United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30613
Research and Development
EPA/600/S3-86/057 March 1987
&EPA Project Summary
GETS, A Simulation Model for
Dynamic Bioaccumulation of
Nonpolar Organics by Gill
Exchange: A User's Guide
Luis A. Suarez, M. Craig Barber, and Ray R. Lassiter
A FORTRAN program that estimates
the absorption and depuration of a
chemical across fish gills is described.
The program is based on a set of dif-
fusion and forced convection differential
equations. Gill morphometric parame-
ters are computed by the program via
its own internal database. This database
spans approximately 20 species. The
program requires that the user input 12
relatively easily obtainable parameters.
This Project Summary was developed
by EPA's Environmental Research
Laboratory, Athens, GA, to announce
key findings ot the research protect that
Is fully documented In a separate report
of the same title (see Protect Report
ordering Information at back).
Introduction
When fish are exposed to dissolved
organic chemicals, such substances are
accumulated within the fish by diffusive
transport across its gills. During acute
exposures, chemical exchange across the
gill is the fish's prevailing route of expo-
sure. During chronic exposure in the en-
vironment, exposure through contami-
nated food can become increasingly
important or even greatly exceed direct
gill uptake. Nevertheless, gill exchange
still reciprocally controls the fish's body
concentrations by determining the fish's
excretory rates of the chemical.
Ultimate levels of organic toxicants in
aquatic organisms can be explained in
large measure as thermodynamic parti-
tioning between toxicant in the aqueous
environment and hydrophobic compo-
nents of the organisms (primarily lipid).
Reports of exceptions to this simple rule
are numerous, however, particularly for
chemicals having high partition coef-
ficients. The frequency with which these
exceptions have been noted has often led
to questions of the reliability and utility of
the relationship between bioconcentra-
tion factors (BCF) and thermodynamic
partitioning. Based on our review of re-
ported results for both laboratory and
field investigations of BCF, uptake, and
depuration, we have concluded that such
results are both expected and explainable
on an essentially thermodynamic (but not
equilibrium) basis. Furthermore, we have
developed a thermodynamically based
kinetic model called GETS, which predicts
whole-body burdens and concentrations
of organic chemicals in fish.
GETS (Gill Exchange of Toxic Sub-
stances) is a FORTRAN simulation model
that predicts a fish's whole body con-
centration (i.e., ppm = M9 chemical [g live
weight fish]'1) of a nonmetabolized,
organic chemical which is exchanged
across a fish's gill by thermodynamic
concentration gradients. These concen-
tration dynamics are calculated algebrai-
cally after simulating a fish's total body
burden of the chemical, Bf = jug chemical
fish'1, and its live weight, W = g live
weight fish'1. The temporal dynamics of
these two quantities are generated by
the system of coupled differential
equations
dBf
= S*kw» (Cw - [PL*KL]-1CF) (1)
dt
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dW
— = gamma * W
dt
(2)
where S is the fish's total gill area (i.e.,
cm2), kw is the chemical mass con-
ductance through the interlamellar gill
water (i.e., cm/day), Cw is the chemical's
environmental concentration (i.e., ppm =
mg 1 ~1), PL is the fish's lipid content as a
fraction of its total live weight, KL is a
lipid to water partition coefficient (i.e.,
[mole chemical/g lipid]/[mole chemical/g
water]), Cf = Bf/W is the fish's whole
body concentration of the chemical, and
gamma is the fish's specific growth rate
(i.e., g/g/day).
Scope of Model
To use GETS, a user must specify 12
relatively straightforward input parame-
ters. These are:
1. the scientific name of the fish to
be analyzed (e.g., Salmo gairdneri)
2. the fish's family (e. g. Salmonidae)
3. the fish life form (i.e., freshwater
or marine)
4. the chemical's molecular weight
5. the chemical's log Kow (i.e. log P)
6. the fish's initial live weight (g)
7. the fish's lipid content as a propor-
tion of its total body weight (g
lipid/g live weight)
8. the fish's specific growth rate (i.e.,
g/g/day)
9. the fish's initial whole body con-
centration of the chemical (i.e., M9
(g live weight fish)'1)
10. the chemical's environmental con-
centration as ppm = mg V1
11. a kinetic adjustment factor that is
discussed below (unitless)
12. the length of the desired simulation
in days
The fish's species name, family, and
life form are used to assign the gill
morphometric parameters that the GETS's
subroutine GILRAT uses to estimate the
fish's net exchange rate (S*kw). These
assignments are made by GETS using a
model data file. This file, which is supplied
with GETS, contains the coefficients and
exponents of the allometric functions ar-
ranged by species,
S = total gill area, cm2 = s1 W82 and
RHO = # lamellae (mm gill filament)'1 =
p'Wp2
where W is the fish's live weight (g).
