OR/18/012 Executive summary

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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.

This report is the product of a joint British Geological Survey (BGS) — Environment Agency (EA) study to develop a methodology for attributing vulnerability of groundwater to pollution from sub-surface oil and gas activities. It follows the UK Government’s guidelines for environmental risk assessment and management (Defra, 2011). The methodology considers:

  • hazards associated with unconventional and conventional oil and gas exploration, coal bed methane (CBM) and underground coal gasification (UCG);
  • near-surface and deeper aquifers as receptors, including;
-  groundwater that is currently used as a resource and/or supports surface water flows/wetlands (designated groundwater bodies);
-  other groundwater that could potentially be used in the future as a drinking water resource or for other beneficial purposes.

From a review of the location of hydrocarbon sources and groundwater in England, and potential contamination pathways, a prototype Tier 1 (qualitative) (‘Guidelines for Environmental Risk Assessment and Management: Green Leaves III’, Gormley et al., 2011[1]) screening methodology has been developed to assess groundwater vulnerability and risk from sub-surface hydrocarbon activities in three dimensions, at a site-specific scale. The methodology identifies and reviews the potential risks to groundwater and provides a means of communication of the outcomes of the vulnerability assessment. Outputs are intended for use by regulators (e.g. the Environment Agency and local planning authorities) to support environmental consultants advising industry and the public.

The vulnerability and risk screening methodology is described in this report and implemented in a supporting spreadsheet tool. A digital data package is also associated with this report for use with the methodology. This comprises the National Geologic Model (UK3D v2015[2]) for England, which can be viewed in LithoFrame Viewer software (downloadable free from the BGS website). Application of the methodology has been demonstrated using generic case studies from five areas across England.

The methodology is based on the DRASTIC model (a standardized system to evaluate groundwater pollution potential using hydrogeologic settings) that uses an overlay/index approach to assess risk where there is insufficient data for detailed numerical modelling. Each component of the assessment is described in different sections of this report:

  • importance and classification of potential receptors; (rock units containing groundwater);
  • intrinsic vulnerability of potential receptors (due to the geological characteristics of the rock units between the hydrocarbon source rock and potential receptors);
  • specific vulnerability of the potential receptors (due to the nature of the hydrocarbon exploitation activity, driving head and intrinsic vulnerability) and;
  • risk group of the potential receptors (the specific vulnerability combined with the potential receptor classification).

The framework for the assessment is a conceptual model for an Area of Interest (AOI), i.e. the area at the ground surface below which hydrocarbon extraction activities in the sub-surface may impact groundwater. The spreadsheet tool can be used to guide development of the conceptual model, and the vulnerability and risk screening process. The LithoFrame Viewer 3D model comprises a series of geologic cross-sections across England. As part of this project, each cross-section has been attributed, where relevant, with: a) potential hydrocarbon source rocks, and b) EA/BGS aquifer designations. This information provides a regional understanding of the 3D spatial relationship between hydrocarbon source rocks and aquifers.

Vulnerability and risk are assessed for all geological units (potential receptors) in an AOI. The potential receptors are assigned a class from ‘A’ to ‘D’, representing progressively decreasing importance or value of groundwater according to the Environment Agency’s aquifer designation schema and the UKTAG guidance on the maximum depth of groundwater bodies (UKTAG, 2011[3]). Evidence is also presented for decreasing water quality with depth and this factor is also considered in the methodology with an option to re-assign the class of the potential receptor according to groundwater quality (total dissolved solids (TDS)) where evidence exists. However, information on water quality at depth is generally sparse.

To determine the qualitative risk group for each potential receptor (low, medium-low, medium- high or high) both intrinsic vulnerability and specific vulnerability have to be assessed. A confidence level is also provided. This reflects the lowest of all confidence levels assigned to each factor in the intrinsic and specific vulnerability assessments. The risk groups and confidence levels can be used to identify sites where further information is required.

The intrinsic vulnerability assessment considers the geological characteristics of the rock units between the hydrocarbon source rock and a potential receptor(s) and their influence on the contaminant pathways between source and receptor. The proximity of the source and receptor, total thickness of low permeability mudstone in the intervening units and predominant groundwater flow mechanism(s) are all considered as part of the intrinsic vulnerability assessment. Preferential flow paths such as faults, mine workings and existing boreholes acting as conduits can allow contaminant transport over large distances in the sub-surface and therefore the presence of these is considered. A rating and weighting is also attributed to each potential receptor for each intrinsic vulnerability factor.

The specific vulnerability assessment combines the intrinsic vulnerability with the hazard (contaminant pathway driving force introduced by the sub-surface hydrocarbon exploitation activity) and natural drivers for contamination (i.e. groundwater head-controlled flow). A hazard factor is calculated from the potential impacts resulting from the hydrocarbon release mechanism (i.e. permeability enhancement, pressure/temperature changes, depressurisation) and natural head gradients (assumed from source to receptor if there is no evidence to the contrary). Generally, little information exists about the latter at a regional scale and therefore a worst-case (precautionary) scenario in which groundwater flows from the source to the potential receptor is assumed in the absence of further information.

The methodology has been tested for scenarios where receptors would be considered to be in the high or low vulnerability/risk groups according to the specific hydrocarbon activities and geological situations considered. It has also been tested in five case studies from different parts of England with different hydrocarbon source rocks/exploitation methods: conventional oil and gas in southeast England, CBM in the East and West Midlands, shale gas in northwest England and shale gas and conventional oil and gas in northeast England.

While there are differences in the technologies used and in geological settings, the case studies demonstrate that contaminant pathways in the sub-surface from hydrocarbon activities can be assessed using a common risk screening approach and parameter sets presented in this report. The case studies also indicate that a methodology based on a Tier 1 assessment, is useful in assessing and communicating risk and highlighting areas where additional information or process understanding may be important to improve risk assessment, management and decision making.

References

  1. GORMLEY, A, POLLARD, S, ROCKS, S, and BLACK, E. 2011. Guidelines for Environmental Risk Assessment and Management: Green Leaves III (London: Department for Environment, Food and Rural Affairs).
  2. UK 3D v2015. UK 3D — 3D geological model for the United Kingdom (online). Available: http://www.bgs.ac.uk/research/ukgeology/nationalGeologicalModel/GB3D.html. [cited 24 May 2016].
  3. UKTAG. 2011. Defining and reporting on groundwater bodies, UK Technical Advisory Group on the Water Framework Directive, working paper V6.22/Mar/2011 [online]. Available from http://www.wfduk.org/resources%20/defining-and-reporting-groundwater-bodies. [cited 12 September 2014].