Environmental Engineering Reference
In-Depth Information
Table 2.9
Ionic radii of several metals and metalloids that frequently occur with iron and some
that don't
Fe
3+
Fe
2+
K
+
Cr
3+
Ni
2+
Co
2+
V
3+
Ti
2+
Cu
+
Cu
2+
Zn
2+
Mn
2+
Pb
2+
Ba
2+
Sb
3+
Sn
4+
0.64
0.74
1.33
0.63
0.69
0.72
0.74
0.76
0.96
0.72
0.74
0.80
1.20
1.34
0.76
0.71
The National Environment Protection Council
(NEPC) Environmental
kg
dw
−
1
under the
assumption the chromium will be hexavalent. However, other ANZECC
6
criteria
are respectively 50 mg
Investigation Level
(EIL)
for chromium is set at 1 mg
·
kg
dw
−
1
for level B (
Environmental Soil Quality threshold
standard necessitating further investigations
) and 400 mg
·
kg
dw
−
1
for NEPC for
level B (
Soil Investigation Level for residential land with substantial vegetable
garden contributing at least 10% of vegetable and fruit intake
). Cr
6+
is not stable
in soils which contain easily oxidisable organic matter, as is always the case in
natural soils. Therefore the NEPC (EIL) is irrelevant. However, Table
2.9
indicates
the Cr
3+
would fit easily in any tri-valent iron compound in the soil and therefore
is likely to be part of the crystalline structure of the ferruginous matter and equally
insoluble.
The samples that proved to have elevated chromium concentrations invariably
were the same where illuviation had caused the accumulation of iron along with
clay in the B horizon. Or, in another case near Broadford, Victoria, where fer-
ruginous nodules had developed at the interface between A and B horizons where
seasonal waterlogging and drying occurs. Apparently the enrichment of the subsoil
with chromium is merely a pedogenetical phenomenon. Biotite is also an iron-rich
mineral and so it is likely the chromium is contributed by the biotite by isomor-
phous substitution for iron. Figure
2.16
illustrates that samples number 1-63, which
are Duplex soil profiles with dark reddish brown B horizons, always have more
chromium in the subsoil than in the topsoil. Samples number 64 and higher all
relate to shallow skeletal soils (Leptosols), without subsoils overlying biotite gneiss
bedrock.
Another proposed development site near Seymour also caused environmental
doubts to arise when a number of soil samples showed high chromium. All these
samples were from shallow topsoil which contained large numbers of ferruginous
concretions (Fig.
2.17
).
Although iron is not considered a contaminant, proper insight into soil chemi-
cal behaviour of chromium resulted in the decision to additionally measure the iron
concentrations in soil, with the purpose of understanding the distribution and mobil-
ity of chromium in the soil. This approach proves that chromium in these soils at
Bandiana and Broadford is practically inert and risk for the ecosystem or to human
health is negligible.
·
6
Australia and New Zealand Environment Conservation Council.
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