Dq 511

Why is the determination of sampling sites a particular problem for contaminated land?

Answer

It may not be obvious how to define the area in which contamination has occurred. A small number of sampling sites may, in fact, miss areas of pollution. Sampling can also often be complicated by the variety of solids which may make up the 'land' on an old industrial site. This could be soil, sand, shale, brick, remnants of concrete buildings and other industrial waste.

Particular care should be taken where there is likely to be small areas of relatively high concentrations of contamination ('hot spots') that the sampling scheme will be the most suitable. It is quite possible that contamination from a single source on to sloping land is in the form of a ribbon from the source. If a regular grid sampling strategy is used (see Table 5.1 above), a herringbone-type grid is sometimes suggested as this would be less likely to miss ribbon contamination than the common square-grid pattern (Figure 5.4).

At this stage, simple surface tests could be performed, sampling with trowels or with one of the soil samplers shown earlier in Figure 5.2. You should remember the potentially corrosive nature of many industrial contaminants and all tools should be either PTFE-coated or made of stainless steel.

A number of field monitors have been developed for rapid site assessment to lessen the need for expensive laboratory analysis. These include the following:

(i) Immunoassay test kits. Did you notice when we were looking at water immunoassay kits (see Section 4.2.5 above) that many were for industrial contaminants? Much of the development for these kits has been for contaminated land analysis. The kits include simple apparatus to extract the contaminant from the soil into solution. Immunoassay of the extract then follows.

(ii) Portable X-ray fluorescence (XRF) spectrometers. XRF is a method for elemental analysis which has the great advantage that it can directly analyse

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Square-grid sampling - hot spots may be located

X X

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Square-grid sampling -ribbon contamination may be missed

Square-grid sampling - hot spots may be located

Square-grid sampling -ribbon contamination may be missed

Herringbone-pattern sampling - better for ribbon contamination

Figure 5.4 Some examples of sampling strategies employed for localized contamination.

solids as well as liquids. Within chemical analytical laboratories, it has been used much less than atomic techniques (see Section 4.3.3 above) due to lower accuracy, often attributable to strong matrix effects. Simplified instruments can be made portable and give readings in the mg kg-1 concentration range. Lead analysis is a typical application. The theory and instrumentation of XRF will be described later in Section 7.4.1.

(iii) Monitors for specific groups of compounds. These are based on spectroscopic properties which can identify the groups without separation. UV fluorescence monitors can be used for PAHs (c.f. Section 4.2.4) and infrared absorption for hydrocarbons (cf. Section 4.2.6). Some other uses of IR for environmental analysis will be discussed later in Chapter 6.

(iv) Gas monitors. These include portable gas chromatographs, methane gas analysers and direct-reading instruments for the analysis of individual gases. These will all be described below in Chapter 6.

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