Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:3:p13
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 3 (pt 13/21)
Character Range: 1378655–1381558

totality of the ecosystem and all species are equally as important to ecosystem functioning (that is, no consideration is given to keystone species).

3.2.3         Assessment factor methods
In AF methods, all available toxicity data for a contaminant is collated. Then the lowest toxicity value is divided by a constant that is variously called an assessment factor, uncertainty, application or safety factor. Typically the AFs are 10, 100 or 1000. The magnitude of the AF used to derive an EQG is inversely related to the perceived environmental relevance of the toxicity data; that is, the more environmentally realistic the toxicity data, the smaller the AF and vice-versa. This approach for deriving EQGs was first proposed by Hart et al. (1995) and was adopted from methods used in human health to derive average daily intakes (Cotruvo 1988; Calabrese & Baldwin 1993). The AF method is used to derive both soil and water quality guidelines in numerous countries.

Depending on the toxicity data available, up to three extrapolations can be made by AF methods, with each extrapolation typically given an AF of 10. The extrapolations are laboratory-to-field, acute-to-chronic, and interspecies, and are designed to compensate for inadequacies in the available toxicity data. The magnitude of the various AFs and the type and magnitude of the extrapolations that are inherently assumed by the AFs used in the modified US EPA (OECD 1992) and CCME (1991) methods are presented in Table 18 below.

The field-to-laboratory extrapolation accounts for the supposition that laboratory studies tend to underestimate the toxicity in the field. Proposed reasons for this include: laboratory tests being conducted on animals that are robust and easily bred/maintained in the laboratory rather than 'sensitive' species, life stages that are not tested in the laboratory may be more sensitive to toxicants (Hart 1996), and all the limitations associated with single species toxicity tests that are discussed in Warne (1998).

It is also possible for laboratory-based experiments to overestimate the toxicity in field situations. This can arise if laboratory experiments use freshly spiked soils with minimal ageing period, which overestimates the bioavailability compared to field bioavailability.

The acute-to-chronic extrapolation is extensively used to derive WQGs because the vast majority of toxicity data is acute whereas chronic data is preferred for environmental protection. The CCME method (CCME 1991), like the original US EPA method (US EPA 1986), uses an ACR derived from another species for the same contaminant in preference to a generic ACR. When a contaminant-specific ACR is not available, then CCME (1991) and the US EPA (1986) use a generic ACR. CCME (1991) uses an ACR of 2 or 10 depending on the environmental persistence of the contaminant, while the modified (OECD 1995) and unmodified US