«LONDONWEST MIDLANDS ENVIRONMENTAL STATEMENT Volume 2 | Community Forum Area report CFA7 | Colne Valley November 2013 ES 184.108.40.206 VOL LondonWest Midlands ...»
8.3.22 The Buckinghamshire Minerals and Waste Core Strategy Development Plan Document (DPD)5, 2012 shows that the route passes through a Minerals Safeguarding Area. Policy CS1 states that development proposals in this area, other than those involving minerals extraction, will need to demonstrate that it will not sterilise the Future baseline 8.3.32 There is currently one identified committed development within the study area which is likely to change the land quality baseline during either construction or operation of the Proposed Scheme. The eastern side of the proposed quarry area at Denham Park Farm is located under an area designated as a temporary earthworks stockpile.
Depending on the timescales this could temporarily sterilise the resource.
Effects arising during construction 8.4 Avoidance and mitigation measures 8.4.1 The construction assessment takes into account the mitigation measures contained within the draft CoCP (see Volume 5: Appendix CT-003-000). The draft CoCP sets out the measures and standards of work that will be applied to the construction of the Proposed Scheme. Its requirements in relation to work in contaminated areas will
ensure the effective management and control of the work. Such requirements include:
Environment Agency (2004) CLR11 Model Procedures for the Management of Land Contamination British Standards Institute (2011) British Standard BS10175Investigation of Potentially Contaminated Sites Sustainable Remediation Forum UK (2010) A Framework for Assessing the Sustainability of Soil and Groundwater Remediation
CFA Report – Colne Valley/No7 | Land quality
8.4.4 Contaminated soils excavated from the site, wherever reasonably practicable, will be treated as necessary to remove or render any contamination inactive and reused within the Proposed Scheme where needed and suitable for use. Techniques are likely to include stabilisation methods, soil washing and bio-remediation to remove oil contaminants. Contaminated soil disposed of off-site will be taken to a soil treatment facility, another construction compound (for treatment, as necessary, and reuse) or to an appropriately permitted landfill.
8.4.5 In addition to the excavation and treatment of contaminated soils, ground (landfill) gas and leachate control systems within affected old backfilled sites, will be installed where necessary on a temporary or permanent basis, to ensure that ground (landfill) gas and leachate migration pathways are controlled.
Assessment of impacts and effects 8.4.6 This section of the route will start in cutting before passing on to a viaduct across the Colne Valley. The route will then pass into cutting, going below Tilehouse Lane and then run onto an embankment approaching the Chiltern tunnel south portal. From here the route will be run within a retained cutting before entering the Chiltern tunnel.
8.4.7 Construction works will include earthworks, utility diversions, deep foundations, temporary dewatering and other activities. In addition, road infrastructure works will also be required within this section of the Proposed Scheme.
8.4.8 There is a sustainable placement area located to the south-east of South Harefield.
This is an area identified as a potential additional placement area for the disposal of surplus excavated material.
8.4.9 The Ickenham auto-transformer feeder station will be located at the eastern end of this route section south of the current HOAC site and an auto-transformer station located at West Hyde close to the Chiltern tunnel south portal. There will be a National Grid feeder station located north-east of the HOAC.
8.4.10 In the Colne Valley area there are two main compounds, used for activities including materials storage, precast works, concrete batching plant and a slurry plant and seven civil engineering satellite compounds and two railway installation satellite compounds (both of which will continue to use compounds previously established for the civil engineering works). (see Maps CT-05-019 to CT-05-023 in Volume 2, CFA7 Map Book) Land contamination 8.4.11 In line with the assessment methodology, as set out in the SMR, SMR Addendum and its appendices, an initial screening process was undertaken (identified in the methodology as Stages A and B) to identify areas of current or historical contaminative use within the study area and to consider which of these areas might pose contaminative risks for the Proposed Scheme. In total, 33 areas were considered during this screening process; 15 of these areas were taken forward to more detailed risk assessments (Stages C and D) in which the potential risks were assessed more fully. The majority of the areas undergoing the more detailed risk assessments were historical landfills and potentially in-filled gravel or chalk pits. All areas assessed are shown on Maps LQ-01-011 to LQ-01a (Volume 5, Land Quality Map Book) and those considered as potentially posing a risk to the Proposed Scheme are labelled with a reference number.
(1) Each area is assigned a unique identification number (See Volume 5, Appendix LQ-001-007).
