OR/15/028 Data sources

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.

The key sources of data and information used to develop the aquifer maps and groundwater bodies in Scotland are described in this section. Digital geological maps were used directly to form the basis of the linework of the aquifer groups and subsequent groundwater bodies. Maps of aquifer productivity, groundwater vulnerability and hydrogeology, and the hydrogeological understanding that they encapsulate, helped to inform the classification of each aquifer.

Geological maps

Digital geological maps for Scotland are produced as part of the BGS DiGMapGB onshore datasets (http://www.bgs.ac.uk/products/digitalmaps/digmapgb.html), which are based on the different series of published BGS geological maps. The DiGMapGB datasets are available as vector data at a variety of scales and in a variety of formats in which they are structured into themes primarily for use in geographical information systems (GIS), where they can be integrated with other types of spatial data for analysis and problem solving in many earth-science-related issues.

The main geological mapping dataset used for Scottish aquifer classification is the British Geological Survey (BGS) DiGMapGB-50, at 1:50 000 scale (bedrock and superficial deposits). The DiGMapGB-50 data typically provide a digital version of the geology as shown on the map face of the published 1:50 000 scale paper map. No topography is shown. Most geological units are represented by polygons in the data and are arranged in up to four themes, as available: Bedrock (formerly ‘solid’ geology); Superficial (formerly ‘drift’ deposits or Quaternary); Mass Movement (mostly landslide); and Artificial (or artificially modified or man-made ground). Geological units such as thin coal seams and fossil bands and other features such as faults, mineral veins and some landforms, which are all shown as lines on the published maps, are held in a fifth, Linear, theme.

The DiGMapGB-50 dataset forms the basis of the Aquifer productivity, bedrock aquifer groups, superficial aquifer groups, and subsequent groundwater body classifications. The larger scale BGS DiGMapGB-625 1:625 000 (bedrock) geological mapping dataset has also been used for a separate project to subdivide bedrock aquifers using slightly different criteria, in order to highlight specific hydrochemical issues and controls.

Aquifer productivity

GIS-enabled maps of aquifer productivity for bedrock and superficial aquifers in Scotland were first developed by BGS in 2004 based on the 1:50 000 scale DigMapGB dataset (MacDonald et al., 2004, 2005), and refined (Version 2) in 2011 (Ó Dochartaigh et al., 2015a). The maps categorise two key physical properties of aquifers: the dominant groundwater flow type in an aquifer, and the aquifer’s potential for sustaining various levels of borehole water supply. The maps are designed for use at a scale of 1:100 000, making them suitable for regional assessments.

The bedrock aquifer productivity map has three groundwater flow categories: significant intergranular flow; mixed fracture/intergranular flow; and fracture flow. In Scotland, most groundwater flow in bedrock aquifers is through fractures; intergranular flow is important in only a few sandstone formations. The bedrock map has five aquifer productivity classes: very high, high, moderate, low and very low, which are based on a judgement of the typical long term sustainable abstraction rate from a properly sited, constructed and developed borehole (Figure 3).

All superficial deposits aquifers in Scotland are assumed to have primarily intergranular groundwater flow. The superficial deposits productivity map has four productivity classes (high; moderate to high; moderate; and a category to signify that a deposit is ‘not a significant aquifer’), based on a judgement of the typical long term sustainable abstraction rate from a properly sited, constructed and developed borehole or a group of boreholes (Figure 3).

Figure 3 Superficial deposits and bedrock aquifer productivity maps for Scotland (from Ó Dochartaigh et al., 2015a). Contains OS data © Crown Copyright and database rights [2015].

Groundwater vulnerability

A digital groundwater vulnerability map for Scotland was first produced by BGS in 2004 (Ó Dochartaigh et al., 2005) and refined (Version 2) in 2011 (Ó Dochartaigh et al., 2015b) (Figure 4). The map shows the relative vulnerability of groundwater at the uppermost water table to contamination. It is based on a number of different datasets, primarily 1:50 000 scale digital bedrock and superficial deposits geology (DiGMapGB-50), aquifer productivity and permeability, superficial deposits thickness (derived from BGS borehole records), depth to water table (from BGS datasets), and aspects of soil thickness, permeability and saturation (from the Hydrology of Soil Type (HOST) dataset produced by the James Hutton Institute).

Figure 4 Groundwater vulnerability map of Scotland (Ó Dochartaigh et al., 2005; Ó Dochartaigh et al., 2015b). Contains OS data © Crown Copyright and database rights [2015].

Groundwater chemistry

The project Baseline Scotland, which was jointly conceived and funded by BGS and SEPA in 2004, is improving data availability and general understanding of the natural chemistry of groundwater in Scotland’s bedrock aquifers. Nearly 650 analyses of groundwater chemistry have been interpreted to characterise the ranges in natural background groundwater quality across Scotland. Parameters measured include major and trace inorganic chemistry, dissolved organic carbon, stable isotopes and residence time indicators. The new groundwater chemistry information is being used in a number of ways, including helping to establish threshold values as a basis for assessing the chemical and quantitative status of groundwater bodies as part of Water Framework Directive requirements.

Aquifer geology is a key control on groundwater chemistry. For the purposes of interpreting and characterising Scottish groundwater chemistry, bedrock aquifers were categorised according to their overall influence on groundwater chemistry, using a very similar methodology to that used to subdivide aquifer groups, but based on 1:625 000 scale geological linework (DiGMapGB-625), and using slightly different criteria to highlight specific hydrochemical issues and controls. One main difference was to maintain the distinction of major igneous intrusions (mostly granitic) across the Highlands in dominantly Precambrian metamorphic or, in Skye and Mull, volcanic terrain, because the available data indicate that groundwater chemistry can vary noticeably between these rock types, although they can have similar physical aquifer characteristics.

A full report on the baseline chemistry of Scotland’s bedrock groundwater is in preparation. A statistical summary of selected chemical parameters for each of the aquifer groups in this report (see Permo-Triassic and Superficial aquifers in Scotland articles).