GGIS No 6 – Using purge and recovery tests in ground gas risk assessment

ABSTRACT

Purge and recovery tests involve removing gas from a monitoring well and recording the recovery in gas concentration immediately afterwards. The tests were often used in the early 1990s to assess gas emission rates but were replaced by direct flow rate measurement from boreholes once accurate flow meters with a low detection limit became available.

They are still used by some but BS8576: 2013 recommends that the tests (which it refers to as recharge tests) are not used to replace flow rate measurements. This is because there are concerns over the accuracy of the results, which need careful interpretation and can give misleading indications of gas risk in certain situations.

This paper identifies the situations where the results are likely to be reliable or unreliable. It describes how the results in appropriate situations can be used to determine diffusion coefficients for use in detailed gas risk assessment.

May 2, 2019

Geoff Card, Richard Lavery

Steve Wilson

MSc BEng CEng MICE CEnv CSci CWEM MCIWEM FGS

Technical Director

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