Assessing the Environmental Persistence of a
Variety of Chemical Substances Including Metals
A report prepared for Environment Canada
Dr. Patrick Doyle, Scientific Authority
November 2001
CEMC Report No. 200104
Prepared by:
Don Mackay, Steven Sharpe, Tom Cahill, Todd Gouin,
Ian Cousins, Liisa Toose
Canadian Environmental Modelling Centre
Trent University
Peterborough, Ontario K9J 7B8
CANADA
Summary
A review is presented of current approaches for evaluating the persistence of chemicals in the
environment. Such evaluations should be based on an adequate quantitative understanding of
chemical fate including degradation rates, partitioning, speciation, availability and an appreciation
of which media are relevant and irrelevant. There is a wide range in the properties of individual
commercial chemicals and they fall into numerous chemical classes. In order to simplify and
streamline the assessment process, a compelling incentive exists to evaluate all substances using a
single, consistent, equitable process which can accommodate this range in properties and classes.
Given this incentive and the overlapping properties of organic and inorganic substances, including
metals, it is suggested that there is no justification for adopting different methods or criteria for
evaluating persistence between these diverse groups.
Persistence in the environment can be evaluated empirically by observing environmental behaviour
or by using mass balance models which simulate chemical fate. The latter is generally preferred
because of the lack of empirical or monitoring data. Methods of calculating persistence in a multimedia environment are reviewed and it is shown that a variety of residence times or half-lives can
be calculated which represent the average time that a molecule of the substance spends in the system
before becoming subject to the specific removal process. The process can be degradation reaction,
advection (i.e. loss by flow out of the system, usually in air or water), intermedia transport (eg
evaporation from water to air or deposition from water to sediment) or any combination of these
processes. A residence time or half-life can be calculated corresponding to each of these processes
singly or in combination. The most appropriate residence time or half-life is that attributable only
to removal by degrading reactions. It is primarily a function of the intrinsic properties of the
chemical and is influenced by the environmental conditions such as presence of oxidants, sunlight
and the prevailing microbial community. A set of half-life ranges is suggested extending from a few
hours to essentially (or actually) non-degradable, i.e. exceeding 11 years, within which all chemicals
can be classified.
In contrast, the use of residence times or half-lives attributable to advection or intermedia transport
is shown to be inappropriate because these times are largely a function of environmental conditions
rather than intrinsic chemical properties. In particular, the proposed use of a residence time criterion
1
in the water column for metal ions is problematic because many persistent organic substances prove
to be non-persistent using this criterion. Likewise the inclusion of availability as part of the
definition of persistence introduces excessive complexity and ambiguity and is inappropriate. A
brief review is included of the salient aspects of metals in the environment, especially fate in the
water column.
A strategy is suggested for evaluating the persistence of all commercial chemical substances
including metal-containing inorganic substances, which it is believed, affords an equitable and
unambiguous treatment of all classes of commercial chemicals. As an illustration, a set of chemicals
including organics and dissolved forms of metals was subjected to mass balance modelling using
two models, the EQC multimedia model and a QWASI model of chemical fate in a lake typical of
conditions in the Canadian Shield. The results are discussed and it is concluded that it is feasible
to use a single mass balance model to evaluate the persistence of all commercial chemicals,
including metal-containing inorganic substances. The persistence so evaluated reflects the inherent
degradability of the substance. It does not address its availability or its rate of intermedia transport
by processes such as sedimentation. These processes, while important, are best addressed as
separate components of the risk assessment process.
2
Contents
Summary
1
Contents
3
List of Tables
4
List of Figures
4
1. BACKGROUND AND OBJECTIVES
5
2. KEY ASPECTS OF CHEMICAL FATE AND PERSISTENCE
8
2.1 Speciation
8
2.2 Availability
9
2.3 Relevant and Irrelevant Media of Partitioning
11
2.4 Degradability or Reactivity
12
3 ENVIRONMENTAL FATE AND PERSISTENCE
16
3.1 Depicting and Quantifying Chemical Fate in the Environment
16
3.2 Calculating Persistence in the Environment
20
3.3 Viewpoints on Persistence
26
4. METALS IN THE ENVIRONMENT
30
5. A SUGGESTED STRATEGY FOR EVALUATING PERSISTENCE
34
6. Illustrative APPLICATION OF THE PROPOSED STRATEGY
38
6.1 Model and Chemical Selection
38
6.2 Results of the EQC model
40
6.3 Results of the Shield Lake Model
45
7. CONCLUSIONS
58
REFERENCES
61
3
List of Tables
Table 1 Classes of Chemicals
6
Table 2 Suggested degradation half-life classes
14
Table 3. Chemical Properties
39
Table 4. Properties of the EQC environment
40
Table 5. Properties of the Shield Lake
40
Table 6. Summary of overall reaction residence times in the EQC model with emissions to air,
water and soil , using reaction half-lives as defined in the handbook
44
Table 7. Process rates and residence times of DDT in a Shield Lake as deduced from
Figure 3A
54
Table 8. Summary of degradation and sedimentation residence times in the water column of a
Shield lake
55
List of Figures
Figure 1. Mass balance diagram of the fate of a hypothetical chemical in a four compartment
environment.
18
Figure 2. EQC outputs for DDT emitted to air, water, soil and all three media
42
Figure 3. Shield Lake results
46
Figure 4. Residence time (t) of Zn in lakes or in aquatic mesocosms as a function of the particle
flux through the water column per unit volume. Couillard (2001), adapted from Santschi
(1984).
57
4
1. BACKGROUND AND OBJECTIVES
Several international initiatives are underway to assess the hazard posed by commercial chemical
substances based on their attributes of persistence, bioaccumulation, toxicity, and potential for longrange transport. Most emphasis has been placed on organic substances, especially hydrophobic
organo-chlorines which tend to be slow to degrade and can be appreciably bioconcentrated in lipid
tissues. In Canada, a systematic evaluation is underway of the some 23000 chemical substances on
Environment Canada’s Domestic Substances List (DSL) as mandated under section 73 and 74 of the
Canadian Environmental Protection Act (1999) ( i.e. CEPA 99). Most of these substances are
organic in nature, but some 2000 are inorganic, many of which contain metals. In addition,
Environment Canada must review the properties of new chemicals which are being introduced to
commerce.
In recent years considerable progress has been made in developing methods for assessing the
persistence of the conventional organic contaminants such as PCBs and pesticides. There are,
however, numerous chemicals which have properties quite different in nature from these relatively
well studied and understood substances. Examples are dyes, pigments, silicones, metals, organometals and surfactants which have unusual chemical structures or functionality that prevents
accurate property estimation using conventional structure-activity-property relationships. Often they
can exist as several species such as elemental, non-ionic, and ionic forms with various oxidation
states.
Table 1 is a list of classes of commercial chemicals which may require evaluation, although not all
are necessarily addressed in the CEPA initiative.
5
Table 1 Classes of Chemicals.
Non-polar organics including hydrocarbons and halogenated hydrocarbons
Polar organics including alcohols, acids and bases
Surface active substances
Pigments and dyes
Metals including native metals, inorganic metal-containing compounds, organic metal salts and
organo-metallic substances
Gases such as freons
Inorganic substances which may or may not ionize
Pharmaceuticals, pesticides and biocides
Fuels, solvents and petroleum distillates
Monomers, oligomers and polymers
Natural substances such as waxes, fats and oils
Radioactive elements
These substances vary greatly in their physical-chemical properties. Thus each substance, or class
of substance, may have its own chemical character which dictates its partitioning and degradability
in the environment.
There is a compelling incentive to assess the hazards posed by all these chemicals in an even
handed manner with comparable rigour thus avoiding the potential problem that certain classes are
over-scrutinised or under-scrutinised. In the present context the key properties are persistence and
bioavailability. A review of the literature was thus undertaken with a view to addressing the
following questions.
