OR/18/012 Vulnerability screening methodology

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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 3D groundwater vulnerability screening methodology for England (3DGWV) is designed to assess the intrinsic and specific vulnerability and assign a risk group to potential receptors, related to hazards associated with conventional or unconventional hydrocarbon exploitation activities in the subsurface. It is a prototype, Tier 1, qualitative risk screening method which can be used to identify whether or not a more detailed assessment is needed to aid risk prioritisation (see https://www.gov.uk/guidance/groundwater-risk-assessment-for-your-environmental-permit#history).

A risk group is attributed to each potential sub-surface receptor (rock unit) in a geological sequence. The risk group takes into account the importance of the receptor, the intrinsic vulnerability of the receptor, and the nature of the hazard (specific vulnerability). As far as possible, the framework is consistent with the terminology and definitions used in current groundwater vulnerability assessment framework for England (EA, 2017a[1]):

Intrinsic vulnerability (IV): considers geological factors related to the intervening units between the potential receptor and hydrocarbon source rock (such as separation distance, mudstone and clay thickness and geological pathways) which may influence potential receptor vulnerability;

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

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

Since this is a Tier 1 methodology (Gormley et al., 2011[2]), likelihood and impact (standard risk factors) are not quantified but are accounted for implicitly in the nature of the hazard. The methodology accounts for potential contamination to groundwater from specified hydrocarbon source units in the sub-surface only. It does not pertain to potential contamination from above ground sources, specific drilling practices or infrastructure (e.g. borehole integrity) failure. However, if a borehole is known to be leaking from a specific depth, the method can be applied to assess the vulnerability of receptors to contaminant release from this point.

The methodology has been developed in the context of current environmental regulations for England, including (but not limited to):

  • The EA position statement on UCG, CBM, shale gas extraction and for oil and conventional gas exploration and extraction that it will ‘normally object to UCG, CBM or shale gas extraction infrastructure or activity within a SPZ1. This includes subsurface SPZ1 areas which are confined by impermeable strata at the surface’. Outside SPZ1s, the EA will also normally object when the activity would have an ‘unacceptable effect on groundwater’ (Table 1 in EA, 2017b[3]);
  • the Infrastructure Act (2015)[4] prohibits high volume hydraulic fracturing at depths of less than 1000 m below ground level (bgl). The Onshore Hydraulic Fracturing (Protected Areas) Regulations (2016)[5] extends this to 1200 m bgl within protected groundwater source areas;
  • drilling is controlled under the Environmental Protection Act (1990)[6], through which protection is emphasised for groundwater that is currently used as a drinking water resource. Best Available Technology is expected to protect groundwater where drilling or operation of boreholes passes through a groundwater resource (EA, 2017b[3]).

It is stressed that the risk group boundaries identified by this screening methodology are preliminary, based on professional judgment, and are expected to be revised with further review, testing and increasing scientific evidence. Therefore, it is not recommended that the initial risk screening be used on its own for site specific decision making by regulators, but that it be used to help guide further investigations. Where there is a lack of data for developing a conceptual model, either further research/investigation should be undertaken to address the knowledge gaps, or they should be identified as areas of high uncertainty where the precautionary principle should be applied in assessing the needs for protecting groundwater quality.

Approach

The methodology uses an overlay/index approach similar to the DRASTIC method, a standardized system model to evaluate ground water pollution potential using hydrogeological settings (Aller et al., 1987[7]). Overlay/indexing approaches are used as an alternative to detailed numerical groundwater modelling, when there are insufficient quantitative data. In DRASTIC, parameters which are considered to be influential to the overall vulnerability of groundwater from surface activities are combined. Each parameter has a range of possible values, indicating the degree to which that parameter protects or makes groundwater vulnerable in a region. Overlay/indexing methods are relatively easy to implement, using readily available data over large areas, and typically produce categorical results (Focazio et al., 2002[8]). DRASTIC and other related approaches have been very widely used (Gogu and Dassargues, 2000a[9]; Kumar et al., 2015[10]; Shirazi et al., 2012[11]). Other approaches, which take into account preferential flow pathways, have been considered for karst environments e.g. EPIK, PI and COP (Andreo et al., 2009[12]; Doerfliger et al., 1999[13]; Gogu and Dassargues, 2000b[14]; Goldscheider, 2005[15]; Vías et al., 2006[16]).

