Evaluation of Partitioning and Persistence of Organic
Substances on Environment Canada’s Domestic
Substances List
A Report to Environment Canada
CEMC Report No. 200105
Prepared by:
David Woodfine and Donald Mackay
Canadian Environmental Modelling Centre
Trent University
Peterborough, Ontario K9J 7B8
CANADA
CONTENTS
Executive Summary
iii
1. Background
1
2. The Nature of Persistence
2
3. Theory
3
3.1 Environmental Volumes and Characteristics
3
3.2 Chemical Properties, namely Z values, Half-lives, Rate constants and D
4
4. DSL Classification
10
4.1 Stage I: Initial classification assessment for persistence and bioaccumulation
10
4.2 Stage II: Classification for Persistence Alone
11
4.3 Stage III: Partitioning
12
4.4 Sample Output
16
4.5 Possible inconsistencies of estimates in the database
18
5. References
19
Acknowledgment
19
-ii-
EXECUTIVE SUMMARY
This report provides an evaluation and recommendations regarding the partitioning and persistence of 11,648
organic substances on the Environment Canada Domestic Substances List. A list of these organic substances
was provided by the Scientific Authority including: substance name, CAS number, molar mass, melting point,
estimated water solubility, measured and/or estimated log KOW, pKa values, and the estimated air, water, soil
and sediment half-lives.
The estimated or measured logKOW values were compared with the Environment Canada criterion for
bioaccumulation of logKOW > 5. A total of 2,812 substances or 24% of the total were found to exceed this
criterion.
The estimated half-lives were compared individually with the Environmental Canada criteria for persistence
or half-life in the four media of air, water, soil and sediment. A total 2,999 chemicals or 26% of the total were
found to exceed at least one of these persistence criteria.
In total 4,061 chemicals or 35% exceed the Environment Canada Criteria due to their bioaccumulation
potential as indicated by KOW or their persistence. Some chemicals exceed both criteria.
The nature of persistence as an attribute of a chemical substance is reviewed, including the merits of
examining persistence or half-life on a medium by medium basis, versus the overall persistence or residence
time in the combined media. Level I, II and III multimedia fugacity models are reviewed briefly as a means of
incorporating partitioning information into the assessment of persistence. It is concluded that a Level II model
is preferred for the present purposes because there is no mode of entry information available. More advanced
Level III models may be used for more detailed, later screening level risk assessments of priority substances
lists.
An assessment was undertaken using a Level II fugacity model of the equilibrium partitioning of the chemicals
in an evaluative environment similar in dimensions and properties to those used in the EQC model. The
fractions, which partition into each medium were calculated. This information was used to determine the extent
to which each medium contributed to the overall persistence and to the overall degrading reactions. It was
found that in many instances a substance exceeded the specified half-life criterion, but only a very small
fraction of the substance actually partitioned into that medium, i.e., it is not “realistically present” in that
medium. It is recommended that in such cases where the “realistic presence” of the substance in a medium
is negligible there is no need to apply the half-life criterion and a hitherto “persistent” substance can be
declared to be “non-persistent” because of a lack of “realistic presence”.
The cut-off used to specify the fraction which determines “realistic presence” is examined, and it is concluded
that a figure of 5% is justified, i.e. if less than 5% of the substance partitions into a medium, then the half-life
in that medium can be safely ignored because the substance is not “realistically present” in that medium. A
very conservative figure of 1% could be used, yielding similar results. The upper limit to this figure is judged
to be 10% and would result in the addition of about 100 substances. Overall it is suggested that the 5% figure
gives the most effective discrimination and should be used to define “realistic persistence”. This concept of
“realistic presence” should be considered as one of several factors influencing decisions on chemical
characterization.
-iii-
A computer program was written to evaluate all the chemicals and designate them as being in one of three
groups: “clearly persistent” or ”non-persistent as a result of lack of realistic presence in a medium” or “clearly
non-persistent”. In addition the program checks the “reasonableness” (i.e. if the chemical parameters are
outside of the normal ranges observed) of the input data and flags those data, which may be suspect. An
individual narrative is produced for each chemical summarizing its input and output data and the conclusions
as regards persistence. If desired the 5% figure can be changed and the chemicals re-evaluated.
Of the 1,299 chemicals which exceed the air criterion of 2 days, if the additional requirement is imposed that
at least 5% of the substance must be in air, then 417 (32%) of these, substances are “clearly persistent” and
satisfy both criteria i.e. they are persistent and realistically present in air. A further 882 (68%) are “ nonpersistent as a result of lack of realistic presence”.
Of the 1,818 chemicals, which exceed the water criterion of 6 months, if the additional requirement is imposed
that at least 5% of the substance must be in water, then 1,406 (77%) of these substances are “clearly
persistent” and satisfy both criteria, i.e., they are persistent and realistically present in water. A further 412
(23%) are “non-persistent as a result of lack of realistic presence”.
Of the 1,818 chemicals, which exceed the soil criterion of 6 months, if the additional requirement is imposed
that at least 5% of the substance must be in soil, then 589 (32%) of these substances are “clearly persistent”
and satisfy both criteria i.e. they are persistent and realistically present in soil. A further 1,229 (68%) are “nonpersistent as a result of lack of realistic presence”.
Of the 1,818 chemicals, which exceed the sediment criterion of 1 year, if the additional requirement is imposed
that at least 5% of the substance must be in air, then none of these substances are “clearly persistent” and
satisfy both criteria i.e. they are persistent and realistically present in sediment. All 1818 are “non-persistent
as a result of lack of realistic presence”. The reason that sediment criteria are not exceeded is that the
estimated sediment half-life in all cases was four times the soil half-life, but the volume of sediment is only
about 2% of the soil. At equilibrium there can never be more than 2% of the chemical in the sediment.