•
How best can we define persistence and bioavailability?
•
Is it feasible to identify or develop an acceptable general procedure or model which can
describe persistence and bioavailability of the full spectrum of substances? If not, how
should different classes be treated?
•
Is persistence best evaluated on a single medium basis, e.g. for air and water individually,
or is a combined or overall multimedia persistence preferable?
•
How should substances be treated which, by virtue of their partitioning properties, do not
partition appreciably (or at all) into certain media? For example ionic metals and certain
surfactants do not evaporate.
The general objective is to explore these issues in the light of the current science, make suggestions
and recommendations, attempt to devise a comprehensive modelling system and conduct a
6
preliminary assessment of the persistence of a representative set of organic and inorganic
substances, including metals, using such a system.
Unless otherwise noted in this document the term metal is understood to apply to the metal (and
metalloid) released into solution when commercial compounds dissolve. Note that even sparingly
soluble inorganic metal compounds release small amounts of metal into solution in contact with
water. It is the persistence of this metal that is evaluated and attributed back to the parent substance.
2. KEY ASPECTS OF CHEMICAL FATE AND PERSISTENCE
The objective of this section is to present and briefly review certain key aspects of chemical
behaviour in the environment, especially those aspects which influence persistence and availability.
2.1 Speciation
A very important attribute of a substance is its tendency to "speciate", especially in aqueous
solution. Many substances, as exemplified by benzene exist in only one form or species, thus mass
balances describing the fate of such substances consider only a single species. Some such as acetic
acid or phenol dissociate to form a hydronium ion and a corresponding anion such as acetate or
phenolate. Organic bases can behave similarly reacting with water to form a hydroxyl ion and an
organic cation such as a charged amine. Metals may exist in several forms. An extreme case is
mercury which can exist in elemental, and free cationic forms as well as variously charged organic
and inorganic complexes.
Much of the interest in speciation derives from the recognition that different species exert different
toxic effects and display very different behaviour in the environment.
A related, but separate issue is the issue of parent and daughter species such as conversion of
DDT to DDE. These conversions are usually irreversible. It is usually necessary to treat these
species individually, however, models describing multi-species systems are under development.
Mass balances can be compiled for a single species, or for a group of species without specifying the
proportions of the various species. For example a balance could be compiled for phenolate anion
or ferric iron recognizing that this is only one of several possible species. Alternatively, it could be
7
compiled for total phenol species (phenol plus phenolate) or various forms of iron (ferrous, ferric,
sulphide etc.). A key issue relating to such speciating substances is that there is potential for species
to species conversion as a result of external environmental factors such as pH, Eh and the presence
of other substances. Speciation is thus usually reversible. A difficulty with mass balances of single
species when multiple species exist is that for each environmental compartment the rates of
formation and conversion of the species must be included as additional terms. This adds complexity
to the mathematics and is the reason that many such mass balance models treat the total species. It
is not then necessary to include these species-to-species conversions.
With regard to speciation there is no fundamental difference between the substances listed in Table
1. Either they do not speciate, or they do speciate (with interspecies conversion being possible in
reasonably measurable times which range from a fraction of a second for ionic dissociation of most
metal salts, to days, weeks and months for conversion between organic and inorganic forms of
substances such as mercury. When speciation occurs, these substances can be treated as individual
species or the sum of all species.
2.2 Availability.
A key concept in environmental toxicology is chemical availability, or in the case of uptake by biota,
bioavailability. It is recognized that not all the substance present is necessarily subject to a
particular process or capable of exerting a specific toxic effect. For example, in the atmosphere,
substances such as PAHs exist in both the gaseous form and sorbed to aerosols. It is important to
determine these proportions because they affect environmental fate (e.g. availability to wet and dry
deposition or sorption to vegetation) and the ultimate uptake by respiring biota. For commercial
metal-containing substances the focus is on the metal released into solution through
dissolution/transformation reactions since in general it is this metal that is taken up by and may be
harmful to organisms exposed at high concentrations.
In the water column, substances (whether metallic or organic) may be in solution as the free neutral
or ionic form, sorbed to filterable particles such as phytoplankton or minerals or sorbed to nonfilterable particles such as fulvic acids, proteins, or fine clay particles. Again it is important to
discriminate between these forms because they differ in availability to processes such as evaporation
or uptake by biota. The concept of including an assessment of bioavailability is well accepted for
organics (e.g. Hamelink et al. 1994) and for metals in the form of the FIAM (free ion activity model)
8
concept (e.g. Morel 1983). Di Toro et al. (2001) and Parametrix (1995) give excellent reviews of
metal availability for environmental receptors.
Fugacity models can be formulated to take availability into account through their definition of the
fugacity capacities or Z values of the chemical in media such as water. The Z-value can contain
terms for the dissolved and non-dissolved fractions. A corresponding procedure is adopted in the
"aquivalence" models which are used for non-volatile chemicals such as metal ions. Indeed, the
FIAM and fugacity concepts as applied to metal ions and organics respectively, are closely related
in that both seek to express the quantity of chemical present in its available form, rather than as the
total concentration. They automatically take into account the available fraction . Although both
approaches have their pitfalls, there is no doubt that they provide an invaluable paradigm for
expressing availability.
In the simplest case one species is available and the others are not, but in some cases each species
may have an individual availability. There may be difficulties in obtaining parameter values and
appropriate mathematical expressions to characterize speciation, but at least a first order
characterization of availability is possible if the speciation is expressed quantitatively and the
interaction of each species with the organism is understood. Again, there is no fundamental
difference between chemical classes.
2.3 Relevant and Irrelevant Media of Partitioning.
Many chemicals have properties which dictate that they do not partition or dissolve at all in certain
media. This is not a problem, indeed the environmental fate calculations usually become simpler
since one medium may be ignored.
Examples include high molecular weight polar organic
substances such as surfactants, certain pharmaceuticals, strongly ionizing substances such as
trifluoroacetic acid which has a pKa several units below environmental pH levels, and metal cations
and related anions. These substances are essentially involatile and they are not detectable in the gas
phase. They may be present in aerosol particles or fog or rain droplets, in which case these media
should be included in any model of chemical fate.
Some organic substances such as certain dyes and pigments, hydrocarbon waxes and inorganic
substances such as silicones and mineral matter do not dissolve in water to any measurable extent.
9
If they are present in the water column it is necessarily in a non-aqueous phase. In such cases the
aqueous phase is ignored, except as a carrier for dispersed particulate and other matter. Some metal
compounds such as sulfides have solubility products which are so low that they are essentially
insoluble in water and they are present in only the "insoluble" sulfide form in a non-aqueous phase.
They may later react to form more soluble and bioavailable species, especially under oxic
conditions.
2.4 Degradability or Reactivity.
The universe of chemicals being evaluated for environmental fate varies greatly in their
susceptibility to degradation to yield substances with a permanently different chemical structure.
This susceptibility is commonly expressed as a degradation half-life, a concept borrowed from
radioactive decay.
The use of a half-life implies first order kinetic behaviour in chemical
concentration or amount. For chemicals in the environment the half-life depends not only on the
chemical’s molecular structure, but also on the nature of the environment, especially the presence
of oxidising or reducing species, acids and bases, the incidence of sunlight and the nature and
abundance of microorganisms. These half-lives differ from medium to medium and are functions
of temperature. The lack of reliable degradation rate or half-life data is a major problem in chemical
fate assessment, however, it is often possible to assign chemicals to classes or ranges of half-lives.
At one extreme is a group of substances which degrade negligibly or not at all. Metals are
inherently stable, i.e. their degradation rate constants are zero and their degradation half-lives are
infinite.
