Likely potential contaminants associated with the former uses of a site are identified during the desk study. Soil and groundwater samples are collected during the ground investigation and scheduled for appropriate laboratory analysis. The number of samples taken will be reflective of the complexity of ground conditions actually encountered.
The UK approach to contaminated land is set out in Contaminated Land Report No. 11 (2004) “Model Procedures for the Management of Land Contamination”. The approach is based upon risk assessment, where risk is defined as the combination of the probability of occurrence of a defined hazard and the magnitude of the consequences of the occurrence.
In the context of land contamination, there are three essential elements to any risk: (1) a contaminant source, (2) a receptor (e.g. controlled water or people) and (3) a pathway linking the (1) and (2).
Risk can only exist where all three elements combine to create a pollutant linkage. Risk assessment requires the formulation of a conceptual site model (CSM), which supports the identification and assessment of pollutant linkages.
Lithos adopt a tiered approach to risk assessment, consistent with UK guidance and best practice. The initial step of a contaminated land risk assessment (Tier 1) is the comparison of site data with appropriate screening values.
In the context of redevelopment within the planning regime, Lithos have derived tier 1 screening values for four different land-use scenarios:
A - Residential with gardens, but no cover (or only up to 300mm)
B - Residential with gardens and 600mm ‘clean’ cover
C – Residential apartments with landscaping (i.e. no home grown produce)
D - Commercial/industrial with landscaping
Lithos adopt a tiered approach to risk assessment, consistent with UK guidance and best practice. The initial step of such a risk assessment is the comparison of site data with appropriate guideline (or Tier 1) values. It should be noted that exceedance of Tier 1 values does not necessarily mean that remediation will be required.
Should any of the Tier 1 criteria be exceeded, three potential courses of action are available:
- undertake statistical analysis of the laboratory results in order to determine whether contaminant concentrations within soil\fill actually present a risk;
- carry out a more detailed quantitative risk assessment;
- based on a qualitative risk assessment, advocate an appropriate level of remediation to 'break' the pollutant linkage - for example the removal of the contaminated materials, or the provision of a clean cover.
Asbestos in Soils
Asbestos fibres are widespread in near-surface soils (albeit often in trace amounts), especially in urban areas, as a consequence of asbestos usage in the UK from the late 1800s, and especially between the 1940s and 1970s. It is therefore not unusual to detect asbestos fibres in soil samples recovered from brownfield sites, or indeed residential gardens in many UK towns & cities.
However, asbestos fibres in soils typically pose a negligible risk to health, because of (a) the low concentrations present and (b) the ability of soil to “bind” the fibres in, especially when damp. Nonetheless, some release may occur in dry conditions if the soil is disturbed by excavation or wind.
Lithos' Desk Study will determine whether or not previous use of the site is likely to have resulted in significant asbestos contamination in the soil.
The majority of soil samples recovered during a Lithos ground investigation are scheduled for asbestos ID (effectively a screening test). For the vast majority of sites this testing is to confirm the absence of a significant risk, rather than because we expect (due to previous use of the land), or have seen (during the excavation of trial pits etc), significant amounts of asbestos.
As experienced geoenvironmental engineers, we are able to recognise larger fragments of asbestos-containing material (ACM) in soil; e.g. asbestos-cement sheeting, or asbestos-insulation board. Any potential ACM is sampled and scheduled for asbestos ID laboratory analysis.
Information and guidance is provided to all Lithos employees via annual Asbestos Awareness Training.