Benthic impact-Theoretical bioaccumulation potential for nonpolar organic compounds (Cont.)

CS = the concentration of contaminant in sediment, mg/kg dry weight

fOC = the fraction organic carbon in sediment, kg organic carbon/kg

dry weight

McFarland (1984) calculated an average equilibrium BSAF of 1.7 for a suite

of compounds, indicating a slight enrichment of chemical in the lipid phase.

Rubinstein et al. (1987) showed that a value of 4.0 is appropriate for calculating

the bioaccumulation potential of field-collected samples. The ITM and OTM

(USEPA/USACE 1991, 1998) calculate the TBP, the concentration of

contaminant in biota in mg/kg wet weight, as:



C

 fL

TBP = 4  s



f

(2)



 oc

The TBP calculation is used in Tier II as a coarse screen to predict the

magnitude of bioaccumulation likely to be associated with nonpolar organic

contaminants in the dredged material. As a follow-up to the conservative TBP

screening approach, sediment bioaccumulation tests are used in Tier III to

measure actual accumulation of contaminants from samples of dredged material

(USEPA/USACE 1991). USACE developed a database from these sediment

bioaccumulation tests and other sources that demonstrates the variability in

reported BSAFs. The BSAF database is available through the Dredging

Operations Technical Support (DOTS) Web page of the USACE Waterways

Experiment Station (http://www.wes.army.mil/el/dots/).

The following uncertainties are associated with BSAFs used in the TBP

calculation:

a. Measured BSAFs are often lower than the equilibrium-based estimates.

Several factors can cause deviations from equilibrium conditions. First,

sequestration of polycyclic aromatic hydrocarbons (PAHs) into soot

particles may reduce partitioning into pore water and bioavailability to

benthic invertebrates. Second, metabolism of organic compounds by the

organisms usually increases the rate of elimination and decreases steady-

state body burden of the compound. Third, in some cases, sufficient time

may not have elapsed for exposed organisms to reach equilibrium with

the sediments.

b. Variability across species, or true population heterogeneity, for reported

values in the DOTS database can be significant. For example, BSAFs

reported for chrysene in the DOTS database range from 0.02 to 0.61.

Other recent work reports a median BSAF for all PAHs of 0.032

(USEPA 1998b). Uncertainty is introduced by using a single calculated

BSAF across a range of species and contaminants in estimating TBP.

Limited validation of the TBP model can be accomplished by comparing the

results of Equation 2 with Tier III bioaccumulation tests (corrected for steady-

state conditions, if necessary). Uncertainty in the factor of 1.7 can be evaluated

Chapter 4 Uncertainty in Tiered Evaluation of Dredged Material