Replacement Dwelling, Billinge, St Helens, WN5

Planning Authority: St Helens Metropolitan Borough Council

Planning Reference: P/2021/0464/FUL and C/2021/0096/CON

Synopsis:

In order to support the application for the erection fo a replacement dwelling as Phase I desk study report as required as the potential for the site being affected by contamination was raised by the contaminated land officer during consultations.

The aim of a desk study report is to assess the potential for a site to be impacted by elevated concentrations of contamination, this is achieved by using a variety of sources to assess the history of the site and the surrounds (Ordnance Survey maps, aerial plates, street level imagery etc.) as well as obtaining information from regulatory bodies (local authority, Environment Agency etc.).

The review of the Ordnance Survey maps indicated the site was not developed until circa 1970 when the site was occupied by a dwelling, marshland was noted in close proximity to the site.

BGS geological maps confirmed the site was underlain by Coal Measures, data obtained from the Coal Authority indicated the site was not within a ‘Development High Risk Area’.

Data in the Groundsure report identified a gravel pit, colliery and a number of ponds within 250m of the site. Information provided by the Environment Agency indicated an inert landfill was present 225m north fo the site.

On completion of the desk bases assessment a site reconnaissance visit was undertaken in order to obtain additional information on the potential sources of contamination identified as well as determining if there were any other potential sources of contamination. The site visiut confirmed the site was occupied by a dwelling and associated gardens.

On completion of the walkover survey a qualitative risk assessment was undertaken in order to determine if any of the potential sources of contamination posed a potential risk to the site, it was concluded that further works were warranted in order to assess the potential risk posed by ground gases being generated from off site sources as well as the potential for made ground to be present on the site.

Given the relatively low risk both site investigation and remediation in lieu of further works were proposed.

The proposals of the Phase I report were accepted and once planning had been granted the Phase II site investigation was undertaken.

The works comprised of the excavation of trial pits and sinking of dynamic sampling boreholes and the installation of monitoring wells in four locations in order to obtain samples of the soils and to assess the risk posed to the site by ground gases.

At the time of the investigation the site had been cleared and the groundworks had commenced, the investigation revealed that made ground was not present on the site.

The quantitative risk assessment using the chemical analysis of the site soils confirmed they did not pose a risk to end users. Ground gas monitoring was complicated by high water levels.

The barometric pressure was monitored and site visits were undertaken during periods of falling barometric pressure, on completion of the gas monitoring the data obtained was assessed and it was concluded that the site was not affected by ground gases and no protection measures were warranted.

The Phase II report was submitted to discharge the contaminated land planning condition (C/2021/0096/CON), additional information as to the affect the high groundwater levels and the gassing risk was requested and the report updated to consider this.

The revised report was submitted and accepted.