Hydrophobic chemical which will partition appreciably to sediment will automatically exceed the soil criterion,
therefore they will not escape identification as persistent.
The overall result of the examination of 11,648 chemicals is that 8,649 (74%) are clearly not persistent by any
criterion. There are 2,999 chemicals that exceed one of the Environment Canada single medium criteria. By
applying the additional “realistic persistence” criterion of 5% a total of 2,190 are classified as persistent and
“realistically present” i.e. “clearly persistent”. The number of chemicals in this class is; 417 for air, 1,406 for
water, 589 for soil and none for sediment. This totals to 2,412 “exceedences” but 222 of these chemicals have
more than one exceedence (i.e. they are persistent and present in both air and water or soil), therefore the
total number of “clearly persistent” chemicals is 2,190. A total of 809 (27%) are classified as being nonpersistent due to a lack of “realistic presence”.
The net result is that of the 2,999 chemicals, which exceed one or more of the single media half-life criteria
only 2,190 can be regarded as clearly persistent. This approach has therefore been successful in removing
27% of the apparently persistent chemicals from further consideration, thus contributing to a more focused
evaluation of the chemicals of greatest concern.
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1. BACKGROUND
The categorization and classification of substances on the Domestic Substances List (DSL) is mandated under
sections 73 and 74 of the Canadian Environmental Protection Act (CEPA 1999). Section 73 obliges the
Ministers of Environment and Health to categorize, within 7 years, the substances that are on the Domestic
Substances List for the purpose of identifying those substances on the List that, in their opinion and on the
basis of available information,
a)
b)
may present, to individuals in Canada, the greatest potential for exposure; or
are persistent or bioaccumulative within the meaning of the regulations, and inherently toxic to nonhuman organisms and to human beings, as determined by laboratory or other studies of non-human
organisms.
The DSL contains approximately 23,000 substances. Of these, there exist approximately 12,000 organic
substances. As identified in Section 73(1)(b), the Minister of the Environment must determine, as part of this
process the persistence of the 12,000 organic substances. The criteria for persistence are defined in the
Persistent and Bioaccumulation Regulations under CEPA 1999.
The persistence (P) criteria described in the Persistent and Bioaccumulation Regulations were selected by
an expert panel of scientists, based on knowledge of the properties that are most characteristic of persistent
organic pollutants. Criteria for describing substances as persistent are listed below. A substance is considered
persistent if its transformation half-life satisfied the criterion in any medium as identified as follows in Table 1.
Table 1: Single media persistence criteria.
Medium
Air
Water
Soil
Sediment
≥
≥
≥
≥
Half-life
2 days
6 months
6 months
1 year
Note: a substance is considered persistent when the criterion is exceeded in any one medium.
A substance may be considered as persistent in air if it is shown to be subject to atmospheric transport to
remote regions such as the Arctic.
A question arises when assessing individual environmental compartments to determine whether a substance
is persistent under these regulations. If a substance exceeds the half- life criterion in an environmental
compartment (i.e. in air, water, soil or sediment) but it is predicted to not partition (i.e. it is not likely to be
present) in that environmental medium, then is the substance really persistent? The Categorization and
Classification Program has sought guidance on how to address the issue of partitioning compared to presence
in an environmental compartment when deciding whether or not a substance is persistent under the
Regulations.
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Accordingly, the purpose of this report is to develop an evaluation system including a fugacity model to
evaluate the partitioning and persistence of the 12,000 organic substances on the DSL using physicalchemical properties of the substances. An assessment of these partitioning predictions along with estimated
half-lives for each chemical in the four environmental compartments of interest was carried out in order to
provide the best discrimination in terms of “realistic presence” of a substance in environmental compartments
to compare with the half-lives criteria.
Environment Canada provided the authors with a list of some 12,000 CAS numbers, chemical names,
predicted half-lives in each environmental compartment, and predicted and measured physical chemical
properties. This report describes the resulting system and makes recommendations on its implementation. In
addition, while examining the input data certain potentially problematic data were identified. A complete
evaluation of the entire set of chemicals is provided separately and contains a categorization and identification
of potentially problematic data. A computer program (in MS Excel format) is also provided.
2. THE NATURE OF PERSISTENCE
It is useful to clarify the nature of persistence, why it is important and how it is quantified. Persistence can be
considered to be the undesirably long continued presence of a chemical. It is usually quantified by a half-life.
If we consider two chemicals, S with a Short half-life of 10 days and L with a Long half-life of 20 days, then
after equal masses of these chemicals have been exposed to environmental degradation for 20 days, half of
L will remain and a quarter of S, thus environmental exposure to L will be greater by a factor which approaches
two after a prolonged time. Concern is increased accordingly.
Another metric of persistence is residence time at steady state. If both S and L are emitted at a rate of 100
kg/day into a medium, then, when steady-state is reached and input rate equals output rate there will be a
mass of S of 100 x 10/0.693 or 1,443 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 that mass is 1,443 kg. It
follows that the residence time of the chemical is 1,443 kg / 100 kg/day or 14.43 days. This is, of course the
half-life of 10 days divided by 0.693. For chemical L the corresponding rate constant is 0.0346 days-1 and the
mass will be 2,886 kg. The concentration of L will thus be twice that of S and exposure will increase by this
factor.
We conclude that persistence can be equally well expressed by three metrics, half-life, residence time, or
quantity present at steady-state in the environment. These quantities are closely related and convey the same
information.
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 et al. (1998) have
discussed this issue 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 environment.
