OR/15/071 Summary and recommendations

Bell, R A, Manamsa, W G K, and Dochartaigh, B É Ó. 2015. The baseline concentrations of methane in Great British groundwater - the National Methane Baseline survey. British Geological Survey Internal Report, OR/15/071.

Summary

The BGS has undertaken a national baseline survey of methane in groundwater, concentrating attention on areas where a shale gas resource is thought likely to exist and which could be developed in the future. The Survey has tried to capture the natural baseline conditions of methane in groundwaters of GB at a regional scale (Figure 26 and 27). The Survey has collated past methane data and merged with these new samples to create a database containing 439 methane data points (Table 16). Approximately 96% of the data have methane concentrations under 100µg/l, 80% are under 10µg/l and 43% are under 1µg/l. Less than 1% of the boreholes sampled had methane concentrations over the LEL of 1,600µg/l and none of these sites are public water drinking supplies. The average concentration across GB in the aquifers sampled is 45µg/l. From the different regions sampled as part of the Survey, South Wales has the highest methane concentrations, followed by Central-Southern Scotland and Lancashire/Cheshire. Although there are a few outliers in the other regions, the methane concentrations are lower in the East Midlands, Northumberland/Cumbria and the Wessex/Weald Basin. In general, methane concentrations in carbonate aquifers (Chalk, limestones, Oolites etc.) are low, similar to those seen in the Permo-Triassic sandstone aquifers, although methane is ubiquitous across all these aquifers. From the temporal data, little variation in methane concentrations has been observed, except in South Wales, where further investigations are needed to determine whether these variations are intrinsic to the aquifer type or are related to the mode of sampling used there.

Table 16    Summary methane concentration data for GB.

Area	CH4 Concentration (µg/L)				Number of samples
	Minimum	Median	Average	Maximum
UK	<0.05	1.2	45	4,720	439

Figure 26    GB methane concentrations.

Figure 27     Summary methane data for GB.

Recommendations for further work

Examination of the data collected as part of the Survey has revealed three main knowledge gaps that warrant further research to develop our understanding of methane in groundwater. The first is the need for a comprehensive study into the impact of different sampling methods on dissolved gas concentrations; this is pertinent for the baseline monitoring of methane and other volatile compounds around potential shale gas sites. There is the potential for certain types of pump (in particular, suction-lift pumps) to impact on dissolved gas concentrations; this is a known issue from sampling carried out in contaminated land settings and the potential degassing of Volatile Organic Compounds (VOC’s) (Nielsen and Yeates, 1985; Parker, 1994; U.S EPA, 2010). This could also therefore impact on methane concentrations in groundwater. In Halwa (2015), a duplicate sample was also collected using a peristaltic pump, rather than the submersible centrifugal pump, the results of which showed a dramatic reduction in apparent methane concentration. The second area of research is the need for further investigation of the elevated concentrations of methane found in parts of the Weald and Wessex basin, to characterise its spatial distribution and develop an understanding of its source(s). An understanding of this is critical to determine how groundwater chemistry and microbiology are affected by these inputs. Thirdly, there is the need to develop techniques for dissolved methane monitoring at a high frequency, as this will be required to effectively monitor the possible effects shale gas operations on aquifers.