Geology Reference
In-Depth Information
NH
3
/NH
+
N
2
O / NO / NO
2
N
2
80
20
10
9
30
4.1
1300
×
90
2
90
120
30
25
10
2
80
fertiliser
biomass
5
×
dissolved N
compounds
3
×
10
5
SEDIMENTS-
Organic
compounds
Inorganic compounds
1.3
×
10
4
biomass
10
8
8
×
30
2
×
10
8
30
SEDIMENT
×
10
9
kg N a
−
1
×
10
9
kg N
fluxes
reservoirs
Figure 11 Schematic representation of the biogeochemical cycle of nitrogen, indicating
the approximate magnitude of fluxes and reservoirs
(After O'Neill.
23
)
atmosphere from leaded petrol upto 1996, since when emissions from
leaded petrol have declined almost to zero.
24
Lead emitted to the
atmosphere has a lifetime of around 7-30 days and hence may be
subject to long-range transport. Concentrations of trace elements in
polar ice provide a historical record of atmospheric deposition. Meas-
urements (Figure 2) have shown a marked enhancement in lead accom-
panying the increase in leaded gasoline usage, and a major decline in
recent years attributable to reduced emissions to atmosphere.
Atmospheric lead is deposited in wet and dry deposition. Lead is
relatively immobile in soil, and agricultural surface soils in the UK
exhibit concentrations approximately double those of background soil,
which contain ca. 15-20 mg kg
1
derived from soil parent materials,
other than in areas of lead mineralisation where far greater concentra-
tions can be found. Local perturbations to the cycle of lead can be
important. For instance, the lead content of garden soils correlates
strongly with the age of the house. This is probably due to the deteri-
oration of leaded paintwork on older houses and the former practices of
disposing of household refuse and fire ashes in the garden. Lead is also
of low mobility in aquatic sediments and hence the sediment may
provide a record of historical lead deposition (see Figure 8).
