Ground Gas Risk Assessments – Getting it Right
Ground Gas Risk Assessments form a crucial part of a Geo-Environmental Risk Assessment to support planning applications and determine risks and liabilities during investment.
The importance of assessing the risks from ground gas emerged following the high profile gas explosions at Loscoe and Abbeystead during the 1980s. With the potential for disaster prominent in peoples mind, a number of reports were published in the 1990s on the measurement of ground gases, the assessment of risk such gases may present along with the measures that can be employed to mitigate these risks.
Since then, guidance and assessment techniques have developed as the understanding of the risks from ground gas have improved. By understanding and adopting the most up-to-date techniques, REL gain a thorough understanding of a site, enabling a more robust and pragmatic assessment of the risk to be made, which results in the ability to recommend more sustainable, time saving and cost-effective outcomes for our clients.
In this blog we will discuss the ground gas risk assessment process, how to choose the right tools for the job and where the benefits are found for developers.
What is a Ground Gas Risk Assessment?
Ground Gas is generated from a number of sources throughout the UK, including:
- Coal Mine Workings
- Infilled Pits, Quarry’s and Ponds
- Man-Made Ground
- Organic Rich Alluvium and Peat
Generally, when we discuss ground gas, we centre on the two main bulk gasses; Carbon Dioxide and Methane. In addition, there are a number of trace gasses such as Hydrogen Sulphide, Carbon Monoxide and Volatile Organic Compounds (vapours).
The first step inground gas risk assessment is to determine if the sources of gas are located at your site or within the surrounding area.
To do so requires a Conceptual Site Model to be developed using the source-pathway-receptor model during a Preliminary Risk Assessment (Desk Study).
A level of risk is then applied based on the anticipated amount of gas generation, likelihood of this gas impacting the development and the consequence of these gasses entering properties.
If no pollutant linkages are identified, there is no need for further investigation or ground gas protection measures. Too often, ground gas monitoring is recommended as a precaution due to a lack of interpretation during the preliminary desk study. This is neither warranted or sustainable and REL would urge developers to challenge if this is required on low-risk sites or whether alternative methods of further assessment would be more suited.
If pollutant linkages are identified, further intrusive investigation is required. Most importantly, the results of the Conceptual Site Model will inform the most suitable techniques for further investigation and classification of the risk. Traditionally, this has been ground gas spot monitoring through the installation of wells, however, as assessment techniques evolve, this is not always the best approach.
Following an intrusive investigation, the data obtained is reviewed, and the risk assessment and Conceptual Site Model updated. It is important to refer back to the Conceptual Site Model and determine whether risks would be present under natural conditions and not solely base the ground gas risk assessment on the in-situ data collected as monitoring wells can often result in artificial conditions, especially within impermeable geologies such as clays and mudstones.
Based on the updated risk assessment, the need for ground gas protection measures can be dismissed or a suitable design specified to mitigate the risk to future site users.
Finally, before the proposed development can be occupied, where protection measures are installed, they must be verified to ensure they have been installed correctly and in-line with the design and specification.
A summary of this process is presented below:
What are the Right Tools for Ground Gas Risk Assessments?
Not all sources of ground gas are created equally. Some sources, such as landfills from post 1990s, pose a significantly higher risk than organic matter within Alluvium or Made Ground.
As discussed above, traditionally, ground gas spot monitoring is carried out to further determine the risk from ground gas during intrusive investigation, however, this is not always the best approach. Below are some scenarios where a different approach should be considered which can result in cost-savings and a more robust assessment.
Scenario 1 – Made Ground Over Non-Ground Gas Producing Strata
Many geology’s in the UK are not considered to be ground gas producing, these include London Clay, Mercia Mudstone, Lias Clay, Gault Clay and Glacial Till.
Importantly, these geologies are relatively impermeable and therefore will restrict ground gas movement onto a development site also.
Should this be the case on your site, the key remaining ground gas source may be Made Ground which can contain degradable organic material, capable of producing gas over time as it breaks down.
