Sampling procedures

Consideration must be given to the different sampling or measurement requirements of the different media present on a site. Radioactive contamination may be restricted to the soil layer;

it may also behave differently in the vadose and saturated layers. Rock or clay layers may be impenetrable to activity or may bind activity. Where rocky inclusions are found in soil, they may be essentially free of activity. As an example, this may cause apparent discrepancies if soil sample measurements are compared with external gamma ray measurements on ground containing substantial amounts of rock. In this case, a soil sample would be unrepresentative of the average near-surface material.

If surface water is present it may contain activity; if activity is likely to have reached the saturated zone, it may also be relevant to sample groundwater. Sediments in lakes, rivers, estuaries and coastal sites may concentrate or fix activity.

Measurements in "secondary" media, i.e. other than those containing the main contaminant, may give useful information on the distribution or presence of activity. Biota which concentrate activity may help in detecting the presence of radionuclides which are otherwise buried or at low concentrations. Emanation of gases (radon, tritium) from buried sources can help determine the presence of activity. Most of these measurements should be regarded as qualitative rather than quantitative, but they prove valuable in early stages of characterization to identify areas for further investigation.

5.2.2.1. Sampling of gaseous products

Elements and compounds to be considered include radon, tritium, (possibly as water vapour), ¹⁴C and other volatile compounds associated with radioactive materials. It has been shown that gas monitoring can be used to locate waste in the ground. Radon itself is a component that needs to be considered because it is of health concern in case it is present in building materials or in populated areas. The presence of above background concentrations of radon in air directly indicates that there is a source nearby of radium or its parent isotopes.

Methods commonly used for the sampling and analysis of radon include tracketch devices (mentioned in an earlier section), and air sampling through filter papers or charcoal packs followed by gross beta or gamma counting to detect radon and its radioactive decay products.

5.2.2.2. Sampling of flora and fauna

Some species of flora and fauna have the ability to concentrate naturally occurring or artificial radionuclides. Iodine, for example, is known to concentrate in certain algae and shellfish, while caesium can exhibit an enhanced uptake in plants like lichens, heather, fir and spruce, as well as mushrooms. It should be noted that in general, radionuclides have stable sister isotopes which are common in nature and are taken up to varying degrees by biota. Natural processes of plant or animal uptake have evolved which ought not to be affected by the nuclear

properties of the element. This results in a broad and mainly still uninvestigated field of promising use as bioindicators and, moreover, for bioremediation.

Some bioindicators have been identified, as shown in Table HI, which does not claim to be exhaustive.

TABLE HI. EXAMPLE BIOINDICATORS FOR SOME KEY RADIOELEMENTS BIOINDICATOR

5.2.2.3. Sampling of soil and subsurface access technologies

Where measurements of the activity distribution with depth are required, it will be necessary to provide subsurface access for instruments or sampling. At the simplest, this could involve digging a hole to carefully remove samples at different depths. More sophisticated approaches would use auger, penetrometer, or borehole technology to provide access to deeper strata.

Soil sampling, both above and below ground level, can provide essential information towards determining the accumulated amounts of contaminants which have been deposited on the ground. It is very important to ensure that the samples taken are seen to provide a realistic representation of both the perceived problem and the area (laterally and/or vertically) over which the contamination is anticipated to exist. For example, the depth at which a radionuclide has penetrated down into the soil is dependant on the age of the release and the mobility of the radionuclide in that particular environment.

Practical methodologies for soil sampling and criteria for methodology selection to assess radioactivity contamination have been compiled [41,42]. Several of the more commonly used

sampling methods like coring, trenching, and core penetrometer testing (CPT) technology are described below.

Coring

While investigating contaminated areas one of the main objectives will be to ensure the acquisition of an undisturbed sample, preferably with a 100% recovery rate. When samples may be taken using coring equipment, caution must be taken that cross contamination of samples below more active strata does not take place. This can occur if activity is carried on the coring bit or if cutting fluids are used during the operation. The influence of cross-contamination on individual samples can be reduced if the outer layer of the core sample is carefully removed before analysis takes place.

Once a core has been recovered it is important to carefully cut open the liner and expose the undisturbed core on a work bench. This should then be photographed, logged and sampled at a constant frequency (0.5 m may suffice in short length cores, although it may be appropriate to analyze at closer intervals if, say, the contamination is believed to have leached downwards from the surface and is concentrated near to the top layer of soil) and, in addition, at any particular features of interest. It is often advisable to confirm the size of the required sample with the laboratory and ensure that a duplicate sample is taken.

Trenching

Trial pits and trenches are often used as a relatively cheap yet quick method of viewing and sampling the subsurface strata. Stratigraphic and structural changes can be seen more clearly than in cored material and samples are easy to obtain. The approximate maximum depth of 4 m is one of the disadvantages of trenching. Sample points at one-half meter intervals are normally sufficient for contaminant analysis, and once the sample has been obtained the procedures prior to laboratory analysis are similar to that for cores. When done with care, trenching can be used to obtain subsurface samples free of cross-contamination, but it is labour intensive and may be unacceptable for environmental or safety reasons. Trenching may generate unacceptable quantities of waste and may expose workers to both physical hazards from unstable ground formations as well as high levels of radiation from the exposed surface.

Cone Penetrometer or Direct-Push Technology

Cone penetrometer testing (CPT) or, more generally, direct-push technology provides an opportunity for subsurface measurement without coring or boring. It depends on hydraulically pushing a small-diameter instrumented probe from the ground surface downward. Depending on the soil conditions and size of the pushing device, the depth of penetration can reach tens of meters.

CPT probes include a variety of sensors to identify different contaminants. They are often used to screen contaminated areas for later placement of monitoring wells. Sensors for radioactivity are presently under development and in testing.

The primary advantages of direct push technology over boring are small disturbances, relatively rapid sampling, low cost, and no creation of waste. The limitations are requirements for site access for the truck-mounted device, resistance of some lithologies to penetration, and semi-quantitative nature of the measurements from present sensors.

5.2.2.4. Sampling of water and sediments

Water and sediments sampling is often a necessary prerequisite in the determination of radionuclide migration. Contaminants could be transported by water in solution on suspended particulates and sorbed into sediments. Lateral movement could be highlighted by analysing surface water and bottom sediment samples. Downward radionuclide migration through the vertical soil profile can be determined in core samples and groundwater samples. Sampling approaches are outlined, for example, in Ref. [42].