Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

AX7.2.1.5 Metal Mixtures
As discussed above, the EPA’s current approach for developing sediment criteria for Pb
and other metals is to consider the molar sum of the metal concentrations (∑SEM). Although a
similar approach has not been applied to AWQC, metal mixtures have been shown to be more
toxic than individual metals (Spehar and Fiandt, 1986; Enserink et al., 1991). Spehar and Fiandt
(1986) evaluated the acute and chronic toxicity of a metal mixture (arsenic, cadmium, chromium,
copper, mercury, and Pb) to fathead minnows (Pimephales promelas) and a daphnid
(Ceriodaphnia dubia). In acute tests, the joint toxicity of these metals was observed to be more
than additive for fathead minnows and nearly strictly additive for daphnids. In chronic tests, the
joint toxicity of the metals was less than additive for fathead minnows and nearly strictly
additive for daphnids. One approach for considering the additive toxicity of Pb with other metals
is to use the concept of toxic units (TUs). Toxic units for each component of a metal mixture are
derived by dividing metal concentrations by their respective acute or chronic criterion. The TUs
for all the metals in the mixture are then summed. A ∑TU > 1.0 suggests the metal mixture is
toxic (note that this is the same approach as discussed above for developing metal sediment
criteria based on pore water concentrations). According to Norwood et al. (2003), the TU
approach is presently the most appropriate model for predicting effects of metal mixtures based
on the currently available toxicity data. However, it should also be emphasized that the TU
approach is most appropriate at a screening level, because the true toxicity of the mixture is
dependent on the relative amounts of each metal. The TU approach is also recommended with
mixtures containing less than six metals.
Lead and other metals often co-occur in sediments with other toxicants, such as organic
contaminants. Effects-based sediment quality guidelines (SQGs) have been developed over the
past 20 years to aid in the interpretation of the relationships between complex chemical
contamination and adverse biological effects (Long et al., 2006). Mean sediment quality
guideline quotients (mSQGQs) can be calculated by dividing the concentrations of chemicals in
sediments by their respective SQGs and then calculating the mean of the quotients for the
individual chemicals. Long et al. (2006) performed a critical review of this approach and found
that it reasonably predicts the incidence and magnitude of toxicity in laboratory tests and the
incidence of impairment to benthic communities increases incrementally with increasing
mSQGQs. However, the authors pointed out some of the limitations of this approach, such as a
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