OR/15/071 Survey methodology

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.

The start of the Survey, in 2011, was prior to any major investigations into the shale gas resources in the UK and the target areas for the Survey have expanded over time (Figure 3). In 2011, the main target areas were based on expert geological knowledge of unconventional hydrocarbon resources and the regional geology based on available geological maps, 3D models, borehole data and geophysical data. Account was also taken of the locations of current hydrocarbon licenses and known operator interest in particular areas. Further information on the geological and geophysical basis for delineating these areas can be found in the DECC report on unconventional hydrocarbons in the UK (2010) and Smith et al. (2010). Survey target areas were based on this state of knowledge, with the focus being on areas where resources were considered to exist but there were limited or no groundwater methane data. These initial target areas were then refined in 2013 with the DECC/BGS assessment of Britain’s unconventional resources (Harvey and Gray, 2013). In 2014, the DECC regional specific reports detailing prospective areas were released, in addition to the release of maps produced by BGS/Environment Agency (EA) which mapped the vertical separation distance between principal aquifer and shale gas source rocks (Bloomfield et al, 2014). As such the target areas have been refined over time to account for emerging information. Samples have only been collected from areas where the shales are deep enough to have achieved sufficient maturation to be a potential shale gas resource. No samples were collected from Northern Ireland; the baseline evaluation of groundwater quality in Ireland and Northern Ireland is being co-ordinated by the Irish Environmental Protection Agency as part of a joint research project on unconventional gas exploration and extraction (UGEE).

Figure 3    Target areas for the Survey a) in 2012 when the Survey started and b) after the work of Bloomfield et al (2014) and the 14th Licensing round.
Contains public sector information licensed under the Open Government Licence v3.0.

To collect methane data from the target areas, sample sites were selected by contacting the appropriate water companies and EA Regional offices, with the aim of having a good spatial distribution of sites and including a variety of aquifers utilised in the specific region. For ease of sampling only sites served by a borehole, not a spring or catch-pit, were chosen.

Sample sites near known high concentrations of methane such as landfill sites were avoided to prevent biased results. The number of samples that were collected per year as part of the Survey is reported in Table 1; the total number of samples collected over the 3 year Survey period was 248. The number and distribution of sites was influenced by the national scale of the Survey; the size of each of the target areas; the availability of sampling points in the area; and the resources available. Finding suitable sampling points was only possible with extensive assistance from public water supply companies and the EA. The majority of Survey sample sites are either water supply boreholes or EA monitoring boreholes (either from the Groundwater Quality Monitoring Network or private supply boreholes). However, assistance from the private supply borehole owners has also been invaluable. In the Wessex and Weald Basin 17 samples were collected by Thames Water and 11 by the Environment Agency; these data have been included in the summary.

Table 1    Summary of methane samples collected per year (including quarterly repeats) per target area.

Survey Area	2012	2013	2014	2015
Lancashire & Cheshire Basins	13	-	5	5
South Wales	12	-	9	4
Southern England	17	20	87	16
East Midlands Province	-	16	26	23
Northumberland	-	-	16	-
Totals	42	24	134	48

As mentioned in the Summary, this is a national scale survey to enable a broad understanding of the distribution of methane in aquifers across relevant areas of GB and cannot replace an understanding of groundwater quality at a local scale. It is also not intended to replace any oil and gas operator’s local monitoring as required by the regulators.

Sampling methodology

To obtain representative concentration measurements, groundwater has to be sampled before it has contact with the atmosphere so that no gases can exsolve. This entails collecting a pumped sample directly from a borehole, either by attaching a hose with a gastight connection to the wellhead for an installed pump, or directly to the outlet for a portable pump (Figure 4a). If an installed pump has no suitable wellhead, the nearest access point should be used. This must be situated before the pumped water enters a storage tank or is treated in any way. Samples collected using a portable pump were purged until field parameters were stable, with the pump placed at the mid-point of the screen, where details were available.

The methane samples are collected into double-valved steel cylinders of known volume, range 47–55 cm³ (Figure 4b). Ideally the samples are obtained at pump pressure via suitable clamped tube connections, but in cases where no suitable offtake point is available other arrangements may have to be extemporised and the sample taken at less than pump pressure. Unless the water is visibly degassing, this should have little or no effect on the dissolved gas concentration.

Water parameters including pH, temperature, specific electrical conductivity and alkalinity are measured in the field at the wellhead or other outlet. Samples for inorganic chemical analysis (ion chromatography and ICP-MS) were collected on site, filtered through a 45µm cellulose nitrate membrane; a sub-sample preserved with 0.5% HCl and 1% HNO₃, then analysed in the BGS Keyworth Laboratories.

Figure 4    a) Collecting unstable parameters during sampling and b) gas collection cylinder and samples for inorganic and organic carbon analysis.

Analysis methodology

The dissolved gas samples are analysed in batches of approximately 20 at the BGS Wallingford laboratories. A headspace technique is used. This involves the transfer of the water and gas in the sampling cylinder to an evacuated glass bulb of known volume (range 117–123cm³). The displacement process uses helium gas from the same source as used by the gas chromatograph (GC). Aliquots of the headspace gas are then expanded into the evacuated inlet system of the GC, from where they are admitted to a 1/8th-inch Porapak-Q packed column at room temperature. Eluting methane and ethane (if present) are detected by a flame ionisation detector (FID). Co-eluting CO₂ is detected by a thermal conductivity detector (TCD). The detection limit for methane varies between 1 and 0.1µg/l depending on GC conditions during batch analysis. Measurement precision is estimated to be ± 5% RSD.

Further details of the method are available in Darling and Milne (1995).

Quarterly monitoring

Quarterly monitoring was established with the aim of checking for possible temporal changes in dissolved methane concentrations over the course of a year. This initiative was intended to demonstrate whether a one-off sampling, conducted at any particular site, could be regarded as representative of conditions at that point in the aquifer over time, or whether methane concentrations were more of a ‘moving target’ in seasonal terms. It was considered unlikely that most groundwaters would show such fluctuations, so in the context of the resources available to the Survey the decision was made to monitor only a representative subset of sites amounting to about 10% of the total. Methane data from the repeat monitoring are included in both the national and regional summary data as additional samples.

Selecting the quarterly sites

In March 2014 a total of 17 sites were chosen for repeat sampling, from sites previously visited, to give a representative geographical spread of the aquifers in GB. The number of quarterly sites in each survey area was proportional to the size of the area, so the larger the target area the more sample sites selected. Within each survey area the quarterly sites represent a range of different aquifers and geographical separation. As the quarterly monitoring involved several visits to the same site, ease of sampling (e.g. a sample tap rather than an ad hoc tubing connection, and logistical constraints due to the nature of the borehole owner) was taken into account. Greatest preference was water company boreholes, followed by Environment Agency boreholes, followed by private sites.