Some organic substances are also essentially non-degradable, examples being
trifluoroacetic acid, highly chlorinated PCBs and highly fluorinated substances. Most of the
“priority” chloro-organics have degradation half-lives in water, soil and sediment measured in years,
but with shorter degradation half-lives in the atmosphere by virtue of attack by hydroxyl radicals.
Polymers such as polyethylene or PVC also react very slowly, as do certain silicones.
At the other extreme are organic substances which are readily degradable with half-lives of the order
of hours. Often these are polar in nature containing reactive functional groups containing oxygen,
nitrogen, sulfur or phosphorus. Most chemicals of commerce probably lie in the intermediate range
with degradation half-lives in the range of a few days to a year.
10
There has been considerable debate about half-life “cut-offs” or “thresholds” or “fencelines”.
Obviously of most concern are those substances which can survive appreciably from year to year
as a result of degradation half-lives exceeding 4 to 6 months. For such substances the rates of
reaction are so slow that, as is shown later in the model calculations, the dominant route of removal
is usually by advective processes or transport to another region or compartment. Whether the
degradation half-life is 1 or 3 years or infinite (as applies to metals and certain organics) is then
largely irrelevant in determining environmental concentrations and exposures.
It is thus concluded that there is a continuum and wide range of degradability or reactivity extending
from zero, or essentially zero reaction, to fast and almost instantaneous reactions. There is no
scientifically justifiable method of classifying substances as falling into classes of persistent or nonpersistent, especially given variability in environmental conditions and the inherent difficulty of
measuring slow reaction rates in the presence of competing loss mechanisms.
Table 2 Suggested degradation half-life classes
Class
1
2
3
4
5
6
7
8
9
10
Mean degradation half-life (hours)
Range (hours)
5
17 (~ 1 day)
55 (~ 2 days)
170 (~ 1 week)
550 (~ 3 weeks)
1700 (~2 months)
5500 (~ 8 months)
17000 (~ 2 years)
55000 (~ 6 years)
> 11 years
< 10
10-30
30-100
100-300
300-1000
1,000-3,000
3,000-10,000
10,000-30,000
30,000-100,000
> 100,000
The times in Table 2 are divided logarithmically with a factor of approximately 3 between adjacent
classes. With the present state of knowledge it is probably misleading to divide the classes into finer
groupings; indeed a single chemical may experience half-lives ranging over three classes, depending
on season and location. Metals and substances such as trilfluoracetic acid are in Class 10 and are
essentially non-degradable under environmental conditions.
One approach is to attempt to assign substances to one of the classes listed below in Table 2. This
classification is a modification of that used by Mackay et al. 2000.
11
Developing methods to assign half-lives to substances is an area of ongoing research. These
methods attempt to relate structure to reactivity. In the simplest form, they are “rules of thumb”
such as a branched octyl chain is less susceptible to biodegradation than a straight octyl chain. The
more complex methods can predict abiotic and biotic degradation half-lives from structure using
sophisticated quantitative-structure activity relationships (QSARS). All available methods tend to
agree qualitatively in that they suggest that the same compounds will be rapidly degraded or
persistent. Problems arise, however, in assigning the half-lives from different environmental media
because of lack of, and variability in, measured environmental half-lives. It is desirable that there
be more international agreement on the methods that are adopted to assign half-lives to substances
because there have already been international initiatives in setting “cut-offs” or “guide-lines” for
assessing which chemicals are persistent in different environmental media. This issue is more fully
discussed in the review edited by Klecka et al (2000).
3. ENVIRONMENTAL FATE AND PERSISTENCE
3.1 Depicting and Quantifying Chemical Fate in the Environment
It is first useful to outline the nature of multi-media models as applied to substances which are
ubiquitous in the environment and are detected in all media. An example is a hypothetical
chlorophenolic substance, designated here for convenience as HCP. This organic substance can be
viewed as a general case, recognizing that other substances are less multi-media in character. In
addition to being present in all media, the HCP speciates by dissociation (loss of proton or H+)
forming a chlorophenolate ion (CP). The proportions of HCP and CP will depend on pH and other
factors. Equilibrium may, or may not apply. Speciation may occur in only some media or in all
media. In some cases there may be multiple species.
One aim of environmental fate studies is to establish the concentrations and masses of the HCP in
various media, including a description of any speciation. In addition, it is valuable to determine
pathways between media, such as evaporation from soil to air, or deposition from water to sediment.
The substance may be subject to irreversible degradation reactions in some or all media by, for
example, photolysis or biodegradation. It is likely that different species partition differently, thus
in the context of bioaccumulation and toxicity it is necessary to define the fractions in each speciated
form.
Attention is then usually focussed on the chemical species most related to chemical
12
availability or toxicity. This availability is also affected by the presence of sorbing phases such as
aerosol particles in air and suspended solids in water. Ultimately, it is desirable to quantify all
masses, concentrations, and input, transport and degradation rates to obtain a complete mass balance
or accounting. In addition there should be consideration and preferably quantification of availability
and speciation of the substance in order that the extent of biouptake and toxicity can be evaluated.
This may be attempted for a specific environment which is contaminated, or for a hypothetical or
evaluative environment in which the aim is merely to determine the general behaviour
characteristics of the substance.
Figure 1 depicts this approach for a typical multimedia environment consisting of air, water, soil and
sediment compartments or media. Within each compartment there may be other phases, for example
the air may contain aerosols, the water may contain suspended matter and biota such as
microorganisms and fish. For some purposes these four compartments are adequate, but for others
it may be necessary to add to them, for example by introducing a groundwater or vegetation
compartment, or sub-divide them into, for example, several water and sediment compartments. For
the present purposes, four compartments are believed adequate and the effect of increasing the
number of compartments can be readily evaluated.
The arrows on Figure 1 represent the rates of processes of chemical discharge or emission, transport
and degradation. Under steady-state or near steady-state conditions as frequently exists in the
environment if the inputs are fairly constant for a prolonged period of time, the rates of these
processes are such that the total rate of chemical input to a compartment equals the total rate of
output. This also applies to the system as a whole. There is a standing inventory in each
compartment which is achieved when inputs equal outputs. The numbers on Fig. 1 are purely
illustrative but give a typical set of inventory and rate data for an organic substance such as HCP.
Such diagrams can be compiled for each species or for the total of all species.
A major task of environmental fate studies is to compile such diagrams as a complete and
quantitative depiction of chemical fate. When this is achieved it becomes apparent which processes
are most important. Certain quantities such as residence times can be deduced. Environmental
13
monitoring programs reveal concentrations from which amounts can be deduced, but it is rarely
possible to measure process rates in the environment. They must usually be calculated from
17
30
E
A
R
AIR
mass 1000
5
17
A
64
11
17
69
70
26
SOIL
mass 30000
2
WATER
mass 20000
104
R
A
180
121
R
E
9
29
20
A
5
SEDIMENT
mass 50000
4
Figure 1. Mass balance diagram of the fate of a hypothetical chemical in a
four compartment environment, showing all fluxes in units such as g/h and
masses in g. E refers to emission rate, A to advection (inflow or outflow)
rate, and R to reaction rate.
14
R
A
concentration information by applying predictive equations.
For a specific chemical and environment there are two general methods of compiling such diagrams.
First is the empirical approach which requires extensive measurements of concentrations, and in
some cases rates. For example it may be possible to measure the emission rate to soil, the soil-water
partition coefficient and the soil-to-water run-off rate. The rate of degradation in soil may be known
to be very slow from laboratory experiments, thus the rate of net evaporation to air may be deduced.
This may actually be the difference between two opposing rates, i.e. air-to-soil deposition and soilto-air evaporation. If some information is available on deposition rates, the actual evaporation rate
may be deduced.
Second is the modelling approach in which the aim is to calculate or compute the quantities, usually
with the aid of a computer program which ensures that a mass balance is achieved. The program
contains estimates of process rates as a function of concentration and physico-chemical properties.
