Greenpark Energy Ltd April 2010
8. Hydrogeology and Land Contamination
8.1 Summary
This chapter assessed the hydrogeological context of the proposed development together with possible impacts resulting from the development and considered the following key issues:
The impact of the development on aquifers and groundwater located beneath the site; The impact of the development on nearby groundwater abstractions; Mitigation measures to be undertaken during site preparation, drilling and production and restoration of the site to prevent potential adverse impacts.
This chapter also considered the current contamination status of the site and the potential for land contamination resulting from the proposed development and considers the following key issues:
Potential contamination of land resulting from the proposed development; Mitigation measures to be undertaken during site preparation, drilling and production and restoration of the site to prevent potential adverse impacts.
The site has variable shallow drift geology including glacial sand and gravel and glacial till to depths of between 0.9 and 2.6m below ground level. The solid geology below the site comprises strata of the Mercia Mudstone Group. The Mercia Mudstones overlie strata of the Triassic Sherwood Sandstone Group which comprises thick sandstones and is classified as a Major Aquifer by the Environment Agency; these strata extend up to approximately 300m in thickness in the area and are locally abstracted by Severn Trent Water for public potable supply. The Sherwood Sandstones lie unconformably above Upper Carboniferous Strata which include the Coal Measures Strata which are the target strata for the CBM gas development. The Coal Measures Strata are separated from the overlying Sherwood Sandstone by the Carboniferous Barren Measures Strata which are classified as a Minor Aquifer. The Coal Measures Strata containing the targeted coal seams generally comprise predominantly argillaceous rocks of low permeability and are considered a Minor Aquifer.
Mitigation measures suggested therefore include: best practice in the storage or fuels, permanent double steel casing of the boreholes that would last for at least 20 years, using standard water well drilling practices and, at the end of the project, ensuring that capping of the boreholes follow the safety standards required by the Department of Energy and Climate Change and the Health and Safety Executive.
By undertaking appropriate mitigation measures, the impacts upon the hydrogeology of the area are assessed as being of no significance.
8.2 Introduction
The proposed CBM gas development has the potential to impact upon the underlying hydrogeology. In order to ensure the environmentally sound development of the extraction site, this chapter details the existing baseline conditions at the site and assesses the potential for these to be adversely impacted by the development. This enables any appropriate mitigation measures to be identified, as necessary.
This chapter assesses the potential impacts associated with the proposed CBM gas
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extraction development on the surrounding hydrogeological conditions, for both shallow drift and deep bedrock groundwater reserves. This chapter does not make any assessment of the reverse scenario, i.e. impacts upon the proposed development by hydrogeological conditions at the site.
During site preparation, drilling, production and restoration of the site there would be potential for impacts to land quality resulting from development activities and use and storage of various materials on the site. This chapter therefore qualitatively assessed the current baseline condition of soils, the contamination status of the site, potential impacts resulting from the development, and recommends mitigation measures that may be required.
8.3 Relevant Legislation, Policies and Guidelines
The hydrogeological and contamination assessment will be conducted in accordance with the following legislation, policies and guidelines:
The Water Framework Directive (2000/60/EC) (WFD), and Controlled Waters (Water Framework Directive) (England and Wales) Regulations 2003; Part IIA of the Environmental Protection Act 1990 (the 1990 Act) as inserted by section 57 of the Environment Act 1995; The Contaminated Land (England) Regulations, 2006 (SI 1380/2006), The Stationary Office Limited; Planning Policy Statement 23 (PPS23). Planning and Pollution Control. PPS23 is intended to complement the pollution control framework under the Pollution Prevention and Control Act 1999 and the PPC Regulations 2000; Pollution Prevention Guidelines PPG21: Incident Response Planning, Environment Agency, March 2009, 19pp. Planning Policy Statement 10 (PPS10). Planning for Sustainable Waste Management. PPS10 sets out the Government's policy to be taken into account by waste planning authorities and forms part of the national waste management plan for the UK; DEFRA, 2008, Guidance on the Legal Definition of Contaminated Land; DEFRA, Circular 1/2006, Environmental Protection Act 1990: Part IIA Contaminated Land; DEFRA, 2004 Model Procedures for Management of Land Contamination (CLR11); British Standard, Investigation of potentially contaminated sites – Code of Practice, BS10175:2001; Department for Communities and Local Government, 2006. Environmental Impact Assessment: A guide to good practice and procedures – A Consultation Paper; Department of Transport, 2003. The Water Environment Sub-Objective; Environment Agency 2008, managing all our water needs. Abstracting water, A guide to getting your license; Environment Agency, 2007, water abstraction getting the balance right. Staffordshire and Trent Valley Catchment Abstraction Management Strategy; CIRIA C665, 2007. Assessing risks posed by hazardous ground gases to buildings; and, US Geological Survey (USGS), October 2000. Fact Sheet FS–123–00. Coal- Bed Methane: Potential and Concerns. CIRIA C502 Environmental Good Practice on Site (1999); CIRIA C532 Control of Water Pollution from Construction Sites (2001); CIRIA C552 Contaminated Land Risk Assessment – A Guide To Good Practice (2001);
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The Borehole Sites and Operations Regulations 1995. Statutory Instrument 1995 No. 2038, HMSO. Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). And; The Control of Pollution (Oil Storage) (England) Regulations 2001
8.4 Methodology
The hydrogeological and land contamination assessment:
Obtained qualitative baseline information on the condition of the site and connected hydrogeological systems, through consultation with relevant statutory organisations, site walkover survey observations and desk based review of published information; Assessed potential impacts, both direct and indirect, on the hydrogeological features and soils at the site including identifying critical pathways between contaminant sources and receptors for potential pollutants; Identified mitigation measures and residual impacts; and Evaluated the significance of impacts.
