In 1992 the National Rivers Authority published its "Policy and Practice for the Protection of Groundwater". The implementation of the policy depends upon the definition of groundwater source protection zones and the preparation of vulnerability maps. This guide is one of two volumes which provide the background to the production and use of these two policy tools and complement the original policy document. The companion volume to this guide is the "Guide to Groundwater Vulnerability Mapping in England and Wales", published by HMSO (ISBN 0-11-310104-X).