From this data file five geometric means
are calculated by GETS for each of the
values, s1, s2, p1, and p2. For example,
GETS calculates a geometric mean for
s1, first using all the data reported in this
file. Concurrently, GETS also calculates
geometric means for s1, using only data
records which have the same life form,
family, genus, and species of the fish
designated by the user. GETS then at-
tempts to assign s1, s2, p1, and p2 using
first the species geometric means. If,
however, the species is not represented
in MORPHO.DAT, GETS then tries to
assign the geometric means that might
have been calculated for the same genus
as the desired species. In like fashion, if
the genus is not found in the model data
file, geometric means for the fish's family
are assigned. If these assignments are
not possible, the geometric means for the
same life form (i.e., freshwater vs. marine)
as the desired fish are used.
GETS is parameterized for a particular
chemical of concern by specifying the
chemical's molecular weight and log Kow.
The chemical's molecular weight is used
to estimate its aqueous diffusivity, which
is needed to estimate the conductance,
kw. The chemical's log Kow is used in the
calculation of the fish's excretion rate, k2
= S*kw*(PL*KL)1.
The physical characteristics of the fish
that are required as input are the fish's
live weight (W), its lipid fraction (PL) and
its specific growth rate (gamma). The
specific growth rate, gamma, specified as
input should be nonnegative. Although
negative growth is exhibited by organisms
under stress in natural ecosystems, it is
not known whether fish that are losing
weight significantly alter their gill
morphometry. Therefore, although nega-
tive growth rates, per se, can be input to
GETS, the resulting simulations, which
depend on gill morphometry, may not be
meaningful.
Because the user can specify any initial.
whole body concentration, Cf, of the
chemical in the fish as well as any en-
vironmental concentration, Cw, the user
can analyze either absolute uptake (i.e.,
Cf = 0 and Cw ^ O), pure depuration (i.e.,
Cf ^ 0 and Cw = 0) or any scenario
between these two extremes.
The user-supplied adjustment factor,
adjust, is used to calibrate the mass
conductance of the chemical that is
estimated by GETS. In general, the value
specified for this parameter should be in
the range of 0.1 > adjust > 0.05 since
the estimated conductance, kw, is gen-
erally between 10 and 20 times higher
than laboratory studies would indicate.
Such overestimation, however, is
expected.
Conclusion
GETS enables internal fish concentra-
tions of an organic chemical to be related
dynamically to the chemical's environ-
mental concentration. This capability is
important when chemical exchange
between fish and their environment is
kinetically limited either by the physico-
chemical properties of the chemical (e.g.,
high log P or low water solubility) or by
allometric properties of the fish (e.g.,
dynamic surface to volume ratios) since it
is actually the chemical's concentration
within the fish at specific or nonspecific
sites of action that elicit potentially
adverse ecological or physiological
responses.
Similarly, the ability of GETS to predict
the dynamics of bioaccumulation of or-
ganic chemicals in fish offers a powerful
tool to assess potential food chain ex-
posure for terrestrial fish-eating or-
ganisms. For example, if the use of a
particular chlorinated organic pesticide
were banned within a river basin, when
might the pesticide's concentration no
longer pose a health risk to man or an
environmental threat to bald eagles or
other piscivorous birds in the region?
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The EPA authors Luis A. Suarez, M. Craig Barber, and Ray R. Lassiter are with
the Environmental Research Laboratory. Athens. GA 30613.
The complete report, entitled "GETS, A Simulation Model for Dynamic Bioac-
cumulation of Nonpolar Organics by Gill Exchange," (Order No. PB 87-132
791/AS; Cost: $13.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA authors can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
Athens. GA 30613
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-86/057
0000329 PS
U S ENVIR PROTECTION AGENCY
REGION 5 LIBRARY
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CHICAGO IL 60604
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