ANZECC water quality guidelines - FAQs
8. I've heard that the default trigger values, especially for toxicants, are more conservative (and possibly less attainable) than the values listed in the previous (1992) Guidelines.
The methodology used to calculate the Guidelines for toxicants is similar to that used in 1992 (ie. applying a factor to laboratory toxicity data), with two exceptions. Firstly, with the new Guidelines, much more data were available and, secondly, a statistical method was used wherever possible to calculate the size of the factors used, based on all of the toxicity data, not just the lowest figure. The reasoning behind this is discussed in detail in Volume 2 of the Guidelines. Some, but not all, of the toxicant figures are more conservative. However, the risk-based scheme for applying the guidelines at specific sites provides the opportunity to modify the figures to take into account factors at the site that modify toxicity. This is particularly important for metals and, although for many organic chemicals there is not the information to allow guidelines to be modified, the scheme will provide the impetus to generate the required data. The scheme also provides the opportunity to undertake direct toxicity assessments to determine how toxic the chemical or effluent is at a specific site.