If all of the substance partitions into soil and the half-life is 100 days then clearly the overall half-life is simply
100 days. But if 50% partitions into water where the half-life is 25 days and 50% partitions into soil, then the
overall half-life can be shown to be 40 days. It is not entirely clear how this partitioning is best calculated. It
could be done from monitoring data but such data are lacking for most chemicals. The simplest approach is
-2-
to assume equilibrium partitioning in a defined environment. Issues then arise concerning the dimensions of
this environment, i.e. the relative and absolute volumes of air, water, soil and sediment. Partitioning data are
also needed. A more rigorous approach is to evaluate residence time or persistence in a non-equilibrium
environment. This also requires partitioning data but in addition it requires “mode of entry” information, i.e. into
which medium the chemical is discharged. This arises because the proportions of the chemical in air, water,
soil and sediment depend on whether the chemical is discharged to air water or soil and how fast it migrates
between the media. This type of analysis can be accomplished by a Level III model, but since the DSL
database does not include mode of entry information it cannot be applied at this stage. It is more likely to be
applied at later more detailed stages of chemical evaluation.
We conclude that given the available data an equilibrium or Level II partitioning calculation is preferred.
3. THEORY
A description is given here of the theoretical basis of Level I and II calculations. Full details are given in the
text by Mackay (1991, 2001) and in a series of papers, notably Mackay and Paterson (1982).
It is not necessary for the purposes of this study for the reader to appreciate the full mathematical details of
these Level I and Level II calculations. For many readers, and especially those who are not familiar with
fugacity models, it may be more convenient to skip the theory and go directly to the summary statement at the
conclusion of this section.
Both Level I and Level II calculations require three classes of information, namely those defining the
environment, the chemicals properties and the quantity of chemical.
3.1 Environmental Volumes and Characteristics
The environment is evaluative in nature, i.e. it is a hypothetical or designed environment consisting of four
primary media, air, water, soil and sediment. The volumes and compositions (e.g. organic carbon contents)
are defined. The earliest models treated an area of 1 square kilometre containing 70% water area, thus
representing a microcosm of the Earth. Later models including the EQC model (Mackay et al., 1996) treated
a larger area of 100,000 km2 with 10% water. Some models such as EUSES, which is used for assessment
in the European Union, have a smaller water area. This is a result of the relatively few large lakes in Europe
compared to North America. In this study the EQC environment is used. The volumes of the four
compartments in the EQC model are given in Table 2.
Table 2: The volume and area used in of each medium used in the EQC model.
Compartment
Volume (m3)
14
Area (m2)
11
Area (%)
Depth (m)
Air
1 x 10
1 x 10
100
1000
Water
2 x 1011
1 x 1010
10
20
10
90
0.10
10
as for water
0.01
Soil
Sediment
9
9 x 10
8
1 x 10
9 x 10
1 x 10
-3-
The soil is assigned a typical organic carbon content of 0.02 g/g and the organic content is treated as being
equivalent to 35% octanol. The net effect is that each 1 m3 of soil is equivalent to 0.017 m3 of octanol in its
partition properties. Similarly for sediment with a typical organic carbon content of 0.04g/g each 1 m3 is
equivalent to 0.034m3 of octanol. The total volumes of “octanol equivalent” soil and sediment are thus 1.5 x108
m3 and 3.4 x 106 m3, Gouin et al. (2000) used these quantities to arrive at a set of volumes ratios in Table 3.
Since the absolute quantity of chemical and the absolute environmental values are not known, it is simpler to
use these ratios in the Level I and II calculations in order to calculate relative partitioning. It is noteworthy that
the volume of the sediment is only about 2% of the volume of the soil
Table 3: Relative volume ratios and octanol equivalent for air, water,
soil and sediment used by Gouin et al. 2000.
Compartment
Relative ratios
Octanol equivalent
Air
650,000
not applicable
Water
1,300
not applicable
Soil
58.5
0.978
Sediment
0.65
0.022
3.2 Chemical Properties, namely Z values, Half-lives, Rate constants and D values
The chemical reactivity data are reported as half-lives in days. The corresponding rate constant k is
0.693/(half-life). The D value or fugacity transformation rate parameter is VZk, where V is the compartment
volume and Z is the Z value (fugacity capacity) for that compartment.
For air the ZA is calculated as 1/RT where R is the gas constant (8.314 Pa m3/mol K) and T is the temperature
at which the data apply, i.e. 25 oC or 297K
For water , Zw is calculated for a non-ionizing substance as Cs/Ps where Cs is solubility (mol / m3) and Ps is the
vapor pressure (Pa). This ratio is also 1/H where H is the Henry’s Law constant. The air water partition
coefficient KAW is H/RT or ZA/ ZW or Ps/ CsRT. Note that the units of CS are mol/ m3 not g/m3.
If the solubility (S) is given in g/m3 then KAW can be calculated as PSM/SRT where M is the molar mass (g/mol).
For soil and sediment the partition coefficients with respect to water are calculated as
KSW = 0.35 N KOW ρ/1000
Where, N is organic carbon content, ρ is the density kg/ m3 and KOW is the octanol-water partition coefficient.
Z values can then be calculated as ZWKSW.
Later, soil and sediment are treated as consisting of equivalent volumes of octanol, namely 1 m3 is treated as
(0.35 N ρ/1000) m3 of octanol.
-4-
Ionizing substances
When an organic acid ionizes, in a simple one-stage process the ratio of ionized (I) to non- ionized (NI) is given
by I/NI = 10 ( pH - pKa )
A typical environmental pH is 6.0 i.e. slightly acid therefore if pKa exceeds 8.0 ionization occurs to 1% or less
of the substances and can be ignored. For a substance that contains a functional group that ionizes twice in
sequence (as occurs with sulfate or carbonate) there are two sequential pKa values and:
I/NI = 10 ( pH – pKa1) [1 + 10 ( pH – pKa2)]
Where pKa1 and pKa2 are the two dissociation constants. The lower of the two is pKa1, and this is the most
readily released proton. In the database this is flagged as MA i.e. most acid.
A similar approach can be applied to the higher degrees of ionization.
