Figure 5. Schematic of TBP process

ERDC TN-DOER-R2

December 2001

The assumptions of the TBP model derive from thermody-

namics. The system, consisting of sediment, organism, and

water, is modeled as being closed. A neutral organic

chemical in the system is given free movement and will

distribute throughout the phases until equilibrium is estab-

lished. The concentrations at equilibrium are determined

by the chemical potentials in each phase. Organic carbon

in the sediment and lipids in the organism are assumed to

be the primary compartments that account for partitioning

of neutral chemicals. Thus, the expected equilibrium con-

centration in an exposed organism of a given lipid content

is a function of the concentration of a chemical in the

sediment (normalized on the basis of its organic carbon

content) and a partitioning coefficient between the sedi-

ment and the lipids (McFarland 1984; McFarland and Figure 5. Schematic of TBP process

Clarke 1987). The model equation is

TBP = BSAF (Cs / foc ) fL

(1)

where the partitioning coefficient is the BSAF and

Cs = concentration of neutral organic chemical in sediment

foc = decimal fraction total organic carbon content of the sediment

fL = decimal fraction lipid content of the target organism

TBP was incorporated into the RECOVERY model to assess the effect of contaminated sediments

on biota (Ruiz and Gerald 2001). The model uses the organic carbon foc of the sediments and the

estimated contaminant sediment concentration Cs to estimate the body burden of biota exposed to

contaminated sediments over a long period of time (years or decades). The user is encouraged to

use site-specific data to estimate BSAFs and biota lipid content. If site-specific data are not

available, the BSAF database has lipid and BSAFs for a number of contaminants.

BSAF. The BSAF database was constructed from numerous field and laboratory observations.

Empirically derived BSAFs were calculated as

BSAF = (Ct / fL ) / (Cs / foc )

(2)

where Ct / fL is the lipid-normalized contaminant concentration in the tissues of the exposed

organism and Cs / foc is the organic carbon-normalized contaminant concentration in the sediment

to which the organism has been exposed. The database contains BSAFs for contaminants of concern

and lipid fractions for a number of organisms.

DREDGE. DREDGE uses empirical and analytical models to estimate the resuspension and

transport of sediments and associated contaminants during dredging operations. DREDGE com-

bines empirical sediment resuspension (near-field) models and simple suspended sediment transport