U.S. ENVIRONMENTAL PROTECTION AGENCY
SCIENCE ADVISORY BOARD
ENVIRONMENTAL POLLUTANT MOVEMENT AND TRANSFORMATION COMMITTEE
Advisory Papers on_ the Development and Calculation of Global
Material Balances for Selected Chemical Substances. £]_
General Background Concepts
This is the first In a series of advisory papers on the problems of
developing global material balances for selected chemical substances
In all parts of the environment and the methodologies needed to
exploit these material balances to make predictions on the movement
and behavior of the chemicals being studied. This paper is based
largely on the material developed in a draft of a consultant paper
prepared by Dr. Robert A, Ouce, University of Rhode Island,
-------
EPA NOTICE
This report was prepared by members of the Science Advisory
Board for the Environmental Pollutant Movement and Transformation
Committee, The Science Advisory Board is chartered to provide
independent advice to the EPA. This report has not been reviewed
or approved by the EPA, and therefore it should not be considered
to reflect official Agency policy, nor should mention of trade
names or commercial products be considered as an endorsement.
The report has been approved by the Environmental Pollutant
Movement and Transformation Committee as an advisory document.
The report has been endorsed by the Chairman of the Executive
Committee for transmittal to the Agency*
-------
TABLE OF CONTENTS
EPA Notice • ...... 2
Table of Contents .,,,,3
Introduction ..... 4
Global Material Balances . ..... 6
-------
INTRODUCTION
A fundamental method of assessing the movement and distribution
of a chemical substance in the environment is to establish a material
balance for that substance. Such a material balance is an inventory,
or method of bookkeeping, which totals the amount of the material
which enters the system, the amount which circulates within the
system, the amount which changes chemical form, and the amount which
leaves the .ystem. The "system" is that part of the environment of
interest at the moment with respect to the chemical substance which
is being inventoried. Thus, the "system" is an expression of "scale"
or "size." One may speak of planetary material balances -- the
inventory of substances that enters and leaves the planet at large.
Here the inputs could be meteorites, the outputs could be space
satellites or rocket boosters, and circulation is movement within
the planet. The chemical substance of interest is assessed in terms
of its concentration within the various input and output materials,
One may also speak of global material balances -- the dynamics of
movement through the atmosphere, aquatic systems {rivers, lakes,
estuaries, and oceans), terrestrial media (soils, then groundwat^rs),
and biota (organisms and tissues) -- and ignore the "planetary"
inputs and outputs as not being relevant. Or one may speak of a
continental oalance -~ the dynamics of input, output and circulation
within a large land mass which has been "intellectually isolated"
from all other land masses for study purposes. Material balance
scales continue downward through regional, urban or rural, local,
individual, cellular, subcellular, etc.
The Environmental Pollutant Movement and Transformation Committee
of the Science Advisory Board is concerned with how pollutants move
through the environment -.nd change. The Committee is concerned with
environmental distributions of natural and manmade chemicals, and with
processes affecting environmental distributions of these chemicals;
with methods of predicting what impacts particular changes in pollutant
distributions have on exposed animal, plant, and human populations;
with the impacts of distributions on methods of control or utilization
of possible pollutants; and with decision making and policy making
which depend on maintaining or changing particular distributions.
This Committee is specifically concerned with advising the EPA on
methods of calculating end utilizing material balances for selected
chemicals in the environment and in predicting effects that modification
of environmental distributions of selected chemicals will have on
regulatory policies. The Committee requested its member, Dr. Robert
A. Duce, to begin to explore how to advise EPA with respect to
material balances.
-------
The importance of this project has been underscored by some
recent problems within the Environmental Protection Agency. Of
note has been the desire of various regulatory groups within the
Agency, especially the Office of Toxic Substances, to establish
guidelines to use the "fate of a pollutant" as a means of predicting
environmental impact and the types of regulatory strategies needed
for control of toxic chemicals.
Unfortunately, of all of the things usually known about a
given chemical pollutant, e.g., its toxicity and physical and
molecular properties, the "fate" in the environment is usually the
least known. In fact, the "fate" of environmental chemicals is
only "known" in a very few cases. One such case is the "nitrogen
cycle," which has been studied by agronomists, limnologists, and
wicrobiologists, for many years, .Although some of the data may be
questionable, there seems to be large scale agreement among scientists
as *o the mechanisms involved.
