Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:3:p8
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 3 (pt 8/21)
Character Range: 1365888–1368663

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 derive their WQGs and SQGs. This reflected the research that the Dutch had undertaken to assess the scientific validity of this method (Emans et al. 1993; Okkerman et al. 1991, 1993). One drawback of the Aldenberg and Slob method compared to the Wagner and Løkke method was its use of the log-logistic distribution. There is no theoretical basis for the sensitivity of species to conform to a logistic distribution (Forbes & Forbes 1993). In fact, Aldenberg and Slob (1993) stated that the log-logistic distribution was chosen because it has 'practical mathematical features that make the calculations of statistical confidence intervals relatively easy'. Aldenberg and Jaworska (2000) overcame the mathematical difficulties associated with using the normal distribution to develop a log-normal equivalent method to the Aldenberg and Slob method. The Aldenberg and Jaworska method has since been adopted by the Dutch to derive their WQGs and SQGs (Crommentuijn 2000a, 2000b). All of the above methods attempt to fit a single statistical distribution to the toxicity data.

The draft Australian and New Zealand WQGs (ANZECC & ARMCANZ 1999) adopted the Aldenberg and Slob SSD method. However, during the derivation of the TVs it was found that in more than 33% of cases where the Aldenberg and Slob method could be used, based on meeting the minimum data requirements of the method, the data did not have a log-logistic distribution. Therefore, strictly speaking, it was invalid to use the Aldenberg and Slob SSD method. This meant that for many contaminants an AF method had to be used. As there is no theoretical reason why species sensitivity must conform to a logistic distribution, there is no reason why other distributions cannot be considered. This issue was first realised by Shao (2000) and he therefore recommended that a family of distributions, the Burr type III (BT III) be used to fit to the toxicity data, rather than a single distribution as with the other SSD methods. Other authors (Maltby et al. 2003; Kwok et al. 2007) have since also adopted a more flexible approach to the statistical distributions being fitted to the data, whereby the distribution that best fits the data is used to derive the EQG or to determine the ecological risk.

The variety of shapes that BT III distributions can have is large (Shao 2000), including the log-logistic distribution and approximations of the log-normal and log-triangular distributions. Thus, attempting to fit a BT III distribution to any given toxicity data set has a