OR/18/012 Introduction

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.

In principle, the EU’s Water Framework Directive (WFD) and the Groundwater Directive (GD) require that all groundwater is protected, though in the UK, groundwater bodies, which require active management under legislation, are defined for aquifers down to a maximum depth of 400 m below ground level (bgl) (UKTAG, 2011). Nevertheless, it is being increasingly recognised that groundwater within rocks that are not traditionally considered not considered to be a usable resource, and at a range of depths, may also be beneficial to society and require protection. Moreover, because of their nature, the impacts from contamination on such groundwater systems may not be observed for a long time, are not easy to predict, and it may not be possible to remediate in such instances. Consequently, any deep potentially contaminating activities, such as the exploration for, and development of, unconventional hydrocarbons should be assessed using a risk-based approach. In this context, a Tier 1 methodology is described in this report to assess risk to groundwater regardless of location (depth) and aquifer status, from the exploration for, and development of, unconventional hydrocarbons. This is consistent with the UK Government guidelines for environmental risk assessment and management (Gormley et al., 2011 ‘Guidelines for Environmental Risk Assessment and Management: Green Leaves III’).

Onshore conventional hydrocarbon exploitation is long established in the UK but in the last five years there has been a renewed interest. In particular, there has been increasing interest in unconventional hydrocarbon extraction e.g. shale gas, coal bed methane (CBM) and underground coal gasification (UCG), where the geological formations require some form of stimulation, such as hydraulic fracturing, to release the gas/oil. Until now, shale gas has received most attention in England, although exploration licences have also been granted for CBM and mine gas, in addition to more conventional hydrocarbon resources. At the time of writing, the UK Government has said that UCG is unlikely to go ahead in the near future. An up-to-date guide to the geological units and areas that are currently licensed or under consideration for hydrocarbon exploration and production can be found on the UK Government’s Oil and Gas Authority website (https://www.ogauthority.co.uk/data-centre/data-downloads-and-publications/licence-data/).

Hydrocarbon extraction may impact the subsurface by introducing new chemicals (potential pollutants), disturbing/mobilizing existing natural contaminants within rocks, or by changing the permeability structure of the rock (introducing new pathways). These changes represent additional hazards which may impact groundwater quality. Hazards to groundwater from development of shale gas resources are summarised by Lefebvre (2017) and include contamination from spills or leaks of fluids at the surface (considered the most probable mechanism leading to groundwater contamination), through leaking wells (the most challenging issue that might lead to groundwater contamination) or via subsurface pathways from the source rock (about which there is ongoing scientific debate). The subsurface hazard is influenced by the exploitation technique, which differs between hydrocarbon activities.

Groundwater may be vulnerable to contamination from these subsurface hazards through subsurface ‘pathways’. However, despite the abundant literature emerging, particularly from North America, there remain great uncertainties as to the vulnerability of groundwater associated with these pathways due to the influence of the geological and hydrogeological circumstances (Harkness et al., 2017).

Factors influencing vulnerability include the geological properties of the rock between the source of contamination (the hydrocarbon source, i.e. where the contaminants exist or are introduced) and the receptor of contamination (geological formations that contain groundwater and require protection), pre-existing fracture and fault networks, and the stress regime.

Another key factor impacting groundwater vulnerability is the proximity of the source and receptor. A joint BGS/EA project (iHydrogeology) mapped the vertical separation distances between key shale units and principal aquifers in England and Wales (maps available at https://www.bgs.ac.uk/research/groundwater/shaleGas/iHydrogeology.html) (Loveless et al., 2018) using the BGS GB3D model (Mathers et al., 2014). This showed large variations in vertical separation between aquifer-shale pairs across England and Wales and even within basins. For example, the separation distance between the Bowland Shale (the hydrocarbon source unit of interest in the north-west) and Triassic Sandstone aquifer ranges from <200 m to >1,500 m. The iHydrogeology project highlighted the need for site specific assessments of vulnerability and risk.

This report describes a prototype, Tier 1 (Gormley et al., 2011), site specific, qualitative 3D risk screening methodology for potential receptor units (i.e. rock units that may contain groundwater) to hydrocarbon exploitation practices. The methodology (3DGWV) has been designed to support decision making and the management of subsurface hydrocarbon activities to ensure groundwater protection.

As far as possible, the framework is consistent with the terminology and definitions used for current groundwater vulnerability mapping and assessment tools (EA, 2017b). The methodology considers:

Intrinsic vulnerability: Characteristics of the intervening units between the potential receptor and hydrocarbon source rock (such as separation distance, thickness of mudstones and clays and geological pathways) which may influence potential receptor vulnerability.

Specific vulnerability: Intrinsic vulnerability * (nature of the hydrocarbon exploitation activity (and associated processes impacting the subsurface) * (driving heads).

Risk Group: Specific vulnerability and receptor classification (i.e. perceived importance of the rock unit for groundwater).

An overview of the vulnerability methodology is provided first. Factors influencing the receptor classification, intrinsic vulnerability and specific vulnerability are then presented with details of the methodology’s scoring system. The 3DGWV methodology is accompanied by a software package containing; the 3DGWV Screening Tool Spreadsheet and the 3DGWV LithoFrame Viewer 3D model and user guide. This report provides guidance on how to undertake the 3DGWV screening and is intended to be used in conjunction with the 3DGWV Screening Tool Spreadsheet and the 3DGWV LithoFrame Viewer (LFV) 3D model.

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. It is designed to be used as part of a dynamic assessment which should be upgraded when additional information becomes available at each site. The risk group classifications are preliminary and used for illustrative purposes, and will be reviewed in the light of comments received, as will the scoring of the parameters within the assessment. Case studies in Appendix 7 demonstrate the effectiveness of the method and the potential use of the risk assessment.