United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30613
Research and Development
EPA-600/S3-84-033 Apr. 1984
Project Summary
Modeling the Transport,
Speciation, and Fate of Heavy
Metals in Aquatic Systems
A.R. Felmy, S.M. Brown, Y. Onishi, S.B. Yabusaki,
R.S. Argo, D.C. Girvin, and E.A. Jenne
Concern about environmental ex-
posure to pollutants has increased the
need for techniques to predict the
behavior of heavy metals entering
natural waters as a result of the mining
of raw materials and the manufacture,
use, and disposal of commercial pro-
ducts. The modeling technique devel-
oped in this research study permits the
user to examine the equilibrium specia-
tion of heavy metals along with trans-
port and fate in various aquatic systems.
Because different species of a metal
cause different biological effects, this
modeling technique should help users
better relate metals discharge and
aquatic chemistry data to observed or
expected effects.
MEXAMS, the Metals Exposure
Analysis Modeling System, allows the
user to consider the complex chemistry
affecting the behavior of metals in con-
junction with the transport processes
that affect their migration and fate. This
is accomplished by linking MINTEQ, a
geochemical model, with EXAMS, an
aquatic exposure assessment model.
MINTEQ is a thermodynamic equi-
librium model that computes aqueous
speciation, adsorption and precipita-
tion/dissolution of solid phases. It has
a well-documented thermodynamic
data base that contains equilibrium con-
stants and other accessary data for
seven priority pollutant metals: arsenic
(As), cadmium (Cd), copper (Cu), lead
(Pb), nickel (Ni), silver (Ag) and zinc (Zn).
Six different adsorption algorithms are
included: 1) an "activity" partition coef-
ficient, Kd; 2) an "activity" Langmuir
equation; 3) an "activity" Freundlich
equation; 4) an ion exchange algorithm;
5) a constant capacitance surface com-
plexation model; and 6) the triple layer
surface complexation model. In addi-
tion, a large number of user-oriented
features such as the ability to handle
alkalinity inputs, an initial mass of solid,
and different analytical input units were
incorporated.
EXAMS is designed for the rapid
evaluation of synthetic organic
pollutants. Given the characteristics of
a pollutant and an aquatic system,
EXAMS computes steady-state distribu-
tion of pollutant concentrations, the fate
of the pollutant in the system, and the
time required for effective purification
of the system (persistence).
To facilitate the use of MEXAMS, a
user interactive program was devel-
oped. This program queries the user to
obtain water quality data for MINTEQ,
then controls the operation of MINTEQ
and EXAMS, passing simulation results
back-and-forth between the models.
As it is currently structured,
MEXAMS can be used in a number of
ways. It can be used like EXAMS to per-
form rapid hazard evaluations for prior-
ity pollutant metals. MEXAMS also can
be used to evaluate the water quality im-
pact of point source discharges and
mine drainage as well as to support the
interpretation of metals bioassay data.
Finally, and perhaps most importantly,
MEXAMS can be used as a framework
for defining what is and what is not
known about the behavior of priority
pollutant metals in aquatic systems.
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This framework will make it possible to
identify the need for and guide the per-
formance of future research.
This Project Summary was developed
by EPA's Environmental Research Labo-
ratory, Athens, GA, to announce key
findings of the research project that is
fully documented in two separate re-
ports (see Project Report ordering infor-
mation at back).
Introduction
One class of pollutants that has received
considerable environmental attention is the
priority pollutant metals. A reason for this
attention is the fact that the current water
quality criteria are based on "total
recoverable" rather than "dissolved" con-
centrations. Historically, only total concen-
trations were reported in the published
results of aquatic bioassays for metals, even
though it was generally known and accepted
that the dissolved fraction is the most
bioavailable and toxic, and that certain
dissolved species are much more toxic than
others. Virtually all modeling studies directed
at examining the migration and fate of metals
have neglected many of the more important
chemical interactions controlling their
behavior in aquatic systems. The Metals Ex-
posure Analysis Modeling System
(MEXAMS) represents an improvement in
metals modeling in that the complex
chemistry affecting the behavior of a metal
and the transport processes affecting its
migration and fate are handled by two
separate, but linked, models. The chemical
interactions are handled by MINTEQ, a
geochemical model that uses fundamental
thermodynamic equilibrium relationships and
data to calculate dissolved, adsorbed, and
precipitated metal concentrations. The mi-
gration and fate of a metal is handled by the
Exposure Analysis Modeling System
(EXAMS), a steady-state transport model
developed primarily as a screening-level tool
by the Environmental Research Laboratory,
USEPA, Athens, Georgia.