To date, the majority of published overlay/indexing models for hydrocarbon exploitation have been developed to assess the risks to groundwater from the surface aspects of the development These include gas exploration in the Karoo Basin (Karoo Groundwater Expert Group, 2013[17]), tar sand extraction in Nigeria (Ojuri et al., 2010[18]), open cast removal of oil shale in Jordan (Mohammad et al., 2016[19]), coal mining in India (Tiwari et al., 2016[20]), tight petroleum exploration in Quebec, Canada (Raynauld et al., 2016[21]), tight shale exploration in Ohio (Thompson, 2012[22]) and shale gas development in South Africa. WorleyParsons (2013)[23] developed a methodology to assess the risk to groundwater and related receptors from the exploitation of coal seam gas (or CBM) using a hybrid approach within a source–pathway–receptor risk model combining an overlay/index method with a process-based model. A number of source hazards were identified and links between these and the pathways and receptors were then separately assessed to inform prioritisation. Each pathway and receptor factor was weighted, rated and scored. Water extraction and gas migration were considered to be the most significant hazards, together with five pathway factors and three groundwater receptors. Application of this risk mapping to each of the individual coal seams enabled identification of seams which presented the greatest risk to groundwater and its receptors.

The 3DGWV screening methodology comprises a series of steps, beginning first, and most importantly, with the conceptual model of the deep to shallow hydrogeological system for the full 3D footprint of the proposed hydrocarbon activity, below the Area of Interest (AOI). The AOI is the area at the surface below which sub-surface hydrocarbon extraction activities could potentially impact groundwater. The AOI includes all boreholes laterals and cavities created as part of the extraction process, in addition to a 2 km buffer zone.

The vulnerability screening should be carried out for every geological unit, or potential receptor, between the hydrocarbon source unit and the surface or, if the proposed activity is <1200 m below ground surface, to a depth of 1200 m, including units below the proposed hydrocarbon activity. The exact resolution of units is dependent on the region, information available and purpose of the screening. ‘Hydrocarbon source unit’ refers to the rock unit from which the hydrocarbon would be extracted, i.e. for conventional oil and gas, the reservoir storing the hydrocarbon, rather than the original source of the hydrocarbons. For shale gas and CBM/UCG the source rock will be the shale and coal units respectively.

The potential hydrocarbon source unit rocks and aquifers can be displayed in the 3DGWV LithoFrame Viewer 3D model. The LithoFrame Viewer 3D model comprises a series of geologic cross-sections across England. 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. This can then be used to aid development of the conceptual model. Additional sources of information should also be utilised, where available, as outlined in this document. Confidence limits are explicitly recorded as part of the assessment process and should be taken into account when reviewing specific vulnerability scores and risk groupings. In AOIs where there is a high degree of geological variability and/or uncertainty regarding the conceptual model, a number of potential geological scenarios may be possible and so each should be assessed in order to understand the sensitivity to changing parameters. As the site is investigated further, the screening can be refined with the additional knowledge, and uncertainty reduced. The stages of the assessment process are outlined below in Figure 2.1 and described in full in Table 2.1 to Table 2.3. An example is presented in Appendix 1 - 3DGWV screening methodology, spreadsheet tool and example for low vulnerability scenario.

Classification of the importance of potential receptors (PR); undertaken for all units within the geological sequence, according to EA aquifer designations and evidence for groundwater quality. These are classified as A to D, representing progressively lower value groundwater.

Intrinsic vulnerability (IntV); assessment of the intrinsic vulnerability of each potential receptor to the proposed hydrocarbon activity. Parameters relating to key factors (and sub-factors) influencing intrinsic vulnerability (e.g. proximity between hydrocarbon source unit and potential receptor) are provided with a parameter rating (rx). Each subfactor is weighted according to its perceived influence on vulnerability (wx). An overall score for each subfactor is calculated (rx*wx). The confidence level is also recorded for each sub-factor.