Water Companies, the Environment Agency, the British Geological Survey and the relevant Local Authority departments have been consulted to obtain surface and groundwater information and matters related to potential land contamination. A site visit was carried out to verify this information and to gain a good understanding of the shallow hydrogeological conditions on site. This included:
Water supplies (whether springs, boreholes or wells) including private water supplies; Surface or groundwater abstractions; Historical land use and the potential of containing contaminated land, including aspects of waste management that may be relevant to the assessment; Borehole records; Rainfall data; and Any existing and/or historical reports relating to the hydrogeology / contamination status of the site.
Information about the following was analysed:
Local geology (superficial deposits and bedrock) and soils; Aquifer status and resource potential; Topography; Climate; Current and historical land use; and, National, regional and local landscape and character designations.
Specific environmental information regarding the history and condition of the site and its catchment area was obtained from Stafford Borough Council’s Environmental Protection Department.
Reference was also made to the EA documents Abstracting Water, A guide to getting your license and the Staffordshire Trent Valley Catchment Abstraction Management Strategy (CAMS) report.
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Lastly, the EA was consulted concerning water features, their attributes and indicators of quality within the site and its catchment area.
Assessment of Significance
Table 8.1 provides guidance on defining the magnitude criteria for potential impacts, with some examples. The magnitude of the potential impact is completely independent of the value of the attribute affected and therefore gives no indication of significance when considered alone.
TABLE 8.1 – CRITERIA FOR DETERMINING IMPACT MAGNITUDE Magnitude Criteria Example Major Results in loss of attribute loss of EC designated Salmonid fishery change in GQA grade of river reach loss of flood storage/increased flood risk pollution of potable source of abstraction Moderate Results in impact on integrity of loss in productivity of a fishery attribute or loss of part of contribution of a significant proportion of the attribute effluent in the receiving river, but insufficient to change its GQA grade reduction in the economic value of the feature Minor Results in minor impact on measurable changes in attribute, but of attribute limited size and/or proportion Negligible Results in an impact on discharges to watercourse but no significant attribute but of insufficient loss in quality, fishery productivity or magnitude to affect the biodiversity use/integrity no significant impact on the economic value of the feature no increase in flood risk
Table 8.2 provides guidance for estimating the importance of an attribute based on criteria such as quality, scale, rarity and consideration of whether water attributes are replaceable over a given time frame.
TABLE 8.2 – GUIDELINES FOR ESTIMATING THE IMPORTANCE OF ENVIRONMENTAL ATTRIBUTES Importance Criteria Example Very High Attribute with a high quality and Aquifer providing potable water to a large rarity, regional or national scale population and limited potential for EC designated Salmonid fishery substitution
High Attribute with a high quality and GQA Grade A reach of river rarity, local scale and limited Aquifer providing potable water to a small potential for substitution, population attribute with a medium quality EC designated Cyprinid fishery and rarity, regional or national scale and limited potential for substitution Medium Attribute with a medium quality GQA Grade B / C reach or river Aquifer and rarity, local scale and providing abstraction water for agricultural limited potential for substitution, or industrial use attribute with a low quality and rarity, regional or national scale and limited potential for substitution Low Attribute with a low quality and Floodplain with limited existing rarity, local scale and limited development potential for substitution Minor or Non-aquifer of low quality
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The significance of a potential impact is estimated by its magnitude (Table 8.1) and the importance (Table 8.2) of the affected attribute. Table 8.3 provides guidance for determining the significance of a potential impact based on its magnitude and the importance of the attribute.