Several such programs exist, the more respectable ones having been validated by demonstrating that
their predictions are in accord with observations. In the Canadian Environmental Modelling Centre
(CEMC) group we develop and provide a family of such programs employing the fugacity concept,
but other programs can give equivalent results. Often, these programs are used in conjunction with
other QSAR programs which estimate the required properties of the chemical substance from
molecular structure.
In practice, a combination of both monitoring and modelling methods is usually most effective. The
key point is that it is possible for a specific substance to obtain a diagram analogous to Figure 1.
How the various quantities are achieved is irrelevant in the present context.
3.2 Calculating Persistence in the Environment
In qualitative terms, persistence can be considered to be the undesirably long continued presence
of a chemical in the environment. Although a chemical may be removed from an environmental
medium by degradation reactions, advection and/or inter-media transport, as is explained below, not
all of these removal processes are considered appropriate for estimation of persistence. Persistence
is usually quantified by a degradation half-life. Excellent reviews of the persistence of organic
chemicals have been compiled by Klecka et al. (2000) as a result of a SETAC-sponsored workshop
15
and in a two-volume American Chemical Society Symposium Series publication edited by Lipnick
et al. (2001). If we consider two chemicals, X with a short half-life of 20 days and Y with a long
half-life of 40 days, then after equal masses of these chemicals are introduced and are subject to
environmental degradation for 40 days, half of Y will remain and a quarter of X, thus environmental
exposure to Y will be greater. In the limit, the factor increase in exposure (expressed as a
concentration-time interval) will be two. There is a corresponding increase in concern.
Another metric of persistence is residence time at steady state. If both X and Y are emitted at a rate
of 100 kg/day into a medium, then, when steady-state is reached and the input rate equals the output
rate there will be a mass of X of 100 x 20/0.693 or 2886 kg. This arises because the rate constant
k is 0.693/half-life or 0.0693 days -1. The mass will rise until its product with k is 100 kg/day, which
occurs when the mass is 2886 kg. It follows that the residence time of the chemical is 2886 kg / 100
kg/day or 28.86 days. This is, of course the half-life of 20 days divided by 0.693. For Y the
corresponding mass will be 5772 kg and the residence time 57.72 days. The concentration of Y will
thus be twice that of X and exposure will increase by this factor.
We conclude that persistence can, in principle, be expressed by two metrics: 1) half-life which
implies examining the first order degradation of an amount of substance, or 2) residence time under
steady-state conditions. As shown above, these quantities are closely related and convey the same
information.
There is a common, and correct, perception that degradation half-life is the time required for half
the chemical to be removed, thus after two half-lives a quarter remains and so on until a negligible
(but non-zero) fraction remains. This applies only in a single medium. If there are two or more
connected media (e.g. water and sediment) with different degradation half-lives then the individual
media and overall system will not follow first order kinetics and a degradation half-life can not be
measured. There is often, in this case, fast initial removal from the water, followed by slow removal
as chemical bleeds from the less reactive sediment. As a result it is preferable to evaluate
persistence using steady-state rather than dynamic models.
An issue that has been the subject of some debate in the scientific literature is whether it is
preferable to assess chemicals on a single medium basis or on an overall persistence basis. Webster
16
et al. (1998) have discussed this issue in detail and concluded that overall persistence is more
meaningful. To obtain an estimate of overall persistence requires additional information on how the
substance partitions in the entire environment as exemplified in Figure 1. It is instructive to examine
this diagram in more detail and draw certain conclusions about different metrics of persistence.
The strategy which appears to be evolving is to obtain and evaluate the single media half-lives
individually, as is done in the CEPA process, then as a second stage, and if necessary, evaluate the
overall or multimedia persistence using a Level II model as described by Gouin et al. (2000) or a
Level III model as described by Webster et al. (1998).
In Figure 1, the air contains 1000 g of HCP and the degradation rate is 17 g/h, thus the rate constant
is 17/1000 or 0.017h-1. The degradation half-life is 0.693/0.017 or 41 hours. The residence time
attributable to degradation is 1000/17 or 59 h and is, of course also 41/0.693 h. The residence time
is the time required to decay to e-1 or 37% of the original amount. The corresponding figures for the
other media can be calculated yielding degradation half-lives in soil of 155 h, in water of 61 h and
in sediment of 1195 h. Here we use the symbol τ for residence time and t for half-life.
The total mass in the system is 101000 g and the total degradation rate is 169 g/h, thus the residence
time attributable to degradation only is 598 h, corresponding to a degradation half-life of 414 h.
This is clearly a weighted mean value of the four-component degradation half-lives. It transpires
that the overall reaction rate constant, which is the reciprocal of the residence time, is the sum of the
individual rate constants weighted in proportion to the fraction of the mass in each medium F. It can
be shown that the overall degradation half-life, tR, is similarly weighted as follows, subscripts A, W,
S and E referring to air, water, sediment and soil (earth) respectively
1 / tR = FA / tA + FW / tW + FS / tS + FE / tE
This obviously applies because the total rate of degradation, R, is
R = 3mÊ kÊ = 3 FÊ M / tÊ = 0.693 M 3 FÊ / tÊ
and tR = 0.693 M/R where mÊ is the mass in compartment Ê, M is the total mass, ki is the rate
constant in compartment i.
The actual residence time in each compartment differs from the degradation residence time because
of losses by advection and intermedia transport. For example, the air loses 17 g/h by degradation,
17
17 g/h by advection and (64 + 17) or 81 g/h by deposition to soil and water, a total of 115 g/h. The
corresponding residence times are 35 h, 35 h, and 7.4 h and the total residence time is 600/(17 + 17
+ 81) or 5.2 h which is of course less than the individual values.
It is thus possible to calculate an advection half-life and residence time for each medium and for the
system as a whole. Sediment burial is treated here as an advective loss. In this case the total
advective loss is (17 + 104 + 4) or 125 g/h, thus the advection residence time is 101000/125 or 808
h and the half-life, tAD is 560 h.
The total rate of loss from the system is thus 125 g/h by advection plus 169 g/h by degradation, i.e.
294 g/h which, of course, equals the total input rate by emission and advection. The overall
residence time JO is thus 101000/344 or 294 h and the half-life, tO is 204 h.
Obviously, τR, τAD and τO are related as
1 / τO = 1 / τR + 1 / τAD
and
1 / tO = 1 / tR + 1 / tAD
The overall residence time depends on the “mode of entry” of the substance to the environment. For
example if the emission was changed to be entirely into water a different distribution and residence
time would be obtained. Intermedia transport process rates do not appear directly in the calculation
of these overall half-lives, but they do influence the masses in each medium.
It is noteworthy that the actual residence times and half-lives in individual media are affected by
intermedia transport. The actual residence time in air is 1000/115 or 8.7 h corresponding to a halflife of 6.0 h. This is related to the sum of the reciprocals of the individual half-lives by degradation,
advection and intermedia transport. Specifically in the air, the half-lives are: degradation 40.7h,
advection 40.7 h and transport 8.55 h. The overall half-life is thus,
1/(1/40.7 + 1/40.7 + 1/8.55) = 6.0 h
We thus conclude that, in principle, when assessing persistence of chemical substances it is
appropriate to use either a residence time or a half-life, these being related by the factor of ln 2 or
0.693. It is not appropriate to use the actual mass or concentration because these quantities depend
also on the local discharge rate of the chemical.
For each medium or compartment in the system, it is possible to calculate degradation, advection
and intermedia transport residence times and half-lives.
18
For the system as a whole, it is possible to calculate overall degradation and advection residence
times and half-lives. This is a function of mode of entry.
