Appendix 3

1
Appendix 3

• Frank Wania Evaluating Persistence and Long
  Range Transport Potential of Organic Chemicals
  Using Multimedia Fate Models

2
Evaluating Persistence and Long Range Transport
Potential of Organic Chemicals Using Multimedia
Fate Models
Frank Wania, WECC Don Mackay, Eva Webster, Trent
University Andreas Beyer, Michael Matthies,
Universität Osnabrück
What? development of techiques that incorporate
multimedia fate models in the process of
evaluating candidate POPs for persistence and
long range transport potential. Why? because the
multimedia distribution of a chemical profoundly
affects its environmental persistence and
potential for long range transport.
3
Evaluating Environmental Persistence
overall persistence
Mtot and NRtot can be calculated using a
multimedia environmental fate model such as EQC
multimedia partitioning, and thus t, is governed
by physical-chemical properties mode of
emission environmental characteristics
Webster, E., Mackay, D., Wania, F. Evaluating
Environmental Persistence. Environ. Toxicol.
Chem. 1998, 17, 2148-2158
4
Calculating Overall Persistence
3-compartment level III model used to estimate an
overall persistence of an organic chemical in the
global environment
Wania, F. An integrated criterion for the
persistence of organic chemicals based on model
calculations. WECC Report 1/98.
5
Calculating Overall Persistence
dependence of overall persistence on physical
chemical properties as expressed by log KAW and
log KOW.
Assumptions Equal fraction of emissions into
air, water and soil. Half-lifes 48 h in air,
1460 h in water and 4380 in soil. Level III.
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Calculating Overall Persistence
overall persistence twater with emission into
water only
overall persistence tsoil with emission into
soil only
fair fraction of emissions into soil
0
0.25
1.0
0.5
0.75
1.0
0.75
0
fwater fraction of emissions into water
0.25
0.5
fair fraction of emissions into air
0.5
0.25
0.75
overall persistence tair with emission into air
only
0
1.0
linear additivity of overall persistence t
fairtair fwater twater fsoiltsoil
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Calculating Long Range Transport Potential
Characteristic Travel Distance
CM
CM0
distance it takes for the concentration in the
moving phase (e.g. air) to fall to e-1 or 37 of
its initial value due to degradation in the
moving phase (e.g. air) and net transfer to the
stationary phase (e.g. soil, water).
CM0/e
distance
LM

• Assumptions
• steady-state between moving phase and stationary
  phase
• no dispersion
• advective transport uni-directional

van Pul et al. 1998, Bennett et al. 1998, Beyer
et al. 1999
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Calculating Long Range Transport Potential
Reformulation for Well-Mixed (or Box) Systems
the distance in well-mixed system over which the
concentration in the moving phase falls to half
its input value. Then the rate of advective loss
equals the total loss by reaction 0.5 NIn
NOut (NRM NRS)
facilitates use of traditional multimedia model
for calculation of L
Example Air Moving Over Soil
NRA
characteristic travel distance in air
NIn
air
LA uMA / (NRA NASF) LA uVAZA / (DRA
DASF) where F DRS / (DSA DRS) (fraction of
chemical retained by soil)
NOut
NAS
NSA
soil
NRS
Beyer, A., Mackay, D., Matthies, M., Wania, F.,
Webster, E. 1999. An evaluation of the role of
mass balance models for assessing the long range
transport potential of organic chemicals. Report
9901, Environmental Modelling Centre, Trent
University, Peterborough
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Relationship Between Characteristic Travel
Distance and Overall Persistence
It can be shown that the general formulation for
the travel distance in moving phase M is LM
uMM / NRtot whereas overall persistence was
defined as t Mtot / NRtot
LM uMMt / Mtot
LM is distance a molecule travels during the
environmental residence time (ut), multiplied by
the proportion of mass in the moving medium (MM /
Mtot)
Example Travel Distance in Air for very volatile
chemicals Mair / Mtot 1, thus Lair ut
(maximum possible) for less volatile chemicals
Mair / Mtot is small, thus Lair is small
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Calculating Long Range Transport Potential
maximum travel distance chemical partitions only
into moving phase (air)
1000000
u.tair
HCB
100000
tetraCB
10000
travel ditance in air in km
chlorobenzene
g-HCH
dieldrin
heptaCB
DDT
1000
decaCB
benzene
OCDD
aldrin
100
1
10
100
1000
10000
10000
1000000
half-life in air in hours
minimum travel distance chemical partitions
completely onto particles and deposition is
irreversible
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Calculating Long Range Transport Potential
using a multimedia model (EQC) to estimate a
characteristic travel distance in air and water
(Beyer et al., 1999)
travel distance in water in km
travel distance in air in km
1000000
10000
u.t
u.t
g-HCH
100000
HCB
1000
a-HCH
HCB
tetraCB
10000
100
chlorobenzene
tetraCB
dieldrin
km
DDE
g-HCH
hexaCB
dieldrin
biphenyl
DDT
1000
DDT
10
OCDD
benzene
TCDD
OCDD
aldrin
100
1
0
1
10
100
1000
10000
100
1000
10000
overall persistence in days
overall persistence in days
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Limitations of These Techniques
1. for many candidate substances, not even the
most basic physical-chemical properties are
available. 2. overall persistence and travel
distance are dependent on environmental
characteristics, e.g. temperature. 3. these
techniques provide a scale to rank chemicals
according to the persistence and LRT potential,
but not cut-off criteria, for what constitutes
persistence/ non-persistence, and LRT
potential/no LRT potential.
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Overall Persistence and Global Distribution
Overall persistence of a-HCH as calculated by a
global distribution model during the time period
1947-1996.
persistence is not fixed value, but dependent on
climate and thus on the zonal distribution of a
chemical
Wania, F., and D. Mackay 1999. Global chemical
fate of a-hexachlorocyclohexane. 2. Use of a
global distribution model for mass balancing,
source apportionment, and trend predictions.
Environ. Toxicol. Chem., in press.
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Effect of Temperature on Travel Distance in Air
A drop in temperature causes two opposing
effects 1. reaction half-lifes increase,
resulting in an increase in persistence 2.
partitioning shifts from air into surface media
(soil, water, etc.)
For chemicals with t lt 550 days, Lair always
increases with decreasing temperature. If
degradation in environment is fast, a short Lair
is determined by a short persistence and not by
small partitioning into air. If T drops, the
persistence of such substances will increase
severely and Lair will also rise.
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Comparative Environmental Chemistry of POPs
There is a need to investigate the influence of
zonal ecosystem characteristics (climate,
vegetation, soils, etc.) on the multimedia fate
of organic chemicals Objective Comparing various
ecosystems with respect to their potential to
cause high exposure of POPs to organisms
fate process ecosystem characteristic degradation
- clearance potential by degradation partitioni
ng - dilution potential intermedia transfer -
clearance potential by export / retention
potential - focussing potential within
ecosystem bioaccumulation - focussing potential
within ecosystem