Research and analysis

Field-scale evaluation of collection methods for dissolved methane samples in groundwaters - summary

Published 8 November 2023

Applies to England

1. Chief Scientist’s Group report summary

This project investigated field methods for sampling methane concentrations in groundwater under different conditions. The project developed a good-practice lookup sheet summarising the findings and identifying the sampling methods that should be used under different conditions. Groundwater monitoring practitioners can use this information to increase confidence in measurements of methane concentrations in groundwater.

1.1 Background

Methane can be present in groundwater due to the natural decay of plant and animal material in rocks and soil, or from human activities such as the disposal of waste in landfills, coal mining, and extraction of oil and gas. Because methane moves easily in the subsurface, presence of methane can be an early indicator of pollution from oil and gas extraction, storage or transport. However, there are challenges measuring dissolved methane concentrations since it is difficult to detect in low quantities and at high concentrations it can form bubbles and be released before it is measured.

1.2 Approach

The project investigated two boreholes, one with high and one with low methane concentrations, to assess the influence of different sampling methods on measured methane concentrations. The project compared different purging methods (the amount of water removed from the borehole before sampling), pump types (which bring water to the surface) and ways of collecting water samples (for example, in a bottle open to air, under water or closed to the atmosphere). A lookup sheet helps practitioners choose appropriate sampling methods for particular sites.

1.3 Results

Purging methods: methane concentrations decreased as the volume of groundwater purged increased. The effect was greater for the low concentration borehole. The reason for the decrease in methane with purging is not clear but suggests that when designing a sampling method, it is important to know how methane concentrations vary with time of purging.

Pump types: pump selection affected methane concentrations when using closed sampling methods. Pumps that maintain pressure (peristaltic and bladder) resulted in higher methane concentrations. In contrast to other studies, using a peristaltic pump did not result in a loss of methane to the atmosphere.

Water sample collection methods: methane concentrations were lowest for open samples and highest for samples closed to the atmosphere. However, due to costs of the closed method, the cheaper semi-closed upright method would be appropriate for use in most conditions.   

1.4 Conclusions

This investigation found large variation in the measured methane concentrations in groundwater from different field sampling methods. These variations highlight the importance of sampling protocols in estimating groundwater methane concentrations and the need to be clear about the methods that have been used. Ideally, investigations should employ a consistent sampling method and if sampling methods are changed, practitioners should use both methods together so that results can be compared to provide greater confidence in changes or trends. Practitioners should choose sampling methods according to specific borehole characteristics and can use the lookup sheet to guide this choice to ensure that results are robust and comparable.

1.5 Publication details

This summary relates to information from the following project:

• Report: SC210014
• Title: Field-scale evaluation of collection methods for dissolved methane samples in groundwaters
• Available from: https://nora.nerc.ac.uk/id/eprint/534181/
• Project manager: Sian Loveless, Chief Scientist’s Group

This project was a joint project between the British Geological Survey and the Environment Agency’s Chief Scientist’s Group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

Enquiries: research@environment-agency.gov.uk.

© Environment Agency