Methods for Deriving Pesticide Aquatic Life Criteria - Reviews of Environmental Contamination and Toxicology - page 90

groups, based on mode of action, are aggregated with the msPAFs by the response
addition calculation method, to yield an overall msPAF for the mixture. It is current
practice to apply the concentration addition calculation method to groups of chemicals
with narcotic modes of action, and to photosynthesis inhibitors and acetylcho-
linesterase inhibitors (Traas et al. 2002). Typically, this aggregation would be per-
formed within taxonomic groups, but Posthuma et al. (2002b) extend this to suggest
that it is possible to aggregate across taxa to derive an msPAF for all species in an
ecosystem. By this method, an msPAF value less than 0.05 would indicate com-
pliance with water quality criteria, whereas, an msPAF > 0.05 would indicate
noncompliance (assuming criteria were derived to protect 95% of species).
Using the above methods, mixture toxicity is addressed at the compliance stage,
not at the criteria derivation stage. Although it would be ideal to actually derive
criteria for mixtures, it would be impossible to develop criteria for all potential pes-
ticide mixtures that could occur in a water body. To determine compliance of criteria
for individual chemicals, an appropriate model should be selected. If little is known
about the actions and interactions of the chemicals in a mixture, then an additive
assumption is reasonable and simple models may be used. However, if interactions
are known to occur that lead to antagonistic or synergistic action, then a more
complex model, such as that of Rider and LeBlanc (2005), should be used.

7.3.2 Bioaccumulation

Bioaccumulative chemicals pose risks that are not measured in standard laboratory
toxicity tests. For bioaccumulative chemicals, many methods exist to incorporate
bioaccumulation data into criteria derivation. Such methods may be simple, such as
adjusting the size of the applied AF (Zabel and Cole 1999; Samsoe-Petersen and
Pedersen 1995; Bro-Rasmussen et al. 1994; Lepper 2002), or using a tissue residue
level to determine a chronic criterion (USEPA 1985). They may also involve con-
verting food-based NOECs for fish-eating predators into water-based NOECs,
which can be combined with other water-effects data, in criteria derivation (RIVM
2001; OECD 1995). Others methods do not address bioaccumulation in aquatic life
water quality criteria, but do so in other ecological effect assessments (CCME
1999; USEPA 2003a). For example, the Great Lakes guidance includes a procedure
for derivation of water quality criteria for the protection of wildlife (USEPA
2003a). The South African methodology does not consider bioaccumulation at all
(Roux et al. 1996).
The FRV used in the USEPA methodology (1985) is intended to prevent exceed-
ance of FDA action levels in recreationally or commercially important species, and
to protect wildlife, including fishes and other animals that consume aquatic organisms,
where adverse dietary effects have been demonstrated. The FRV is a water concen-
tration derived by dividing a maximum permissible tissue concentration by a BCF
(uptake directly from water) or BAF (uptake from water and food). BAFs are
preferred for the FRV calculation, but because BAFs are generally not available,
BCFs are often used. The maximum permissible tissue concentration may constitute