Innovative Use of Radiocarbon Dating to Assess Ground Gas Risks at an Operational High Pressure Gas Site by Atkins
Shortlisted Brownfield Awards Category 2 Best Scientific/Technical/Digital Advance
Atkins conducted a site investigation in 2018 at this operational high pressure gas distribution site, which identified elevated ground gas flow and high methane (CH4) concentrations, exceeding 62%v/v in two monitoring wells. It was unclear whether these readings were being generated from leaking gas mains, anthropogenic material in Made Ground or from the underlying alluvial deposits. The site investigation quantitative risk assessment had identified a potentially unacceptable risk to on-site workers in excavations due to the asphyxiation and explosive risks.
We used an innovative technique, radiocarbon dating analysis, to date the decay of Carbon-14 isotopes and the percentage of modern gas (pMC) within gas samples. This ascertained whether the gas was biogenic and occurring on-site from the underlying stratum, or was a potential product of leaking natural gas mains on-site.
This innovative method met the client’s objectives through using a lines of evidence and application of current guidance, ensuring the transmission system remained in safe operation. The project was a cost-effective solution – allowing identification of the methane source without recourse to expensive surveys of pipeline assets, requiring valuable operational personnel and financial resources.
The client was very happy with the innovative method used: a conclusive result was achieved on a quick turnaround, which satisfied the gas network that there were no integrity concerns at site, and resulted in a significant reduction in the site capital risk provision.
Conceptual Site Model
The groundwater level varied between 1.74 m and 2.15 m bgl and was struck in the Alluvium.
Potential ground gas sources identified for the site included organic material in the Alluvium (up to 11.1% soil organic matter) and biodegradable materials reported within the Made Ground. However, with methane concentrations exceeding 62%v/v in two locations and steady gas flows reported as high as 16.6l/hr, it was considered that on-site high pressure gas mains could also be a source.
Published geological maps and site-specific geological information indicated that the site was underlain by Made Ground and Alluvium comprising silty, peaty, sandy clay over the London Clay. The site is 900 m from the coast.
Previous site investigation reported a layer of granular Made Ground across the site (up to 3.0 m bgl depth), underlain by Alluvium was encountered to a maximum unproven depth of 7.85 m bgl. The Alluvium comprising bands of sandy clay, gravelly sands and sandy gravels with pockets of organic material.
High pressure above ground gas mains and equipment
The Route to Innovation and Quality Assurance
The methane and flow readings gathered over three rounds of ground gas monitoring were considered to present an unacceptable risk to on-site workers in excavations due to the asphyxiation and explosion risks.
The client’s preference was to undertake an investigation which would not interrupt the high-pressure gas mains supply at the site, due to implications on the local network. This ruled out the option to test the integrity of the on-site pipework for leaks. The investigation would need to be sufficiently robust due to the consequences of a undetected weakness / leakage in high pressure below ground gas pipelines.
Determination of the source of elevated methane concentrations and flows was complicated by the presence of anthropogenic and biogenic sources of methane identified within the natural and Made Ground soils at the site.
The results collected from the previous investigation were subject to additional scrutiny, ensuring that there was no erroneous or overly conservative data that would skew the risk assessment. This included ensuring that: volatile organic compounds (VOCs) in the gas monitored were not falsely elevating the methane concentrations reported; and confirming whether the influence of the potentially tidal hydrogeological regime was affecting the flow and generation of ground gas.
The selected approach comprised an innovative technology which would identify the main source of the gas, could be undertaken on a quick timescale, would not interrupt the operation of the site, and was considered sustainable in terms of time, cost and environmental impact.
To determine the source of the methane we elected to use radiocarbon dating to differentiate between different sources –
Modern: Made Ground or landfill material, recently (<60 years old) generated methane through decomposition; and,
Ancient: methane which originates from sources of geological age (e.g. natural “North Sea” gas).
Carbon dating works by measuring ratios between three naturally occurring isotopes of carbon:-
Carbon-12 (12C) – comprising 99% of carbon atoms (stable)
Carbon-13 (13C) – comprising about 1% of carbon atoms (stable)
Carbon-14 (14C) – represented by 1 carbon atom per trillion (unstable)
We firstly assessed samples for the presence of the unstable carbon-14 (14C) isotope, which is continuously generated in the atmosphere through the action of cosmic rays on nitrogen and was also produced as a result of open air nuclear testing in the 20th Century. The “Ancient” sources of methane contain very low concentrations of 14C as the half life is 5,570 years, and concentrations will have diminished over the millennia.