TABLE 8.3 – CRITERIA FOR ESTIMATING THE SIGNIFICANCE OF POTENTIAL IMPACTS Magnitude of Impact Importance of attribute Very High High Medium Low
Major Very Significant Highly Significant Low Significant Significance
Moderate Highly Significant Low Insignificant Significant Significance
Minor Significant Low Insignificant Insignificant Significance
Negligible Low Insignificant Insignificant Insignificant Significance
Where the predicted potential impact is highly uncertain as a result of lack of information or insufficient design details or a more significant, but less probable impact is identified, the impact assessment has determined whether the potential risks identified justify invoking the precautionary principle, or whether it is sufficient to flag them up as issues for more detailed consideration at a later stage, based on the relative probability of the possible outcomes and their significance.
8.5 Baseline Conditions
Fieldwork
A site walkover was undertaken on the 27th October 2009 to determine current land use, setting and site condition.
At its greatest, the site would cover an area of 0.8ha and be approximately rectangular in plan. The site area presently comprises roughly vegetated ground of level topography adjacent to the M6 motorway, at an approximate elevation of 126m OD.
Drift Geology
The British Geological Survey (BGS) 1:50,000 sheet 139 for Stafford (Drift Edition) shows that the site is not overlain by significant drift deposits. An area of glacial sand and gravel is shown approximately 500m to the east of the site area.
Four borehole logs for the immediate vicinity of the site have been obtained from the British Geological Survey. The locations of the borehole records relative to the site are shown on Figure 8.1. The logs indicate that superficial deposits were encountered to depths of between approximately 0.9m and 2.6m bgl comprising glacial sand and gravel and glacial till deposits.
Information obtained from the landis soil database (http://www.landis.org.uk/soilscapes) indicates that predominantly loamy and clayey soils with impeded drainage underlie the site. These are likely to be associated with weathering of the underlying Mercia Mudstone Strata. Freely draining sandy and
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loamy soils underlie the area to the north of the site associated with underlying sandstone geology. Subsequent interrogation of existing borehole data supported this appraisal of ground conditions.
Solid Geology
Published geological plans for the area show the site to be underlain by strata of the Triassic Mercia Mudstone Group. These strata comprise mainly red and occasionally green and grey mudstones and siltstones. Gypsum, anhydrite and sandstone beds are common at some stratigraphical levels. This formation is designated a non- aquifer by the Environment Agency. However, some sandstone and siltstone horizons are present which can transmit limited quantities of groundwater.
The Mercia Mudstones are underlain by Strata of the Wildmoor Formation of the Triassic Sherwood Sandstone Group. These strata comprise thick sandstones and are classified as a Major Aquifer by the Environment Agency and extend up to approximately 300m in thickness on the area.
The Sherwood Sandstones lie unconformably above Upper Carboniferous Strata which include the Coal Measures Strata which are the target strata for the CBM gas development. The productive coal seam bearing strata are separated from the overlying Sherwood Sandstone by the Carboniferous Barren Measures Strata which are generally non coal bearing Carboniferous strata comprising sandstones, siltstones and mudstones, dominantly reddish, brownish or purple grey in colour. The Barren Measures are classified as a Minor Aquifer although they contain sandstones which can be good sources of groundwater. A borehole record has been obtained for Darlaston Borehole at Hem Heath Colliery located approximately 2.1km east north east of the site (SJ83NE Darlaston). This borehole was drilled to a depth of 1208m bgl and indicates that Strata of the Barren Measures incorporating the Keele Formation, Newcastle Formation and Etruria Formation are present to approximately 468m bgl, beneath which are Strata of the Productive Carboniferous Coal Measures.
The Coal Measures Strata containing target coal seams generally comprise predominantly argillaceous rocks of low permeability and are considered a Minor Aquifer.
The Swynnerton Fault is conjectured to be located very near to and possibly just within the northern boundary of the site. The fault is downthrown to the south with Strata of the Sherwood Sandstones outcropping at the surface to the north of the fault line. The geometry of the fault is not indicated on the geological map although as it is a normal fault, it is likely to dip to the south at an angle of approximately 60 to 70 degrees. Therefore if the CBM boreholes are drilled vertically down at this location it is likely that the boreholes would intersect the fault and pass into the Sherwood Sandstone strata at a depth of between 50 and 100m bgl.
