Geology Reference
In-Depth Information
where C
o
is the chemical concentration in octanol (i.e. 'soils/sediment')
(g m
3
). Note that the volume-concentration product (VxCx) gives
the mass of a chemical for a particular environmental medium. The
right-hand side of Equation 6.19 illustrates how using the partition
coefficients, the concentration of a chemical can be deduced for one
compartment with knowledge of the chemical concentration in an
adjacent compartment (e.g. K
aw
C
w
¼
C
a
). The mass fractions of a
chemical in each medium (F) are therefore
Air
ð
a
Þ
: F
a
¼
K
aw
V
a
=ð
V
w
þ
K
aw
V
a
þ
K
ow
V
o
Þ
ð
6
:
20
Þ
Water
ð
w
Þ
: F
w
¼
V
w
=ð
V
w
þ
K
aw
V
a
þ
K
ow
V
o
Þ
ð
6
:
21
Þ
Octanol
ð
o
Þ
: F
o
¼
V
o
K
ow
=ð
V
w
þ
K
aw
V
a
þ
K
ow
V
o
Þ ð
6
:
22
Þ
Figure 4 displays the plot of log K
aw
vs. log K
ow
including points that
represent all 233 chemicals used in the study. The chemicals were selected
to represent environmental contaminants of concern and include a broad
range of different chemical classes with varying properties. Figure 4 is
effectively a partitioning map that illustrates the proportion of a chemical
within each of the compartments. The 45
o
diagonals are lines of constant
log K
oa
,asK
oa
¼
K
ow
/K
aw
(or log K
oa
¼
log K
ow
log K
aw
). Lines of
constant F
a
, F
w
and F
o
were drawn using the volume ratios outlined
above for this particular study. The 1% and 99% lines divide the K
aw
/K
ow
space into regions into which chemical partitioning is predominantly into
one medium. For example, the region to the upper left of the figure is
where more than 99% of a chemical is in air, to the lower right (beyond
the 1% air line) is where more than 99% of a chemical is in the octanol
phase and to the lower left (beyond the 1% octanol line) is where more
than 99% of a chemical is in water. For chemicals that fall into these
'areas' then their half-lives in the respective medium will likely control
their overall persistence and the half-lives in the other media are largely
irrelevant. For example, a chemical present in the lower right of the figure,
within the octanol 'area', will be strongly sorbed and only degradation
data for soils/sediments is likely to be needed. Importantly, half-lives in
air are generally shorter than those in water or organic phases due to
rapid reactivity with the hydroxyl radical (OH
), so that even 0.5%
partitioning to air may result in appreciable degradation with respect to
overall loss. It is therefore useful to include a line that represents the 0.1%
air. The lines corresponding to one third (33%) in each compartment,
converge where log K
ow
is 3.1 and log K
aw
is
2.74; chemicals that fall
within this area ('A & W & O') are truly multi-media and their half-lives
