- 30th September 2016
- Posted by: Sabina Gubbin
- Category: Projects
An extension to Wood Lane Quarry, Ellesmere, was proposed adjacent to the White Moss peat body and upgradient of the Cole Mere Site of Special Scientific Interest (SSSI)/Ramsar site. A habitat impact assessment, supported by a robust hydrogeological and eco-hydrological conceptual model, was required as part of the planning process.
In this study, the potential impact of the proposed quarry extension was assessed in relation to the adjacent peat body and its associated habitats, as well as the surface water network and the internationally-designated downgradient receptors.
The key eco-hydrological receptors which have the potential to be impacted by the development are:
- White Moss – this is a peat body and wetland immediately east of the site. The main National Vegetation Classification (NVC) habitats are M23b, MG13 and MG7. The peat body which forms White Moss was identified as being up to 6m thick in places. The surface watercourses which cross White Moss are also receptors.
- Cole Mere SSSI/Ramsar further east of the site has a direct surface water link between White Moss and Cole Mere. As a result, there is the potential for the quantity
and quality of flows entering Cole Mere to be affected
by the development.
The sedimentary sequence at Wood Lane Quarry consists of a series of cross-stratified gravels, laminated fines and rippled sands overlying till, interpreted as ice-marginal and ice-contact sediments in a trough sandur. This can lead to multiple hydraulically unconnected perched aquifers, separated both vertically and horizontally by low permeability deposits. Understanding these complex and local groundwater body interactions were central to the impact assessment.
Detailed investigation of the area included a peat survey of 68 auger holes and installation of 21 monitoring boreholes, as well as topographical and hydrological analysis of LIDAR data. The hydrogeological conceptual model which resulted from these studies indicated that the neighbouring 6m deep peat body was contained within, and supported by, a perched aquifer formed from sandy deposits which had infilled a glacial meltwater channel.
Using NVC survey data, an eco-hydrological conceptual model was then developed to identify the Wetland Water Supply Mechanisms (WETMEC), that support the distribution of wetland habitats bounding the site.
Site conditions are deemed to be analogous to the groundwater flushed slope (WETMEC 17). A risk assessment methodology determined the impact significance by first clarifying both the potential receptors and the potential mechanisms for impacts, alongside the likelihood of an effect occurring, and accounting for any mitigation included within the scheme design.
The assessment identified a number of potential impact mechanisms which could affect the receptors. The main potential mechanism identified would involve the draining of the local perched groundwater body which supports the White Moss peat body. This would change the habitats on White Moss and lead to the degradation of the peat body. Consequently, flows entering Cole Mere could be reduced and polluted by carbon released via peat degradation on White Moss.
A mitigation through design process was undertaken to limit the spatial and vertical extent of the scheme and thereby limit the potential for the identified impact mechanisms to occur. The scheme developed by this process has been subject to an impact assessment.
Based on the current understanding, the first phase of the scheme could be implemented and is assessed as having negligible impact on the identified eco-hydrological receptors.
Local groundwater conditions are complex and additional characterisation of the local groundwater conditions is required to improve the eco-hydrological conceptual model and refine the existing impact assessment in order to move forward for the second and third phases.
It is possible that, for these later stages, no practical mitigation strategy can be developed, which would lead to potential significant effects to receptors. Nonetheless, it may be possible to mitigate impacts using ongoing monitoring or using engineering solutions such as a low permeability cut off bar.
To find out more about our range of hydrogeology related disciplines including quarry assessments, water resources assessment, and site investigations email Mike McDonald.