For radiological environmental risk assessments, the benchmark may be in the form of a dose rate or back-calculated using the available assessment tools to medium environmental concentrations for each radionuclide that would give rise to the predicted no effect dose rate. These environmental concentrations [Environmental Media Concentration Limits (EMCLs) in the ERICA Tool, or Biota Concentration Guides (BCGs) in the USDOE Graded Approach] can be compared directly to measured or model-predicted environmental media concentration values and subsequently used to determine a ‘‘risk quotient’’. Calculated environmental media concentration benchmark values are usually
applied at earlier tiers of a risk assessment for identifying (or screening out) sites where there is negligible risk of potential impact. The assumptions used in the calculation of environmental concentration benchmark values are usually conservative with respect to transfer to the organism, exposure scenario and in some instances geometry.

A risk quotient (RQ) provides a simple means of assessing risk by integrating the exposure and effects data to determine the likelihood of an ecological risk occurring. It is calculated from the quotient of the estimated exposure and a numeric benchmark (in the form of a dose rate or activity concentration). The benchmark dose rate is a dose rate which is assumed to be environmentally ‘‘safe’’. The RQ is defined as:

Where the resulting RQ is less than one, then no further effort or action would normally be required. Where the RQ is greater than one, then the assessment would likely need further work (such as collecting more data, refining the exposure assessment, or taking action to reduce the risk).

There are three methods commonly used to derive numeric criteria in ecotoxicology:

(i) deterministic — based on the application of assessment (or safety) factors to the most restrictive single sensitivity value observed;

(ii) probabilistic — based on Species Sensitivity Distribution (SSD) model­ling; and

(iii) a weight of evidence approach — typically using data from field expo­sures, such as in situ measurements of biodiversity indices co-occurring with stressor levels.

Over the last few years, the first two approaches have been applied to radiological assessment59 63 and are based on the guidance provided by the European Technical Guidance Document (TGD)64 for chemical risk assess­ment. The benchmark produced by both approaches is designed to ensure protection of ecosystem structure and function.

The third method has not been widely used to derive benchmarks for use in radiological assessments of the environment although there are examples for specific sites (e. g. uranium mining).65

The deterministic approach, takes the lowest dose rate observed to give a significant biological effect available for any tested species and divides it by a predefined assessment/safety factor ranging from 10 to 1000 (10000 for marine ecosystems) according to the quality and quantity of the data available. The assessment/safety factor is intended to account for uncertainty and guidance on what value to apply is set out in a technical document supporting EC Directive 93/67/EEC.64

In contrast, the probabilistic approach uses the available (quality-assured) ecotoxicological data to determine the dose rate, giving a 10% effect resulting distribution for chronic exposure in the ecotoxicological data (the so called ‘‘effective dose rate for a 10% effect’’; EDR10). The EDR10 value is used to compensate for the influence of experimental design. For instance, the lowest no effect concentration or the highest no effect concentration may be con­siderably different to the true no effects concentration dependent upon experimental replication. These EDR10 values are then plotted together for all species for which information exists and are used to identify (usually) the fifth percentile from the species sensitivity distribution (SSD). To account for any residual uncertainty, an assessment factor of between 1 and 5 is applied to the fifth percentile value based on the available quality and quantity of the data in the SSD to produce the predicted no effect dose rate. This approach, as applied to radiological assessment, is described in full by Garnier-Laplace et al.60 62

The SSD approach has been used to derive a screening dose rate of 10 pGyh-1 using different data selection criteria;59,60 this value is used as the default screening dose rate in the ERICA Tool.13 The screening dose rate is to be applied to incremental (i. e. above background) exposure. Currently, it has only been possible to derive a generic screening value applied to all ecosystems using this approach due to the lack of appropriate quantitative data across a sufficient number of different wildlife categories. Thus, it is not possible to derive screening values by the SSD approach sub-divided into different wildlife groups due to statistical constraints.

A screening dose rate is for application in regulatory assessments of planned releases and is not a useful benchmark for use in accidental situations such as that ongoing at Fukushima. Consideration of effects on wildlife after the Chernobyl accident with reference to Fukushima is presented by Beresford and Copplestone.70