Records of five boreholes drilled in the vicinity of the site have been obtained from the BGS. The locations of the borehole records are shown on Figure 8.1. Boreholes 170 and 172 located within the site encountered mudstones and siltstones of the Mercia Mudstone strata to their maximum drilled depths of 18.35 and 20.2m bgl. These strata were found to comprise reddish brown to greenish grey silty sandy mudstones sub-horizontally interlaminated and occasionally calcite cemented. Borehole 173, located within the north western corner of the site encountered a thin band of Mercia Mudstone overlying Sherwood Sandstone at a depth of approximately 5.0m bgl. Sherwood Sandstone was found to comprise reddish brown medium grained sandstone with closely spaced sub-horizontal discontinuities. Occasional calcite filled voids were noted. This borehole may be located to the northwest of the
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Swynnerton Fault.
Borehole Ref SJ83NE285 relates to the existing abstraction well located approximately 80m to the north of the site. This borehole encountered Sherwood Sandstone Strata from approximately 1.0m bgl, beneath drift deposits, to the maximum drilled depth of 180m bgl.
Groundwater
The site is immediately underlain by very thin shallow superficial glacial soils often comprising glacial sand and gravel. These deposits are generally freely draining soils and have the potential to permit infiltration of rainfall recharge or liquid contaminants.
The site itself is recorded as being underlain by a Non-Aquifer as a result of the thickness of Mercia Mudstone Strata overlying the Major Aquifer located at depth. The Sherwood Sandstone located at depth beneath the Mercia Mudstones and to the north of the Swynnerton fault is a Major Aquifer.
The Sherwood Sandstone Group comprises a series of fine to medium grained sandstones with thin mudstone lenses and pebble beds. The sandstone is a Major Aquifer, confined by the Mercia Mudstone where this is present. Based on Environment Agency data for the local source protection zones regional flow is anticipated to be generally towards the south east and the River Trent. Locally groundwater flow direction is likely to be influenced by the public groundwater supply abstraction from three wells located immediately to the north of the site.
Information obtained from current Environment Agency observation boreholes indicates that the potentiometric surface for groundwater within the Sherwood Sandstone to the north of the Swynnerton Fault is between 72 and 80m AOD.
Details of groundwater abstractions within 5km of the site area have been provided by the Environment Agency. Based on this information there is one licensed groundwater abstraction within 5km of the site. Groundwater Abstraction points seen on the map provided by the EA show abstraction licence referenced 03/28/01/0142. This licence belongs to Severn Trent Water Ltd for public water supply and includes the following abstraction points:
Swynnerton borehole No 1: SJ 864 359 Swynnerton borehole No 2: SJ 868 355 Swynnerton borehole No 3: SJ 869 360
Licensed quantities: 3740909m³/year; 10557m³/day.
Swynnerton borehole No 1 is located 80m northwest of the boundary of the site and is recorded at the location of the Former Royal Ordnance factory Borehole Ref. SJ83NE285. The abstraction draws water from the Sherwood Sandstone Aquifer.
The site is located within zone III of the Source Protection Zone (SPZ) associated with the above abstraction The boundary of zone II of the SPZ is located along the line of the Swynnerton Fault immediately to the north west of the site. The location of the SPZs is shown on the plan provided by the Environment Agency and contained within Appendix 5.
The Sherwood Sandstone is the only Major Aquifer within the Staffordshire Trent Valley Catchment Abstraction Management Strategy (CAMS). The site is located within the Tittensor Groundwater Management Unit within this CAMS area. According
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to the CAMS strategy this groundwater management unit is currently ‘over abstracted’. The target status of the unit is ‘over licensed’. This means that no new abstraction licenses would be granted in this area and that time limit restrictions may be applied to renewals of existing licences.
Due to the presence of the Swynnerton fault, the groundwater regime immediately below the site is likely to be very different to that to the northwest of the fault line where the groundwater abstractions are located within the Sherwood Sandstone. Hydraulic connectivity between the Mercia Mudstone Group strata beneath the site and the abstracted strata at relatively shallow depth is likely to be limited as inferred by the shape of the SPZ zone II towards the south. Given the likely dip of the Swynnerton fault it is likely that a vertical borehole drilled from the site would cross Swynnerton fault at between 50 and 100m bgl and at this point would pass into zone II of the SPZ as considered in three dimensions.
No abstractions are recorded within the Mercia Mudstone or the underlying Carboniferous Strata in the vicinity of the site.
