Geology Reference
In-Depth Information
exceedence of specific environmental half-lives i.e. Z6 months in soils/
sediments, Z2monthsinwater,Z20 days in air. The latter is partic-
ularly relevant for those chemicals, which may be released to the air and
are resistant to atmospheric degradation and/or deposition, and hence
able to undergo long-range atmospheric transport, with the potential to
contaminate remote areas. This multiple half-life approach is one adopted
by several countries including Canada,
24
whereby a chemical that exceeds
the criteria for any of these compartments may be declared persistent.
However, if the chemical is unlikely to reside in that compartment then
this may unfairly penalise a substance, because it fails the half-life
criterion for a medium into which it does not appreciably partition.
To examine this issue, Gouin et al.
32
proposed a screening level method
to allow the quick assessment of a large number of chemicals, based on
their media-specific half-lives and their partitioning characteristics. An
evaluative environmental fate model, the Equilibrium Criteria Model
(EQM), was used for this study. EQM comprised of four hypothetical
compartments air, water, soil and sediment, where octanol represented
the organic matter found in soils and sediments. Each compartment was
given a volume, in this case to represent the regional or country scale. For
example, the air volume (V
a
) was 10
14
m
3
, that is a land surface area of
100,000 m
2
, with an atmospheric height of 1000 m. 10% of this area was
considered to be covered with water to a depth of 20 m, resulting in a
water volume (V
w
)of2
10
11
m
3
. Soil was considered to have a depth of
0.1 m with the soil volume (V
soil
)calculatedas9
10
9
m
3
, converted to an
equivalent volume of octanol (V
o
) according to
V
o
¼
V
soil
F
0
:
35r
ð
6
:
18
Þ
where F is the fraction of organic carbon in the soil (0.02), r is the density
of soil (2.4 kg L
1
or 2400 kg m
3
), where 0.35 represents the constant
relating the K
oc
to K
ow
(see Equation 6.5). Similarly for sediment, the
equivalent V
o
was 3.4
10
6
m
3
calculated using Equation 6.18, but with
V
sed
as 10
8
m
3
, F as 0.04 and r the same as soil. For this scenario the ratios
of the air/water/octanol volume was
B
650,000:1300:1. The EQM model is
a steady-state, equilibrium model into which a chemical was constantly
discharged and allowed to attain equilibrium between the various media
with no advective losses. Steady state was reached when total reactive losses
equalled the discharge rate and the model was run for 233 chemicals using
respective physical-chemical property data including K
aw
, K
ow
and K
oa
as
well as reactivity data in the various media (i.e. water, air, soil/sediment). At
equilibrium the total mass of a chemical (M, g) can be given by
M
¼
V
w
C
w
þ
V
a
C
a
þ
V
o
C
o
¼
C
w
ð
V
w
þ
K
aw
V
a
þ
K
ow
V
o
Þ
ð
6
:
19
Þ
