OR/15/028 Groundwater bodies in Scotland

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Ó Dochartaigh, B É, MacDonald, A M, Fitzsimons, V, and Ward, R. 2015. Scotland’s aquifers and groundwater bodies . British Geological Survey Internal Report, OR/15/028.

Introduction

A groundwater body is a groundwater management unit required by the EU Water Framework Directive (2000/60/EC), defined according to Article 2.12 of that Directive as ‘a distinct volume of groundwater within an aquifer or aquifers’. Delineating and characterising groundwater bodies requires knowledge of the hydrogeology and groundwater resources of the relevant aquifers.

This report describes the second revision of groundwater bodies in Scotland since 2005, as part of the ongoing River Basin Management cycle. The changes shown in this report incorporate lessons learned from the management process to date, and the availability of new and improved data and information. The most significant new development is the separation of groundwater bodies into two vertical layers: a shallow layer of superficial groundwater bodies, and a deep layer of bedrock groundwater bodies. This is important in order to help target action. Shallow groundwater bodies are more at risk from activities such as agriculture, while deeper bodies are more at risk from activities such as mining.

The groundwater bodies have been grouped to form larger areas — termed ‘groundwater body groups’ — for the specific purpose of assessing the risk to groundwater from nitrate. This has been done by grouping groundwater bodies with similar bedrock and superficial geology, soil, land use pressure, and water quality monitoring data, such that the groundwater bodies in a groundwater body group all show similar concentrations of nitrate in groundwater. Aggregating a large number of water quality monitoring results across a groundwater body group allows for more confidence in subsequent risk assessments.

For the latest revision, described in this report, the primary basis of the groundwater bodies is the definition of aquifer groups and their hydrogeological characteristics, as defined and described in Delineating aquifers in Scotland and aquifer characteristics, using the latest 1:50 000 digital geological maps (DigMap-50, Version 5.18), supplemented by other datasets described in data sources and by geological expert knowledge from BGS geologists. Where necessary, the aquifer groups have been subdivided or grouped together to allow for more effective groundwater management, as described in bedrock groundwater bodies and superficial groundwater bodies.

Bedrock groundwater bodies

General methodology for delineating groundwater bodies

The bedrock aquifers shown in the map in Figure 5 are the starting point for the delineation and characterisation of bedrock groundwater bodies in Scotland. Groundwater bodies and groups of bodies are shown in Figure 7. The aquifers are described in detail in aquifer characteristics.

The way these aquifers are defined reflects key groundwater flow characteristics which are, in turn, the main drivers for differences in groundwater management approaches. The aquifers are therefore the key building blocks of groundwater bodies, and in most cases, groundwater bodies follow aquifer boundaries. Aquifers have only been subdivided into smaller groundwater bodies, or amalgamated into larger bodies, where necessary or appropriate for particular management reasons. The criteria for further subdivision that were adopted in this revision were as follows:

  • Areas of higher pressure from human activity, mostly driven by diffuse pressures from mining and agriculture.
  • Mining: BGS and Coal Authority information were used to identify deep coalfields, extensive areas of historical oil shale mining, and clusters of open cast mining sites. In some parts of the Central Belt, Carboniferous sedimentary rocks that have been extensively mined for coal occur at depth below Carboniferous rocks that don’t contain significant coals and have not seen extensive coal mining. The geological and bedrock aquifer maps only show the rocks that crop out at the surface. In such cases, where the 2D maps do not reflect the 3D geology, and that difference has significant impacts for groundwater and groundwater management, groundwater bodies have been delineated so as to also reflect the deeper geology. Additionally, in some areas significant oil shale mining, which can have significant impacts on groundwater, is known to have occurred in Carboniferous rocks which have not been extensively mined for coal, and this has been incorporated into the groundwater body delineation.
  • Agriculture: relative differences in the degree of nitrate loading, soil type, agricultural land capability, and topography were taken as broad measures of farming intensity. Soil type, land capability and nitrate information were obtained from the James Hutton Institute. Decisions were also influenced by SEPA’s knowledge of the location of clusters of agricultural groundwater abstraction and significant point sources of pollution.
  • Differences in management of pressures, which were driven by River Basin boundaries and by Advisory Group areas of responsibility. Area Advisory Groups are a key administrative component underpinning River Basin Planning. More details can be found in the River Basin Management Plans for Scotland. Scotland’s first river basin management plans were published in 2009, and the second are due to be published by the end of 2015 (SEPA, 2015)[1].