For a substance, which has two distinct functional groups i.e. a di-hydroxy compound, the two extents of
dissociation simply add and the equation becomes:
I/NI = 10 ( pH – pKa1) + 10 ( pH – pKa2)
A similar approach can be applied to the higher degrees of ionization.
The effect of ionization is to increase ZW above that of the parent compound but not other Z values i.e. the
water has an increased capacity for the chemical. The factor increase is (NI + I)/ NI or 1+ I/NI where I/NI is
calculated as above.
This “correction” can be applied when needed, but any pKa exceeding 8 can be ignored.
The solubility in water reported in the database is that of the neutral or non-ionic species. A correction for
ionization was therefore included. For bases such as amines a pKa or pKb can be defined such that pKa is (14 pKa). An acid with a low pKa (e.g. 4) ionizes appreciably as does a base with a high pKb (e.g. 10). In the
interest of simplicity the bases are treated using a pKa equation. The same equation is thus applied to bases
since they are also characterized by pKa.
Level I Calculations
The chemical properties required for Level I are molar mass, solubility in water, vapor pressure, octanol-water
partition coefficient (KOW ) and if applicable, the pKa of dissociating substances.
For Level I the total quantity of chemical present is required, usually 1000 kg. The equilibrium distribution of
this quantity of chemical is calculated including masses, concentrations and percentages. A common fugacity
applies. There is no input or output of chemical. The fugacity f (Pa) is calculated as follows:
f =
M
ΣVZ
-5-
Where M is total amount (mol), V is medium volume (m3), Z is the chemical’s Z value or fugacity capacity in
each medium (mol/m3 Pa). Concentrations (C mol/m3) are calculated as Zf and amounts (m mol) as CV or VZf.
The sum of amounts m must equal M.
Level II Calculations
For Level II the equilibrium distribution of the amount resulting from a steady-state input balanced by output
by reaction is calculated. The percent distribution on Level I and II are equal but the absolute amounts differ.
Output rates are expressed as D values (mol/Pa h) defined as VZk where k is the reaction rate constant or
0.693 / half-life. The fugacity is calculated from:
f =
E
ΣD
Where E is the emission rate (mol/h) usually set at 1,000 kg/h. A separate D is deduced for each medium. The
amounts present in each medium and in total are calculated as before. The rates of reaction are Df mol/h.
An invaluable quantity is the overall persistence or residence time
τO =
M
E
or f
∑ VZ
E
The corresponding overall half-life is 69.3% of this residence time.
For the purposes of this study it is useful to expand on the factors influencing the contribution of each medium
to the overall persistence.
The overall rate constant k0 =
1
τ
O
or
E . Substituting for both E and M yields:
M
k0 =
fΣ D
ΣVZk
ΣD
=
=
fΣVZ ΣVZ
ΣVZ
For example if there are two media, 1 and 2
k0 =
where, F1 =
(V1Z1k1 + V2 Z 2 k 2 )
= F1k1 + F2 k 2
(V1Z1 + V2 Z 2 )
V1Z1
V2 Z 2
and F2 =
V1Z1 + V2 Z 2
V1Z1 + V2 Z 2
-6-
Fi is the fraction of the amount of chemical present in each medium. Therefore, the rate constants are weighted
by the mass fractions to give the overall rate constant, which in turn can be used to calculate the overall
persistence.
Clearly if F1 is zero, the reaction rate constant, k1 is irrelevant because none of the chemical will be subjected
to this reaction. It is unlikely that k1 could be measured because of the difficulty of creating an amount in
medium 1 with which to measure a rate or half-life.
A more difficult issues which arises when only a very small quantity is present in medium 1, e.g. F1 = 0.01. It
is then very unlikely that reaction in this medium is important.
The exception could occur when k1 is 100 times faster than other media rate constants. Intuitively it seems
unwise to declare a substance persistent on the basis of k1 being small (half-life long) if virtually none of the
chemical is present in medium 1. It is clearly a matter of judgment what fraction F1 is deemed to be
insignificant. A major task of this project is to shed light on this issue.
One approach could be to decree that if degradation in a specific medium (as dictated by Fi and ki) is found
to contribute negligibly to the overall persistence then that medium could be ignored in the evaluation. This
will occur when Fi is small. Now the overall persistence can be expressed as:
τ0 =
1
M
Σm
ΣVZf
ΣVZ
=
=
=
=
k 0 ΣVZk ΣVZkf
fΣ D E
Therefore for the two- medium system:
τ0 =
m1 m 2 MF1 MF2
+
=
+
= τ1 + τ 2
E
E
E
E
Where τ1 = F1 τ0 and τ2 = F2 τ0
Each medium contributes to the total persistence in proportion to the mass fraction in that medium. This
applies even if there is no reaction in the medium, i.e. even if ki and Di are zero.
Persistence is thus derived from the mass present at steady state and each medium contributes to the total
persistence regardless of whether or not reaction occurs in it.
It is then possible to assert that, for example, if a medium contains 5% of the chemical mass or less, it must
contribute 5% or less to the persistence.
It is important to discriminate between contributions to persistence or overall half-life and contributions to
reaction or degradation rate. The overall degradation rate can be expressed as
-7-
k0 =
ΣVZk
= ΣFk
ΣVZ
Therefore for a two compartment system
k 0 = F1k1 + F2 k 2
The individual rate constants ki add in proportion to their mass fractions in each medium to give the total or
overall rate constant k 0. We designate the contribution of each medium to the total reaction rate as R percent
where
Ri =
100 × Fi ki 100 Fi 100 Fiτ O
=
=
τi
k0
τi
(1 / τ 0 )
Here J is the reciprocal of k. Since half-lives are 0.693 / k, R can also be calculated directly from half-lives as
follows where T is half-life and is 0.0693 J.
