Issue Date: September 2026
In the 1990s, it became clear that gas monitoring in monitoring wells in the UK, installed in recently placed earthworks materials such as London Clay, could result in elevated methane concentrations that were not representative of hazardous gas emissions. In 2005, extensive monitoring and assessment of earthworks sites in San Diego County in the USA showed that gas monitoring wells installed in compacted earthworks materials on development sites gave elevated methane concentrations but there was no risk of hazardous emissions.
From 2013 onwards, extensive work on remediation of old landfill sites in the UK has provided further evidence of the problems caused by post earthworks gas monitoring in wells installed in the fill materials. The monitoring frequently encounters elevated flow rates and gas concentrations, but when surface emissions are measured, they are minimal. A significant amount of time and resource has been expended in demonstrating, via other lines of evidence, that there is low or negligible ground gas risk.
To reduce contractual risk and provide much better risk management, other lines of evidence should be used to control gas risk in earthworks. This document provides guidance on good practice when managing earthworks to minimise the ground gas risk on completion of the works and removes the need for gas monitoring in boreholes and the associated problems it brings.
