Environmental Engineering Reference
In-Depth Information
tool. Some suggest that EqP-derived predictions combined with aquatic
toxicity databases can be used as stand-alone pass/fail predictors of sedi-
ment quality whose implementation can be modeled after EPA's Water Qual-
ity Criteria. A review of this issue is beyond the scope of this chapter. The
reader is referred to articles promoting the use of EqP model predictions in
sediment management decisions (DiToro et al., 1991; Ankley et al., 1996;
Burkhard, 2000) and those that argue for more limited use of the models in
sediment management decisions (Iannuzzi et al., 1995; Driscoll and Lan-
drum, 1997; O'Connor et al., 1998; van Beelen et al., 2001; Condor et al.,
2002). Instead, we will focus the remainder of this discussion on technical
issues relevant to two factors that can have major effects on the dosage of
recalcitrant compounds realized by benthic macrofauna that are not
addressed in K oc -based TBP or EqP models: recalcitrant compounds' seques-
tration in sediment and microbial degradation of recalcitrant compounds in
sediment.
2.4 Recalcitrant compounds in sediments
Luthy et al. (1997) characterized matter in soils and sediments as geosor-
bents. Soils and sediments are heterogeneous at the scale of samples, aggre-
gates, and particles. Structurally or chemically different constituents of sed-
iments interact differently with recalcitrant compounds in terms of binding
energies and associated rates of sorption and desorption. Complex assem-
blages of the components can cause complex mass transfer phenomena. The
term sequestration refers to some combination of diffusion limitation, adsorp-
tion, and partitioning. Sorption and desorption rates for recalcitrant com-
pounds in geosorbents occur on timescales ranging from fast (e.g., minutes
to days) to slow (e.g., weeks to years). Although their relative proportions
vary greatly, most recalcitrant compounds' contaminated sediments to date
have both rapidly and slowly desorbing recalcitrant compound fractions.
Desorption rate differences are thought to be due to processes such as
intra-aggregate diffusion, releases from micropores, or different forms of
geosorbent organic matter.
Two of these proposed geosorbant domains, soft amorphous organic
matter and soot, have been shown to be particularly important when
attempting to predict recalcitrant compounds' equilibrium partitioning and,
ultimately, exposure and toxicity. Decaying plant material (case A in Figure
2.2) is a major source of sediment organic matter and a major food source
for detritivores. This low-density fraction of sediment organic matter from
a New York-New Jersey estuary contained 10 times the level of PAH pre-
dicted by organic carbon normalized equilibrium partition coefficients (K oc )
(Rockne et al., 2002). This fraction readily released PAH into the aqueous
phase and was the controlling factor in whole-sediment PAH release. Drift-
ing plant detritus has also been shown to be a major contributor to the total
annual load of organochlorine contaminants (including polychlorinated
biphenyl (PCB)) in the Detroit River (Lovett-Doust et al., 2002). These recent
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