The scores for each subfactor (rx*wx) are then added together to obtain an overall intrinsic vulnerability (V=Σ (rx*wx)) for each potential receptor in the geological sequence

Specific vulnerability (SpecV); hazard factors are ranked according to the nature of the hazard(s) resulting from the hydrocarbon activity and contaminant release mechanism (H1) and local hydraulic gradient(s) or driving force(s) (H2). The rankings are not weighted. H1 and H2 are both multiplied with the intrinsic vulnerability.

Risk group (RG); the receptor classification and specific vulnerability score are combined to assign potential receptors as low, medium-low, medium-high or high risk, according to Table 2.4.

Figure 2.1    Flow chart showing the screening process (full process in Table 2.1 to Table 2.4).
Table 2.1    Receptor classification. (Classifications are currently preliminary).

Receptor classification

Potential receptor classification EA aquifer designation and depth to top of unit below surface Total dissolved solids (TDS)
A Principal aquifer <400 m <1000 mg/l
B Principal aquifer >400 m, secondary aquifer <400 m 1000–3000 mg/l
C Secondary aquifer >400 m 3000–10 000 mg/l
D Unproductive > 10 000 mg/l


Table 2.2    Hazard ranking. (Ranking is preliminary).

Hazards

Hazard factor Hazard parameter Ranking (r) Weighting (w) Confidence Maximum score
Release mechanism of hydrocarbon (H1) Permeability enhancement and increase in pressure and temperature (UCG) 5 N/A H 5
Permeability enhancement from high volume hydraulic fracturing (e.g. shale gas) 4
Permeability enhancement from low volume hydraulic fracturing (e.g. conventional oil and gas with hydraulic fracturing) 3
Water table lowering and depressurisation (CBM) 2
No permeability enhancement (passive) for conventional oil and gas. 1
Head gradient driving flow (H2) Head gradient from hydrocarbon source to receptor (or unknown) 2 L, M or H 2
No head gradient from hydrocarbon source to receptor 1


Table 2.3    Intrinsic vulnerability. (Rating and weighting are preliminary only).

Intrinsic vulnerability

Assessment for intervening zone between top of hydrocarbon source unit and base of the potential receptor unit

Intrinsic vulnerability factor Intrinsic vulnerability subfactor Intrinsic vulnerability parameter range Rating (r) Weighting (w) Confidence Maximum score
Proximity of hydrocarbon source unit and potential receptor Vertical separation of hydrocarbon source unit and potential receptor >1200 m 1 1.5 M or H 12
900–1199 m 2
600–899 m 3
400–599 m 4
300–399 m 5
200–299 m 6
100–199 m 7
<99 m 8
Lateral separation of hydrocarbon source unit and potential receptor > 2000 m 0 3 M 12
1000 to 1999 m 1
500 to 999 m 2
200 to 499 m 3
<199 m 4
Mudstones and clays in intervening zone between hydrocarbon source unit and potential receptor >250 m mudstone or clay 1 3.5 M or H 17.5
>100 m mudstone or clay 2
>50 m mudstone or clay 3
>20 m mudstone or clay 4
No intervening strata or <20 m mudstone or clay 5

Assessment for intervening zone between top of hydrocarbon source unit and top of the potential receptor unit

Factor Sub-factor Range Rating (r) Weighting (w) Confidence Maximum score
Groundwater flow mechanism Only units designated 'Unproductive Strata' by EA 0 3 M or H 9
>50% principal or secondary aquifers (EA designation) with intergranular flow (e.g. sands) 1
>50% principal or secondary aquifers (EA designation) fractured, poorly connected or mixed fracture and intergranular flow (e.g. well fractured sandstones, multi-layered Carboniferous rocks) 2
>50% principal or secondary aquifers (EA designation) fractured, well connected (e.g. limestone) 3
Preferential flow pathways Faults Faults not known in the area of interest 1 4.5 L, M or H 18
Known faults within 2 km of the hydrocarbon activity 2
Known faults within 0.5 km, or transmissive fault within 2 km of the hydrocarbon activity 3
Faults known to be transmissive within 0.5 km of the hydrocarbon activity 4
Solution features No potential solution features 0 2 L or M 6
Potential for solution in evaporite minerals 1
Potential for karst or known solution features in evaporite minerals 2
Known karst features in area of interest 3
Anthropogenic features mines No known mine (and assumed to be absent) within 2 km of maximum lateral extent of hydrocarbon activity, or 600 m vertically 0 8 H 16
Known mine within 0.5–2 km of the maximum lateral extent of hydrocarbon activity, and/or 600 m vertically 1
Known mine within 0.5 km of the maximum lateral extent of hydrocarbon activity, and/or 200 m vertically 2
Anthropogenic features boreholes No known boreholes (and assumed none present) within 600 m vertically or 2 km laterally of hydrocarbon activity 0 4 M or H 8
Known boreholes extending to within 600 m vertically, and/or 0.5–2 km laterally of hydrocarbon activity 1
Known boreholes extending to within 200 m vertically, and/or 0.5 km laterally of hydrocarbon activity 2
TOTAL 98.5