From a purely scientific viewpoint there is no ambiguity or controversy about these definitions. It
is certain that the overall (system-wide) degradation and advection residence times and half-lives
exist and can be measured or estimated. Likewise, there is certainty that for a medium which is part
of a system of connected media, there are degradation, advection and intermedia transport residence
times and half-lives, which again can be measured or estimated. For some chemicals, some
processes may effectively be ignored thus the half-lives are essentially infinite. Examples are water
to air transport of dyes and pigments, degrading reactions of metallic ions or recalcitrant organics
such as TFA.
The regulator thus has a set of potential half-lives which can be applied and tested against criteria.
This is not a scientific issue, but science can contribute by examining the implications of applying
various assessment processes to a variety of substances. An attractive option is to compile all these
half-lives for a number of chemicals which vary greatly in properties and determine how they
compare. This is done in the next section in which a set of chemicals is selected, all are evaluated
using a common multimedia model and the half-lives estimated and tabulated. These results are
then discussed with a view to establishing a method of assessment common to all substances,
regardless of properties or chemical class.
3.3 Viewpoints on Persistence
It is useful to digress at this point to note the following issues which surround the definition of
persistence, especially as it applies in regulations.
Inclusion of Advective Losses
A consensus has developed that although advection losses result in lower local masses and
concentrations, they should not be considered as a mitigating factor because they merely relocate
the contaminant, they do not destroy it. They are therefore irrelevant in the context of persistence.
An argument can be made that permanent burial of contaminant in inaccessible sediment is a valid
removal process, if it can be shown that it is indeed irreversible. It is, however, usually a very slow
process. For example in a Canadian Shield lake with a sedimentation rate of 1 mm/year and an
19
active depth of 50 mm the corresponding residence time is 50 years which is considerably larger
than degradation times of persistent organic pollutants.
Inclusion of Intermedia Losses
Again, there is a consensus that loss by intermedia transport should not be treated as a mitigating
factor because the transported chemical can return to its source, i.e. the transport is irreversible. For
example, a fraction of the quantity deposited from the air may return to the air. This fraction would
be estimated from a mass balance model in which there are zero inputs to soil, water and sediment.
There is, however, an issue surrounding the equitable treatment of substances such as metals and
polymers which do not degrade at all or in reasonable times. They can have infinite degradation
half-lives, and the only mechanisms of loss are advection and transport. They will obviously exceed
any half-life criterion.
Inclusion of Availability
The definition of persistence based on degradation residence time or half-life advanced earlier is
viewed as being a scientifically valid expression of one feature of chemical fate. Other authors
choose to define persistence differently. For example Fairbrother and Kapustka (2000) in a review
for the International Council on Metals and the Environment (ICME) state:
“For metals, persistence needs to be defined as the continued presence of a bioavailable toxic form.
If the metal persists in a form that readily dissociates, but is not quickly leached from the soils, it
will remain in a bioavailable state for a relatively longer time, thus increasing the potential hazard.
On the other hand, a substance that is readily leached from the soil or remains in a sorbed or
otherwise non-available state will not pose a hazard to soil organisms for very long. Therefore, the
concept of persistence is really one of transformation and bioavailability”. (Their emphasis)
Di Toro et al. (2001) in another ICME review also couple availability in their definition.
“Persistence is a characteristic of a metal that is indicative of the constancy and duration of exposure
of the available metal forms in a particular medium”. (Our emphasis)
20
If, in the assessment of persistence, the concept of availability is expanded to include consideration
of the fate of these ions in receiving environmental media, this greatly complicates the evaluation.
It is often difficult to assess availability. It can change with temperature, DOC, pH and many other
environmental factors. Chemical available to one organism may not be available to another. For
example, chemical bound to sediment may be ingested by a soil or benthic organism such as a
worm, but it is not taken up by a mammal or fish. In our view bioavailability, while important, is
an entirely different issue from persistence and should be addressed separately. There is a
compelling incentive to evaluate persistence as simply and unambiguously as is possible. Finally,
if the environmental fate or availability of metal ions is included in the evaluation of persistence,
considerations of equity require that it also be considered for organic substances. Metals and certain
organic substances are then totally persistent and should, in our view, be treated as such.
We suggest that availability be considered only in the sense that it is understood that persistence
criteria apply to the metal (and metalloid) released into solution when commercial compounds
dissolve. Note that even sparingly soluble inorganic metal compounds release small amounts of
metal into solution in contact with water.
Ecological Risk Assessment of Organics and Metals
In a thoughtful discussion of issues relating to the Ecological Risk Assessment (ERA) of inorganic
metals and metalloids, Chapman and Wang (2000) suggest that “a generalized ERA process
applicable to organic substances is inappropriate for metals”. Justifications include the fact that
metals are naturally occurring, organisms can adapt to metal concentrations, some metals are
essential elements, availability is often limited and lipophilicity is irrelevant. While it is true that
there are certain ERA issues which are unique to metals, these issues primarily relate to assessment
of exposure, effects and risk and not to persistence, or indeed to environmental fate. It is, we
maintain, possible to develop a generalized process for assessing fate and persistence applicable to
metals and organics, provided that it addresses the issues of speciation and availability. Chapman
and Wang (2000) state that for metals “the term ‘persistence’ is meaningless” because “metals of
course persist in the environment forever...”. We agree that it is "meaningless" in the sense that the
half-life of metals is obviously infinite and its significance in the ERA process can not be
determined without evaluating whether a particular metal is acutely toxic. Magnesium and cellulose
are "persistent", but that does not imply that they are hazardous. It is only when a substance is a
21
concern from a toxicity perspective that meaning can be attributed to the fact that it is "persistent"
– specifically that persistence may be interpreted as an indicator of the potential to cause chronic
toxicity. It is true that persistence does provide discrimination for substances that do not meet
persistence criteria (i.e., it can be concluded that they are unlikely to cause chronic effects because
they are not persistent). However, the fact that persistence alone does not provide discrimination for
substances that do meet persistence criteria is a limitation that applies to all substances, not just to
metals.
The usual perspective is that long persistence is one of several criteria used to assign a high level
of concern to a chemical. The corollary is that short persistence is a criterion for assigning a
chemical to a class of low concern. Indeed, most organic chemicals fall into this latter class.
Viewed in this context, persistence is a valuable indicator that an organic chemical is unlikely to be
of concern, but it fails in this respect for metals since they are totally persistent. Whereas
assessment of persistence provides an “escape route” for many organics, it fails to do so for metals.
4. METALS IN THE ENVIRONMENT
A complete review of metal fate in the environment is beyond the scope of this report. The reader
is referred to the many excellent texts and reviews including Morel (1983), Morel and Herring
(1993), Stumm and Morgan (1996), Di Toro et al. (2001), Parametrix (1995), Allen (1995), Allen
et al. (1998), Campbell et al. (1988). Only the more salient points are discussed here.
With few exceptions metals do not partition into the atmosphere, although they may be discharged
into the atmosphere. They thus tend to be encountered in soils, water and sediments. There is ample
evidence that metals in soils are to some extent labile and can be taken up by plants and are subject
to run off in water. Of most interest is the water-sediment system in which the tendency is often for
the metal to associate primarily with the sediment. This does not imply irreversible transport or a
total loss of availability. As DeGroot (1995) has pointed out “as a consequence of changing
environmental conditions, heavy metals can be released from sediment” and “suddenly occurring
intense mobilization processes will be marked as chemical time bombs”.
Examples are the
mobilization of cadmium as a result of a salinity increase, the degradation of organic matter with
consequent loss of its sorptive capacity, changes in carbonate levels, and erosion events which may
be episodic in nature resulting in changes in redox status. The onset of biological activity can
22
increase biomethylation rates and may cause an increase in the activity of a benthic community
which can serve as a food and energy source for higher trophic level organisms.
Human
interventions such as dredging can also result in mobilization. Drainage of wetlands can result in
pyrite oxidation to sulfuric acid, a drop in pH and dissolution of metals rendering them more
available in the soil and in the receiving waters.