In summary, there are a number of distinct hydrogeological units beneath the site which would be penetrated during drilling, including the shallow drift deposits, Mercia Mudstone (Non-Aquifer), Sherwood Sandstone (Major Aquifer – abstracted), Carboniferous Barren Measures (Minor Aquifer – not abstracted) and Carboniferouus Coal Measures (Minor Aquifer – not abstracted). Due to the location of the Swynnerton Fault on the northern site boundary, there may be limited hydrogeological connectivity between the drilled strata and nearby abstracted aquifer as inferred by the shape of the SPZ. However, a vertical borehole is anticipated to cross the fault at an approximate depth of between 50 and 100m bgl at which point it would be effectively within zone II of the SPZ.
Site History and Contamination Status
In order to establish the history of the site a selection of available published historical plans for the site has been reviewed. The earliest published map of 1891 shows the site to be open farmland with peripheral field boundaries.
Generally no significant changes in land use have been identified over the period since 1891 and the site has been in use as agricultural land throughout this period. The land has most recently been used as pasture and now appears to be of limited use being covered with rough vegetation. The nearest residential properties to the site are located approximately 150m to the west of the site beyond the M6 motorway.
Based on the known historic uses of the site it is considered to be a greenfield site and the potential for significant historic contamination is considered to be negligible. Potential contamination is considered to be very limited to spoil associated with the construction of the adjacent motorway and residues of historic fertilisers or pesticides applied at the site.
Staffordshire Borough Council Contaminated Land Department was contacted. It confirmed that it has no records of contaminated land, pollution incidents or authorised processes within the vicinity of the site.
There are no records of landfilling within or within 250m of the site held by the EA or the Local Authority and no records of pollution incidents, private water abstractions, discharge consents or environmental permits are held. Therefore a pre-development contaminated land survey is not likely to be necessary.
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8.6 Impact Assessment
Potential Receptors
Potentially sensitive receptors noted to be present, for the purpose of this chapter, are:
nearby residents/property; 150m west and 390m northeast. natural soils; groundwater contained within the Sherwood Sandstone Major Aquifer; groundwater contained within the deeper Carboniferous Westphalian Coal Measures strata Minor Aquifer.
The most sensitive of these is considered to be the Sherwood Sandstone Major Aquifer locally abstracted for potable water supply 600m north of the site. Severn Trent Water has confirmed that the borehole and pumping station 100m north of the site is not currently used to abstract potable water.
Potential Impacts
The following activities have the potential to generate impacts upon land and hydrogeological receptors and therefore require consideration within each of the four development phases:
1. Potential contamination of surface soils during use and storage of drilling fluids and fuels on site; 2. Potential cross contamination of stockpiled natural soils with construction materials (e.g. Tarmac and cement); 3. Whilst drilling the boreholes there is the potential to create pathways between hydrogeological units causing cross contamination; 4. Creation of hydraulic fractures during drilling; 5. Contamination of groundwater resulting from surface spillage of fuels; 6. Contamination of groundwater by drilling flush medium; 7. Escape of gas into the overlying permeable strata (e.g. Sherwood sandstone) during the drilling and production phases; 8. Abstraction of Sherwood sandstone groundwater during drilling / borehole construction; 9. Dewatering of the target coal seam could lower the water table in the Westphalian Coal Measures strata; and 10. Dewatering would generate abstracted waters for treatment/disposal/discharge.
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TABLE 8.4 – ASSESSMENT OF POTENTIAL IMPACTS Potential Impact Potential Significance of Impact (as defined in Table 8.3) 1. & 2. Contamination of surface soils Insignificant
3. & 4. Pathways created between vertical geological Highly Significant units resulting in cross contamination
5. Contamination of groundwater from surface spillage Highly Significant of fuels, sewage or drilling fluids
6. Contamination of groundwater by drilling flush Significant
7. Escape of CBM gas Low significance
8. Over abstraction of groundwater (Sherwood Significant Sandstone)
9. Lowering of water table in Carboniferous Coal Insignificant Measures
10. Production of groundwater from Carboniferous Insignificant Strata at depth
Further discussion of these potential impacts and associated mitigation is provided below.
8.7 Mitigation
The Borehole Sites and Operations Regulations 1995 (HMSO) is the key document specifically relating to the drilling phase of CBM boreholes. Greenpark always prepares a full health and safety document together with a drilling plan which is provided to the HSE (Offshore Safety Division) in advance of drilling, as well as advising the EA and the Coal Authority. This document details plans for prevention of fire and explosions and detection and control of flammable / toxic gas. A site specific emergency escape plan is drawn up and informed to all staff, showing escape routes and muster points, together with fire precautions and first aid provisions. All staff and visitors are inducted with site specific information, procedures and rules together with general health and safety information. Members of staff are provided with appropriate training in health and safety. General and specific risk assessments are carried out and communicated as appropriate. These measures would mitigate any potential risks to on site personnel during the operation of the scheme.
