Appendix B: Food Chain and Toxicity Models

Appendix B

Food Chain and Toxicity Models

This appendix provides brief descriptions of several food chain and toxicity

models for use in ecological risk assessment. These include:

a. Gobas Food Chain Model

b. Great Lakes Methodology for Predicting Fish Tissue Concentrations from

Water Concentrations

c. Sum polycyclic aromatic hydrocarbon (PAH) Model

d. Narcosis Model

Gobas Food Chain Model

The Gobas model estimates the bioaccumulation and biomagnification of

organic contaminants (except PAHs) in aquatic food web due to surface water,

sediment, and food web exposure. The model is most useful for predicting the

concentrations of organic compounds that are not readily metabolized. It uses

compound-specific information, including the octanol-water partition coefficient,

Kow, and Henry's Law Constant to predict the disposition of contaminants in an

aquatic food web.

A few compounds, for which Henry's Law Constants are not available, cannot

be modeled with the Gobas model (PCB-183, -184, and -185, o,p-DDD, o,p-

DDE, and trans-nonachlor).

The Gobas model consists of five major compartments: phytoplankton,

zooplankton, benthic invertebrates, forage fish, and piscivorous fish.

Concentrations of chemicals of concern (COCs) in the phytoplankton,

zooplankton, and benthic invertebrates (polychaetes and mysid shrimp) are

estimated using equilibrium partitioning from water and sediment to biota

(BSAF=1). Concentrations in forage fish and piscivorous fish are estimated using

mass-transfer coefficients that describe uptake of chemical from water and

ingestion of organisms from lower trophic levels, elimination of contaminant by

excretion, and dilution by growth.

Appendix B Food Chain and Toxicity Models