Rather than undertaking traditional spot monitoring, an assessment of the Total Organic Carbon (TOC) can be undertaken by collecting soil samples of the Made Ground during a Site Investigation. This should be supplemented with detailed forensic descriptions of the Made Ground constituents and an understanding of the depth profile across the site.
The depth of Made Ground, constituent make up and TOC content can be directly related to research presented in CL:AIRE RB17 A Pragmatic Approach to Ground Gas Risk Assessment, determining the ground gas protection requirements.
Scenario 2: Post 1990’s Landfill
Where high risk ground gas sources are identified, for example an on-site landfill from post 1990, traditional spot monitoring would need to be undertaken over a significant time period (6-12 months) at bi-weekly intervals.
This duration of monitoring may not fit within the development schedule and as such alternatives need to be considered.
One approach REL recommend is Continuous Ground Gas Monitoring. Unlike traditional spot monitoring, taking readings at bi-weekly intervals, continuous monitoring records ground gas concentrations and borehole flows every hour. Due to the increased frequency, the period over which the monitoring is required can be significantly reduced.
Furthermore, gas monitoring needs to cover a range of barometric pressures, including periods of falling pressure which would be considered “worst-case” conditions, forcing gas out of the ground and into future developments. During summer months, these falling pressure events are fewer and therefore it is harder to accurately determine “worst-case” conditions. If not identified, the duration of spot monitoring may need to be extended or regulators may not grant approval to the application.
Finally, continuous monitoring provides a significantly larger data set for consultants to interpret. This enables a more detailed assessment of the ground gas regime, identifying outliers in the results and ensuring that the correct ground gas classification and associated mitigation measures are proposed.
What are the Benefits of Better Ground Gas Risk Assessments?
Reduced Site Investigation Costs and Time on Low Risk Sites
By undertaking TOC sampling rather than ground gas monitoring on low risk sites, clients can save on costs for monitoring visits and installations, reducing the overall cost of a Site Investigation.
In addition, the results of ground gas monitoring are usually complete 2 months after completion of site works for commercial developments and 3 months after site works for residential developments. Where TOC sampling replaces ground gas monitoring the results of the ground gas assessment can be provided within usual reporting timescales of 6 weeks.
Reduced Costs and Carbon Associated with Installing Protection Measures
By adopting a multiple lines of evidence approach, incorporating a detailed Conceptual Site Model and results of either TOC testing or ground gas monitoring, the data can be more robustly interrogated, ensuring that gas protection measures are not recommended unnecessarily as a precaution.
The cost difference between protection requirements for lower and higher risk sites can be significant. As such, accurately determining the requirements through detailed assessment can result in significant cost savings for the development.
Finally, the greenhouse gas footprint resulting from the production of polymers associated with gas membranes would be significant if they were they to be installed by default in all new developments. High Density Polyethylene (HDPE), the material used in gas membranes, has a carbon footprint of 1.93 kg CO2/kg, so a typical 2000 gauge HDPE membrane with a density of 0.45 kg/m2 over the footprint of a typical UK house of 60m2 will result in the release of approximately 52.1kg of CO2 per house. On a development of 20No. properties this is 1,042 kg of CO2 – roughly the equivalent of a return flight from London to Paphos.
Since the importance of assessing the risks from ground gas emerged in the 1980s significant improvements in guidance and assessment techniques have developed as the understanding of the risks from ground gas have grown.
The industry is moving towards a more pragmatic approach to assessments and away from recommending protection measures as a precautionary approach. To do so requires a robust Conceptual Site Model to be developed during desk-based assessments and an understanding of how best to further assess the risk and determine mitigation requirements following intrusive investigation.
Doing so ensures that the most cost and time effective method of investigation can be undertaken, and protection measures are not unnecessarily installed, saving costs and carbon for developers.
Roberts Environmental have extensive experience in ground gas risk assessments and providing accurate and pragmatic solutions to designing, specifying and verifying protection measures.
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