Also of note is the feeling among those Agency regulators
Who are concerned with proposing effluent guidelines for selected
chemicals in industrial dischargers that knowledge of the "fate
of the chemicals" from the discharge may be of considerable value
in choosing ambient standards in the aquatic environment without
having to rely solely on the-safety factors applied to limited
toxicological data with selected aquatic species. It would be nice,
for example, to be able to state that a dilution factor of 100 or 500
was applicable to certain discharge situations and that mixing zones
are either usable or unusable without having to cite toxicological
data for aquatic species not indigenous to the waterbodies of
interest.
This first paper is adapted from Dr. Duce's consultant paper.
It deals with'global material balance concepts and is primarily
introductory. Future papers in the series will deal with specific
methodologies and cases, and in order to be of maximum advisory-
benefit to the Agency, will consider topical inputs from the Agency.
-------
GLOBAL MATERIAL BALANCES
Over the past 10-20 years there has been a growing realization
that man can affect and sometimes significantly change the global
environmental cycles of many substances. Public concern relative
to global-scale problems has arisen because of (1) possible climate
modification due to carbon Jioxide and atmospheric particle
increases, (2) human health effects due to possible changes in the
ozone levels in the stratosphere as a result of increased aircraft
operations at high altitude and chlorofluorocarbon and nitrogen
fertilizer use, and (3) the impact of pollutants on marine and
terrestrial ecosystems. These three problems have probably been
the primary driving forces behind increased research directed
toward understanding global material balances, but they have also
led us to an awareness of how poorly we understand biogeochemital
cycles and global material balances in general. None r.f these
problems can be adequately addressed unless the mass flow and
accumulation of the substances of interest in various compartments
of the environment are known and unless the natural and anthropogenic
source strengths of the substances are quantified.
Geochemists have been concerned with global chemical balances
for decades. Only recently, however, have they had adequate
analytical tools and sampling techniques to evaluate the importance
of the marine and atmospheric environments in the global material
balances of many substances. On the basis of recent data, it
appears that anthropogenic sources may be dominating, or at least
measurably altering, major components of the natural geochemical
cycles of a number of substances, specifically elemental lead,
sulfur dioxide, carbon monoxide, carbon dioxide, nitrous oxide,
ozone, and atmospheric particulates.
An understanding of the complete environmental cycling of
certain classes of toxic substances is necessary to develop adequate
models for the environmental behavior of similar substances and to
plan for their control and containment. Evaluation of global
balances identifies environmental compartments in which individual
substances are being concentrated. Research emphasis can then be
directed toward evaluation of the effects of these substances on
the ecosystems residing in these compartments. In addition, information
can be obtained about the general features and mechanisms of transport
processes using the distribution of flux of certain chemical tracers.
For example, the extent of tropospheric air mass exchange across the
Equator has recently been evaluated from atmospheric carbon monoxide
measurements in both hemispheres.
-------
For global scale considerations,, decades to centuries are
involved for marine transport. The atmosphere is the primary
transport path for global transport processes on time scales of
less than a few>years. • In evaluating the global balance of any
substance, we must consider its distribuiton, sources and sinks,
fluxes and transport, and reactions and transformations. In
general, the discussion below will relate to the atmospheric
part of global biogeochemical cycles and material balances. It
is only used as an example of the kinds of questions we must ask
about all components of the environment, be they marine, fresh.
water, terrestrial, or atmospheric.
Obviously,, a measurement of concentrations or burdens in
various compartments of the environment is the first step in
evaluating a biogeochemical cycle for any substance over any
significant geographic scale. For many substances, however, we
are not really "off the ground," literally and figuratively,
with this first step. For example, atmospheric information on
the vertical distributions of chemicals, information which is
critical to assessing atmospheric burdens, is virtually nonexistent.
for most substances, measurements are restricted to near surface
urban regions, with little surface data and almost no vertical data
from remote marine and continental regions. Although advances in
analytical methodology have been remarkable in the past'few-years,
determining of global material balances for many substances is
still analytically limited. For example, satisfactory methods are
still in the development stage for measuring atmospheric nitric
oxide in remote regions. Reliable measurements for DDT in open
ocean waters are still not available.
