United States
Environmental Protection
Agency
Environmental Research
Laboratory
Duluth MN 55804
Research and Development
EPA-600/S3-84-007 Feb 1984
4>EPA Project Summary
Generalization of Water Quality
Criteria Using Chemical Models:
Development of the REDEQL-
UMD System of Computer
Programsfor Aqueous Equilibria
Donald K. Harriss, Sara E. Ingle, David K. Taylor, and Vincent R. Magnuson
The primary objective of this research
was to develop site-specific water
quality criteria for metals through
correlating aquatic toxicity with activi-
ties of metal species. The intent of the
project was not to conduct experiment-
al studies but use the existing literature
on metal toxicity to generate the
correlations. Because knowledge of the
number of metal species present and
the relative amounts of each was
necessary to the research, a set of
computer programs for finding equili-
brium concentrations of metal com-
plexes, REDEQL-UMD, was developed,
along with a stability constant data base
and associated reference-footnote data
bases. Literature values of stability
constants were adjusted to 25°C by
application of thermodynamic principles.
Once speciation and toxicity data were
available, a combination of factor
analysis and multiple regression analy-
sis was used to develop correlations. A
detailed example involving toxicity of
copper to Daphnia magna is included in
the full report.
This Project Summary was developed
by EPA's Environmental Research Labo-
ratory, Duluth, MN, to announce key
findings of the research project that is
fully documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
Correlations between aquatic toxicity
and activities of metal species are
important in determining water standards,
because although legal limits for pollutants
are most often stated as allowable total
concentrations, biological activity com-
monly correlates best with specific forms
or species of contaminants. This project
makes use of existing literature reports of
metal toxicity to generate the correlations.
The three basic areas of activity were:
developing computer programs that
predict speciation in a wide variety of
aqueous environments; assembling a
data base of appropriate stability constants;
and designing predictive modeling proce-
dures to determine the desired correlations
between aquatic toxicity and metal
species concentrations.
REDEQL-UMD, the computer program
used to determine the equilibrium
concentrations of species of metals, was
modeled after REDEQL2, a program
developed at the California Institute of
Technology (McDuff, er a/., 1975).
Parameter storage was placed in linear
rather than rectangular arrays in order to
reduce storage requirements This allowed
treatment of complexes with mixed
ligands or metals and more than one
precipitated form of a metal-ligand pair to
be present at one time. The program was
also modified to allow the redox state of
the system to be specified by providing
values of pE, Eh or concentration of
dissolved oxygen Two options have been
given for correction for ionic strength
the Davies equation may be used for
ionic strengths up to 0.1 molal. A new
equation, the Magnuson equation, was
developed as part of this studytoallow for
correcting of ionic strengths up to 0 5
molal (Sun, et a/., 1980). Provision was
made to determine total carbonate from
alkalinity or total inorganic carbon
Finally, concentrations may be input m
either mg or moles per liter.
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Formation constants were stored in a
data file made up from 43 metals and 65
ligands. Constants stored were for 25°C,
but could be adjusted to temperatures
other than 25°C with the use of a special
program containing enthalphy data. All
bibliographic data and conditions under
which the thermodynamic data were
measured have also been stored in a
separate program and are available in
REDEQL-UMD.
The application of data obtained from
REDEQL-UMD m toxicity studies was
demonstrated in a study of the toxicity of
copper to Daphnia magna (Andrew, et a/.,
1977) Nineteen different combinations
of concentrations of components, yielding
19 sets of species concentrations and 19
measures of toxicity, were applied.
Carbonate, phosphate and pyrophosphate,
as well as copper, were varied. The
system was found to contain 25 complexes
of copper with these three ligands and
hydroxide. Since the number of depend-
ent variables exceeded the number of
independent variables (LCso values),
factor analysis was used to reduce the
number of dependent variables, and the
problem was solved by the application of
multiple regression analysis (Magnuson,
et a/., 1979). Application of factor analysis
is given in detail in an appendix to the full
report.
Two manuals for the use of the
REDEQL-UMD program are available
separately, a user's manual and a
programmer's manual (Harriss, era/, in
press). The programmer's manual would
be necessary only for those using the
program with a different computing
system or wishing to modify the program.
Conclusions
A practical procedure for the correlation
of aquatic toxicity with chemical species
was developed and demonstrated The
REDEQL-UMD program is a versatile one
with features not available in other
speciation programs It was also prepared
in a novice-level interactive form. Semi-
empirical equations were derived that
allow ionic strength corrections to be
extended to higher concentration ranges.
Stability constant and enthalpy data
bases were obtained from the literature.
Reference and footnote data accompanies
each of these.
Recommendations
Correlation of aquatic toxicity with
metal speciation requires the use of
formation constants of both simple and
mixed complexes formed in the system.
Carefully designed experiments are
needed to establish these correlations
Temperature corrections should be used
to adjust formation constants to the
temperature at which toxicity is being
determined. The Magnuson equation for
correcting for ionic strength should be
used in calculations to determine equili-
brium concentrations of the various
metal species. Factor analysis followed
by multiple regression is recommended
for determining toxicity in complex systems.
References
Andrew, R.W., K.E. Biesinger and G.E.
Glass, 1977. Effects of inorganic
complexing on the toxicity of copper
Daphnia magna. Water Res. 11, 30!
315.
Harriss, O.K., S.E. Ingle, V.R. Magnusc
and O.K. Taylor. Programmer's manu
for REDEQL-UMD.
Harriss, O.K., S.E. Ingle, O.K. Taylor ar
V.R. Magnuson. A user's manual fi
the aqueous chemical equilibriui
modeling program REDEQL-UMD.
Magnuson, V.R., O.K. Harriss, M.S. Su
O.K. Taylor and G.E. Glass, 197!
Relationships of activities of meta
ligand species to aquatic toxicity. li
Chemical modeling in aqueous system
ACS Symposium Series #93, edited t
E.A. Jenne, American Chemical Societ
Washington, D.C. pp. 635-656.
McDuff, R.E. and F. Morel, 1975. Descri)
tion and use of the chemical equilibr
um program REDEQL2. W.M. Kec
Laboratory, California Institute c
Technology, Pasadena, CA, Tech. Re
EQ-73-02.
Sun, M.S., O.K. HarrissandV.R. Magnuso
1980. Activity corrections for ion
equilibria in aqueous solutions. Can.,
Chem. 58, 1253-1257.
Donald K Harriss, Sara E. Ingle, David K. Taylor, and Vincent R. Magnuson are
with the University of Minnesota, Duluth, MN 55812.
Gary E. Glass is the EPA Project Officer (see below).
The complete report consists of two parts, entitled "Generalization of Water
Quality Criteria Using Chemical Models—Parts I and II"
"Part I," (Order No. PB 84-135 508; Cost: $ 17.50)
"Part II," {Order No. PB 84-135 516; Cost. $20.50)
The above reports are available only from: (cost subject to change)
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
6201 Congdon Blvd.
Duluth, MN 55804
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