(2) CSMs have been prepared as part of the detailed land contamination methodology (refer to Volume 5) for baseline, construction and postconstruction (3) The moderate or high risks identified reflect the uncertainty in existing baseline information. Whilst there are unlikely to be properties or receptors that experience the reported high or moderate existing baseline risk in the absence of site investigation a precautionary, worst case risk is reported in the table.
Temporary effects 8.4.14 An assessment of the effects of contamination has been undertaken by comparing the CSM developed for potential contaminated areas at baseline, construction and post construction stages. The baseline and construction CSM have been compared to assess effects at the construction stage.
8.4.15 Table 14 presents the summary of the construction effects obtained from a comparison of the baseline and construction impacts. The construction risk assessment takes into account the implementation of the mitigation measures set out within the draft CoCP. The details of these comparisons are presented, Volume 5 (Appendix LQ-001-007).
8.4.16 The baseline and construction CSM have been compared to determine the change in level of risk to receptors during the construction stage, and thus to define the level of effect at the construction stage. Where there is no change between the main baseline risk and the main construction risk, the temporary effect significance is deemed to be negligible even if the risk is assessed to remain as high. This will be the case where the construction of the Proposed Scheme does not alter the risks from an existing potentially contaminated site that is outside the construction boundary.
Table 14: Summary of temporary (construction) effects
(1) The low/moderate main construction risk identified in the above table does not necessarily imply an unacceptable risk. Application of the processes and measures within the CoCP will ensure that site risks during the construction stage are controlled.
(2) The high risks identified reflect the uncertainty in existing baseline information. Whilst there are unlikely to be properties or receptors that experience the reported high risk in the absence of site investigation a precautionary, worst case risk is reported in the table. Application of the processes and measures within the CoCP will ensure that site risks during the construction stage are controlled.
8.4.17 Table 14 indicates that based upon the assessment, the construction phase is expected to have a minor beneficial to minor adverse effect on the receptors overall.
These effects are not considered to be significant in relation to potential land contamination.
8.4.18 Risks to receptors are considered to be potentially high during construction from 7-18, 7-19 and 7-20 as these are all sites that have been identified as recorded landfills and these sites are located within the footprint of the construction areas in advance of the Chiltern tunnel south portal. In accordance with the draft CoCP ground investigations
CFA Report – Colne Valley/No7 | Land quality
and risk assessments will be undertaken in advance of construction works commencing.
The risks that have been identified (7-18, 7-19 and 7-20) are within land that will be used for construction activities associated with the compounds in this area and landscape mitigation at the portal. Therefore the level of disturbance to natural soils underlying the earthworks will in fact be limited. The disused sand and gravel and chalk pit (7-16) is partly located within an area of ground treatment and therefore any contamination encountered will be remediated. There will remain a potential risk that mobilisation of contamination could occur as contaminated soils are exposed and excavated at construction stage resulting in a minor adverse effect on controlled waters.
8.4.19 Where sites are located beneath the viaduct there will also be relatively limited potential for disturbance of soils. However, there will be substantial piling associated with the viaduct piers that will have the potential to disturb contaminated soils and create new pathways. This is particularly likely at the former sewage works (7-11) at Denham Green, located directly beneath the viaduct where sewage sludge was historically deposited and associated organic and inorganic contamination may exist.
Any contaminated land will be remediated in accordance with the draft CoCP including excavation, if appropriate. In addition, to manage the potential vertical migration of poorer quality surface water into the Chalk aquifer the piling method will be selected to provide an appropriate seal and to ensure that there are no significant effects to controlled waters. Sites located outside the footprint of the viaduct, such as the in-filled quarries but within landscape mitigation areas, are unlikely to be disturbed and impacts will be limited.
8.4.20 There will be a minor beneficial effect to 7-32 during construction as the landfill will be covered by a temporary stockpile which will reduce infiltration.
8.4.21 The Chiltern tunnel south portal will lie at the western end of this section of the area.
Here the route will run below ground level as it passes into CFA8. This means it is unlikely that there will be any impact in the Colne Valley study area from the landfill lying at Warren Farm in the CFA8 area.
8.4.22 Construction compounds located in the area will include the storage of potentially hazardous substances, such as fuels and lubricating oils and may also be used for temporary storage of potentially contaminated soils. The measures outlined in the CoCP will manage risks from the storage of such materials.
