Ground Gas Risk Assessment at Woodend Farm, Falkirk
Title: Ground Gas Risk Assessment at Woodend Farm, Falkirk
Client: Falkirk Council
To determine whether the initial generic screening risk assessment and characterisation of the gas rime (CS5) at the site was appropriate.
Services performed by GGS:
- Continuous ground gas monitoring at six locations over a period of three weeks. This included continuous flow and groundwater level monitoring.
- Water sampling at select locations to determine gas provenance.
- Surface walkover survey and flux box tests to detect any methane emissions from the surface soils.
- Periodic monitoring at seven monitoring locations.
- Summary of previous site investigations.
- Provision of a detailed ground gas risk assessment using a multiple line of evidence approach and in line with best practice guidance (Ciria C665 and BS8485:2015).
Ground Gas Risk Assessment at Woodend Farm, Falkirk
The site was currently farmland that was under consideration to build over one hundred social residential houses. Initial environmental due diligence by the client had identified elevated ground gas concentrations and flow.
The Generic Screening Risk Assessment based upon the initial observations of ground gas concentrations and flow placed the site within the CS5 classification using Ciria C665 guidance. This classification was based upon ‘worst possible’ conditions at the site and the outcome was largely driven by the observations made at a single monitoring installation. The source of ground gas was undetermined, with preliminary work suggesting former coal mine workings.
The site is overlain with former coal workings and geological faults are known to be present in the area. A water main traverses a section of the site and the site was subject to a past of changing farming activities. Thus, the setting of the site offered several potential gas sources and pathways.
Previous monitoring suggested a worst case CS5 characterisation of the site, yet the results took little account of the site specific conceptual site model or other lines of evidence. The challenge was to assess how representative previous monitoring results were of the true site conditions and design a robust monitoring scope to provide sufficient evidence to accurately characterise the site.
There was a known security issue at the site due to open access and there was a history of vandalism and fly tipping. As such, all monitoring equipment had to be housed in secure headworks to prevent any interference and data loss.
The monitoring period captured the first ‘Beast from the East’ weather event. GasSentinel® instruments are designed to work in harsh environmental conditions of fluctuating temperature and high humidity. However, battery performance can suffer when operating in exceptionally low temperatures. Fortunately, owing to the telemetry enabled instruments, GGS could remotely monitor the performance of the instruments and react to prevent data loss from issues such as depleted batteries.
The multiple lines of evidence approach allowed GGS to determine that the high flow rates (>40 l/h) observed during initial monitoring were likely a function of the ground conditions and importantly not an indication of gas regime. Continuous flow and groundwater monitoring provided significant lines of evidence, with GGS PRT tests corroborating the results and demonstrating negligible ground gas generation.
Groundwater was high across the site throughout the monitoring period and fluctuated between 0 -1.5m below ground level. Rising groundwater confines gas in the standpipe to relatively little headspace, creating a pressure differential and this is detected as flow when undertaking periodic monitoring; often termed the ‘piston effect’. As such, GGS demonstrated that the piston effect was the key driver of the high flow rates found during the initial periodic monitoring.
The dissolved gas analysis suggested minimal risk for hazardous gases from dissolved phase. Importantly, there was found to be little carbon dioxide saturation, despite the groundwater levels being so high and carbon dioxide dissolving more readily then methane. Isotopic analysis strongly suggested that the gas was derived from biogenic sources (produced microbially) as opposed to a thermogenic source (such as coal mine gas).
The initial CS5 classification was based upon high flow rates and gas concentrations encountered during the initial programme of periodic monitoring. GGS found the initial monitoring, especially flow rates, to not be representative of the sites gas regime. As a result, the lower flow rates and gas concentrations observed during the GGS monitoring period, supported by multiple lines of evidence and resulted in a revised CS2 classification for the site.
- GGS designed and undertook a thorough and appropriate monitoring scope to support a comprehensive and robust ground gas risk assessment.
- GGS mobilised quickly to facilitate the clients tight time scales.
- GGS successfully characterised the site and suggested the most appropriate ground gas protection, in line with best practice guidance.
- GGS saved the client time and money.
- GGS used the latest technology including continuous flow and telemetry.
- GGS combined several tests and monitoring techniques, both new and established, to provide a robust multiple lines of evidence approach.
Regulators across the UK rely on GGS for continuous ground gas monitoring and accurate risk assessments.
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