The role of benthic animals on the availability and mobilization of metals from sediments has been
reviewed in detail by workers such as Fisher (1982) and McCall and Tevesz (1982). Matisoff
(1995) has compiled a more recent review revealing the many complexities of this issue. As is
pointed out in that and other reviews, sediments are inhabited by a variety of animals which by
burrowing, feeding, excretion, respiration, and locomotion influence the physical and chemical
properties of sediments and any chemical present in them. The implication is that metals and
organic contaminants present in sediments cannot be assumed to forever remain buried.
The factors influencing the fate of metals in estuaries has been reviewed by Church and Scudlark
(1998) who discriminate between Type A metals such as iron, chromium and manganese which
readily associate with particles, and Type B metals such as copper, nickel and zinc which complex
with organic matter. Although this is a very broad generalization and conditions can be very sitespecific, Type A metals tend to be removed primarily by sedimentation while Type B metals are
more likely to be transported out of the region by advection.
Di Toro et al. (2001) have reviewed the residence time of metals in the water column and conclude
that half-lives vary, generally, from less than 10 to 22 days depending on depth, particle
sedimentation rates and particle-water partitioning.
In 1988 the National Research Council of Canada commissioned a comprehensive report on
“Biologically Available Metals in Sediments” (Campbell et al. 1988). This report reviews the
geochemistry of metals in sediments, factors affecting bioavailability, trophic transport from
sediment, as well as measurement and bioassay techniques. A total of 17 recommendations were
made with the general intent of improving the understanding of bioavailability of metals in
sediment. In the 12 years since that report much has been accomplished. For example, Bergman and
Dorward-King (1997) have compiled a recent review of the environmental speciation, fate,
23
toxicology and regulatory aspects of metals in aquatic systems. Among the recommendations
regarding metal fate and transport was that models should be designed “to protect both the water
column and sediment” and further research on “understanding the mechanism and rates of metals
released from resuspended sediment is needed” (Di Toro 2001).
Clearly metals in sediments are less available than those in the water column, but they are available
to some extent and given the “right” conditions they can become readily available. It is therefore
concluded that sedimentation is not an unqualified mitigating factor.
A considerable body of quantitative predictive capability exists on the speciation of metals, their
association with particulate matter, their reaction in sediments with sulfide and on rates of transport
between sediments and the water column. Di Toro (2001) has compiled models describing sediment
water fluxes in a variety of settings in a comprehensive text on the subject. As a result of extensive
research in recent decades, behaviour and availability in water-sediment systems is thus well
understood and can be quantified. Concepts such as the FIAM, the ratio of SEM (simultaneously
extracted metal) to AVS (acid volatile sulfide) and the BLM (biotic ligand model) are powerful tools
for evaluating metal availability and potential to cause adverse effects on organisms. Much remains
to be done and the FIAM has limitations (Campbell 1995). It is not yet fully explained how uptake
of metals by fish by respiration resulting in binding to gill surfaces compares to uptake by ingestion
as a contaminant source (Fisher and Hook 1998). Generally, in water column exposures, the free
ion in the dissolved phase appears to be the best predictor of the toxicity of the metal ion. Despite
the inevitable exceptions and uncertainties, the science of metal fate and effects is well advanced
and quantification of behaviour using mass balance models is entirely feasible.
5. A SUGGESTED STRATEGY FOR EVALUATING PERSISTENCE
On the basis of the discussion to date, it is suggested that the following multi-stage strategy be
adopted when evaluating the persistence of all substances.
(1) Characterization of media of chemical partitioning
This involves determining the relevant media for evaluation. These are media in which the chemical
is present in a significant quantity. One method of defining “significant” is to assess if inclusion or
exclusion of the medium results in a change in the calculated persistence by an amount such as 5%.
24
Obviously if there is significant exposure or risk relating to the chemicals’ presence in a medium,
then that medium should be included regardless. For many substances it will be obvious that a
specific medium is irrelevant. For example, in the case of metal ions, air can obviously be ignored.
(2) Characterization of chemical speciation
This involves determining if the substance exists in more than one species which will be present in
significant quantities or proportions. Significance may be determined by quantity or toxicity.
Further evaluation will then involve the total quantity of all species, and if desired assessment of
individual species which are deemed to be significant.
For example, in the case of benzene only one species applies. For phenol which dissociates, albeit
to a small extent at environmental pH values, total protonated and ionic levels need only be
evaluated. For pentachlorophenol which dissociates appreciably it will be necessary to treat total,
protonated and ionic forms. For metals it will be necessary to treat total and various ionic forms,
and in some cases organometallic forms. The approach adapted thus depends on circumstances
including the availability of speciation data.
(3) Selection of model configuration and conditions
A model is then selected which treats the relevant media defined in (1) and the species in (2) and
conditions are defined relevant to the likely environmental exposure conditions.
The most general model will treat a multispecies chemical in a multimedia environment, both as
total and individual species. Often a single species model will be adequate. For metals and
involatile organics the atmospheric compartment can be ignored, unless there is emission to air, in
which case atmospheric deposition must be considered. In some cases only one compartment need
be considered. For example for freons only the atmosphere is likely to be relevant. For metals
which are discharged to the aquatic environment it may be adequate to treat a water-sediment
combination. Conditions of volumes flowrates, phase compositions, temperature etc. are also
selected on the basis of likely environmental conditions.
Numerous models are available for use at this stage including atmospheric models, water quality
models, sediment-water flux models, soil leaching and run-off models and general multimedia
25
models. A basic requirement is that the model be fully documented and transparent to the user so
that inherent assumptions and parameter values can be inspected. The model should, if possible,
have been validated by application to similar chemicals under similar conditions.
(4) Model Calculations
The model is then run for a hypothetical or realistic discharge of the chemical into each relevant
medium individually, and if desired into all media simultaneously. The simplest and preferred
approach is to apply a constant discharge under steady-state conditions since this involves only
manipulation of algebraic equations. As was discussed earlier, problems arise if a pulse input is
used and there is intermedia transport because an unequivocal half-life cannot be easily determined
as a result of the non-first- order decay in total quantity.
Assuming that steady-state conditions are used, the individual media residence times are calculated
as (mass inventory)/(rate of loss) for losses by degrading reactions, advection, intermedia transport,
and total losses as described earlier. The corresponding half-lives can also be calculated. The
overall residence times are also calculated similarly for each mode of entry individually and any
desired combination of modes of entry. If desired an overall half-life can be calculated as 69% of
the overall residence time.
The resulting residence times or half-lives are tabulated to provide a basis for regulatory judgement.
For some substances which are stable the degradation values will be infinite. It is also desirable to
tabulate the rate constants or reciprocal residence times since the rate constants for degradation,
advection and intermedia transport add to give a total rate constant which is the reciprocal of the
total residence time. An infinite half-life corresponds to a zero rate constant. This conveys more
clearly which processes are most important.
(5) Interpretation of persistence
The obvious key quantities are the residence times or half-lives attributable to degrading reactions.
Residence times attributable to advective and intermedia transport losses are generally accepted as
not relevant in this context because they merely relocate the chemical to a “downstream”
environment, or temporarily move it to another medium.
26
It is a matter of judgement if the individual or overall residence times or half-lives should be
evaluated, but probably both should be considered.
It is noteworthy that if the model is linear in concentration the overall residence time or half-life for
a multimedia mode of entry (e.g. to air plus water) is the sum of the individual residence times or
half-lives for air only and water only, weighted according to the proportions to each. It is thus not
necessary to run a multiple mode of entry condition since it can be deduced by adding contributions
from individual values. Further, residence time or half-life is an intensive property of the system
and is independent of the actual emission rate. For example, doubling the emission rate doubles the
inventory and the rates of loss, thus the ratio, which is the residence time, is unchanged.