Site Preparation and Drilling Phases
The following mitigation measures would be implemented during the preparation and drilling phase in order to reduce potential impacts resulting from the development.
1 & 2 - Contamination of surface soils
An incident response plan would be prepared and maintained following best practice set out in Pollution Prevention Guideline ‘Incident response planning’: PPG 21 before works commence on site. It is acknowledged that the EA strongly discourage the storage and use of loose drums of fuel on site. If required, Greenpark would forward
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the incident response plan to the EA before works commence on site, and take into account any relevant comments.
All fuels used on the site would be stored in accordance with necessary legislative requirements and best practice guidelines including use of appropriately bunded storage tanks and designated fuelling areas. Reference would be made to Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) and the Control of Pollution (Oil Storage) (England) Regulations 2001. This would help to eliminate potential impacts resulting from spillage of fuels on site. In addition, plant undertaking enabling works would be appropriately maintained and free from leaks which could result in contamination of surface soils during these works. Storage of drilling materials such as drilling muds would be suitably controlled in order to avoid surface spillage of these materials. Appropriate control of construction materials such as tarmac and cement would be maintained to ensure that no mixing or cross contamination of stockpiled natural soils can occur. Sewage from temporary sanitation facilities to be used on site would be self contained and stored in sealed tanks and emptied at regular intervals by licensed service providers.
3 & 4 - Creation of pathways between hydrogeological units
The drilling would breach low permeability Mercia Mudstone horizons overlying and protecting the Sherwood Sandstone from potential surface contaminants. This could potentially generate a preferential pathway into the underlying aquifer of the Sherwood Sandstone. A permanent steel casing would therefore be cemented into place through the drift deposits and the upper weathered units of the sandstone strata as a mitigation measure.
The drilling of a borehole to a depth of up to 1000m bgl at the site would involve passing through the hydrogeological units of the Sherwood Sandstone, and Carboniferous Westphalian Barren and Coal Measures. Drilling activities could result in the breach of these units and allow upward migration of poorer quality groundwater into the base of the Sherwood Sandstone and subsequent local deterioration of the groundwater body in the vicinity of the site. Groundwater contained within the Carboniferous strata is likely to contain high concentrations of sulphates and heavy metals and upward migration of this groundwater into the base of the Sherwood Sandstone could result in an adverse impact to groundwater quality within the sandstone.
In order to protect the drinking water supplies, it is proposed that the initial stages of drilling be undertaken using standard water well drilling practices. An indicative cross section of the geological strata through which a borehole would pass at this site is shown in Figure 8.2.
The first stage would be the setting of the surface casing. This would involve driving a 317.5mm diameter steel pipe between 0.5 and 2m into the ground as a conductor pipe, as shown in Figure 8.2 (Section C). This pipe would be cemented into place with Class G cement.
Approximately 100m of the borehole would then be drilled at a nominal 195mm diameter. This will be done using air-flush or mist drilling with down-hole air hammers. Returns and pressures will be monitored to maintain negative pressure. The use of air or mist under negative pressure means that there would be no danger of the drilling medium entering the Sherwood Sandstone Aquifer. Instead, any flow would be from the aquifer into the borehole.
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An appropriate dust suppression technique would be employed for down-hole air hammer drilling. This could include foam capture, mist control, water bath, and/or other filter systems.
A 140mm (outside diameter) casing would then be installed and cemented in place with Class G cement (as shown in Section C of Figure 8.2). This would be done using the float shoe method, where the cement is forced under pressure up the outside of the casing. Class G cement is a silica based, fast setting, dense, low porosity, strong cement, with no calcium carbonate.
The next 100m – 300m would then be drilled at a nominal 130mm diameter to the top of the Coal Measures. If possible this would again be done with air hammer drilling. If air hammer drilling is not possible a mud-flushed rotary drill would be used.
Casing (125mm outside diameter) would be installed and cemented in place, again using Class G cement and the float shoe method. This casing and cement would preclude cross contamination between aquifers by sealing the hydraulic units. This casing would continue all the way to the surface, protecting the aquifer with double layers of casing and cement.
Both the casing and the cement would have a design life in excess of 20 years.
