Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:3:p10
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 3 (pt 10/21)
Character Range: 1371047–1373845

chosen percentage of species to experience toxic effects. To do this, the cumulative frequency that corresponds to the percentage of species to be protected is entered into the equation for the distribution that best fitted the toxicity data. Thus, the 5th percentile of the selected distribution becomes the concentration that, if not exceeded, will protect 95% of species and the 10th percentile will protect 90% of species, and so on. The resulting concentrations are generally referred to in Europe as hazardous concentration (HC) values, while in Australia and NZ, Hong Kong and South Africa they are termed protective concentration (PC) values. The number following HC or PC indicates the percentage of species that should be harmed or should be protected respectively. More detailed information on each SSD method can be obtained from the original documents cited above and in the thorough review of SSD methods by Posthuma et al. (2002).
Figure 5. A typical SSD plot. The example provided is output from the BurrliOZ program using EC10 values for plants (field data NBRP)
The toxicity data used to derive a PC value is only a sample of the total species in the ecosystem being protected. As with any sampling program, different distributions could be obtained depending on the species that form the sample. Therefore, different samples could lead to different PC values for the same contaminant being calculated. Aldenberg and Slob (1993) overcame this problem by developing two confidence limits: 95% and 50% for the HC or PC values respectively. These confidence limits indicate the degree of certainty that the calculated HC value will protect the selected percentage of species. Thus, a HC5 95 value means that there is a 95% certainty that the concentration will protect at least 95% of species in an ecosystem. The Dutch used the HC5 95 values as their long-term aspirational goal for water quality. In the Australian and New Zealand WQGs (ANZECC & ARMCANZ 2000) and in jurisdictions that have adopted their methodologies (that is, Hong Kong and South Africa in its marine water quality guidelines) confidence intervals are not used. This was developed because the 95% confidence limits were not deemed to be statistically robust (Fox 1999). Additionally, if the sample size is large, the 50th percentile will approximate the median of estimates of the PC value. Thus, the 50th percentile should equal the HC5 50.

3.2.2.1         Criticisms
All SSD methods make a series of assumptions. In the early 1990s, the SSD methods received considerable criticism from Calabrese and Baldwin (1993), Forbes and Forbes (1993), Schudoma (1994), and Smith and Cairns (1993), and some doubted whether the SSD methods were in fact better than the AF methods.

The key criticisms were:
    * whether