Index > 3 Characterisation of radioactively contaminated sites >

3.3.5 Establishing background environmental quality

Contents Introduction No reference area needed Second area as reference area Site under investigation as reference area Background chemical quality Introduction

For the reasons discussed earlier, it is important to establish the background level of radioactivity in soils and waters at the site. Once the background concentrations have been measured, then the definition of “background” has to be agreed with regulators before decisions can be taken on land management.

It would be possible to define background as the average activity of all samples analyzed; however, the disadvantage of this approach on a heterogeneous site (i.e., where natural radioactivity and fallout-derived radioactivity vary spatially) is that it could be unnecessarily cautious. For example, it could lead to a recommendation to remediate an area that had not been contaminated by site activities.

A more pragmatic approach may be to define “background” in terms of the activity below which a certain percentage of the distribution lies. Clearly, the percentile chosen would need to be justified.
As discussed in Section 3.3.1, background levels of radioactivity will vary (i) from site to site and (ii) spatially in 3-dimensions within a site. Concentrations of naturally occurring radionuclides will be strongly influenced by the composition of the rocks and soils, and by the extent of near-surface weathering effects.

Anthropogenic radionuclides derived from global fallout are (with the exception of tritium) unlikely to penetrate significantly below surface soils; it would therefore be inappropriate to use the background levels of such radionuclides in surface soils to derive a background for deeper soils and rocks.

Background levels can be established by applying a reference area. Reference areas provide a location for background measurements which are used for comparisons with survey unit data. The radioactivity present in a reference area would be ideally the same as the survey unit had it never been contaminated. If a site includes physical, chemical, geological, radiological, or biological variability that is not represented by a single reference background area, selecting more than one reference area may be necessary.

The following approaches exist:

  • No reference area is needed;
  • A second area – reference area – that has similar physical, chemical, geological, radiological, and biological characteristics as the survey unit being evaluated;
  • The site under investigation can, under certain conditions, also serve as reference area. No reference area needed

No reference area is needed if the radiological contaminant of concern is not present in the background or the radiological contaminant is present in such a small fraction of the derived concentration guideline level (DCGLW) (e.g., < 10%) value as to be considered insignificant.
The survey unit radiological conditions may be compared directly to the specified DCGL and reference area background surveys are not necessary. If the background is not well defined at a site, and the decision maker is willing to accept the increased probability of incorrectly failing to release a survey unit (Type II error), the reference area measurements can be eliminated and a one-sample statistical test performed as described in Section 3.10.2. Second area as reference area

In order to determine background levels of radioactivity at a site, it is therefore necessary to characterizes a second area that has similar physical, chemical, geological, radiological, and biological characteristics as the survey unit being evaluated, e.g., rock and soil compositions to the site under investigation, and to evaluate any depth-dependent changes in the background activity of naturally occurring and fallout-derived radionuclides. These background reference areas are normally selected from non-impacted areas, but are not limited to natural areas undisturbed by human activities.

Typically, this would involve collecting samples from an area sufficiently close to the site that its natural radioactivity characteristics are similar to those of the site but sufficiently far away that site-derived radioactivity will not have significantly enhanced the background levels. In site investigations where data are collected across large areas, some of which may never have been used for operations that deal with radioactive materials, it may be possible to obtain on-site information on background levels of radioactivity. However, it is desirable to supplement this information with data from off-site areas. For heterogeneous sites, it may be possible to define different background levels for different soil types and at different depths; for example, to distinguish between made ground and different natural strata. Site under investigation as reference area

Background reference areas are normally selected from non-impacted areas, but are not limited to natural areas undisturbed by human activities as may be the case in heavy industrialized or urban areas (e.g., harbours). In these areas it may be difficult to find a reference area within an industrial complex for comparison to a survey unit if the radio-nuclides of potential concern are naturally occurring. Background may vary greatly due to different construction activities that have occurred at the site. Examples of construction activities that change background include: leveling; excavating; adding fill dirt; importing rocks or gravel to stabilize soil or underlay asphalt; manufacturing asphalt with different matrix rock; using different pours of asphalt or concrete in a single survey unit; layering asphalt over concrete; layering different thicknesses of asphalt, concrete, rock, or gravel; and covering or burying old features such as railroad beds or building footings. Background variability may also increase due to the concentration of fallout in low areas of parking lots where runoff water collects and evaporates. Variations in background of a factor of five or more can occur in the space of a few hectares.

It is unlikely that areas can be found that fulfil the (minimum) requirements of a reference area. In order to determine – assess – background levels of radioactivity at the site under investigation, it can be assumed that some parts of this site may never have been used for operations that deal with radioactive materials. Therefore, it may be possible to obtain on-site information on background levels. This on-site information can be obtained from a preliminary investigation combined with the information from a historical site assessment and adjusted with information gained from an exploratory investigation.

Example 3.2: Samples for background measurements

For example, background measurements may be taken from core samples of soil, pavement, or asphalt, a building or structure surface.

This option should be discussed with the responsible regulatory agency during survey planning. Generally, reference areas should not be part of the survey unit being evaluated.

There are a number of other possible actions to address these concerns. Reviewing and reassessing the selection of reference areas may be necessary. Selecting different reference areas to represent individual survey units is another possibility. More attention may also be needed in selecting survey units and their boundaries with respect to different areas of potential or actual background variability. More detailed scoping or characterization surveys may be needed to better understand background variability. Using radio-nuclide-specific measurement techniques instead of gross radioactivity measurement techniques may also be necessary. If a background reference area that satisfies the above recommendations is not available, consultation and negotiation with the responsible regulatory agency is recommended. Alternate approaches may include using published studies of radio-nuclide distributions. Background chemical quality

For chemical contamination it is important to understand the background quality for soils and ground gases because, in some locations the natural background may contain elevated concentrations of a compound or element.

“Background” may also be elevated due to contamination from a neighbouring site and it is necessary to establish the concentrations to apportion liability. However, in both these media the risks from naturally elevated concentrations need to be assessed, and if necessary managed and controlled.

With groundwater, establishing the background quality is necessary, particularly where no quality objectives exist. Deriving background concentrations are integral to the decision-making process for both risk assessment and risk management purposes.