Differentiation between natural and anthropogenic sources and
sinks is often extremely difficult but is necessary to evaluate
global material balances. For some substances, such as many synthetic
organics, this is not a problem,, as nature does not (or we often
think it does not) produce many of these materials. For substances
which have both natural and anthropogenic sources, several approaches
can be taken-.
Isotopic .studies are often useful. Ou^ to differences in
source materials or exchange processes, .certain natural sources
may produce substances with altered isotope ratios relative to
anthropogenic sources. This has proven useful for elemental
lead, elemental carbon, and elemental sulfur, some compounds of
nitrogen, and recently for hydrogen and oxygen isotopes in the
molecules of water itself.
-------
Within a given elemental cycle, certain sources may emit
different chemical forms, or species, of the element,. For example,
smelters usually emit arsenic as the trivalent arsenic trioxide,
while the biosphere, through microbial activity, converts elemental
arsenic and its oxides to methylated species such as methyl arsine.
For atmospheric particulate matter, SEM/EMP can be helpful,
as particles from specific anthropogenic sources often have
morphology, composition, and mineralogy different from particles
from natural sources.
For a broad scale effort to determine whether the overall
cycle of a particular substance is dominated, or at least
significantly affected, by man on a global or hemispheric scale,
an historical record may be of great use. This includes both
glaciers and major icecaps in the north and south polar regions
as well as fresh water or even near shore marine sediments.
Detailed meteorological analysis, particularly air parcel
trajectory analysis coupled with simultaneous atmospheric
measurements, can allow one to relate substances to certain
source areas.
Sophisticated statistical techniques such as factor
analysis, pattern recognition, and hierarchal clustering
allow one to relate the distributions of substances of unknown
origin in a sample to substances of known origin, thus suggesting
common sources which should be evaluated further.
Finally, one can simply compare the global or regional fluxes of
material from (or to) natural and pollution sources (or sinks). In
most cases, our knowledge of the numerical values for these fluxes is
so uncertain that this approach gives us only a crude and often
misleading approximation of the importance of pollution sources.
Quantitative information on direct emissions from anthropogenic sources
is only now becoming available for the U.S. and is largely missing for
the rest of the world. This problem is complicated by second and third
order processes not directly related to the primary emission process.
These include changes in land use patterns, chemical transformations in
the atmosphere, etc. Nevertheless, it is safe to say that estimates of
the input of most chemical substances to the atmosphere from pollution
sources are considerably more accurate than estimates from natural
sources. Our understanding of the importance of the terrestrial and
marine biosphere, volcanoes, forest fires, surface weathering, etc, as
sources for most trace substances in the global atmosphere is abysmal.
-------
Fluxes are usually more difficult to measure than are ambient
concentrations. As formulated in material balance equations these
fluxes are often linearized by equating them to a ratio of inventories
over residence times. This approach assumes that the rate of removal
(or input) of a substance is independent of the quantity of that
substance already in the reservoir. This is rarely the case for
removal processes and is not that common for source functions.
However, until more is known about the factors controlling the fluxes
of substances in and out of reservoirs, this assumption is the easiest
to make. If possible, fluxes should be measured as a function of
ambient concentration so that the relationship between these two
quantities may be evaluated. With respect to removal mechanisms
from the atmosphere., the correct relationship will probably involve
exponentials in most cases.
The real action in most cases occurs at interfaces in the
environment. An understanding of the chemistry, physicsj and biology
of surfaces is fundamental in all biogedcheinical cycles. Sources,
sinks, and associated fluxes are all concerned with transport across
some interface, be it air/sea, air/leaf, .river/sea, gas/particle,
soil/water, etc. For simple physical and chemical exchange processes
we must know the specific form of the exchanged substances at the
interface (which may be different from its form away from the
interface), the effects of temperature, pressure, light, moisture,
and other chemical substances on the exchange process and the fundamental
surface forces and properties which1control exchange across each
interface. The presence of organisms on a surface (and in many
cases we are concerned with a biological surface to begin with)
will often affect chemical fluxes across the surface. In this
case, not only must the parameters affecting simple physical and
chemical exchange be considered, but also such factors as the state
of organisms development, growth, and season.
