Page 665 - The Toxicology of Fishes

14

Exposure Assessment and Modeling

in the Aquatic Environment

Donald Mackay and Lynne Milford

CONTENTS
Introduction............................................................................................................................................645
A Historical Note...................................................................................................................................646
Fundamental Concepts...........................................................................................................................647
Illustration of a Mass Balance Model in a Sediment–Water System...................................................650
Anthracene....................................................................................................................................652
Pyrene...........................................................................................................................................653
Bioaccumulation ....................................................................................................................................654
Discussion and Conclusions..................................................................................................................656
References..............................................................................................................................................657

Introduction
In aquatic and marine systems, the ultimate toxic effects exerted by chemical substances or toxicants
on fish and other organisms are determined by a combination of chemical concentration, exposure time,
and conditions. These stresses are, in turn, determined by a sequence of events starting with the loading
or discharge of the toxicant to the system which can be expressed in kg or kg/year and may originate
from point sources such as industrial or municipal effluents, non-point sources such as atmospheric
deposition or terrestrial runoff, and advective inflow from rivers or tributaries. Also, in-place toxicants
originating from previous discharges may be found in sediments. The total quantity of introduced toxicant
is then subject to fate processes, including transformations and partitioning between all the phases
present, resulting in concentrations being established that dictate doses and thus cause effects. In this
chapter, we focus on techniques for estimating the masses, concentrations, and fate of chemicals in
aquatic systems. In principle, if an undesirable effect occurs as a result of a high concentration, the
obvious approach is to reduce inputs of the substance with the expectation that concentrations will fall.
The problem is often to determine the magnitude of the desired reduction and how long it will take for
improvements to be evident.
In Chapter 2 of this book, Ankley et al. emphasized not only the need for information on the total
quantity of toxicant present in an aquatic system but also the importance of quantifying where it will
be found (i.e., the extent to which it partitions between the water column, suspended solids, sediment,
and possibly to dissolved or colloidal organic and inorganic material). The toxicant may also speciate
chemically into ionic and non-ionic forms and generate other chemical complexes. The net result is that,
whereas it is obviously essential to ascertain the total concentration of toxicant in the environment, it is
also necessary to determine how the toxicant is distributed among a variety of states with differing
bioavailabilities (van Brummelen et. al., 1998). This information helps to determine the rates of uptake

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