Geoscience Reference
In-Depth Information
Table 12.1 Lead compositions (mg/kg) in cultivated soils (Zhang
2003
)
1913
1923
1933
1944
1955
1967
1974
1985
1995
CC
a
26
28
32
35
38
34
42
38
42
2
b
2
2
2
2
2
3
2
3
CO
26
28
30
35
36
35
34
37
34
2
2
2
2
2
2
2
2
2
COH
20
23
29
41
33
33
34
38
34
1
1
2
3
2
2
2
2
2
19
c
-
d
20
19
19
23
26
-
34
1
1
1
1
1
2
2
21
e
20
19
18
18
20
19
-
20
1
1
1
1
1
1
1
1
1
a
Subplots with continuous corn (CC), rotation of corn and oats (CO) and rotation of corn, oats,
and hay (COH)
b
Number in italics is the uncertainty for the value above
c
Data for soils with depth from 0.18 to 0.33 m
d
Data not available
e
Data for soils with depth from 0.33 to 0.51 m
12.1.1 Heavy Metals
An example of heavy metal deposition and transport in soils with time is given by
Zhang (
2003
), who surveyed 100 years of atmospheric lead contamination of
Mollisols in the Morrow plots of Champaign, Illinois, USA. Data on Pb content of
agricultural soils between the years 1900 and 2000 under different cropping sys-
tems are presented in Table
12.1
. Based on these data, the authors report that Pb
content in surface soils increased with time during the first half of the twentieth
century and slowly decreased in the second half.
Lead disposed on the land surface began to be transported downward into the A
horizon, in appreciable amounts, in the 1950s; the background value of 20 mg/kg
in the upper soil B horizon, at *60 cm depth, remained relatively constant
(Fig.
12.1
). These data show that more than 50 % of the atmospherically derived
Pb was transported into the soil-subsurface system. However, it is not clear what
precise mechanisms affected the Pb mobility.
Often, heavy metals and metalloids reach the land surface and subsurface
regions not as isolated compounds but as a mixture of compounds with different
properties. For example, while Pb transport occurs mainly in colloidal forms, other
heavy metals such as Zn and Cd exhibit downward movement in dissolved forms.
The characteristics of the porous medium solid phase, the chemistry of trans-
porting water phase, and biogeochemical redox processes affect heavy metal and
metalloid transport in the soil-subsurface system. Here, we present several par-
ticular cases of heavy metal and metalloid aqueous transport in the soil-subsurface
system, where the driving forces are well defined.
In the soil-subsurface system, colloid-facilitated transport can be a major
pathway for strongly adsorbed contaminants. Operationally defined as particles
