ABSTRACT
The first use of SSDs was in the derivation of National Ambient Water Quality Criteria (NAWQCs) by the U.S. Environmental Protection Agency (EPA). As discussed in Chapter 11, EPA staff members decided in 1978 to replace the use of expert judgment to derive criteria with a formal method based on protection of a percentage of species. The new method was based on the insight: “We can see that the species sensitivity (LC
or LD
) distributes itself in a rather consistent way for most chemicals. The distribution resembles a lognormal one. Thus, each species we test is not representative of any other species but is one estimate of the general species sensitivity” (Mount, 1982). The method for calculating criteria based on 5th percentiles of SSDs (HC
) was repeatedly revised until the U.S. EPA (1985a) method, which is still in use (Chapter 11). This method calculates two criteria for each chemical, a final acute value (FAV) and a final chronic value (FCV). The FAV is the HC
of acute LC
and EC
values for at least eight fish and invertebrates, divided by 2 to correspond to a lethality rate much lower than 50%. The FCV is derived as the HC
of chronic values if sufficient data are available; otherwise, it is
derived by multiplying the acute HC
by a chronic-acute ratio. The EPA has continued to derive NAWQC values for additional chemicals and to update old NAWQCs using this method.
