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TO ASSIST YOU WITH RESUBMISSION
Earlier comments during the application stage identified the information that would be required to address this condition including:
Environmental and historical information and risk assessment
Intrusive site investigation information and/or ground gas protection measures
Radon protection measures
The assessment was to be undertaken in line with the Council's CLC3 guidance.
The following report has been provided for the discharge of Condition 12:
Chelmer Global Limited, Ground Gas & Vapour Investigation, 78 Portland Road, London, W11 4LQ, June 2022, GVIR/12545
The desk study review indicated the site and the surrounding area was generally residential since the 1800s. A historical investigation at the adjacent site (No. 76) recorded Made Ground to 1.3m depth followed by the London Clay Formation. Monitoring at one borehole (installation to 9m depth) recorded carbon dioxide up to 12.1% (no elevated concentrations of methane, carbon monoxide or hydrogen sulphide were recorded), volatile organic compounds (VOCs) up to 0.7ppm and a flow rate up to 0.5l/hr. Groundwater was recorded in the London Clay at depths between 6.5m and 7.48m below ground level (bgl).
The ground investigation for the subject site (No. 78) comprised 4 boreholes up to 3m depth, with three locations installed with monitoring wells. Six rounds of ground gas and vapour monitoring was completed at the boreholes and at locations within the existing basement. The ground conditions recorded during the investigation at the site comprised Made Ground to 1.1m, with reworked material to 1.5m, followed by the London Clay Formation. No groundwater was recorded.
Seven soil samples were analysed for heavy metals, polycyclic aromatic hydrocarbons (PAHs), total petroleum hydrocarbons (TPH), BTEX (benzene, toluene, ethylbenzene, xylene) and soil organic matter (SOM).
During the 6 rounds of monitoring the report states a maximum carbon dioxide concentration of 5.6% was recorded along with a maximum hydrogen sulphide concentration of 1ppm. Concentrations of methane and carbon monoxide were below the instrument's detection limit. A maximum positive flow rate of 0.3l/hr was recorded. The site was classified as Characteristic Situation 1 and no ground gas protection measures were recommended. A final monitoring visit, during low pressure conditions, is outstanding and is to be reported in an addendum letter report, with recommendations/proposals adjusted accordingly, if required.
Both preliminary and detailed unexploded ordnance (UXO) threat assessments were undertaken. The site was classified as a medium risk and mitigation measures were recommended including having an emergency action plan and safety awareness briefings on site and an EOD engineer on call should suspicious items be identified.
There are various errors and inconsistencies in the report. The main points that will need to be addressed for Condition 12 are as follows:
The environmental information from the Pollution Regulatory Team should be obtained and the desk study and risk assessment updated accordingly.
Reference is made to CLR11 and British Standard which have superseded. The assessment should be undertaken in accordance with current guidance and such guidance referenced in the report (e.g. Environment Agency Land Contamination Risk Management guidance which replaced CLR11, BS5930: 2015 +A1: 2020, BS8485:2015 + A1: 2019)
The Executive Summary states that commercial generic assessment criteria (GACs) were used but section 9.25 states the residential criteria was used during the assessment. This should be clarified. The commercial criteria would not be considered appropriate for this site.
The report has stated that radon protection is not required as the site is in an area where statistically <1% of properties are above the radon action level. However, the report does also state that consideration should be given to the advice from Public Health England regarding radon monitoring in basements proposed for long term occupancy. I reiterate, as stated by the previous Contaminated Land Officer at application stage, that BRE guidance states that basement are at an increased risk from elevated levels of radon regardless of geographical location. Therefore, as requested by BRE and in the Council's CLC 3 guidance radon protection needs to be considered for basement developments. It may be that for the new basement, protection from radon is already inherently provided through the basement design or waterproofing, but this should be confirmed.
Section 9.18 'The proposed end-use is Residential and as such represents a Low sensitivity type of end-use.' - I would consider a residential use would be a high sensitivity end-use.
The monitoring dates and gas concentrations stated in Section 10.7 are completely different to those quoted in Section 9.27. Clarification is required.
It should be confirmed that the client for No. 78 has permission to use the report commissioned for No. 76. The ground gas concentrations from the adjacent site, particularly the 12% carbon dioxide concentration, has not been discussed in the risk assessment. Clarification is required on if the concentration/flow from the adjacent site may pose a potential risk to the subject site. This needs to be further discussed given the report for the adjacent site indicates a Characteristic Situation 2 classification for the adjacent site due to the carbon dioxide concentration.
Section 5.10 states that six locations (G1-G6) were monitored in the basement for potential accumulation of methane and carbon dioxide. However, in the appendix locations referenced G1-G8 are listed. Clarification is required on the number of internal monitoring points in the basement. The report does not appear to discuss the results from these internal monitoring points particularly the 8.8% methane concentration that was recorded. Given the relatively high methane concentration recorded in the existing basement further discussion/assessment is required on the potential risk this may pose to site occupants.
It is noted that a monitoring visit during low pressure conditions is outstanding, in order to capture 'worst case' conditions. This is generally acceptable, although the visit should fall during low and falling pressure conditions. However, for Part A of condition 12 to be discharged either these results are required, or the consultant should demonstrate that the Characteristic Situation 1 classification is unlikely to change. If construction of the basement commences before the ground gas regime is confirmed it may be difficult to retrofit gas protection measures if required.
The recommendations should include details on a watching brief and discovery strategy during basement excavation works for unexpected contamination.
The last line of the limitation section states 'q) In addition, CGL will not be liable for any loss whatsoever arising directly or indirectly from any opinion within this report.' This is not considered acceptable as the consultancy undertaking the contamination assessment should hold responsibility/liability for their assessment, and should be satisfied that sufficient investigations and assessments have been undertaken to appropriately inform the potential risk.