User instructions for operating the com-
plete metals exposure model are provided in
MEXAMS—The Metals Exposure Analysis
Modeling System. The mathematical and
chemical concepts of the MINTEQ compo-
nent are described in MINTEQ—A Computer
Program for Calculating Aqueous Geochem-
ical Equilibria. Although it is not necessary
to master these concepts in order to use
MEXAMS, it is important that the user be
familiar with the basic theory behind
MINTEQ. A basic understanding of the
MINTEQ program will allow experienced
users to solve a broad range of chemical
equilibrium problems. Exposure Analysis
Modeling System (EXAMS): User Manual
and System Documentation, EPA-600/3-82-
023, was published in 1982.
Conclusions
Using MEXAMS, much of the complex
chemistry affecting the behavior of heavy
metals in aquatic systems can be explicitly
considered, including chemical speciation
and its effect on the adsorption and
precipitation of metals. MEXAMS, therefore,
should provide more accurate predictions of
the metal concentrations likely to be found
in different aquatic systems. It should also
overcome some of the limitations inherent
in earlier attempts to model the behavior of
metals.
MEXAMS is applicable to a range of prob-
lems associated with the impacts of priority
pollutant metals on aquatic systems. It can
be used to perform both screening-level and
site-specific analyses of metals problems
such as industrial discharges and mine
drainage. It also can be used as a framework
for guiding the collection and interpretation
of aquatic bioassay data.
There are several limitations that the user
must be aware of before applying MEXAMS.
First, the thermodynamic data base asso-
ciated with MINTEQ only contains equi-
librium constants and accessary data for
seven priority pollutant metals: As, Cd, Cu,
Pb, Ni, Ag and Zn. Some data on the other
metals exist in the literature. Before they can
be included in the data base, however, the
data should be carefully evaluated.
The second limitation relates to organic
complexation. In many natural waters this
phenomenon can have a major impact on the
speciation of metals. While MINTEQ is com-
putationally capable of considering organic
complexation, the thermodynamic data base
does not contain the necessary equilibrium
constants and accessary data. Again, the
literature does contain some thermodynamic
data on organic complexation of selected
metals. These data need to be reviewed and
evaluated before they are included.
Another limitation of MINTEQ and most
other geochemical models is that it treats
precipitation/dissolution, oxidation/reduc-
tion and adsorption as equilibrium processes
when in fact they may not be in equilibrium.
In the area of precipitation/dissolution,
literature data on the rates of formation and
dissolution of selected solids need to be in-
cluded in the data base and supplemented
with experimental work and a kinetic
algorithm. The kinetics of oxidation/reduc-
tion reactions are not well understood.
Because these reactions are frequently
biologically mediated and rarely in equi-
librium, the equilibrium approach can only
provide boundary conditions towards which 4
a system is proceeding. In addition, the im- "
portance of the kinetics of adsorption is
unclear for metals. Most constituents tend
to adsorb quite rapidly (i.e., within hours),
but desorb less rapidly. The limited data
available on the kinetics of adsorption for
selected metals need to be included in the
data base and supplemented with experi-
mental work.
A final limitation is the degree of testing
MEXAMS has received. Although both
MINTEQ and EXAMS have been tested on
and applied to a number of problems, the
linked system of models has received limited
testing. For this reason, users should exer-
cise extreme care in the early stages of ap-
plying MEXAMS. MEXAMS is being more
rigorously tested on a series of hypothetical
and site-specific problems.