Ri =
100 Fi / Ti 100 Fi × TO
=
ΣF / TO
Ti
It is important to note that the percentage contribution of a medium to persistence, designated here as P is
100 Fi as stated above. The percentage contribution to the overall reaction rate (R) is 100 Fi τ0/τi. Thus Ri is
Pi( τ0/τI). A medium will have a large R and a small P when it has a short half- life compared to the overall halflife. For a dominant medium responsible for much of the reaction and persistence R and P will approach a
common value.
These concepts are best illustrated by an example, which is the result of a Level II calculation, which yields
the data in Table 4.
Here the air criterion of 2 days is exceeded, but other media satisfy the criteria. The air contains only 2% of
the chemical.
The overall rate constant k0 is 0.0066 days –1 and the overall residence time is the reciprocal of 151 days. The
overall half-life is 105 days. The overall half-life consists of contributions as follows: Air, 2 days; Water, 8 days;
Soil, 84 days; Sediment, 11 days.
Ignoring air reduces the persistence by only 2 days or 2%, thus air contributes little to the persistence. But air
is responsible for 30% of the chemical loss, thus removing it as a reaction medium results in an increase in
overall persistence or residence time to approximately 217 days.
This is probably not a particularly persistent substance because in the course of a year (which is 3.5 half-lives)
the amount of chemical surviving will be 1 / 23.5 or 9.0%.
-8-
Table 4: An example of a calculation of R and P for a multimedia chemical.
T (days)
τ (days)
F
k (day-1)
Fk(day-1)
F/T
R(%)
P (days)
P(%)
Air
6.9
10
0.02
0.1
0.002
0.0029
30
2
2
Water
139
200
0.08
0.005
0.0004
0.00058
6
8
8
Soil
139
200
0.80
0.005
0.004
0.00575
61
84
80
Sediment
278
400
0.10
0.0025
0.0002
0.00036
3
11
10
0.0066
0.00959
100
105
100
Total
1.0
T is half-life,
F is fraction in each medium,
k is the rate constant (i.e. 0.693 / T),
τ = persistence or residence time
Overall rate constant (kO) = 0.0066 days –1
Overall residence time (τO) = 1 / 0.0066 = 151 days
Overall half-life (TO) = 0.693 / 0.0066 or 105 days
Note: overall half-life is also = 0.693 * 151 days = 105 days
or = 1 / 0.0096 = 105 days
Ri = 100 * Fi ki / ko , This is the percentage contribution to the overall rate of reaction.
Peri = FiTO and is the absolute contribution (days) that each medium has to the overall half-life
P the percentage contribution to persistence is simply 100 F and is also 100 Peri / TO
Note that the contribution of each fraction to overall persistence is equal to the medium’s mass fraction of
chemical.
Summary
Using the physical chemical properties and assumed (EQC) environmental volumes and properties it is
possible to calculate the equilibrium partitioning of a substance between the media of air, water, soil and
sediment. These fractions or percentages are designated F. Using the Level II model, which involves the four
half-life estimates, it is possible to calculate an overall persistence (τ), which proves to be a weighted mean
of the individual media persistences. The fraction or percentage of the overall persistence attributable to the
chemical’s presence in each medium designated P can also be calculated. The value of P for each media
proves to be equal to 100 F, i.e. persistence can be assigned directly on the basis of the fraction present in
a medium.
Another percentage R can be defined as the percentage of the total (overall) reaction, which occurs in each
medium. This value R is closely related to P because both depend on F, but P and R are not equal.
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These quantities F, P, and R and the overall persistence (τ) can be used to explore how partitioning can be
taken into account when evaluating persistence against the criteria. The approach, which is taken in the next
section is to explore “cut-off” values for P and F below which the substance can be considered to be “not
realistically present”. For example, if this “cut-off” is set at 5% and the substance is declared persistent
because it exceeds the criteria in only one medium, but P for the medium is 4%, then that half-life can be
discounted and the substance can be declared to be “realistically not persistent” because so little of it resides
in the medium in which the exceedence occurs.
4. DSL CLASSIFICATION
Given the theory presented earlier the DSL list was screened in several stages.
4.1 Stage I: Initial classification assessment for persistence and bioaccumulation
The list was first screened to determine what fractions are:
(1) persistent according to the criteria, but not bioaccumulative
(2) bioaccumulative according to a log KOW of 5 criterion, but not persistent
(3) are both persistent and bioaccumulative
(4) are neither persistent or bioaccumulative
This was done by simple comparison with the criteria. No model was needed. All 11,649 chemicals were
“scored” as follows:
If the estimated log Kow $5.0 score = 1
If the measured log Kow $5.0 score= 1
If the half-life in air was $2 days score = 1
If the half-life in soil/water $ 182 days score = 1
If the half-life in sediment was $ 365 days score = 1
TOTAL SCORE sum of scores maximum 5
The results of this assessment are shown in Table 5.
-10-
Table 5: Number and percentage of the chemicals screened that exceed bioaccumulation
and/or persistence criteria.
TOTAL number of chemicals with
Number of chemicals
Percentage
2,803
24.1%
136
1.2%
SCORE OF 5
3
<0.1%
SCORE OF 4
64
0.5%
SCORE OF 3
864
7.4%
SCORE OF 2
1,019
8.8%
SCORE OF 1
2,973
25.5%
SCORE OF 0
6,724
57.7%
TOTAL number of chemicals
11,648
100.0%
Est. log KOW >5
Meas. log KOW >5
There were 6,725 or 58% of the chemicals from the list would be unlikely to exceed either the bioaccumulation
and/or persistence criteria. These were eliminated from further consideration.
4.2 Stage II: Classification for Persistence Alone
Chemicals were screened for persistence using the following criteria:
If the half-life in air was $2 days score = 1
If the half-life in soil/water $ 182 days score = 1
If the half-life in sediment was $ 365 days score = 1
The results are shown in Table 6.
Table 6: The total number of chemicals with half-lives that exceed the persistence criterion for each
medium.