Permanent effects 8.4.23 Baseline and post-construction CSM have been compared to assess the permanent (post-construction) effects. The post-construction CSM assumes that all the required remediation has been carried out and validated.
8.4.24 Table 15 includes the summary of the permanent (post-construction) effects obtained from a comparison of the baseline and post-construction impacts and whether these are significant. The details of these comparisons are presented, Volume 5 (Appendix LQ 002-007).
CFA Report – Colne Valley/No7 | Land quality Table 15: Summary of permanent (post-construction) effects
(1) The high risks identified reflect the uncertainty in existing baseline information. Whilst there are unlikely to be properties or receptors that experience the reported high risk in the absence of site investigation a precautionary, worst case risk is reported in the table. Application of the processes and measures within the CoCP will ensure that site risks during the construction stage are controlled.
8.4.25 The magnitude of the permanent effects and their significance have been determined by calculating the change in risk between the main baseline risk and the main postconstruction risk. Therefore, where there is no change between the main baseline risk and the main post-construction risk, the permanent effect significance is deemed to be negligible even if the risk is assessed to remain as high. This will be the case where
CFA Report – Colne Valley/No7 | Land quality
the construction of the Proposed Scheme does not alter the risks from an existing potentially contaminated site that is outside the construction boundary.
8.4.26 Table 15 shows that the Proposed Scheme results in either a reduction or no change in the level of risk already existing at each site for both on site and off site receptors.
8.4.27 Table 15 indicates that, following remediation, there will be an overall minor adverse to major beneficial effect. These effects may be significant at 7-16 and 7-19.
8.4.28 Beneficial effects will occur where the route will pass directly through sites such as disused chalk pits (landfills) resulting in removal or remediation of contaminated soils and therefore removing a contaminant linkage both to human receptors and to the sand and gravel secondary A aquifer and potentially to the Chalk principal aquifer.
8.4.29 A minor adverse effect will occur at 7-11 as the increase in hardstanding surrounding the site may increase surface water run-off into the site and potentially increase leaching of residual contamination to groundwater.
8.4.30 There are anticipated to be no significant cumulative permanent effects.
Mining/mineral resources 8.4.31 Construction of the Proposed Scheme has the potential to impact existing mineral resources and proposed areas of mineral exploitation. This could occur by sterilisation of the resource, direct excavation during construction of the Proposed Scheme or through temporary and/or permanent severance78 that may occur during the construction phase of the Proposed Scheme, possibly continuing through to the operation.
8.4.32 Parts of the route within this study area will fall within a Mineral Consultation Area/ Mineral Safeguarding Area designated by Buckinghamshire County Council for sand and gravel extraction. These resources are not currently being worked in this area.
8.4.33 There are currently two mineral extraction sites within the study area at Denham Park Farm and Moorhall Road (not currently extracting).
Permanent effects 8.4.35 Construction of the Proposed Scheme will affect the designated Mineral Safeguarding Areas, resulting in a minor adverse impact. The effect is assessed as not significant 78 In this context, severance refers to the Proposed Scheme splitting an actual or proposed mining/mineral site into two or more areas, such that separate accesses would be required to work the whole site.
Other mitigation measures 8.4.39 The CoCP details the approach to managing potential land contamination matters. No additional mitigation measures are considered necessary to mitigate risks from land contamination at construction phase beyond those set out in the draft CoCP and instigated as part of required remediation strategies.
8.4.40 Mitigation of the effects on mineral resources can include prior extraction of the resource for use within the project or elsewhere. Extraction may be limited to landscaped areas within the Proposed Scheme adjacent to, rather than beneath the trackbed which will require good founding conditions. A plan will be discussed and agreed in advance of the construction works with the landowner, the appropriate mineral planning department at and any other interested parties to assist in achieving an effective management of minerals within the affected location of the MSA.
Summary of likely significant residual effects 8.4.41 No likely significant adverse effects are anticipated with the application of the mitigation measures detailed above.
Effects arising from operation 8.5 8.5.1 Users of the Proposed Scheme (i.e. rail passengers) whilst within trains, are will at all routine times be within a controlled environment and have therefore been scoped out of the assessment.
Avoidance and mitigation measures 8.5.2 Maintenance and operation of the Proposed Scheme will be in accordance with environmental legislation and good practice whereby appropriate spillage and pollution response procedures will be established.