Description of MEXAMS
MEXAMS consists of three components:
1) a geochemical model, 2) an aquatic ex-
posure assessment model, and 3) a user in-
teractive program. The geochemical model
simulates the complex chemical interactions
that affect metal behavior in natural waters.
The exposure assessment model simulates
the transport processes affecting metal
migration and fate in aquatic systems. The 4
user interactive program links the two "
models and aids in the application of the
overall system. Figure 1 shows how these
three components are linked, and each com-
ponent is discussed in greater detail below.
MINTEQ, the geochemical model in
MEXAMS, is a thermodynamic equilibrium
model that computes aqueous speciation,
adsorption and precipitation/dissolution of
solids. Speciation is calculated using an
equilibrium constant approach wherein a
series of mass action expressions are solved
subject to mass balance constraints on each
chemical component.
In MINTEQ, adsorption is treated as
analogous to aqueous speciation. As a
result, mass action expressions can be for-
mulated for adsorption reactions. MINTEQ
contains six algorithms for calculating ad-
sorption. The first is a single valued partition-
ing coefficient or Kd that has been corrected
for the activity of the metal species binding
to the surface. The corrected value is called
an "activity Kd." The second algorithm is an
"activity" Langmuir isotherm. The third
algorithm is an "activity" corrected
Freundlich isotherm. The fourth algorithm is
for simple ion exchange reactions where the
activity ratio of the exchanging species is
assumed to remain constant. The constant
capacitance model and triple layer model are M
the other two options. They are more theo- ™
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User Interactive
Program
(MISP)
Water
Chemistry
Geochemical
Model
(MINTEQ)
Metal
Concentrations
Metal
Concentrations
Exposure
Assessment
Model
(EXAMS)
Metal Speciation
Figure 1. Structure of MEXAMS.
retically based approaches that consider the
electrostatic potential at the surface of the
sorbing media and the effect of pH and ionic
strength changes on surface properties.
MINTEQ can compute the mass of metal
transferred into or out of solution as a result
of the dissolution or precipitation of solid
phases. While this calculation is limited by
the fact that it is made for equilibrium con-
ditions and precipitation/dissolution reac-
tions may be kinetically controlled, it is
possible to obtain reasonable results if the
solids considered by MINTEQ as possible
equilibrium phases are properly selected.
That is, the user must permit MINTEQ to
consider only those solids whose formation
is not limited by kinetic barriers.
As with any geochemical model, MINTEQ
requires both thermodynamic and water
quality data. The thermodynamic data are
equilibrium constants, enthalpies of reaction,
and other basic information required to
predict the formation of each species or solid
phase. The water quality data are the
physical and chemical properties of the water
body being analyzed. The user only has to
generate the water quality data in order to
use MINTEQ. The thermodynamic data are
contained in a data base that accompanies
the model.
EXAMS is the aquatic exposure assess-
ment model in MEXAMS. It is a steady-state
model for screening-level exposure assess-
ments that is applicable to rivers and lakes.
The model was developed primarily for use
Metal Migration
and Fate
with organic compounds, and it provides
estimates of persistence and ambient water
quality.
The processes considered by EXAMS can
be divided into four categories: 1) ionization
and sorption, 2) transformation, 3) transport,
and 4) chemical loading. For ionization and
sorption, EXAMS can consider up to 15
molecular species of a given pollutant. These
include the uncharged parent molecule and
its singly- and doubly-charged cations and
anions. Each of these can occur in a dis-
solved, sediment-sorbed or biosorbed form.
Equilibrium sorption is calculated using
equilibrium distribution coefficients. The sec-
ond category, transformation processes, in-
cludes photolysis, hydrolysis, biolysis and
oxidation. Rates of transformation for each
process can be assigned to each of the 15
molecular species. The third category, trans-
port processes, includes volatilization and the
movement of dissolved, sediment-sorbed,
and biosorbed fractions. Since EXAMS does
not explicitly compute water and sediment
movement, these data must be obtained
from field measurements or other models.