Each borehole would then be completed using mud-flush based drilling methods in an open borehole. “Mud-flush” would, if possible, use water with xanthan gum as the only additive. Xanthan gum is a biodegradable polysaccharide used as a food additive. It is completely non-toxic and safe for human consumption. However, it may be that potassium salt is also required as a pressure control additive.
Potentially water soluble evaporates of gypsum and anhydrite are present within the Mercia Mudstone and Barren Measures strata and there is the potential for release of groundwater of very high sulphate content and increased dissolution of these during drilling. However, it is considered that the use of casing to seal and stabilise the borehole and drilling using drilling mud would sufficiently mitigate these potential impacts.
5 & 6 - Contamination of groundwater due to surface spillage or drilling flush
Within the site area the Sherwood Sandstone Aquifer is protected from any shallow spillages by a significant thickness (>20m) of low permeability Mercia Mudstone.
Methods of borehole drilling using sealed casing and pressure controlled drilling muds would mean that risk of spilled materials at the surface migrating into deeper seated aquifer units via the borehole is very low. In addition all of the mitigating measures to prevent spillage and ensure control of substances noted in 1 & 2 above would apply.
During drilling, hydrostatic pressure within the borehole would be carefully controlled using circulating drilling fluid (mud) in balance with or just below in-situ hydrostatic pressure. This would prevent the formation of hydraulic fractures resulting from drilling pressure and would maintain a slight negative pressure within the borehole mitigating loss of flushing medium and pollution of the aquifer. Mud weight controls hydrostatic pressure in a borehole. The weight of the mud also prevents collapse of casing and the borehole. Mud weight (density) test procedures using a mud balance have been standardised and published by the American Petroleum Institute (API). The true vertical depth (TVD) is the vertical depth of the borehole independent of its
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path. In the case of a vertical borehole, measured depth is the same as true vertical depth.
A blowout preventer would be used at the wellhead during drilling to prevent uncontrolled fluid loss.
The careful balance of slightly negative hydrostatic pressure of the drilling fluid together with the borehole casing would prevent contamination of the aquifer. These measures would act to seal the borehole during drilling and appropriate additives would be available should heavily fractured or permeable strata be encountered.
7 - Escape of CBM gas
The drilling and production activities by their nature would result in the liberation of CBM gas from the coal seams and this requires control to ensure that it is not lost to overlying permeable strata such as the Sherwood Sandstone through the borehole annulus. The installation of casing as previously detailed would protect the permeable hydrogeological units from gas migration. In addition, the use of a drilling mud to stabilise the borehole would provide a positive pressure on the Coal Measures strata during drilling and thereby restrict the ability of CBM gas to migrate in an uncontrolled manner. Should pockets of CBM gas be encountered during drilling through the sandstone strata, mitigating wellhead equipment (blowout preventers) would be used to immediately seal the wellhead.
In addition, the use of a drilling mud to stabilise the hole would help restrict uncontrolled CBM gas migration via the borehole.
The target Coal Measures strata are separated from the Sherwood Sandstone by a significant thickness of the Carboniferous Barren Measures which comprise mainly argillaceous rocks although the presence of localised siltstone and sandstone beds is likely. Such strata are therefore of low overall permeability and are not subject to significant fracturing, often self sealing in fractures. The risk of coal bed CBM gas escaping through these strata into the Sherwood Sandstone or to the surface external to the borehole due to disruption or fracturing caused by drilling is considered to be negligible.
8 – Over abstraction of Major Aquifer
No groundwater abstraction from the Sherwood Sandstone is proposed during the development. Use of hydrostatically controlled drilling muds, casing of this strata and the wellhead blowout preventor would be used to control discharges from the Sherwood Sandstone during the drilling process such that there should be no significant abstraction of the Aquifer due to penetration of the CBM bore holes.
9 & 10 - Dewatering of the Carboniferous Coal Measures strata
Should the initial testing of the coal show that CBM gas production is viable, modern directional drilling techniques would be used to turn the borehole parallel to the target coal seam when the required depth is achieved so that the lateral ('horizontal') borehole is embedded within the coal seam. Horizontal drilling would then be undertaken potentially to a maximum distance of 1000m from the vertical borehole point within the target seam. During this drilling phase, small scale dewatering of the coal seam would temporarily occur and as such could potentially lower the water table within the Coal Measures strata. This aquifer is considered to be of low sensitivity and there are no recorded abstractions from this hydrogeological unit within 5km of the site. As such, the potential impact is minimal and mitigation is not
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considered necessary.
