Guide to Permeability Indices

This report describes how the BGS Permeability Index dataset has been prepared and includes guidance on how it can be used. It discusses what the indices mean and how the data have been derived. It describes the methodology used to classify each geological unit in the Digital Geological Map of Great Britain at 1:50 000 scale (DiGMapGB-50) according to its predominant flow type and probable permeability. The Permeability Index is a qualitative classification of estimated rates of vertical movement of water from the ground surface through the unsaturated zone, the zone between the land surface and the water table. The Permeability Index codes have been allocated to every lithology (or combination of lithologies) for each named rock unit that has been mapped in DiGMapGB-50. This has been carried out for all four types of deposit shown as separate layers in the DiGMapGB-50 (artificial ground, mass movement deposits, superficial deposits and bedrock) dataset. The Permeability Index consists of a three-part code representing: • Predominant Flow Mechanism • Maximum Permeability • Minimum Permeability The Predominant Flow Mechanism code indicates how fluid will migrate from the ground surface through the unsaturated zone of each rock unit and lithology combination and has three classes, intergranular, fracture or mixed (intergranular and fracture). The second and third codes (Maximum and Minimum Permeability) indicate the range of flow rates likely to be encountered in the unsaturated zone for each rock unit and lithology combination. Five classes have been used for the Maximum and Minimum Permeability codes: very high, high, moderate, low and very low. The Maximum and Minimum Permeability values represent a likely permeability range for the specific named rock unit and lithology combination at, and immediately below, outcrop (rather than at any significant depth). The Maximum Permeability represents the fastest potential vertical rate of migration through the unsaturated zone likely to be encountered. The Minimum Permeability represents the minimum, and in some cases more normal, bulk rate of vertical movement likely to be encountered. Where a widely variable lithology combination occurs within a rock unit this value reflects the probable movement rate likely to be encountered in the least permeable horizons The coding was based on expert judgement but with the following assumptions: • the lithological component(s) for a particular named rock unit mapped within DiGMapGB-50 was correct, whether or not this was expected or normal for the given formation. • the order of the deposits in a lithological ‘string’ was of relevance; it was assumed that the dominant lithology was placed first, with the other lithologies in order of their occurrence, e.g. gravel, sand, silt and clay was different to, and more permeable than, clay, silt, sand and gravel. • that all of the possible geological layers (artificial, mass movement, superficial deposits and bedrock) that could be present at a site were mapped. It is, however, known that this is not the case where the maps are old and the presence of superficial deposits was not always recorded. Similarly the presence of artificial deposits is constantly changing and only those present at the time of survey were recorded