OR/15/028 Delineating aquifers 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

An aquifer is a geological layer that is porous and permeable enough so that it can yield a significant quantity of water to a borehole, well or spring, or provide significant baseflow to rivers, lochs or other surface water bodies. The characteristics of an aquifer — e.g., how productive it is, or the baseline chemistry of groundwater within it — are largely controlled by geology. Therefore, as a precursor to delineating groundwater bodies, Scotland’s geology was first classified into broad aquifer groups, according to their physical aquifer properties and controls on natural groundwater chemistry.

Aquifer groups were classified consistently across Scotland based on 1:50 000 scale 2D digital geological maps produced by the British Geological Survey (DiGMap-50 Version 5.18). These show the lateral extent and boundaries of the uppermost geological units: for superficial deposits, this is the ground surface; for bedrock, it is either the ground surface (if bedrock is exposed at the ground surface) or rockhead (if bedrock is overlain by superficial deposits).

These 2D maps don’t show how geological units change with depth. By using both superficial and bedrock maps, we can incorporate a 3D element to aquifer and subsequent groundwater body classification. However, this doesn’t show where bedrock geology changes with depth. At present, 3D geological models are not available for most of Scotland, and so it is not possible to develop a consistent fully 3D methodology for aquifer classification. However, across most of Scotland, depth changes in bedrock geology do not have significant impacts on groundwater, as they do not cause significant changes in aquifer properties or controls on groundwater chemistry. There are a few cases where bedrock changes with depth are known to have significant groundwater impacts, and therefore to have significant implications for groundwater management, and these have been accounted for in the classification of groundwater body boundaries.

The classification of aquifer groups is described in bedrock aquifers and superficial deposits.

Bedrock aquifers

Scotland’s bedrock geology was classified into broad aquifer groups which have distinctly different physical aquifer properties and/or controls on natural groundwater chemistry. The aquifer groups are shown in Figure 5 and listed in Table 1. The characteristics of the aquifers are described in the Aquifer characteristics article.

Bedrock aquifers were grouped first according to their rock type — calcareous rocks; dominantly noncalcareous sedimentary rocks; and fractured igneous or metamorphic (‘hard’) rocks (Table 1). Within these categories, they were further subdivided based on the aquifer productivity and other hydrogeological maps and data available for Scotland (see data sources). Across Scotland, the age of the rocks is a key control on aquifer productivity, and geological age was therefore the main criterion on which aquifer boundaries were drawn. There were two exceptions:

  • In some cases, aquifers of particular ages have been subdivided. This was either because parts of the aquifer have been subject to significant human alteration (in particular, treating separately Carboniferous aquifers which have been extensively mined for coal), or because the rocks of the same age and type are significantly different in different parts of Scotland (for example, Old Red Sandstone rocks, which are divided between northern and southern/central Scotland; and Precambrian rocks, which are also divided into a northern and a southern group) (Table 1).
  • In some cases, areas of complex geology have been simplified in order to define aquifers suitable for effective management. Some small areas with very complex geology have been combined and categorised as mixed aquifer types. These are shown on the map in Figure 5. In other areas where many small outcrops of one rock type are surrounded by a dominant rock of another type, a minimum area of 10 km2 was defined as the lower limit of a hydraulically significant aquifer, following UK-wide guidance (UKTAG, 2011[1]). Any small outcrops of less than 10 km2 in area have been incorporated into the surrounding aquifer.

The rocks underneath large lochs and other surface water bodies are generally not mapped on British Geological Survey geological maps. For the purposes of defining aquifers and groundwater bodies, geological boundaries were extrapolated across lochs.

