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3.6.3 Non-intrusive drain surveys

Drains and sediments within them may be radioactively and/or chemically and/or microbiologically contaminated. Further, leaks from drains are a potential source of contamination of the surrounding ground. The current and past uses of drains on a site should be determined in order to identify those drains that may have been used to carry chemically or radioactively contaminated liquids. In addition, historical incidents or past practice on a site may have resulted in contamination of drains that were not designated to carry contaminated effluents. The desk study (see Section 2.4) should be designed to obtain such information.

Drain surveys comprise:

  • Radiological surveying of selected manhole chambers and the collection and analysis of drain sediments.
  • Surveying of drain runs using in-drain devices.
  • Closed-circuit television can be used to identify breaks in the drains.
  • Radiological surveying (typically total gamma probes) can be used to identify areas of increased levels of radioactivity.

Various in-drain devices can be used for drains surveys. Remotely operated vehicles (ROV’s) are suitable for larger diameter drains; probes manually pushed along the drain using rods are used for smaller-diameter drains.

Some issues that should be considered when designing drain surveys are listed below:

  • Sediment build-up in drain runs may prevent deployment of in-drain devices. There may be a requirement for washing down the drains prior to the survey. Facilities should be available to handle, and if necessary treat, the sediments washed out during this process.
  • The impact of continued use of the drains after the survey should be considered (in particular, the impact of connections to drains outside the survey area should be established).
  • Calibration of in-drain gamma devices is not straightforward, and depends on the size of the drain and the distribution of any radioactive contamination. The confidence in the quantification of radioactive contamination should be established. If necessary, in situ sampling may be undertaken using in-drain devices.

The results from the drains survey should be used to determine (i) whether the drains and sediments within them are radioactive substances and (ii) whether drains may be sources of contamination of the surrounding ground. In the latter case, targeted sampling of the ground along the drain run should be undertaken using trial pits or boreholes.