In general, the spatial scales over which one might begin to
attempt a mass balance are related to substance residence times in
the system. The shorter the residence time, the smaller the spatial
scale one may wish to cover. In such cases as elemental sulfur and
nitrogen, hemispheric (i.e., northern and southern) mass balances
may be most appropriate. For substances whose atmospheric residence
time is one to two years or less and whose biogeochemical cycle is
-------
suspected to be significantly altered by man, a comparison of northern
and southern hemisphere mass balances, inventories, etc, etc., can
often be extremely valuable. This has been particularly useful for
carbon monoxide, some halogenated organic species, and certain heavy
metals.
Chemical transformations and reaction rates in the atmosphere and
aqueous systems must be evaluated in any material balance. Many harmful
substances are rapidly transformed to less harmful species; others are
not. With an understanding of reaction rates and transport processes,
the appropriate time and distance scales can be evaluated for any
substance. Information on the physical and chemical transformations
of a substance is sometimes available before the distribution of the
substance is well known. Modelling, based on laboratory reaction
kinetics, can predict, sometimes fairly accurately, the concentrations
of certain species in the atmosphere, but these models must be verified
by actual ambient air measurements. In many cases, the real world
measurements have lagged behind the models.
The presence of unmeasured or poorly measured species can ^ometimes
be deduced from conservation of mass considerations. This is difficult
at present in most biogeochernical cycles because the overall uncertainties
in the various components of the cycles are usually so great that it is
hard to identify any "holes." Certainly, there appears to be such a
"hole" in the global atmospheric sulfur cycle, and many forms of sulfur
from several sources have been suggested to fill this hole. It still
has not been filled. As measurement techniques and area coverage
improve, the information on sources,, sinks, and reservoirs will improve,
thus decreasing the cycle uncertainties. This will undoubted reveal
"holes" in other cycles.
It should be pointed out that important intermediate species will
often be completely missed in a simple mass balance approach, and thus
important mechanisms involved in material fluxes or transformations
can be overlooked. For example, a substance with a very short residence
time in the atmosphere may be transformed into a longer residence
time component, and the longer lived component may be the only one
measured. It is likely that the primary flux of arsenic into the global
troposphere is via a vapor phase, However, the v«.por phase comprises
only about 21 of the total arsenic burden due to its very rapid
conversion to or uptake by particles. A simple mass balance in which
only particulate arsenic is considered would be quite accurate relative
to the total tropospheric burden of arsenic, but would miss the very
important fact that this burden is apparently controlled by a short-lived
arsenic vapor phase.
10
-------
The' time scales over which quasi-steady states may be
expected in the atmosphere vary tremendously depending upon the
particular substance evaluated. For substances dominated by
certain rapid photochemical reactions, the presence or absence
of clouds could affect the steady state, and thus a time scale
of minutes would be important. Diurnal time scales are appropriate
for other photochemically controlled cycles, e.g. NQX, and
seasonal cycles may be dominate for biologically controlled atmospheric
cycles. For .cycles of other substances, e.g. the freons, the time
scale is likely to be decades. Determining these critical time
scales is one of the fundamental questions to be answered in any
global mass balance or biogeochemical cycle,
The cycles of many substances interact and couple. Obvious
examples are cycles for nitrogen, chlorine and ozone. We are constantly
being surprised in this area, however. The relationship between
the methane cycle and the carbon monoxide cycle through the hydroxyl
free radical is a good example. The "discovery" of the hydroxyl
radical has perhaps more than any other single event, catalyzed
our .thinking toward watching for and seeking out this coupling of
cycles.
Finally, we must know the ultimate sinks of the substances of
interest, particularly pollutants. Ultimate must be used in a
relative sense here — on a time scale of hundreds to thousands
of years, perhaps. Many substances undergo chemical reactions, are
destroyed, and thus do not accumulate indefinitely in the environment.
Others may be destroyed but only- after accumulating to potentially
harmful levels in certain compartments, e.g., the freons in the
atmosphere. Others may be deposited in the ocean sediments over
various time scales and become largely decoupled from the dynamic
part of biogeochemical cycles.
For any substance with a significant pollution source, the
determination of an accurate global material balance and a realistic
evaluation of its biogeochemical cycle will not be easy. It will
require close communication between the field and laboratory scientists
studying the fundamental processes involved in pollutant transport
and distribution in the environment and the national and international
agencies responsible for pollution control and source strength evaluation.
Tl
------- |