Volatilization is calculated using the two-
resistance or "two-film" model. The final
category, chemical loadings, includes exter-
nal pollutant loadings from point sources,
non-point sources, dry fallout or aerial drift,
atmospheric washout and groundwater
seepage. The user's manual and system
documentation report for EXAMS provides
extended discussions of how each of the
above processes are modeled.
The coupling of EXAMS with MINTEQ re-
quired several mod ' cations to the code and
the way it is used. Code modifications were
designed so that all of the original EXAMS
options and capabilities were retained and
no additional input data would be required.
MISP, the MEXAMS Interactive Software
Program and the third component in
MEXAMS, has several important functions.
First, it helps the user input data to MINTEQ.
(Input data for EXAMS must be prepared
separately using the procedure outlined in
the EXAMS user's manual.) Second, MISP
queries the user to determine simulation and
output options. Finally and most importantly,
MISP links MINTEQ with EXAMS and con-
trols the operation of each model.
Operation of MEXAMS
MEXAMS can be operated in three
modes: 1) the MINTEQ-only mode, 2) the
EXAMS-only mode, and 3) the coupled
MINTEQ and EXAMS mode. The MINTEQ-
only mode allows the user to analyze how
changes in water chemistry will affect the
behavior of a metal without regard for the
effect of transport processes. The EXAMS-
only mode functions exactly like the original
EXAMS model. The coupled MINTEQ and
EXAMS mode allows the user to consider
also the effect of transport processes and
chemical interactions. MINTEQ offers op-
tions that provide flexibility in the way the
user defines the chemistry of the system be-
ing modeled and make it possible for the user
to apply MINTEQ to a large and diverse pro-
blem set. Thus, while not all of the options
available require the use of MINTEQ to
evaluate the behavior of metals, it is impor-
tant that the user be aware of these options
when preparing input data files.
The chemical species in MINTEQ are
assigned one of six different species type
designations:
• Type I — Components: the chemical
species chosen to represent each
chemical constituent in the water
analysis (e.g., Zn!+ is the component
for zinc). A complete list of MINTEQ
components is given in Table 1.
• Type II — Complexes: all aqueous
species that are combinations of two or
more components. Examples of com-
plexes are shown in Table 2.
• Type III — Fixed Species: any species
with a fixed activity, such as solids
infinite supply or gases at a fixed par-
tial pressure.
• Type IV — Precipitated Solids: solids
that have a finite mass and can dissolve
completely.
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Table 1. Components in MINTEQ
Component
E
H20
Ag"
A^
H^sO,
HsASO,
H3B03
Ba^
Br
co}-
Fulvate
Humate
Ca2*
Crf2*
67-
Cs*
CV*
Cu2*
fe2*
Fe3*
«*
/-
/C*
Li"
Mg*
Mn**
Mn*
I.D.
Number
001
002
020
030
060
061
090
100
130
140
141
142
150
160
180
220
230
231
280
281
330
380
410
440
460
470
471
Component
NH;
N02
NO;
/Va*
/V/2*
POf
Pb2*
/?/>*
HS-
S
S02i
HtSiO,
s/-2*
U3t
U"
uoi
UO?
Zn2t
SOH1
SOH2
XPSIO
XPSIB
XPSID
SOHB
I.D.
Number
490
491
492
500
540
580
600
680
730
731
732
770
800
890
891
892
893
950
990
991
992
993
994
995
Table 2. Aqueous Complexes
Complex ID
PbCOJAQ)
PbCr
CdHCOl
MSOJAQ)
6001401
6002800
1601400
5407320
• Type V - Dissolved Solids: solid
phases that can precipitate if they
become oversaturated.
• Type VI — Species Not Considered:
species not included in equilibrium com-
putations but checked afterwards.
