G.4 Case study — Integrated contamination and geotechnical investigation for planned residential development
Planning permission had been granted for the erection of three detached houses with individual gardens on the plot currently occupied by a single vacant pre-1938 detached house, which is to be demolished prior to construction of the new houses. The existing and planned layouts are shown in Figure G.1 and Figure G.2 respectively. The planning permission did not contain a condition relating to contamination but it was deemed desirable to carry out an investigation in view of the history of the site (old residential gardens are often contaminated by, for example, bonfire residues or fuel storage impacts and consequently have elevated concentrations of contaminants at shallow depths) and in order to provide information to satisfy NHBC requirements (see also G.4.5).
The overall investigation comprised:
- a desk study and field reconnaissance (termed a "preliminary investigation" in BS 10175) carried out in accordance with BS 10175 and BS 5930 and followed by preparation of a preliminary [environmental] risk assessment;
- a combined geotechnical and contamination intrusive field investigation in which samples of soil were taken for chemical analysis and geotechnical testing.
The principal objectives of the investigation were to:
- determine whether there is anything in the history of the site or that of neighbouring land that might have resulted in the site becoming contaminated;
- collect information on actual contamination (if any) on the site through an intrusive investigation;
- make an assessment of the implications for the proposed development including development of a "conceptual site model" describing site conditions and the potential for any contamination to affect future site users and other potential receptors;
- provide an outline remediation strategy if required by the findings of the investigation; and
- determine the nature and properties of the ground at the site to permit the design of foundations for the proposed houses.
NOTE "Contamination" is used here to mean the "presence of one or more potentially harmful substances or agents at above natural background concentrations as a result of human activity" (this is similar to the definition in BS 10175). The use of the term in this way does not imply that harm is being, or might be, caused by the contamination, i.e. one of the conditions for land to be statutorily designated as "contaminated land". It is the purpose of the assessment to be carried out on completion of the field investigation to evaluate the potential significance of any contamination that might be present.
G.4.2 Desk study
Documentary information was obtained in the form of:
- Ordnance Survey maps for the period 1875 to 2011;
- geological maps for the area;
- the Environment Agency's groundwater vulnerability map for the area;
- the regional hydrogeological map; and
- borehole logs from the BGS.
Information on the Environment Agency's website was also taken into account.
Potential sources of information not consulted include the local planning authority's historical planning records (other than via the council's website), old street directories and aerial photographs.
G.4.3 Field reconnaissance
An initial site visit was made by the geotechnical engineer who had participated previously in many integrated investigations. He was able to make observations on the layout and general appearance of the site, consider the best way to investigate the site for both purposes, consider such factors as ease of access, and make a photographic record of the site. His observations were supplemented by the environmental specialist during the course of the fieldwork.
G.4.4 The site
The site was in a semi-rural residential area with a large detached pre-1938 house surrounded by a large level garden about 0,27 ha in size (about 75 m long by 42 m at its widest point). Most of the ground was laid to lawn but there was evidence of old flower beds and an old greenhouse. The house appears to have been heated by gas although solid fuel might have been the main source in earlier years (the remnants of a coal bunker were found near to the house).
The old greenhouse was located at the end of the site furthest from the house. The old OS maps showed the greenhouse as being present from before 1938 until at least the 1980s. There was little direct evidence for its former presence apart from a small amount of broken glass on the surface of one of the old flower beds. A neighbour reported that the wood-framed greenhouse had been demolished about twenty years previously and that it might have had a commercial use. He thought that the wood from the frame had been burned on site.
There was also anecdotal information that there had once been a garage built from asbestos-cement sheeting close to the entrance of the site but there were no obvious signs of this to be seen.
No nearby (i.e. off-site) activities that might have led to contamination of the site were identified.
The geological map indicated that the site was underlain by Beaconsfield Gravel comprising sand and gravel which in turn overlies the Seaford Chalk Formation and Newhaven Chalk Formation (undifferentiated). The site is in an area in which solution features have been encountered.
The site was concluded to be in a "sensitive" area in terms of any potentially contaminating activities due to the chalk being classified as a Principal Aquifer and the overlying soils classified as soils of High Leaching Potential (HU). The site is within a Total Catchment Source Protection Zone. There are no nearby surface water features and the site is not in an area subject to flooding.
