long at depths of up to 50 metres.
The decision to build a tunnel, which is intended to remove the site and sound of traffic from the vicinity of Stonehenge, remains controversial, not least because both portals will be inside the Stonehenge World Heritage Site.
Much attention has focused on the impact on the local water table, including the unique site of Blick Mead, where human habitation can be traced back to around 8,000BC, which has been preserved in damp peat and silt. Last November, Highways looked at the potential impact on the site of dewatering during the tunnel’s construction but the structure itself could well disrupt groundwater flows.
Andy Rhind-Tutt, chairman of Amesbury Museum and Heritage Trust and a former mayor of Amesbury, is concerned about the huge impact of such a large tunnel on both Blick Mead and the historic cathedral at Salisbury.
He explains: ‘Stonehenge is on a chalkland aquifer, so basically the rain that lands north of it travels south and eventually ends up in the River Avon. It heads primarily towards West Amesbury but also to Blick Mead. Underground fissures send water towards the River Avon.’
Mr Rhind-Tutt says a 3km concrete tunnel, which will be between 16m and 50m deep, would impact this groundwater flow, especially at the long portal entrance and flyover nearest Amesbury, adding that when the current A303 was constructed in the 1960s the water table at Blick Mead changed significantly.
He also points out that the magnificent 800-year old cathedral is potentially vulnerable to changes in the water levels in the ground on which it is built, so much so that there are regular dip tests to establish this.
Because of its size, the tunnel scheme is categorised as a Nationally Significant Infrastructure Project, which means that a Development Consent Order (DCO) is required. This will require an Environmental Statement (ES), setting out the results of an Environmental Impact Assessment (EIA).
In the meantime, the raft of documents published alongside the main consultation booklet includes a Preliminary Environmental Information Report (PEIR), which gives Highways England’s current assessment of a range of issues including air quality, cultural heritage, biodiversity, and ‘road drainage and the water environment’.
On the last point the PEIR concludes that with the help of a Construction Environmental Management Plan there would be no likely significant temporary or permanent adverse effects during construction activities or the operational phase.
However, the detail of the document suggests that it is not that simple and Highways England does not – yet – have all the answers.
The PEIR identifies a number of issues that the design of the proposed scheme should address, including no ‘unacceptable’ detrimental impacts ‘on the hydrology and water quality of the River Avon or River Till nor on the designated habitats and species for the Special Area of Conservation (SAC) and Sites of Special Scientific Interest (SSSI) associated with those rivers’, or ‘that could affect the integrity (hydrology) of the Blick Mead spring’.
Asked by Highways whether these represent red lines for the project and who will make the judgement, a Highways England spokesperson said: ‘The issues set out here are a combination of statutory requirements and issues raised by the Environment Agency as the relevant statutory stakeholder. The determination of significance and acceptability of effects will be made by the competent experts undertaking the assessment in consultation with the Environment Agency.’
The conclusion on the issue of road drainage and the water environment states explicitly that ‘the integrity of the River Avon SAC (incorporating the River Till SAC) would not be adversely affected by the proposed scheme’ but makes no explicit statement in relation to Blick Mead. Highways England said that this statement encompasses the immediately adjacent Blick Mead area, adding: ‘Our assessments to date indicate that there would not be any significant effects on the Blick Mead area.’
Known unknowns
In addition the PEIR states that a Water Framework Directive (WFD) Assessment will be undertaken and a WFD compliance assessment report produced alongside the ES.
It adds: ‘This report will consider the extent to which the proposed scheme could impact on the current and future target WFD status of the water bodies (the River Avon, the River Till and the Upper Hampshire Avon groundwater body). Where potential adverse effects are identified, an assessment of these will inform what mitigation measures need to be incorporated into the design and construction methods of the proposed scheme to remove or minimise the effect. The results will be presented in the ES.’
This suggests that Highways England is confident that an assessment that it has yet to complete will not identify impacts that cannot be mitigated. It told Highways: ‘The scheme will ensure it is WFD compliant.’
With regard to Blick Mead, the PEIR notes: ‘This is situated along a dry valley to the east of the West Amesbury dry valley, on the northern bank of the River Avon north of the Abbey. This may also be an area of groundwater discharge and will be confirmed within the ES.’
Potential permanent impacts arising from the construction of the proposed scheme identified in the PEIR include ‘impacts caused by lengths of the tunnel below the groundwater level in the chalk interfering with groundwater flow’. It notes: ‘There are a number of springs in the area down hydraulic gradient of the tunnel including the spring system around Amesbury Abbey, which could be affected.’
However, the document’s list of potential mitigation measures does not appear to contain any specific measure relevant to such an impact. Highways England told Highways: ‘In this particular case, our assessment to date indicates that there will be not be any adverse impact that requires any mitigation above that provided by the design of the proposed scheme.’
It is clear that a degree of uncertainty remains. Highways England said it plans to submit a development consent application in the autumn, by which time many of its current assumptions will need to have been thoroughly tested.