This approach is best illustrated by a series of examples in which a variety of chemicals is evaluated
using a number of models.
6. ILLUSTRATIVE APPLICATION OF THE PROPOSED STRATEGY
6.1 Model and Chemical Selection
In consultation with the scientific authority a set of chemicals was identified for evaluation. These
are listed in Table 3 which includes key property data used in the models which are taken largely
from Mackay et al. 2000, Di Toro (2001), Di Toro et al. (2001), and Diamond et al. (1990).
Two models were selected for use in the evaluation. First, is the well-established EQC model which
is widely used for assessing the fate of a variety of chemicals. The Level III version was used in this
study. It is a general multimedia model primarily designed to treat organic substances and is fully
described in a series of papers by Mackay et al. (1996a, b, c). Table 4 gives some relevant
conditions. Second, is a water quality model based on the QWASI approach as described by
Mackay (2001).
This model was originally formulated using fugacity, which requires that the
vapour pressure or Henrys Law Constant of the substance is known. It was later modified to be
applicable to non-volatile substances by replacing fugacity by an analogous equilibrium criterion,
aquivalence which is essentially fugacity divided by Henrys Law Constant. The model used here
has the option of using either fugacity or aquivalence, and can thus treat organics and metals. In this
application the model treats a connected air-water-sediment system and is parameterized to
27
correspond to a Canadian Shield Lake. The relevant dimensions and conditions are given in Table
5.
The reason that two models were used is that the second model is tailored to describe conditions
which are of primary interest when assessing the influence of chemicals, including metals in
Canadian freshwater lakes. It is expected that the residence times or half-lives in water and
sediment calculated by the two models will differ because of different hydrological conditions. It
is of interest to explore the magnitude and effect of such differences.
28
Table 3. Chemical Properties
Physical-Chemical Properties of Organic Chemicals (KOW is the octanol-water partition
coefficient). Metal ion partition coefficients were selected as typical of pH 6 conditions.
Higher pH values tend to increase the partition coefficients and will reduce residence time in
the water column as a result of faster sedimentation. PCP is treated as an inonizing organic
acid at pH 6. Data are from Mackay et al. (2000)
Chemical name
Molar
Mass
Solubility
3
(g/m )
(g/mol)
Vapor
Log
Pressure
KOW
pKa
(Pa)
Half-lives (hours)
Air
Water
Soil
Sediment
DDT
354.5
0.0055
1.35 x 10-4
6.19
170
5500
17000
55000
PCB 155
360.9
0.002
0.003636
7
5500
55000
55000
55000
PCB 52
292
0.03
0.002
6.1
1700
55000
55000
55000
HCB
284.8
0.005
0.2447
5.5
17000
55000
55000
55000
naphthalene
128.19
31
36.81
3.37
17
170
1700
5500
pyrene
202.3
0.132
0.0119
5.18
170
1700
17000
55000
benzo[a]pyrene
252.3
0.0038
2.13 x 10-5
6.04
170
1700
17000
55000
phenol
94.1
88360
67.66
1.46
9.89
17
55
170
550
pentachlorophenol
266.34
14
0.12
5.05
4.92
550
550
1700
5500
Physical-Chemical Properties of Metals including KPW the particle-water partition coefficient and
KSW, the sediment-water partition coefficient (L / kg). Data are from Woodfine et al. (2000),
Diamond et al. (1990), Di Toro (2001) and Allen et al. (1998).
Substance name
Atomic Mass
KPW
Copper (Cu)
63.55
5.49 x 10
Nickel (Ni)
58.7
1440
KSW
4
4.58 x 104
1200
6
7.80 x 105
Iron (Fe)
55.85
1.02 x 10
Lead (Pb)
207.2
4.16 x 105
3.48 x 105
Zinc (Zn)
65.38
9.31 x 104
7.60 x 104
Arsenic (As)
74.92
1.80 x 105
2.00 x 105
29
Table 4. Properties of the EQC environment. See Mackay et al. 1996 for full details
Area
Air
Water
Soil
Sediment
100,000 km2, 25oC
104 m3 containing 2000 m3 aerosol
2 x 1011 m3 containing 107 m3 particles and 2 x 105 m3 biota
18 x 109 m3, 20% air, 30% water, 50% solids
5 x 108 m3, 80% water, 20% solids
Organic carbon contents; particles 0.2, soil 0.02, sediment 0.04 g/g
Advection residence times; air 100 h, water 1000 h, sediment 50,000 h
Table 5. Properties of the Shield Lake
Temperature 25oC, pH 6 but adjustable
Water area 2 x 107 m2, depth 10 m, volume 2 x 108 m3
Sediment area 2 x 107 m2, depth 0.01 m, volume 2 x 105 m3
Sediment 90% water, 10% solids, organic carbon content 0.05 g/g
Particles 2 mg/L, organic carbon content 0.30 g/g
Aerosols 30 µg/m3 deposition velocity 10 m/h
Sediment deposition rate 2 g/m2"day
Sediment resuspension rate 0.5 g/m2"day
Sediment burial rate 0.5 g/m2"day
Water inflow and outflow 10000 m3/h
Rain rate 1 m/year, scavenging ratio 200000
Both models were run in steady-state form with illustrative emissions of 1000 kg/h to the EQC
model and 10 kg/h to the lake model, the lake being much smaller in volume.
6.2 Results of the EQC model
The results of the EQC model can be given as tables but the mass balance diagram is of most interest
in this context since it contains the essential information. Figure 2 gives the EQC output for DDT
when emitted to air, water, soil and all three media. A set of individual media residence times and
half-lives can be calculated which apply to all simulations, as can the overall residence times and
half-lives which depend on mode of entry. In all cases the residence time is a ratio of mass in the
30
source medium to flux from that medium. The degradation residence times are merely the input
degradation half-lives divided by 0.693.
It is clear that a large number of residence times and half-lives can be deduced and are available for
interpretation of persistence. In addition to the individual media degradation half-times (which are
input quantities), the most illuminating quantities are the overall degradation residence times or halflives.
Table 6 gives the residence times attributable to degradation for the entire set of chemicals for three
modes of entry. For metals the degradation rate constants are zero and the degradation residence
times infinite. The advective residence times and half-lives (which are not given) are identical for
all chemicals, but the intermedia residence times and half-lives do vary from chemical to chemical
as a function of chemical properties. For ionizing organics such as phenol and PCP an estimation
of speciation is included as described by Mackay et al. (2000).
31
Figure 2. EQC outputs for DDT emitted to air, water, soil and all three media
32
33
Table 6. Summary of overall reaction residence times in the EQC model with emissions to
air, water and soil , using reaction half-lives as defined in the handbook (Mackay et al.
2000)
Reaction residence times (in days)
Chemical
DDT
PCB 155
PCB 52
HCB
naphthalene
pyrene
BaP
phenol
PCP
Cu
Ni
Zn
Pb
Fe
As
Emission to air
805
2950
1191
2058
1
150
901
2
82
infinite
infinite
infinite
infinite
infinite
infinite
Emission to water
2421
3298
3115
3233
8
702
1809
3
81
infinite
infinite
infinite
infinite
infinite
infinite
Emission to soil
1023
3307
3305
3306
88
1021
1023
10
102
infinite
infinite
infinite
infinite
infinite
infinite
The persistence as expressed by degradation residence times or the corresponding half-lives varies
greatly from a few days to infinity. The task of categorizing these into classes of environmental
concern is the task of the regulator, but we offer the following suggestion.
An overall half-life in classes 1 to 5 from Table 2, i.e. up to 1000 hours or 42 days represents rapidly
reacting substances for which persistence is unlikely to be of concern. They will experience 8.7
half-lives per year thus only 0.2% will survive a year.
