Geoscience Reference
In-Depth Information
Case study 1.2 Temporal changes in sediment composition as a result of pollution:
archives of lead pollution
Sediments that have accumulated in aquatic systems may be good records of the external inputs
affecting sediment composition, and can be used as both indicators of pollution impacts upon
sediment composition, and as archives of local and global pollution changes. Extensive studies
on lake, reservoir and river basins have been undertaken. This example focuses on lead in lake
and reservoir sediments. Lead (Pb) is a natural element, which is supplied to sediments through
geological weathering of rocks and mineral deposits. However, Pb has also been sourced to
sediments through two anthropogenic processes: the mining and smelting of Pb ores, and the
combustion of fossil fuels, especially vehicle fuel with added Pb. Levels of Pb in the environ-
ment are a concern because Pb can act as a powerful neurotoxin.
Studies of sediment composition in lakes in northern Scandanavia have documented long-term
(over 2000 yr) records of sediment-Pb composition (Brännvall et al. 1999). Sediment composi-
tion in these studies (Case Fig. 1.2) shows consistent changes in Pb deposition to these sediments
over a wide area. An early peak at around 2000 yr BP was related to lead smelting during the
Greek and Roman cultures. This was followed by peaks in Pb deposition around AD 1200 and
1530, again related to lead smelting and coinciding with known peaks in metal production
in Europe. The presence of significant Pb in sediments of these ages indicates that sediment
composition was being markedly altered prior to the Industrial Revolution, and that pollution
levels in the environment are not all a recent phenomenon. These lake sediments also show a
clear input of Pb in the latter half of the twentieth century (Case Fig. 1.2), related to the burning
of fossil fuels and the use of Pb in petrol. This late twentieth century impact upon sediments has
also been documented by numerous other studies from a wide range of sedimentary environ-
ments (e.g. Renberg et al. 1994; Shotyk et al. 1998; and see Chapter 6). The use of stable isotopes
of Pb (
206
Pb /
207
Pb) further clarifies the increases of anthropogenic Pb pollution from natural Pb
inputs from soil weathering (Case Fig. 1.2a).
Shorter time periods of Pb-pollution impacts on sediment composition have been studied
in reservoir sediments, as these systems commonly display faster sediment accumulation rates,
thereby allowing increased temporal resolution. For example, Callender & Van Metre (1997)
studied sediment cores from water reservoirs in the south-west and Midwest USA (Case Fig. 1.2b).
This study clearly documented a high Pb peak in sediment between 1970 and 1980, linked to
atmospheric Pb input from leaded-fuel combustion. More recently deposited sediments display
a clear, and rapid, reduction in Pb levels, as a result of the Clean Air Act of 1970 in the USA,
and also the phasing out of leaded-fuel.
Similar records of Pb pollution have also been documented from Scottish freshwater lakes
(Eades et al. 2002). Sediments in Loch Lomond show an increase in Pb past 1800 as a result
of industry and fossil-fuel burning (Case Fig. 1.2c). A large increase in the latter half of the
twentieth century, coupled with a significant drop in the
206
Pb/
207
Pb of this Pb was a result of
vehicle combustion of fuel with Pb additives. A significant fall in Pb levels since the 1980s was
a result of the increasing use of unleaded petrol in vehicles.
Relevant reading
Brännvall, M.-L., Bindler, R., Emeteryd, O., et al. (1997) Stable isotope and concentration records of atmospheric
lead pollution in peat and lake sediments in Sweden.
Water, Air and Soil Pollution
100
, 243-52.
