Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:3:p7
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 3 (pt 7/21)
Character Range: 1363378–1366131

6.

3.2              Methods to calculate soil quality guidelines
In general, there are three main methods to derive SQGs. These are in order of increasing complexity: the geometric mean method, AF methods and SSD methods. They are discussed below.

3.2.1         Species sensitivity distribution methods
The SSD methods are statistical methods to calculate a soil concentration that protects a specified number of species and/or soil processes. Briefly, all SSD methods use toxicity data obtained from tests on individual species and fit a statistical distribution to the data to derive a concentration that should protect any selected percentage of species in the ecosystem being considered.

There are essentially four different SSD methods that have been used to derive EQGs:
    * the Stephan et al. (1985) method, which fits a log-triangular distribution to the data
    * the Aldenberg and Slob (1993) method, which is an enhancement of the Kooijman (1987) and Van Straalen and Denneman (1989) methods, which fits a log-logistic distribution to the data
    * the Wagner and Løkke (1991) and Aldenberg and Jaworska (2000) methods, which fit a log-normal distribution to the data
    * the Burr type III (Shao 2000; Campbell et al. 2000) method, which fits the best of the Burr type III family of distributions to the data.

The Stephan et al. (1985) method was the first SSD method developed. It is used by the USA to derive its WQGs (US EPA 1986) and was adopted by South Africa to derive freshwater guidelines (Roux et al. 1996).

Limitations of this method are that by using a log-triangular distribution it assumes there is a threshold toxicity value, below which no detrimental effects will occur, and the scientific literature and risk assessment theory does not support such an concept (Okkerman et al. 1991; OECD 1992; Emans et al. 1993; Pedersen et al. 1994; NZ Ministry of the Environment 1996) and it uses an arbitrary AF of two without any justification (Hart et al. 1995; NZ Ministry of the Environment 1996). As early as 1995, the US EPA recognised that the method required updating (Delos 1995). For the above reasons, this method was not considered for the derivation of the Australian and New Zealand WQGs (Warne 1998). At least partially due to the limitations of the Stephan et al. (1985) method, South Africa has adopted the more advanced Burr type III SSD method (Shao 2000) for its marine water quality guidelines (Warne et al. 2004a, 2004b).

In the late 1990s, the Aldenberg and Slob (1993) method was viewed as the preferred and most scientifically defensible SSD method. It was recommended over the Wagner and Løkke method by the OECD and subsequently adopted (OECD 1995). The Dutch used the Aldenberg and Slob method to