In all cases, groundwater bodies were subdivided based on hydraulic criteria, in accordance with UK-wide guidance (UKTAG, 2011[2]). This means that groundwater body boundaries represent groundwater flow boundaries. Two methods of subdivision were used:

Surface water catchment boundaries. This was used for aquifers where groundwater flow is usually controlled by topography (Table 3). The main river catchment boundaries were used as the basic subdivision. In areas with more pressures on groundwater, these were further subdivided on the basis of tributary catchments. Where the intersection of catchment and geological boundaries produced very small, ‘mosaic’, subdivisions, these were simplified so that the minimum groundwater body size is 10 km2, to define hydrologically sensible groundwater bodies of manageable size. These situations were dealt with on a case-by-case basis. Geological criteria usually took preference, meaning that some groundwater bodies cross catchment boundaries. There are also a very small number of ‘multipart’ groundwater bodies, in which separate outcrops of similar geology were assigned to the same body, although they may be separated by a few kilometres.

Geological and structural features, such as faults and folds. This was used for aquifers where groundwater flow is usually controlled by geology (Table 3). Appendix 1 provides more information.

Where surface water catchment boundaries were used to define aquifer subdivisions, SEPA carried out the subdivision; where geological criteria were used, BGS carried out the subdivision.

Where there is no reason for subdividing bedrock aquifers, groundwater bodies can be large. Some bodies in areas of lower risks to groundwater are over 500 km2. In areas of higher risk, 100 to 200 km2 is more typical. A minimum area of 10 km2 was used for all groundwater bodies, except for:

  • islands smaller than 10 km2 which have a population of more than 50 people, or
  • geological units smaller than 10 km2 which support a key feature such as a large drinking water abstraction or a wetland.
Figure 7 Bedrock groundwater bodies in Scotland. Contains OS data © Crown Copyright and database rights [2015].

Superficial groundwater bodies in scotland

The boundaries of superficial aquifers were used as the basis for defining superficial groundwater bodies in Scotland. Superficial groundwater bodies are shown in Figure 8.

Figure 8 Superficial groundwater bodies in Scotland. Contains OS data © Crown Copyright and database rights [2015].

The superficial aquifers are defined based on their hydraulic properties (permeability and storage) and 3D extent. These characteristics drive differences in the approach to groundwater management and to delineating superficial groundwater bodies. A key feature of superficial aquifers, in contrast to bedrock aquifers, is their scale and distribution: they typically form smaller outcrops which are more widely distributed (Figure 6). The main issue during the process of defining superficial groundwater bodies was to exclude small and/or narrow and/ or thin outcrops of superficial deposits, which are unlikely to form significant aquifers which require active management. This includes narrow linear deposits along minor valleys, even where these are connected to more extensive deposits in major valleys. This process is summarised as follows:

  • All superficial aquifers with an area of less than 1 km2 were removed. Where the total area of a superficial body in a catchment was less than 10 km2, all areas were deleted unless there was a clear feature dependent on groundwater such as a large drinking water abstraction or a wetland.
  • Where superficial deposit outcrops are so small or their aquifer productivity so uncertain that they were not considered to function as a separate aquifer from the bedrock, the bedrock and superficial bodies were merged. This was particularly the case in parts of Aberdeenshire where alluvial and glaciofluvial deposits and weathered bedrock were merged with the underlying bedrock. In effect, this removed the superficial bodies from these areas, leaving only the bedrock body underneath.
  • Superficial aquifers were subdivided using surface water catchments. The main river catchments boundaries were used as the basic subdivision. In very large catchments, or areas of higher risk to groundwater, these were further subdivided on the basis of tributary catchments.
  • Where the resulting bodies crossed bedrock groundwater body boundaries, the superficial bodies were usually split at or near the bedrock body boundaries, in order to maintain a relationship between each bedrock body and its overlying superficial bodies. However, this last step was not undertaken where this process would have produced unrealistically small groundwater bodies or many small unconnected portions of a larger body.

References

  1. SEPA. 2015. River basin management planning publications. http://www.sepa.org.uk/environment/water/river-basin-management-planning/publications#RBMPplan. Accessed 1 June 2015
  2. UKTAG. 2011. UK Technical Advisory Group on the Water Framework Directive: Defining and Reporting on Groundwater Bodies. http://www.wfduk.org/sites/default/files/Media/Characterisation%20of%20the%20water%20environment/Defining%20Reporting%20on%20Groundwater%20Bodies_Final_300312.pdf. Accessed 18 March 2014.