An overall half-life in classes 8 to 10, i.e. exceeding 10000 hours or 417 days react very slowly or
not at all and are of high concern in the context of persistence. This includes metal ions.
Overall half-lives in class 6 i.e. 1000 to 3000 hours will be of low to intermediate concern,
especially if they are also bioaccumulative or toxic.
34
Half-lives in class 7 i.e. 3000 to10000 hours will be of intermediate to high concern.
6.3 Results of the Shield Lake Model
The results of the lake model are presented as a set of diagrams a series of which are given in Figure
3. Using these results it is straightforward to deduce water column and sediment residence times
and half-lives and the overall residence times and half-lives and include values for advection,
intermedia transport, degradation and total losses. Again the degradation residence times and halflives of metals are infinite, but the advection and intermedia transport values are finite.
35
Figure 3. Shield Lake results
36
37
38
39
40
41
42
43
Table 7 gives detailed results for DDT showing the 13 residence times of each compartment process.
Of these, the water column degradation and sedimentation residence times and the corresponding
half-lives are of most interest. These quantities are tabulated for all the chemicals in Table 8.
Table 7. Process rates and residence times of DDT in a Shield Lake as deduced from
Figure 3A. Half-lives can be calculated as 69% of the corresponding residence
times.
Compartment
Loss (kg/year)
Residence time (year(days))
Water
Degradation
1.99
0.90 (328 days)
1.80 kg
Advection
0.79
2.28
Evaporation
1.10
1.64
Sedimentation
13.50
0.13 (47 days)
Total
17.38
0.10 (36 days)
Sediment
Degradation
2.31
9.04
20.89 kg
Burial
3.81
5.48
Resuspension
7.38
2.83
Total
13.5
1.55
Overall
Degradation
4.3
5.27
22.70 kg
Adv/Bur
4.6
4.93
Evaporation
1.10
20.63
Total
10
2.27
44
Table 8. Summary of degradation and sedimentation residence times in the water column
of a Shield lake, in years (days).
Chemical
Reaction (years (days))
Sedimentation (years
(days))
DDT
0.9 (328 days)
0.13 (47 days)
PCB 155
9.05
0.04 (14 days)
PCB 52
9.1
0.03 (11 days)
HCB
9.03
0.34 (123 days)
naphthalene
0.03 (10 days)
2.7
pyrene
0.28 (102 days)
0.60 (221 days)
BaP
0.28 (102 days)
0.12 (45 days)
phenol
0.01 (3.4 days)
2.9
PCP
0.09 (33 days)
0.76 (276 days)
Cu
infinite
0.30 (108 days)
Ni
infinite
2.29
Zn
infinite
0.19 (70 days)
Pb
infinite
0.06 (24 days)
Fe
infinite
0.04 (16 days)
As
infinite
0.12 (42 days)
The degradation residence times and half-lives range from a few days to infinity as is the case for
the EQC results. The sedimentation residence times range from about 14 days (half-life 10 days)
to several years depending on hydrophobicity or particle-water partitioning. The metal ion residence
times lie in the range 16 to 108 days (half-lives 11 to 75 days) and overlap the range of the organics.
Substances which are highly particle-associated regardless of organic or metallic character tend to
be removed from the water column fairly rapidly, i.e. with a residence time or half-life of weeks by
sediment deposition. This is consistent with the review by Di Toro et al. (2001).
45
It is apparent that highly sorbed chemicals are approaching an asymptotically short residence time
of about 10 days or a half-life of 7 days. If all the chemical is associated with particles, the
residence time of the chemical approaches that of the particles. Since the mass of particles in the
water column is 4 x 108 g and the deposition rate is 4 x 107 g/day, i.e. 2 g/m2"day times an area of
2 x 107 m2 the residence time of the particles (and associated chemical) is 10 days and the half-life
7 days. Less sorption leads to reduced fluxes and a longer residence time or half-life. DiToro
(2001) has given the equations describing water-sediment dynamics of such systems in an appendix
to his text.
It is misleading to suggest that a specific number of days can be assigned to the sedimentation halflife or residence time as a characteristic of a chemical, because this quantity depends on the
sedimentation rate and the lake depth. Di Toro (2001) quotes a number of short half-lives measured
by Diamond et al. (1990) but these were in a 2 m deep water body and are thus very short. Couillard
(2001), using an approach suggested by Santschi (1984), has compiled data for Zn residence times
and has shown in Figure 4 that they can vary from 20 days to 10000 days depending on the lake
characteristics, especially the particle flux per unit volume. For regulatory purposes, residence time
or half-life in the water column is thus an inappropriate metric because it is so variable
46
Figure 4. Residence time (t) of Zn in lakes or in aquatic mesocosms as a function of the
particle flux through the water column per unit volume. Couillard (2001), adapted
from Santschi (1984).
47
7. CONCLUSIONS
It has been shown that using steady-state mass balance diagrams deduced from models or empirical
data it is possible to derive a number of residence times or half-lives from which a selection can be
made for the evaluation of persistence for regulatory purposes. If unsteady-state or dynamic
conditions are used the derivations of persistence are more complex.
Advective residence times and half-lives are viewed as being of no regulatory significance since
they merely express the residence time of the flowing medium in the volume of interest. They are
not properties of the chemical. They are hydrological features of the water body or atmospheric
conditions.
Intermedia transport residence times and half-lives depend on rates of deposition and diffusion, but
since they merely relocate the chemical within the system they cannot be regarded as eliminating
the chemical from concern permanently. They are certainly important as determinants of chemical
fate and exposure in a specific medium such as a water column, but they do not address the larger
picture of environmental fate. Since, like advective residence times and half-lives they vary
geographically, they can not be used effectively in a regulatory context for chemical hazard
assessment.
Degradation reaction residence times and half-lives are properties of the chemical and its reactive
environment and are of primary interest in a regulatory context for assessment of a chemical’s
persistence. For individual media these are directly related to the input degradation half-lives, but
for the overall system they depend on partitioning properties, the relative volumes of the media and
the mode of entry.
Di Toro et al. (2001) have suggested that the intermedia residence time or half-life in water be used
as a descriptor of metal persistence instead of degradation persistence (which is infinite). Such an
approach will inevitably lead to distortion of the chemical evaluation process. If, for example, a
criterion of a few weeks is used, many priority organic substances which degrade only very slowly,
but sorb appreciably to sedimenting particles will have residence times or half-lives less than the
criterion and may be classified as non-persistent according to this approach. The approach, if
applied to organics, also lacks discriminating power in that a hydrophobic organic substance with
48
a long degradation half-life may be classified as less persistent than a less hydrophobic one with a
short half-life.
Such a persistence criterion is essentially use of the particle-water partition
coefficient and particle deposition rate. It is not a true persistence. Nor, for reasons discussed
earlier should availability be confused with persistence. Persistence and availability are entirely
different properties and should be addressed separately.
The fact is that metals released into solution from commercial inorganic substances by dissolution
or transformation processes, like some very recalcitrant organic substances, are essentially totally
persistent and inevitably fall into a long persistence class. It is misleading to imply otherwise, by
invoking a combination with availability or treating only residence time or half-life in one medium
and ignoring others.
This is not to imply that residence time or half-life in the water column or availability should not
be considered as mitigating factors in the risk assessment of metals. These factors must be
addressed, but separately from persistence.
It is concluded that it is entirely feasible to use a single model to assess all chemicals for persistence,
including metals. The definition of persistence suggested here is simple and unambiguous. Issues
such as availability and media-specific residence times are best addressed separately in the hazard
or risk assessment. The principal scientific challenges are to obtain accurate data on chemical
reaction rates in the environment and to treat speciating chemicals with sufficient detail and rigour.
Existing models can treat total concentrations adequately, at least approximately, but much remains
to be done in the development of improved models for assessing this challenging class of substances.
49
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