However, the abstracted water is likely to have high levels of dissolved metals, sulphate and acidity and would require appropriate treatment, disposal or discharge. At present, the exact quality/quantity of abstracted waters is not known and would be determined by this phase of the site operations, but it is estimated to be at a rate of three tanker loads each week with the frequency of tankers visiting the site rapidly declining within a few months to about one every other week. The water would be disposed of under the appropriate authorisation from the Environment Agency.
Proper storage of this water would be undertaken to prevent spillages to ground.
Production Phase
The development of the borehole would proceed with the installation of surface equipment, including a buried wellhead, water separators and a water collection tank. All drilling equipment, drilling fluids and fuel supplies would then be removed from site.
Many of the activities and associated mitigation measures discussed in the context of the drilling phase also apply to this production phase. No fuels would be stored on site during this period, however, appropriate precautionary measures as detailed above would be considered for use of fuels on site during any maintenance work including use of fuel powered plant and vehicles on the site during its operational life.
Full details of long term abstraction rates for groundwater from the coal seams and number of bores at the production site are not confirmed at present as this would be subject to the findings of the drilling phase (see above). Additional information made available at a later date may require consideration in relation to potential impacts on the underlying hydrogeology and would be resubmitted to the planning authority, if appropriate.
The system of extraction is closed, with CBM gas transferred to the compressor by underground pipework.
Should the drilling phase determine that the site is not viable, the borehole would be abandoned and restoration would be undertaken as outlined below.
Capping and Restoration Phase
During the restoration phase, the borehole would be plugged ('capped') to safety standards required by the DECC and the HSE. Following the borehole being plugged, the site would then be landscaped and restored to its previous condition of an agricultural field using the stripped soils from the storage bunds. These activities are not considered to have a significant impact on the hydrogeology as the borehole is cased. However, if this restoration phase were not undertaken, this could result in a direct pathway for the contaminants at ground level (e.g. contaminated liquids) to groundwater at depth in the future when the site became vacant.
If the borehole were not suitably capped, there would also be the potential for artesian groundwater from within the lower confined aquifer units to well up to ground level which could result in an adverse impact upon surface water quality or result in local flooding. It is therefore essential that this phase is completed and documented in order to prevent these potential impacts from occurring following restoration.
Swynnerton Environmental Statement 8-14 FINAL Greenpark Energy Ltd April 2010
All relevant mitigation measures regarding use and storage of materials as detailed above would continue to apply.
Summary of residual impacts
Table 8.5 shows the resulting impact of the various phases of the development, incorporating mitigation.
Swynnerton Environmental Statement 8-15 FINAL Greenpark Energy Ltd April 2010
TABLE 8.5 – ASSESSMENT OF SIGNIFICANCE Phase Potential Impact Proposed Significance (as defined in Mitigation Severity Likelihood Residual Table 8.1) (as Impact (as and Importance defined in defined in of Table 8.1) Table 8.3) Environmental Attribute (Table 8.2) Site Contamination of Appropriate fuel Minor Unlikely Insignificant preparation shallow soils and and material groundwater by storage strategy fuels and drilling and site surfacings materials. Medium Contamination of Appropriate control Minor Unlikely Insignificant shallow soils due of plant and to spillage / materials used leakage from during construction plant / (e.g. tarmac and construction cement). Stripping materials. of topsoil and Medium stockpiling in peripheral bunds Drilling Creation of Borehole casing. Negligible Unlikely Low preferential Hydrostatic control Significance pathways and use of drilling between mud. hydrogeological units . Very High Contamination of Borehole casing Negligible Unlikely Low groundwater in and use of drilling Significance Sherwood mud with controlled Sandstone (negative) Aquifer by drilling hydrostatic medium. pressure Very High Escape of gases Borehole casing Negligible Unlikely Low to overlying and use of drilling Significance permeable strata. mud Very High Over abstraction Casing of this unit, Negligible Unlikely Low of Sherwood hydrostatic control Significance Sandstone. of drilling mud Very High Production Temporary Not required – Minor Likely Insignificant impact on water negligible impact table in Coal Measures strata. Low Impact from Control of fuels or Negligible Unlikely Low surface other substances Significance contaminants on (for maintenance) groundwater in used on site during Sherwood the life of the plant Sandstone via wells. Very High
Swynnerton Environmental Statement 8-16 FINAL Greenpark Energy Ltd April 2010
8.8 Statement of Significance
By undertaking appropriate mitigation measures such as in best practice in the storage of fuels, casing and sealing of the borehole, and capping of the borehole on completion, any impact from this CBM gas development is assessed as being of no significance.
Swynnerton Environmental Statement 8-17 FINAL