Page 92 - Partition & Adsorption of Organic Contaminants in Environmental Systems

BIOCONCENTRATION OF ORGANIC CONTAMINANTS 83

inability of the system to reach true equilibrium state. Similarly, the BCF
values obtained with short exposure times before steady-state concentrations
in both biotic and water phases are reached could differ significantly from
equilibrium values. For compounds with very large K ow or very small S w (e.g.,
DDT and some PCBs), the times for establishment of equilibrium would be
very extended because of their large BCF values with fish, which require a
much greater amount of water solution to be transported through fish gills
than for compounds with a considerably smaller K ow or larger S w. The more
limited diffusion rates for larger molecules might also prolong the time for
equilibrium. Whereas experimental BCF values for polar solutes are scarce,
the (BCF) lipid of these compounds would probably be significantly higher than
their respective K tw (or K ow) values because of their additional partition or spe-
cific interactions with polar biological components (e.g., protein). As shown
later, certain dissolved macromolecular materials (e.g., humic substances)
present at low concentrations in natural water can significantly enhance the
water solubility of some extremely insoluble compounds (e.g., DDT and some
PCBs), thus decreasing their apparent BCF values.
The field BCF data are expectedly more complicated because the contami-
nant concentration may vary significantly with time and with location and
because many biotic species (e.g., fish) are not confined to a fixed local envi-
ronment. Contaminant concentrations and apparent BCFs determined with
fish and water samples collected at a given time from a specific site are
therefore the integrated results of these variables. As such, there would be
large uncertainties concerning the achievement of equilibrium of contami-
nants between fish and water in natural systems. Pereira et al. (1988) studied
field (BCF) lipid data for a number of chlorinated compounds on four fish
species (Atlantic croaker, blue crab, spotted sea trout, and blue catfish)
sampled from selected sites of the Calcasieu River estuary in Louisiana.
Swackhammer and Hites (1988) conducted a similar field BCF investigation
for chlorinated compounds on lake trout and white fish from the Siskiwit Lake,
Isle Royale, Lake Superior. The (BCF) lipid data of Pereira et al. are given in
Table 5.7 and a corresponding plot of log(BCF) lipid versus logK tw is presented
in Figure 5.8.
As noted, although the field (BCF) lipid values exhibit significant scattering
between fish species in response to the dynamic nature of the ecosystem and
other variables, most data points are within one order of magnitude of the
equilibrium correlation line, (BCF) lipid = K tw . The data scattering is virtually
random, showing no obvious pattern with a specific fish species. The overall
trend of field-based (BCF) lipid is surprisingly consistent with that found in well-
controlled laboratory studies. Although true equilibrium is probably rarely
achieved between water and fish in a dynamic estuarine system, the results
are clearly supportive of the lipid model for bioconcentration of relatively
nonpolar organic compounds.
From the information presented, a good approximation for the BCF of a
relatively water-insoluble compound at equilibrium with a biotic species can