Methods for Deriving Pesticide Aquatic Life Criteria - Reviews of Environmental Contamination and Toxicology - page 47

first by mode of action, and then by chemical structure. The second approach is
similar to that used in the Dutch methodology (RIVM 2001), except that the OECD
provides QSARs for four classes of toxic modes of action (inert/baseline, less inert,
reactive, and specifically acting chemicals), as defined by Verhaar et al. (1992).
When using the OECD methodology (1995), if no toxicity data are available,
QSARs may be used to derive MTCs. For inert chemicals, QSARs may be used to
estimate toxicity for fish, Daphnia and algae. For chemicals that are not inert,
estimates may be made for fish. QSARs are not used to derive OECD MTCs for
reactive and specifically acting chemicals, because its application for this purpose
has not been adequately evaluated (OECD 1995). If some toxicity data are available,
then QSAR estimates of toxicity for inert chemicals are compared to experimental
values. If the values agree within a factor of 5, then the QSAR values may be used
to extend the database for MTC derivation. MTCs derived solely from QSAR data
are used only for priority setting purposes; they are not used to set EQSs.
When toxicity data are unavailable, QSARs may be used to estimate toxicity and
fill data gaps for polar and nonpolar narcotic chemicals. However, existing criteria
derivation methodologies do not endorse the use of QSARs to estimate the toxicity
for chemicals with specific modes of action.

6.4.2

Ecotoxicity Data

Many types of ecotoxicity data exist in the literature. Results of short-term acute
and long-term chronic tests are available, as are tests on sensitive life stages. The
data from such tests may be used as predictors of chronic toxicity (USEPA 2002b).
Some studies are designed to assess lethality, while others address sublethal end-
points, including inhibition of growth or reproduction. Results of other studies are
used to evaluate effects of toxicants on biochemical endpoints, such as inhibition of
acetylcholinesterase or upregulation of glutathione S- transferases. Some tests are
performed on only one species, while others simultaneously test multiple species in
microcosms or mesocosms. Some tests, performed under laboratory conditions, are
tightly controlled, and others are conducted in field or semi-field settings. Each
study type generates a value, or series of values, such as LC/EC x or NOEC. Results
may also be reported as a lowest observed effect concentration (LOEC), the lowest
concentration of toxicant that causes a response that is different from the control,
or a maximum allowable toxicant concentration (MATC), which is the geometric
mean of the NOEC and LOEC values (USEPA 1987). The following discussion
addresses the many different approaches employed to define and use different kinds
of data among existing criteria derivation methodologies.
Water quality criteria need to protect aquatic life exposed for short or long periods,
and those either transiently or continuously exposed. Long-term exposures are
generally considered chronic exposures, while short-term exposures are considered
acute. However, a relatively short exposure to an organism with a relatively short
life span may be equivalent to a chronic study. What constitutes acute or chronic
durations for toxicity tests varies with the test species. Thus, clear guidance on the