Quintessa collaborated with the British Geological Survey (BGS) in undertaking a project for the Health and Safety Executive (HSE) to examine the potential for leakage of natural gas from Underground Gas Storage (UGS) in salt caverns and depleted oil/gas reservoirs. Leakage scenarios were developed and scoping calculations carried out to evaluate the likely significance of any leakage. A database of Features, Events and Processes (FEPs) was used to audit the issues identified by the BGS as potentially influencing whether or not gas leakage will occur. Three basic scenarios were identified as relevant to existing or potential onshore UGS facilities in the UK. For each scenario, scoping calculations were carried out to evaluate limiting gas fluxes and surface emission areas. The BGS component of the work is reported in HSE RR605, which provides context, background and data for the work reported by Quintessa in HSE RR606. Quintessa has subsequently provided support to UGS developers to assess site-specific leakage risks.