In summary, MINTEQ is structured to
allow the user the flexibility of using as many
data as are available. If the system being
studied has a relatively constant pH and ionic
strength, the metal concentrations are
relatively low, and only limited adsorption
data for that solid or sediment are available,
the "activity" Kd provides an adequate ap-
proach to model metal adsorption. If,
however, metal concentrations can be
relatively high and variable due to changes
in metal loading, then one of the "activity"
isotherms, either Freundlich or Langmuir,
should be used. If the solution pH and ionic
strength vary significantly, data for the con-
stant capacitance or triple layer models
should be obtained.
Data Requirements
Interpretation of the results predicted by
MINTEQ becomes more reliable as the user's
knowledge of the system increases. In the
case of input data, the more data the user
has on the water chemistry of the system,
the more accurate will be the predicted
results. This does not mean that the user
must have data for all of the components
listed in Table 2. Many components do not
react with other components or are present
in such low concentrations that they do not
alter the geochemistry of the particular com-
ponents being studied. Important chemistry
data to consider include:
• pH—the most important parameter re-
quired by MINTEQ.
• Eh (pE)—an important parameter for
elements with oxidation states linked by
redox reactions (e.g., Fe, Mn, Cu, As).
Seldom measured, Eh must usually be
estimated indirectly.
• Temperature—required, but not sen-
sitive.
• Ionic Strength —optional, correctly
computed by MINTEQ given dominant
cations and anions.
• Major Anions—most important are C0§~
or alkalinity, and SO?,'; sometimes im-
portant are Cl~ and H4Si04.
• Major Cations—most important are Ca2*
and Mg2*; sometimes important are Na+
and K*.
• Trace Constituents —hydrogen sulfide,
H2S (for trace metals); orthophos-
phorus, POf (for trace metals, Fe, Ca,
Mg); iron and manganese (under low
pH or Eh conditions); and others.
Applicability
MEXAMS was developed to provide EPA
with a predictive tool capable of performing
screening-level analyses. The user can ex-
amine a broad range of water quality condi-
tions and evaluate how a specific priority
pollutant metal will speciate, adsorb, or
precipitate. Using a range of generalized en-
vironments, the user can also rapidly evalu-
ate fate and persistence of metals,
identifying which processes are important in
different types of aquatic systems and which
types of systems are most likely to be af-
fected by metals.
MEXAMS can also be used on a more site-
specific basis to investigate the potential im-
pacts of different metal sources like industrial
discharges or mine drainage. Such applica-
tions can include the use of MINTEQ alone
or in conjunction with EXAMS.
Another application of MEXAMS relates
to improving the analysis and interpretation
of information derived from bioassays. If the
chemistry of the dilution waters is known,
MEXAMS, more specifically MINTEQ, can
be used to estimate the dissolved concen-
tration of metal present during the bioassay,
as well as the species of metal present. The
former would provide a means of adjusting
the current standards to a dissolved metal
basis. This would provide more reasonable
standards, because the dissolved fraction is
likely to be the most toxic and bioavailable.
Estimates of the concentration of aqueous
species of metal present during the bioassays
would begin to provide a foundation for set-
ting standards based on the toxic species.
A final application involves the use of
MEXAMS as a framework for identifying
what is and what is not known about the
behavior of priority pollutant metals in
aquatic systems. One of the overriding ob-
jectives in developing MEXAMS was to pro-
duce a tool that is not only applicable with
existing data sources but also helps guide the
collection of data in the future.
A. Ft. Felmy. S, M. Brown, Y. Onishi, S B. Yabusaki, R. S. Argo, D. C. Girvin, andE.
A. Jenne are with Battelle, Pacific Northwest Laboratories, Richland, WA
99352.
R. B. Ambrose is the EPA Project Officer (see below}.
This Project Summary covers two reports, entitled:
"MEXAMS—The Metals Exposure Analysis Modeling System," (Order No.
PB 84-157 155; Cost: $17 50)
"MINTEQ—A Computer Program for Calculating Aqueous Geochemical
Equilibria," (Order No. PB84-157 148; Cost: $11.50)
The above reports will be available only from: (cost subject to change)
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
College Station Road
Athens, GA 30613
4
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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