TOTAL number of chemicals with
Number of chemicals
Percentage
AIR T1/2 $2
1299
11.2%
SOIL or WATER T1/2 $182
1818
15.6%
SED T1/2 $365
1818
15.6%
118
1.0%
2999
25.6%
OVERLAP (number of chemicals that exceed both soil and air T1/2)
TOTAL chemicals that exceed one or more persistence criteria
*Note: 2999 is 1299+1818-118 because 118 chemicals appear in both totals.
-11-
Conclusions
There were 2,999 chemicals that exceeded one or more of the persistence criteria. This is 26% of the total
number of chemicals. Air, water/soil and sediment half-lives appear to be approximately equal in importance.
Some 74% of the chemicals are rated as being non-persistent.
4.3 Stage III: Partitioning
The physical-chemical properties were used to estimate the equilibrium (Level I and II) partitioning into air
water, soil and sediment. The volume fractions used were those suggested by Gouin et al. 2000 and based
on the EQC fugacity model mentioned earlier.
The fraction in each medium can then be calculated as follows; the subscript being A –air, W-water, E-soil and
S-sediment.
FA = (650000×KAW) / [(650000× KAW)+(1300×KWW)+(0.978×KOW)+(0.022×KOW)]
FW = (1300×KWW) / [(650000×KAW)+(1300×KWW)+(0.978×KOW)+(0.022× KOW)]
FE = (0.978×KOW ) / [(650000×KAW)+(1300×KWW)+(0.978×KOW)+(0.022× KOW)]
FS = (0.022×KOW ) / [(650000×KAW)+(1300×KWW)+(0.978×KOW)+(0.022× KOW)]
In order to include the effect of ionization KWW was calculated as 1+ I/NI as described earlier, using a pH of
6, which is regarded as typical of environmental conditions.
An overall degradation rate constant (ko) can be calculated as ko = FAkA + FWkW + FEkE + FSkS
An overall persistence (JO) can then be calculated using the following equation that assumes equilibrium
partitioning:
JO = 1/ ko
The contribution, as a fraction, of each medium to the overall persistence is equal to the mass fraction in that
medium.
The contribution in days for each medium (Peri) is Peri = Fi JO and
P, the percentage contribution to persistence is P = 100 Fi
The contribution of each medium to the overall degradation rate (R as a percentage) can also be calculated
using the following equations:
RA = 100 FA kA / ko
RE = 100 FE kE / ko
RW = 100 FW kW / ko
RS = 100 FS kS / ko
-12-
Note that R is largest i.e. degradation is the most important for a medium when the F is large (i.e. much of the
chemical mass resides in the medium) and the rate constant in the medium is a large proportion of the overall
rate constant.
The Chemicals were then “binned” into various ranges of R as shown in Table 7.
Table 7: Number of chemicals in each range of R values in air, water, soil and sediment.
Number of chemicals by medium
Contribution to overall reaction rate (R%)
Air
Water
Soil
Sediment
>90%
213
1,224
383
0
50-90%
111
73
85
0
10-50%
124
72
76
0
5-10%
33
29
35
0
2-5%
60
37
33
0
1-2%
38
13
20
0
0-1%
720
370
1,186
1,818
1,299
1,818
1,818
1,818
TOTAL
Clearly sediment reaction rate contributes negligibly to the total degradation. This is because of the relatively
low volume of the sediment compared to the soil and the typically slow degradation rates.
Water degradation rates are the most important determinants of the overall degradation, thus it can be argued
that water degradation half-life is a critical parameter. There are 1,224 (67%) chemicals for which the water
degradation contributes over 90% to the overall reaction rate.
Soil degradation is less important with 544 (30%) substances having soil degradation rates that contribute 50%
or more of the total degradation rate. For over half (65%) of the chemicals the reaction rate in soil contributes
less than 1% to the overall reaction rate.
Air is similar to soil with 448 (34%) of the chemicals having an air degradation rate contributing 50% or more
to the overall reaction rate. As was the case for soil over half (55%) of the chemicals do not contribute
appreciably (<1%) to the overall reaction rate.
The fractional or percentage contribution of each medium to the overall persistence (P) was calculated using
the following equations:
PA = 100 FA
PE = 100 FE
PW = 100 FW
PS = 100 FS
Note that P is largest i.e. degradation is the most important for a medium when the Fi is large (i.e. much of the
chemical mass resides in the medium). It does not depend on the half-life.
-13-
The Chemicals were then “binned” into various ranges of P as shown in Table 8.
Table 8: Number of chemicals in each range of persistence (P) values in air, water, soil and
sediment.
Number of chemicals by medium
Persistence (P)%
Air
Water
>90%
Soil
Sediment
186
1,226
394
0
50-90%
81
72
87
0
10-50%
109
76
75
0
5-10%
41
32
33
0
2-5%
74
35
32
399
1-2%
35
12
17
91
0-1%
773
365
1,180
1,328
1299
1818
1818
1818
TOTAL
The proportions in Table 7 and 8 are generally similar because both depend heavily on F.
Water is the most important with 1,298 chemicals or (71%) contributing over 50% of the persistence. If a 1%
criterion cut off criterion is used 365 chemicals or (21%) are rejected.
For sediment 1,328 (73%) are below 1% and all are well below 10%.
Finally for air, only 376 chemicals are responsible for greater than 50% of the persistence. If the 1% criterion
is applied 773 (60%) are such that air contributes negligibly to persistence.
Table 9: Number of chemicals that are persistent (P>5%) and non-persistent due to lack of realistic
presence (P<5%) in air, water, soil and sediment and all media combined.
Number of Chemicals
Reaction rate (P%)
Air
Water
Soil
Sediment
All Media
5-100%
417
1,406
589
0
2,190*
<5%
882
412
1,229
1,818
809
1,299
1,818
1,818
1,818
2,999
TOTAL
*Note: Total is not sum of all values because some chemicals have multiple exceedences.