G.4.5 Potential for contamination and related problems
The probability of significant contamination of the land where the houses were to be built was considered to be low except possibly where the greenhouse had been. However, the garden had been in use for over 70 years and it was considered likely, therefore, that some contamination would be present as a result of aerial deposition, deposition of soot and coal ash, burning of domestic wastes on bonfires, including, for example, painted and chemically treated wood and linoleum (pigments containing, for example, zinc were commonly used), corrosion of galvanized metals (zinc) and flaking lead paint, etc. The most common contaminants in domestic gardens are arsenic, lead, copper, zinc and polycyclic aromatic hydrocarbons (e.g. present in ash and soot). The older the property, the greater the amount of contamination that is likely to be present. In a large garden there might be small areas heavily impacted by such activities and events but overall the impact (in terms of relative scale) is probably less than in a small garden.
The environmental specialist identified through past experience that the burning of the wooden-frame of the old greenhouse might have led to contamination of the ground in the vicinity with lead and zinc. There was also a possibility that it had been heated by a coal-fired boiler providing another source of ash that might have been distributed around the garden to form paths etc.
As noted in G.4.4, there was anecdotal information that there had once been a garage built from asbestos-cement sheeting close to the entrance of the site but there were no obvious signs of this to be seen.
No evidence was found that natural concentrations of potentially harmful substances (e.g. arsenic) are elevated in the area.
G.4.6 Preliminary assessment
The hazards posed by the most probable potential contaminants are summarized in Table G.1. Those considered to be most likely to be relevant for the planned development were considered to be potentially toxic elements such as lead, polycyclic aromatic hydrocarbons (PAHs) and phytotoxic zinc.
|Contaminant(s) [sources]||Principal hazard/reasons for concern (including potential receptors)||Comments|
|PAHs and toxic elements including arsenic and lead||Potentially harmful to human health||Near-surface sampling required in garden area.
Potential risks to future site occupants and other human receptors.
|Copper, nickel and zinc||Toxic to plants||Concentrations might be elevated.
Near-surface sampling required.
|PAHs and toxic metals||Potential groundwater and surface water contaminants||Negligible risks|
|Petroleum hydrocarbons||Potential groundwater and surface water contaminants||No evidence for oil tanks etc. found.
Investigation not required.
|Asbestos||Some forms are proven carcinogens||Anecdotal information that there had been an asbestos-cement structure on the site — sampling required in this part of the site.|
The fieldwork was carried out with both the geotechnical engineer and the environmental specialist on the site. All logging was carried out by the geotechnical engineer.
Ten trial pits were formed. The trial pit locations were chosen taking the proposed layout into account so that samples for chemical analysis could be taken as far as practical from both the future front and rear garden areas of each house (there was limited access in places). Two pits were located within the footprint of the old greenhouse and one in the gravel drive at the front of the existing house.
Five of the trial pits at locations chosen by the geotechnical engineer were formed to about 2,5 m to enable the ground to be inspected and samples taken for geotechnical testing. The remaining five pits were formed to depths of up to about 1,0 m. The trial pit locations are shown in Figure G.2.
The trial pits exposed varying thicknesses of topsoil over sandy clay with sand and gravel at greater depths. Overall, twenty four samples were taken for chemical analysis from the topsoil and underlying sub-soil from depths of 0,1 m to 1,2 mbgl from the ten trial pits. Samples were taken from the near-surface top-soil and from within the range 0,4 m to 0,6 m from each trial pit.
In addition, several surface and near-surface samples were taken by hand from the assumed location of the old asbestos-cement garage.
G.4.8 Analytical strategy
The samples were analysed for acidity (pH), a range of potentially harmful elements, and polycyclic aromatic hydrocarbons (PAHs).
The samples from the assumed location of the old asbestos-cement garage and a number of other of the top-soil samples were screened for asbestos fibres (no asbestos-cement fragments were seen during sampling).
G.4.9 Geotechnical testing
Three samples were taken for geotechnical testing. Tests were carried out for natural water content, liquid and plastic limit, pH, sulfate content (total and water soluble).
NOTE If a windowless percussive sampling rig had been used it would have been possible to obtain SPTs.
|1 Approximate site of asbestos cement garage||2 Approximate position of old greenhouse|
|1 Surface sampling for asbestos||2 Approximate position of old greenhouse|