Explanatory notes to accompany the Groundwater Vulnerability Index GIS for Aberdeen City Council

These notes are designed to accompany the ArcView geographical information system (GIS) format groundwater vulnerability index map produced by the British Geological Survey (BGS) for Aberdeen City Council. The map is based on digital geological information for both bedrock and superficial (drift) deposits. It covers the whole of the Aberdeen City Council area plus a ‘buffer zone’ around the landward boundaries to account for peripheral data and allow for more meaningful interpretation. The purpose of the GIS map is to indicate, in broad terms, the vulnerability of groundwater to pollution. Groundwater is contained within aquifers of various types. Abstractions from these aquifers provide water for potable supplies and various domestic, industrial and agricultural uses. Some highly permeable aquifers are very productive and of regional importance as sources for public water supply; other, less permeable formations, are of local importance for domestic, agricultural and industrial supplies. Groundwater also provides the baseflow to surface watercourses. Groundwater is typically of high quality and often requires little or no treatment before use. However, it is vulnerable to contamination from both diffuse and point source pollutants, from direct discharges into groundwater and indirect discharges into and onto land. Aquifer remediation is difficult, prolonged and expensive: therefore, the prevention of pollution is important. The approach and classifications used in the production of the groundwater vulnerability index can also be used in the assessment of specific land use practices, proposed developments and land use changes over aquifers where these could have an impact on groundwater quality. More detailed site specific assessment of vulnerability will be required where it is considered that development may have an impact on groundwater quality. This GIS and printed maps are a compromise between the representation of natural complexity and the simplicity of interpretation at a scale of 1:50 000. This places limitations on the resolution and precision of map information. There is a wide variety of geological strata and potential pollutants, and the vulnerability index classification is, of necessity, generalised. Individual sites and circumstances will always require further and more detailed assessment to determine the specific impact on groundwater resources. The map coverages in the GIS only represent geological conditions (bedrock or superficial) as mapped at their upper surface. Where these formations have been disturbed or removed, for example, during mineral extraction, the vulnerability class may have been changed. Hence, where there is evidence of disturbance, site specific data need to be collected and used to determine the vulnerability of the groundwater. The overall permeability of each geological unit has been interpreted to produce an index of the vulnerability of groundwater occurring in and around Aberdeen, and provides a broad-based view of both the vulnerability of groundwater and the location of the more permeable aquifers in the area. The vulnerability index classification does not follow the methodology devised for published groundwater vulnerability maps used by the Scottish Environment Protection Agency (SEPA). The latter methodology includes an assessment of soil leaching potential; in addition, whereas the GIS-based methodology gives equal weight to bedrock and drift aquifers, on the earlier printed maps the bedrock formation takes precedence if it is highly permeable. The GIS- based methodology provides a broad-based view of both the vulnerability of groundwater and the location of the more permeable aquifers. The data used to interpret the groundwater vulnerability index are derived from the 1:50 000 DigMap bedrock and drift geology coverage. The GIS and associated maps should not therefore be used at scales larger than 1:50 000. Locations of thick clays have been interpreted and drawn based on BGS borehole records. Information on water boreholes is derived from the British Geological Survey Scottish Water Borehole database