Table 9 shows that by applying the 5% criterion for realistic presence, 417 chemicals are clearly persistent in
air 1,406 chemicals are clearly persistent in water and 589 are clearly persistent in soil giving a total of 2,410
-14-
exceedences. This represents only 2,190 chemicals because some chemicals have multiple exceedences.
A total of 809 chemicals are classified as non-persistent by virtue of lack of “realistic presence”.
The net effect of the 5% criterion is to reduce the total of 2999 chemicals to 2190, a “loss” of 27% and a
retention of 73%. The most convenient method of categorizing is to assign the chemicals to one of three
groups using a computer program to evaluate all the chemicals and designate them as being “clearly
persistent” or “”non-persistent as a result of lack of realistic presence in a medium” or “clearly non-persistent”.
A figure of 5% was used but If desired the 5% figure can be changed and the chemicals re-evaluated.
From an evaluation of the effects of changing the cut-off criterion for “realistic presence”, a figure of 5% is
judged to be reasonable for the following reasons:
A value of 5% or 1 in 20 is generally viewed in scientific circles as representing a probability on the boundary
between accidental and improbable. An event, which occurs but has an inherent probability less than 5% is
unlikely to be accidental and is significant. Confidence limits are often expressed at the 95% level, which is
an acceptance that one point in 20 may be outside these limits as a result of random events. In environmental
science, errors in quantities such as half-lives are probably factors of 2 or 3 and thus much larger than 5% by
an order of magnitude or more. The additional error introduced by neglecting a process, which contributes 5%
of the total persistence is thus swamped by this much larger error. A 5% figure is thus regarded as having
some basis in scientific usage and is conservative.
An examination of Table 8 shows that in the cut-off range 2% to 10% the numbers of chemicals included or
excluded do not change greatly. The 5% figure thus lies in a “valley of sensitivity” region where there is
relatively little movement from one category to another, i.e. it is a region of low sensitivity and it would not
matter greatly if the selected figure was 2% or 10%.
In our judgment a figure of 1% is too low and is close to a sensitive region. There is no possibility that a 1%
error is influential in this assessment. A value of 2% is a feasible limit but is also low. With 10%, a considerable
number of substances is eliminated, but it too is feasible. A value of 50% would be unacceptably high given
that there are only four media. It thus seems reasonable to adopt a figure of 5%. It is, however, emphasized
that this is ultimately a matter of judgment and Environment Canada can readily change this number in the
spreadsheet program. It is unlikely that a chemical will escape control as a result of applying this criterion
because the most persistent substances will be identified in the most critical media.
The sediment numbers require some discussion. If a 5% figure is used, no single chemical is deemed
persistent by virtue of its sediment half-life. The sediment volume is about 1/44th of the soil volume in octanol
equivalents thus the contribution of the sediment to the overall persistence is a maximum of 1/44 or 2.3% of
the soil value. It can never exceed the 5% value. The estimation program always predicts a sediment half-life
four times that of soil and water (which are equal). As a result the maximum sediment reaction rate is 1/(4 x
44) or 1/176th of the soil rate and is negligible. This arises because of the equilibrium assumption. In reality
sediment is more important in situations where it has become an “in-place “ contaminant. Such substances
of potential future concern will be identified using this process. They will not escape identification because they
will be identified as present in soil.
-15-
A computer program was written which accepts the database as input then evaluates the chemicals for all the
relevant properties as discussed above and gives a categorization. The result is either printed out on the
screen or on paper.
In addition the program checks the reasonableness of the input data and flags those data which may be
suspect. An individual narrative is produced for each chemical summarizing its input and output data and the
conclusions as regards persistence.
Those that rank highly in water tend to have a low KAW and high solubility in water (or miscibility with water)
and low values for KOW. Those that rank highly in soil tend to be hydrophobic and involatile.
4.4 Sample Output
Figure 1 illustrates the output of the calculation performed in a printed form for two chemicals. The input data
are reproduced in their entirety including any pKa values. Half-lives are given in days, vapour pressure in mm
Hg and water solubilities in mgL-1. The Henry’s law constant H was calculated in units of Pa.m3mol-1 and KAW
is H/RT. The ratio I/NI (ionic to non-ionic) is calculated from the pKa values. The variable KWW cor is the
“corrected” water partition coefficient and is (1+ I/IN). The percentages in each medium are calculated (100F).
This value is also the percentage contribution to persistence. The contribution to reaction R in each medium
is also calculated, as is the overall persistence and overall half-life. The chemical is the classified as to its
exceedence of the criteria. With regard to persistence in each medium the chemical is identified as clearly
persistent (Yes), not persistent due to lack of “realistic presence” (No due to RP) or clearly not persistent (No).
