Methods for Deriving Pesticide Aquatic Life Criteria - Reviews of Environmental Contamination and Toxicology - page 87

is no weighting of important species; (9) sensitive species may be overrepresented;
(10) important species may fall in the unprotected range; and (11) ecosystem func-
tions are not represented. Ecological issues discussed by Posthuma et al. (2002a)
include the problem of using data from a few species in laboratory conditions to
represent responses of many species under field conditions. The authors note that
laboratory data are often biased toward very sensitive or very tolerant species, and
are from studies conducted under conditions that do not account for bioavailability
and multiple routes of exposure. Statistical issues include choice of toxicological
endpoint, data set distribution type, choice of percentile level to represent a no
effect, and methods of quantifying uncertainty.
Despite violations of some of the assumptions, and despite the disadvantages,
SSD methods have many advantages over AF methods in criteria derivation.
Particularly, important is the ability for risk managers to select appropriate percentile
levels and confidence levels, which is not possible by the AF method. So far, criteria
derived from SSDs have proven to be protective of ecosystems. Further future vali-
dation will take place as the database of field studies expands (OECD 1995).

7.3 Other Considerations in Criteria Derivation

7.3.1

Mixtures

A recurring criticism of deriving water quality criteria from singles-species, single-
chemical laboratory toxicity tests is that such tests do not account for the multiple
stressors facing organisms in the field. In the environment, organisms must deal
with chemical mixtures, physical stressors, and interactions with other organisms.
Methods to incorporate the effects of temperature, pH, and other environmental
factors into criteria derivation have been discussed. Species interactions can only
be addressed in multispecies toxicity tests. This section specifically addresses the
effects of contaminant mixtures.
Results of stream monitoring in the US revealed that more than 50% of samples
contained five or more pesticides (USGS 1998). The California DPR reports that
over 175 million pounds of hundreds of different pesticides were commercially
applied in California in 2003 (California DPR 2005b). It is, therefore, probable that
various mixtures will be present in surface waters as a result of transport processes
such as drift and runoff. Studies of the effects of mixtures are few and represent an
extremely small portion of the number of mixtures that could potentially occur in
the environment. Water quality criteria, derived from single-chemical exposures,
have proven to be protective of ecosystems, but a key question is, if chemical A and
B show additive (or synergistic or antagonistic) toxicity, then what level of each is
environmentally acceptable. Lydy et al. (2004) discuss the challenges of regulating
pesticide mixtures, considering our limited knowledge of pesticide interactions.
Alabaster and Lloyd (1982) report that joint toxicity of pesticide mixtures is more
than additive in a high proportion of cases; moreover, they demonstrate this