Table 2.4    Risk groups based on potential receptor classifications and specific vulnerability scores. Note: classifications are preliminary.

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

Development of a geological conceptual model

Key to the screening methodology is the development of a conceptual geological and hydrogeological model of the proposed hydrocarbon extraction site and surrounding area (Figure 2.2; Appendix 1 - 3DGWV screening methodology, spreadsheet tool and example for low vulnerability scenario). This is used to inform the classification of the importance of potential Receptors, Intrinsic vulnerability and Specific vulnerability. In the model, all units across the footprint of the proposed hydrocarbon activity and within 2 km of the lateral extent of the hydrocarbon infrastructure (e.g. lateral and deviated boreholes or cavities), i.e. the ‘Area of Interest’ (AOI), should be identified.

The 3DGWV LithoFrame Viewer (LFV) 3D model, in conjunction with the 3DGWV screening tool (in associated digital media), can initially be used to guide this process and identify aquifer designations and hydrocarbon source units (in addition to other digital media). The LFV model comprises 195 intersecting cross-sections about 20 to 30 km apart, and therefore is not a full 3D volume. However, it gives a good general indication of the stratigraphic sequence at a specific location along, or close to, the lines of section. Where the closest cross-section is some distance away from the proposed site care must be taken as it may not be representative of the geological succession. In these cases other information should be examined. Additionally, due to the vertical exaggeration of the 3D sections provided as part of the accompanying LFV project sections are likely to be more accurate in the vertical direction than in the horizontal.

The model is based on 1:625 000 scale geological mapping and hence there has been some generalisation. Most geological units in the model are identified at the group level, whereas the original aquifer designation was carried out at 1:50 000 scale (formation or member scale), and only for units that occurred at the ground surface. Similarly, potential hydrocarbon source and reservoir units refer only to particular formations within a group, but the whole group will have been identified as such in the model (Appendix 2 - Oil and gas formations in England). Where available, more site-specific information should be obtained from regional geological guides, memoirs, borehole logs and geophysical logs, as detailed in the ‘sources of information’ below to improve the conceptual site model.

As part of this process geological faults and structure should be identified. It is important to understand the location and hydraulic properties of geological faults and the uncertainties associated with their precise position at the surface as well as at depth. However, there are significant uncertainties regarding the spacing and character of faulting at depth in the UK (Monaghan, 2017[24]). Faults are not currently indicated explicitly on the 3DGWV cross-sections in the LFV, although larger ones can be identified by the obvious offset of beds. Faults are often portrayed as a single line on geological maps whereas, in reality, they consist of zones of several tens of metres, or greater, in width, containing several fractures and fault rock. Whether or not small faults are shown on geological maps depends on the map and fault scale, the presence of superficial deposits (since the presence of thick superficial deposits overlying bedrock strata make it more difficult to accurately map the surface expression of faults), the date of the mapping, the lithology and thickness of the formation affected and also the economic importance of any minerals associated with the rocks. For example, historically more faults have been mapped in the Coal Measures, due to the effect that even small throws can have on the underground mining of coal seams. Faults are also occasionally recorded in borehole logs.

If there is significant geological variability across an AOI, either the most sensitive location or a number of locations could be used for the vulnerability/risk screening.

Figure 2.2    Schematic conceptual model. VS is the vertical separation and HS is the horizontal (lateral) separation. The pink unit is the hydrocarbon source unit. Purple indicates units designated as principal aquifers by the EA and yellow secondary aquifers. Not to scale.

Sources of information

References

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Footnote

  1. Water (management protect) boreholes are generally for public water supply and will be known about and licensed by the Environment Agency.