Conversely, “Modern” sources typically contain a higher percentage of 14C isotopes, as limited breakdown will have occurred. The 14C concentrations are reported as a Percent Modern Carbon (pMC).
The second method of assessment uses the ratio of 13C to 12C (reported as δ13C), which varies depending on the source of the methane. This is typically between -50 and -70 δ13C ‰ for landfill and -25 to -40 δ13C ‰ for natural gas.
The table below shows the typical values for pMC and δ13C for a range of gas generation sources. The majority of the UK gas supply, is from the North Sea and Norwegian (Groningen) gas fields.
We therefore undertook radiocarbon dating on samples of gas obtained from our site: reported pMC and δ13C were then used to determine whether gas was likely to originate from a Modern or Ancient source.
Results and Interpretation
The 2019 results from the radiocarbon dating reported the following:
The 14C activity of 106 pMC recorded in the methane from VW18-02 indicated that the source of the gas was most probably biogenic ‘modern’ material that grew in the recent past and had been buried within Made Ground. The 14C activity of 34.9 pMC recorded in the methane from BH18-01B is equivalent to an age of 8,500 years. This suggests that the source of the material generating the gas likely originates from a mixture of gas generated from Made Ground sources and geologically “younger” organic material laid down within the Alluvium.
The 14C data is supported by the δ13C values, which indicate the gas sources are unlikely to be of ancient origin (i.e. natural gas).
Effective Stakeholder Engagement
We engaged with the key client stakeholders – network integrity gas engineers – during the design, implementation and reporting of the work. Upon receipt of the report, network integrity were satisfied that the methane concentrations reported were unlikely to be due to leakage of natural gas, and that further pipeline surveys / inspections were not warranted.
A Robust and Defensible Solution
The radiocarbon dating provided robust evidence that the source of methane at the site was unlikely to be from leaking high pressure gas pipes. Once we were satisfied that the methane is being generated from biogenic sources, we completed the ground gas risk assessment.
The bulk gas sampling identified that VOCs noted in the soil and groundwater were elevating the ground gas results collected by the gas monitoring, and a correction factor of 0.12 in BH18-01B and 0.13 in VW18-02 was used to correlate all the 2018 and 2019 results for the risk assessment.
Based on the evidence from the ground gas monitoring and the bulk gas sampling, the Characteristic Situation for the site has been assessed as CS2, which implies low risk from ground gas.
Application of best practice and compliance with legislation, codes and guidance
All works have been undertaken with care and diligence in relation to current best practice and legislation as follows:
Due diligence was undertaken on the specialist laboratory used to undertake the testing, with their state of the art equipment, clientele history and communication skills a key factor in their selection;
Supplementary gas monitoring was carried out from the wells constructed according to the current best practice given in BS8576:2014 and with full compliance with critical health and safety requirements. To date there have been no reportable incidents or disruption of the current site operation; and
The ground gas risk assessment compared the results of assessments undertaken in accordance with the CIRIA best practice guidance.
The project caused limited disruption and administrative complexities to the client and was more cost effective than physically surveying gas pipelines, which would require a shutdown of the gas equipment.
The outcome of the assessment demonstrated no requirement to undertake further costly remediation and/or potential gas main replacement (conservatively estimated at up to £230,000).
The scope of work and the sustainable, innovative and cost-effective outcome meant site operations could continue safely without the need to disrupt the gas supply or hold a provision for future ground gas remedial works or longer-term control measures and protocols.
We feel this is best summarised by the client contact for the project who has supplied this statement.
‘High concentrations of methane on a live gas site isn’t exactly the best thing to discover outside of the pipes! Atkins quickly put together a plan of action that progressed our land contamination objectives whilst ensuring the transmission system remained in safe operation and avoided tying up valuable operational resource investigating this issue. The lines of evidence approach allowed us to both satisfy the gas network that there were no integrity concerns at site and achieve a big risk reduction’.