CAS#
58866
NAME
D-Xylose
Molmass
150.13
KOW
1.05E-02
R%air (C)
0.0
pKa1
15.53
pKa9
Est.logKOW
-1.98
KOC
1.00E+01
R%water (C)
100.0
pKa2
14.63
pKa10
H (C)
1.92E-08
R%soil(C)
0.0
pKa3
13.67
pKa11
12.46
pKa12
Mes.logKOW
t1/2air
0.1
KAW (C)
7.74E-12
R%sed (C)
0.0
pKa4
t1/2 soil,water
8.67
%inair (C)
0.0
t1/2overall (C)
8.67
pKa5
pKa13
t1/2sed
34.68
%inwater (C)
100.0
pKa6
pKa14
pKa7
pKa15
Persistence(C)
EstVP
9.59E-07
%insoil (C)
0.0
EstWatsol
1.00E+06
%insed (C)
0.0
EstlogBAF
-2.67
KWWcor (C)
1.00E+01
12.5
pKa8
I:NI (C)
3.71E-07
Exceed any Criteria
No
Persistent in Air
No
Log KOW >5
No
Persistent in Water
No
Persistent in Soil
No
-16-
CAS#
58899
NAME
Cyclohexane, 1,2,3,4,5,6-hexachloro-, (1a,2a,3B,4a,5a,6B)-
Molmass
290.83
KOW
1.82E+04
R%air (C)
49.7
pKa1
pKa9
Est.logKOW
4.26
KOC
3.38E+03
R%water (C)
3.4
pKa2
pKa10
Mes.logKOW
3.72
H (C)
7.51E+00
R%soil(C)
46.6
pKa3
pKa11
t1/2air
18.7
KAW (C)
3.03E-03
R%sed (C)
0.3
pKa4
pKa12
t1/2 soil,water
180
%inair (C)
9.2
t1/2overall (C)
101.18
pKa5
pKa13
t1/2sed
720
%inwater (C)
6.1
pKa6
pKa14
pKa7
pKa15
Persistence(C)
EstVP
7.83E-04
%insoil (C)
82.9
EstWatsol
4.04E+00
%insed (C)
1.9
EstlogBAF
4.06
KWWcor (C)
1.00E+01
146.0
pKa8
I:NI (C)
0.00E+01
Exceed any Criteria
Yes
Persistent in Air
Yes
Log KOW >5
No
Persistent in Water
Yes
Persistent in Soil
Yes
CAS#
58902
NAME
Phenol, 2,3,4,6-tetrachloro-
Molmass
231.89
KOW
1.23E+04
R%air (C)
1.4
pKa1
Est.logKOW
4.09
KOC
2.00E+03
R%water (C)
26.1
pKa2
pKa10
Mes.logKOW
4.45
H (C)
5.86E-01
R%soil(C)
72.1
pKa3
pKa11
t1/2air
38.7
KAW (C)
2.36E-04
R%sed (C)
0.4
pKa4
pKa12
t1/2 soil,water
60
%inair (C)
0.9
t1/2overall (C)
60.43
pKa5
pKa13
t1/2sed
240
%inwater (C)
25.9
pKa6
pKa14
pKa7
pKa15
Persistence(C)
87
5.63
pKa9
pKa8
EstVP
3.39E-04
%insoil (C)
71.6
EstWatsol
1.79E+01
%insed (C)
1.6
EstlogBAF
3.79
KWWcor (C)
3.34E+00
I:NI (C)
2.34E+00
Exceed any Criteria
Yes
Persistent in Air
No due to RP
Log KOW >5
No
Persistent in Water
No
Persistent in Soil
No
Figure 1: Sample output for chemicals 58866, 58899 and 58902
-17-
4.5 Possible inconsistencies of estimates in the database
When examining the physical chemical properties in the database we became aware that many entries
appeared to be outside the normal ranges of values, which are expected for chemicals of this type. It was
decided to flag these chemicals, with the suggestion that they be scrutinized in more detail. This was not part
of the contract and our exploration of this issue is only preliminary. It may be useful to conduct a separate
evaluation of this issue. In addition there are a number of zero values, which are also flagged. Most serious
is a zero half-life, which implies an infinitely fast reaction and is not feasible. It may be that this represents a
zero rate constant or infinite half-life.
Three parameters were selected. First, if KAW exceeded 30, the chemical was flagged. With the exception of
freons, most volatile organic chemicals have KAW values in the range 0.1 to 1.0. Oxygen has a KAW of about
30 and is of course very volatile. Any organic chemical, which has a KAW exceeding 30, is therefore flagged.
A high value is likely attributable to a very low estimate of solubility in water and/or a high estimate of vapor
pressure. 172 chemicals were flagged.
Second, are log KOW values exceeding 8, which is close to the highest measured values. The reason is
probably the addition of numerous fragment contributions to large molar mass molecules. Such molecules tend
to become sparingly soluble in both water and octanol, thus KOW tends to “plateau” at about 8 or 9. A total of
956 chemicals were flagged.
Third, few solubilities less than 0.001 mg/L have been measured. For example DDT is about 0.003 mg/L.
Solubilities in this range can only be measured with extreme difficulty thus estimates in this range are flagged.
A total of 1,702 chemicals fell into this class.
In addition, if a solubility is estimated to exceed 100,000 mg/L (which is 10%) the substance is very soluble
and probably totally miscible, as is the case with ethanol. The calculation of KAW from such solubilities is very
suspect. A total of 1,427 chemicals fall into this class.
These flags are given in the spreadsheet and in the printout.
The results are summarized in Table 11.
Table 11: Summary of results.
Parameter
Number of Chemicals
KAW > 30
172
Log KOW > 8
956
Estimated Water solubility < 10
-3
1,702
Estimated Water solubility > 10
5
1,427
Total number of Exceedences
4,257 *
Total number of Chemicals
3,136 *
*Note that some chemicals are included in more than one criterion
-18-
6. REFERENCES
Webster, E., Mackay, D., Wania, F.1998. Environ. Toxicol. Chem 17: 2148
Mackay, D. 1991. Multimedia Environmental Models: The Fugacity Approach. Lewis Publishers Inc. Boca
Raton, Florida.
Mackay, D. 2001. Multimedia Environmental Models- The Fugacity Approach. 2nd Edition Lewis Publishers
Inc. Boca Raton, Florida
Gouin,T., Mackay, D., Webster, E., Wania, F. 2000. Environ. Sci. Technol. 34:881-884
Mackay, D., Paterson, S. 1982. Fugacity Revisited. Environ. Sci. Technol.16: 654a-660a
Mackay, D., Paterson, S., DiGuardo, A., Cowan, C.E. 1996. Environ. Toxicol. Chem. 15(9): 1627-1637
Acknowledgement
The authors wish to thank their colleagues at the Canadian Environmental Modelling Centre for their help in
preparing this report.
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