Methods for Deriving Pesticide Aquatic Life Criteria - Reviews of Environmental Contamination and Toxicology - page 88

characteristic more often than do other kinds of toxicants. In contrast, Mount et al.
(2003) believe that very few cases of extreme antagonism or synergism are observed
in environmental mixtures and, therefore, an assumption of only additivity is appro-
priate in most cases of chemical mixtures. Warne and Hawker (1995) proposed the
funnel hypothesis, which states that deviations from additivity in mixtures, decrease
with increasing numbers of components in the mixture, thus for very complex mixtures,
additivity models are likely to be valid.
There are two models used to assess additive toxicity. One is the response addition
model, in which the chemicals have different modes of action and do not interact
with each other. The second is the concentration addition model, in which chemi-
cals have the same mode of action, but do not interact with each other (Plackett and
Hewlett 1952). According to Mount (2003), the response addition model is not
widely accepted, because it cannot be readily tested. However, the concentration
addition model has been successfully tested for several modes of action and may be
used to derive technically defensible criteria.
The concentration addition model is applied in the Water Quality Control Plan
(Basin Plan) for the Sacramento River and San Joaquin River basins (CVRWQCB
2004). In the Basin Plan, if multiple chemicals with similar modes of action are
present in a water body, WQOs are met if the following is true:

C
O

∑

n

i

≤ 1.0

(2.14)

i = 1

i

where
C i = concentration of toxicant i in water
O i = WQO for toxicant i.
In reviewing proposed Basin Plan amendments, Felsot (2005) noted that, for
diazinon and chlorpyrifos, in particular, this additivity analysis is not appropriate
because the denominator is based, not on actual toxicity values, but on an objective
that includes a safety factor of 2. He proposes that a better way to determine added
toxicity compliance is to use the relative potency factor (RPF) approach, which is
analogous to the toxic equivalency factor (TEF) approach used in assessing the
toxicity of dioxin and dioxin-like compounds. Using the RPF approach, one chemical
(usually the most toxic one) is chosen to be the reference chemical, and the potency
of other similarly acting chemicals is expressed as a ratio to the reference chemi-
cal's toxicity. This ratio, the RPF, is multiplied by measured concentrations of each
nonreference chemical to produce concentrations expressed as equivalents to the
reference chemical. Compliance with the objective for the reference chemical is
based on the sum of the measured reference chemical(s) plus the concentrations of
those expressed in its equivalents.
The USEPA guidelines (1985) do not incorporate mixtures or multiple stressors
into aquatic life criteria derivation. However, the regulators of the Central Valley
RWQCB may use mixture models to assess compliance with objectives. Similarly,
the Australia/New Zealand guidelines do not derive mixture criteria, but determine
compliance using the following formula (ANZECC and ARMCANZ 2000):