User guide : BGS Permeability version 8

Permeability data is often used in studies of groundwater such as investigations of aquifer pollution or contamination. BGS has prepared permeability information based on the 1:50 000 Digital Geological Map of Great Britain (BGS Geology 50k). The Permeability Indices are based on geological considerations as follows: • the Predominant Flow Mechanism: intergranular flow, fracture flow, or a mixture of intergranular and fracture flow • a Maximum Permeability value • a Minimum Permeability value The BGS Permeability Index dataset 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). Permeability is the capacity of a rock to transmit a fluid and hence this dataset provides an indication of the rate of water movement between the ground surface and the water table. The Permeability Indices assigned are not actual values, derived from field tests or that have been tested in a groundwater flow model. They are a heuristic assessment based on knowledge of fluid movement through rocks of differing lithologies based on their dissolution potential, age, degree of cementation, and fracturing (see Appendix 3 for more details). They indicate where rapid infiltration through the unsaturated zone could occur, potentially causing pollution of groundwater or where water could pond on the ground surface due to slow drainage rates through poorly permeable material. The dataset covers Great Britain, and is presented as a map at a scale of 1:50 000, based on the geological data at the same scale. It is for use at the regional scale and is not recommended for use at the site-specific scale. The information provided in this User Guide is intended to provide a quick guide to using and understanding this BGS data product.