|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 has described a Tier 1 (qualitative) methodology (3DGWV) for attributing vulnerability of groundwater to pollution from sub-surface oil and gas activities. The method considers a range of geological, hydrogeological and industry specific factors which influence vulnerability and risk to groundwater at a particular site. It is accompanied by a digital package, including the attributed UK3D2015, GIS data and a spreadsheet tool to guide development of a conceptual model and the assessments.
The methodology has been tested for scenarios where receptors would be considered to be of high or low vulnerability/risk according to the specific hydrocarbon activities and geological situations. 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.
The case studies have demonstrated that contaminant pathways in the sub-surface from hydrocarbon activities can be assessed using the common vulnerability and risk screening approach and parameter sets presented in this report. By this means the relative vulnerability between sites and/or hydrocarbon development scenarios can be compared and used to assist in decision-making processes and risk communication.
The advantage of this approach is that it can be applied as a rapid, initial screening of possible vulnerability and risk scenarios for a particular development (with assessment of receptors at geological group scale, for example). It can also be used for more detailed assessment (for example, assessment of receptors at the geological formation scale) if required as well as identifying where information is lacking. One such area, highlighted by all the case studies is a need to improve the 3D understanding of the geological systems.
Confidence in the vulnerability and risk assessment improves as more information is considered. Consequently it is recommended that it is used in a dynamic manner.
The risk group (qualitative indicator of risk) is the most informative category since it takes into account the sensitivity of the potential receptor along with intrinsic and specific vulnerability. The examples considered in this report produced a range of vulnerability and risk outcomes that were consistent with the scenarios tested.
Refinements to the methodology are required in terms of the factor parameters and weightings, in addition to risk groupings. The methodology is only concerned with risks to groundwater from hydrocarbon activities in the subsurface and does not include any considerations of either the effects of surface spillages or the integrity of boreholes which are dealt with through surface/near-surface groundwater vulnerability assessment tools and drilling regulation.
Development of the methodology has also pointed to a number of topics that need further research in order to reduce uncertainty in such assessments. These include;
- a better understanding of the location and processes within deep aquifer flow systems and their behaviour, including volumes of water recharging to deep basins and regional and local head flow directions,
- attenuation capacities of mudstones/clays at depth and in relation to particular contaminants and the critical thicknesses,
- the impact of particular contaminants in potential receptor units,
- an improved understanding of time scales of contamination breakthrough and ‘safe’ separation distances,
- improved understanding of the pathway behaviour of faults and
- a more in-depth understanding of the occurrence of groundwater that should be protected.
Further development might also explicitly include more detail about the nature of the hazard including chemical characteristics and concentrations.
- 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].