OR/18/012 Risk group

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Loveless, S, Lewis, M A, Bloomfield, J P, Terrington, R, Stuart, M E, and Ward, R S. 2018. 3D groundwater vulnerability. British Geological Survey Internal Report, OR/18/012.

The risk group (RG) takes into account the receptor importance and the specific vulnerability to produce a classification of either low, medium-low, medium-high or high (Table 6.1). The risk group classifications are preliminary, and will require adjustment, as will the scoring of the parameters within the assessment.

A confidence level is also assigned — the lowest of all confidence levels assigned to each factor in the intrinsic and specific vulnerability assessments. The combination of risk groups and confidence levels can be used to identify sites where further information is required for assessment.

For a receptor classified as ‘D’ the risk group is always low because the unit is classified as unproductive and/or has a TDS of >35 000 mg/l. It is therefore a low priority for protection. However, receptors that are classified as ‘A’ and ‘B’ may be principal/secondary aquifers and the impacts of contamination would be greater. Receptors classified as ‘A’ are so important that even low specific vulnerability scores would result in these receptors being in a medium/low risk group.

Table 6.1    Risk groupings based on specific vulnerability and potential receptor classification. Classifications are preliminary only.

Specific vulnerability score

Potential receptor classification <250 250–500 500–750 >750
A Medium/Low Medium/High High High
B Low Medium/Low Medium/High High
C Low Low Medium/Low Medium/high
D Low Low Low Low

Baseline methane

Methane (natural gas) is commonly found at trace levels as a dissolved component in groundwater. Methane is produced by both natural processes and anthropogenic activities. Thermogenic natural gas is produced through thermal decomposition of organic matter at significant depth. Methane can also be produced by biogenic processes (bacterially) at much shallower depths. Because natural gas is buoyant in geological environments, if a pathway exists, it can move upwards and accumulate at shallower depths. Natural gas found in small, uneconomic quantities in shallow zones may have originated in place or may have migrated upwards, and is often referred to as stray gas. Anthropogenic activities that can produce or release methane include coal mining or landfill operations.

There are certain environments in which methane might be naturally high, such as peat bogs, wetlands, lake sediments and landfills or even confined groundwater bodies (Bell et al., 2016[1]; Ward et al., 2017[2]). Geochemical analysis can indicate whether the methane is from a shallow, biogenic source or a deeper, thermogenic source. In the latter case, it is possible that there exists a pathway for contamination from depth to the surface or groundwater body of interest and this should be noted as part of the vulnerability assessment. Where this is not clear, it should also be considered as a possible release from depth since this may be evidence for a higher vulnerability area with a potential driving force and pathway. This should be considered in conjunction with the risk assessment.

Bell et al. (2016)[1] consider the concentration of methane in water of 1600 µg/l as high. This concentration represents the Lower Explosive Limit (LEL) (5% by volume) in a confined space.

Source of information
Information is available from the National Methane Baseline report (Bell et al. 2016[1]) and http://www.bgs.ac.uk/research/groundwater/shaleGas/methaneBaseline/home.html. Because of the scale of the National Methane Baseline survey there are many AOI’s where there will not be data. It is therefore recommended to consider regional datasets, for e.g. on the basin/sub-basin scale (e.g. Weald/Wessex basins).

References

  1. 1.0 1.1 1.2 BELL, R A, DARLING, W G, MANAMSA, K, and O’DOCHARTAIGH, B E O. 2016. The baseline concentrations of methane in Great British groundwater: the National Methane Baseline Survey. British Geological Survey Report, OR/15/017.
  2. WARD, R, SMEDLEY, P L, ALLEN, G, BAPTIE, B J, DARAKTCHIEVA, Z, HORLESTON, A, JONES, D G, LEWIS, A, LOWRY, D, PURVIS, R M, RIVETT, M O. 2017. Environmental Baseline Monitoring Project. Phase II, final report. (Nottingham: British Geological Survey).