Figure 5 Bedrock aquifer groups in Scotland. Contains OS data © Crown Copyright and database rights [2015].
Table 1 Summary of bedrock aquifer groups in Scotland.
Aquifer type Aquifer group
Sedimentary aquifers extensively mined for coal (dominantly noncalcareous) Carboniferous — extensively mined for coal
Sedimentary aquifers (dominantly noncalcareous) Carboniferous — not extensively mined for coal
Old Red Sandstone (North and South)
Permo-Triassic
Mixed Old Red Sandstone South/other sedimentary (dominantly Carboniferous— not extensively mined for coal, or Silurian–Ordovician)
Mixed Igneous/Sedimentary
Fractured igneous and/or metamorphic (‘hard’) rock aquifers Igneous Intrusive — where distinguished (only large outcrops surrounded by aquifers with significantly different hydrogeological properties — e.g. Permo-Triassic — are distinguished. Smaller outcrops and those surrounded by aquifers with similar hydrogeological properties — e.g. Precambrian — are not distinguished)
Igneous Volcanic
Silurian–Ordovician
Precambrian (North and South)
Shetland low permeability (Precambrian, Highland calcareous, igneous intrusive and volcanic)
Ayrshire basic complex (Silurian–Ordovician ophiolite complex with Silurian–Ordovician, igneous intrusive and volcanic)
Calcareous aquifers Highland calcareous

Superficial aquifers

Virtually all superficial deposit — or unconsolidated — aquifers in Scotland were deposited in the last 20 000 years during the Quaternary geological period, during and after the latter part of the last glacial period. The only exceptions are small areas in Aberdeenshire where significant thicknesses of in-situ weathered bedrock have been preserved. This kind of preservation is rare in northern Britain, where more typically glacial processes during the Quaternary eroded any weathered bedrock. The principal superficial aquifers in Scotland are deposits of gravel and coarse sand, including alluvial sand and gravel, raised beach and blown sand deposits, and glaciofluvial sand and gravel.

The productivity of superficial aquifers is primarily defined by their lithology, which both controls aquifer permeability, and is one of the main controls on aquifer storage potential. The aquifers can also be defined by the different processes of aquifer sediment transport and deposition (e.g. glaciofluvial, alluvial or marine). These processes are a major control both on aquifer lithology and on 3D geometry (the lateral extent and thickness), which is the other main control on aquifer storage potential. Some thick, permeable superficial deposits deposited in deeply weathered bedrock channels are overlain by low permeability nonaquifer deposits, and are therefore not mapped at the ground surface, but may form highly productive aquifers.

Three superficial aquifer groups are therefore defined in Scotland, according to their age and provenance (Quaternary unconsolidated or pre-Quaternary weathered) and whether they crop out at the ground surface or are hidden. The extent of these aquifers is shown in the map in Figure 6, and their characteristics are summarised in Table 2.

The superficial aquifer map is largely based on the superficial deposits productivity map for Scotland (Figure 1) which shows the distribution and productivity of all superficial aquifers cropping out at the ground surface. Only superficial deposits categorised as having high or moderate to high productivity (Figure 3) have been classed as superficial aquifers. Included with these aquifers are significant buried superficial aquifers, which are not shown on 2D geological maps, and the significant areas of in-situ weathered bedrock in north-east Scotland. This map of superficial aquifer groups was used as the starting point for delineating superficial groundwater bodies.


Table 1 Summary of bedrock aquifer groups in Scotland.
Aquifer group Description
Quaternary aquifers cropping out at ground surface Significant outcrop/thickness of unconsolidated Quaternary deposits that have been eroded and transported, generally by rivers, ice or the sea.
Buried Quaternary aquifers (‘buried channels’) A subset of Quaternary aquifers, typically forming thick sequences, generally of glaciofluvial gravels and sands, which have infilled deep bedrock channels, but are covered by nonaquifer deposits and therefore not shown on 2D maps. Only significant in parts of the Central Belt.
Weathered bedrock Significant thicknesses of in-situ weathered bedrock. Found in Aberdeenshire where Tertiary weathering of intrusive igneous and Precambrian rocks has not been eroded and removed by Quaternary glacial processes.
Figure 6 Superficial aquifer groups in Scotland. Contains OS data © Crown Copyright and database rights [2015].

Reference

  1. 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.