MINIMUM AREA OF CONTAMINATION LOCATED BY BSI DD175 SAMPLING FREQUENCY

Area of site

(hectares) Recommended number of sampling points Minimum contaminated area to provide one contaminated sample (at a 95% confidence level) _ ,2 m 0.5 15 905 1.0 25 1,129 5.0 85 1,732

The ground investigation should be designed in such a way as to characterise the contamination present within the site with respect to:

• chemical composition and concentration

• physical characteristics (e.g. volatility, solubility)

• biological characteristics (e.g. pathenogenicity, degradation potential) (Source: Welsh Development Agency, 1993, pp3.11-3.12) Design of the ground investigation must therefore take account of any constraints which may prevent adequate characterisation. This is one aspect to which the surveyor should pay particular attention when analysing site investigation proposals and reports. For example, if the preliminary investigation has indicated the presence of fill material with depths of up to five metres and a trial pit investigation is to be undertaken, then it is of no use employing a wheeled excavator with a maximum reach of only three metres but even leading consultants have been known to make such an error. Trial pits in fill material need to be excavated down to natural, undisturbed, ground so as to facilitate the taking of samples from below the fill, to determine whether or not the contamination is mobile. A report which contains phrases such as ‘natural ground was not found in trial pits numbers ...’ or ‘the excavator was unable to penetrate to the full depth of the fill material’ is of limited use and it may be necessary for further investigation work to be undertaken, at further expense, and a possibility that further disturbance of the ground will cause a more widespread distribution of the contaminants. Similarly, design of the sampling and testing procedures must be sufficiently rigourous so as to maximise the likelihood that any possible contaminants will be identified.

4.4.2 Sampling and analysis

In many cases the site investigation for contamination will be undertaken at the same time as, or even as part of, a wider investigation designed to ascertain the geotechnical, geological, hydrological and hydrogeological attributes of the site.

Whilst it may be possible to combine the different aspects into one investigation, the sampling requirements and techniques may be different. The actual method used to carry out the ground investigation may be determined by criteria related to the other aspects of the investigation, rather than the nature of contamination, and some compromises may be necessitated. For most forms of contamination, a trial pit or trench investigation is likely to be most appropriate, as these methods facilitate visual inspection of both the contaminants and the media within which they are contained. For volatile contaminants, however, trial pits and trench methods are inappropriate, due to the problems of sample collection, and a borehole investigation will produce better results. Other forms of investigation include hand sampling of surface materials (to depths of about 0.5 metre), dynamic probes and soil gas surveys.

At least three samples should be taken at each sampling location, from near the surface (150-200mm depth), the depth of greatest concern and a random intermediate sample (BSI, 1988) and the consultants’ proposals should include for testing at least this number. In addition, it is advisable, on sites which have been subjected to fly tipping, to take at least one sample from each discrete tipped load and, from within trial pits to sample any materials which appear not to conform to the general nature of the ground. “It is therefore essential that, at the design stage, an anticipated testing programme is formulated and the appropriate sample sizes selected for each material to be sampled. This will of course need to be refined as the investigation proceeds,” (Hobson 1993, p51). Health and safety regulations must be adhered to, especially when taking samples from trial pits and trenches, which may need to be shored up if the sampling

method requires the investigator to enter the pit or trench. “Data recorded during the investigation must be accurately recorded, in a manner that can be subsequently understood. If this is not done, the value of the entire exercise will be seriously impaired.” (Hobson, 1993, p57) The sampling method must ensure that material of interest is sampled, which includes any contaminants and the background media, and that the sample remains stable until analysed. The state of the sample must also be compatible with the method of analysis to be used.

Consideration of the methods of analysis available for the testing of potential contaminants is beyond the scope of this research but Lord (1987), McEntee (1991), Smith (1991) and Hobson (1993), provide a comprehensive review of techniques. Analysis may take place both on-site and in the laboratory, and the location of testing will quite often be determined by site specific circumstances. For example, on-site testing may be required:

(a) To support health and safety remedial measures by indicating the presence say o f toxic vapours or acidic liquids etc.

(b) Because immediate measurement o f contaminant is the only way to quantify its concentration i.e. because o f its instability, volatility etc.

(c) To speed up analysis o f site materials i.e. by having immediate analytical response.

(d) To reduce the costs o f analysis i.e. by avoiding transport costs.

(Lord, 1987, p89)

Throughout all stages of the site investigation quality control is of the utmost importance. “The methods and techniques used must be standardised so that reproducible results are obtained both between sites and within the same site; reference to published sampling and analytical protocols and the use of NAMAS (National Measurements Accreditation Service) accredited laboratories will help to ensure these requirements are met.” (Welsh Development Agency, 1993, p3.17)

4.5 SUPPLEMENTARY INVESTIGATION

“Good practice during the main investigation will generally limit the need for supplementary investigation, although logistical and phasing constraints may dictate that further work is undertaken.” (Welsh Development Agency, op cit, ) The extent of any supplementary work will be site specific and may arise as the result of the discovery of contaminants, during laboratory analysis, which were not previously expected. It may also be the case that part of the site was unavailable during the main investigation, due to the fact that it was still operational or in another ownership, although it should normally be possible to allow for this type of situation through phasing of the main investigation.

Any supplementary investigation must be subject to the same quality and safety standards as the main investigation, and may comprise some or all of the same types of works carried out in the main investigation. In situations where supplementary investigations are unavoidable, the additional work should be specifically targeted, for example with the objective of obtaining further information about a specific contaminant or to investigate a particular area of the site in more detail.

Supplementary investigations, for example the obtaining of larger samples of contaminants and media, may also be required in order to assist in the design and selection of remedial works. They may also involve pilot studies of treatment processes, or in-situ testing of ground bearing capacities (see McEntee, 1991).

4.6 THE REPORT

The outcome of the investigative process should be a report which is readily understandable by the ‘non-technicaP client, the other members of the professional team and the appropriate regulatory authorities. A high standard of presentation is therefore important and the report should describe the various stages of the work undertaken, together with the findings obtained and any assumptions which may have been made.

Within the constraints of the site and available information the report should indicate the location of those parts of the site affected by contamination and identify the nature of those contaminants. Potential pathways and targets should be described and the report should clearly define the options available to the client. Potential risks should be evaluated and, when environmental harm is noted during the investigation still to be occurring, recommendations should be made in respect of immediate actions required to prevent further harm

The report should conclude with firm recommendations, regardless of whether or not immediate redevelopment is proposed, and advice in respect of future monitoring. An executive summary should be provided, together with a table of contents, a schedule of documents consulted and a bibliography.