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3.5.7 Depth-dependent sampling

Contents
3.5.7.1 Key considerations for depth-dependent sampling of soil
3.5.7.2 Downhole radiological measurements

3.5.7.1 Key considerations for depth-dependent sampling of soil

The sampling approaches described above consider only a 2-D (area) distribution of contaminants. It is essential to understand the 3-D structure of the site and the distribution of contaminants within that volume if valid conclusions are to be drawn from the survey. To achieve this, the soil sampling strategy needs to address the required depth of boreholes and trial pits and the approach to collecting samples from them.

The required depth of boreholes/trial pits and the strategy for collecting soil samples from them depend on the reason for characterising the site, and take into account issues such as:

  • The expected depth distribution of contaminants in the source areas. This is dependent on:
    • The mechanism(s) of contamination (e.g., surface deposition, depth of made ground sub-surface leakage from storage tanks).
    • The geological and hydro-geological properties of the soils and rocks (e.g., the presence of major fracture zones, which may act as pathways for deeper penetration, or of low-permeability horizons, which may act as barriers to contaminant migration).
    • The water balance at the site (e.g., the effective infiltration rate or the presence of rising groundwater).
    • The physical properties of the contaminant (e.g., dissolved in groundwater, light or heavy non-aqueous-phase liquids, colloids/particulates).
    • The chemical properties of the contaminants (e.g., its solubility and sorption characteristics in the sub-surface environment at the site).
  • The potential contaminant migration pathways identified in the conceptual model:
    • Analysis of the immediate surface layer of soil would invariably be required, because of human health issues such as ingestion and inhalation of soil. This surface layer should be defined on a site specific basis related to the conceptual model. Sampling depths may vary between the surface and 0.5 m, and may require sampling at more than one level.
    • Samples from each distinctive horizon of made ground, fill and natural strata should be collected.
    • The focus placed on sampling deeper soils would depend upon the expected significance of subsurface pathways in transporting contaminants from the source area to potential receptors, particularly off-site.
    • Any additional testing requirements (e.g., geotechnical characterisation of the site).

3.5.7.2 Downhole radiological measurements

Downhole radiological measurements complement non-intrusive radiological surveys (see Section 3.6) and radiological monitoring during intrusive investigations (see Section 3.7). The technique, which gives information on the distribution of radioactivity along the borehole axis, can be used in three situations:

  • In conjunction with permanent monitoring points (for example, downhole logging of groundwater monitoring boreholes).
  • During construction of conventional temporary sampling boreholes from which soil and/or water samples are being collected (see Section 3.4.8.4).
  • In conjunction with temporary percussive holes from which no waste or samples are produced at surface (for example, cone penetrometer testing).

Downhole radiological measurements can be used to improve targeting of samples taken for subsequent laboratory analysis or to provide interpolation between sparse data from borehole samples (for example, where contamination of bedrock is focused in fractures that may be difficult to sample, or where drilling conditions lead to depth intervals where no solid material is returned to surface for sampling). In addition, the third situation, above, is useful for characterising areas where there is relatively high contamination by gamma emitting radionuclides, because measurements can be made without the need to produce waste.

In all applications of downhole measurements, it is necessary to consider the following:

  • The penetrating power of the ionising radiation in the soil or rock around the borehole, in any borehole construction materials (such as casing) and in the air or water filling the borehole. Downhole logging is most appropriate to determining the distribution of gamma emitting radionuclides.
  • Calibration of results. The technique provides information on the distribution of areas of elevated radioactivity. Accurate calibration to derive specific activities (e.g., Bq/g of soil) requires information on source-detector geometry, on the spatial distribution of the radionuclide and on the attenuation characteristics of the radiation. If quantitative information on specific activities is required, laboratory analysis of samples will be needed to build confidence in the calibration.
  • The susceptibility of the approach to any external contamination of the detector assembly. It is important to monitor for surface contamination on the detector at frequent intervals and to evaluate results with caution if surface contamination is detected.

It is also necessary to consider the consequence of repeated purging of groundwater monitoring boreholes on downhole radiological measurements. Purging leads to some of the fine-grained material from the formation being drawn into the filter materials placed around the well screen (if these are present) or into the borehole itself. In the latter case, the material settles to the bottom of the borehole (silting up the well). Because radioactive contamination is often concentrated on the fine-grained fraction of the soil or rock, this redistribution of material can have a significant effect on downhole radiological measurements. In the extreme case, downhole measurements may be dominated by radioactivity from contaminated silt at the bottom of the borehole. For this reason, it is best practice to undertake downhole radiological measurements prior to groundwater sampling. Where this is not possible, data from downhole radiological measurements should be interpreted with caution.