Chemistry of Freshwaters - Principles of Environmental Chemistry - page 139

escape to surface waters and groundwater. While cyanide quickly
decomposes in oxygenated, acidic surface water, it can persist at toxic
levels for much longer periods in groundwater and thus pose a longer-
term threat.
3.3.3 Historical Pollution Records and Perturbatory Processes in Lakes
3.3.3.1 Records - Lead in Lake Sediments. Under ideal circumstances,
freshwater lake sediments can preserve the record of temporal variations
in anthropogenic input of contaminants via atmospheric deposition,
catchment runoff, euent inflow and dumping from industrial, transpor-
tation, mining, agricultural, and waste disposal sources. 104 Prerequisites
are transfer of contaminants associated with settling inorganic particu-
lates and/or biotic detritus from the water column to the sediments, no
disturbance of sediments by physical mixing, slumping or bioturbation
after deposition, no post-depositional degradation or mobility of the
contaminants, and the establishment of a reliable time axis (e.g. via the use
of the naturally-occurring radionuclide 210 Pb, half-life 22.35 years, and
the nuclear fallout radionuclide 137 Cs, half-life 30.2 years).
The derived records of lead deposition to sediments in Loch Ness and
the northern and southern basins of Loch Lomond, Scotland, over the
past few hundred years since the onset of the Industrial Revolution are
shown in Figure 6. 105 The noticeable change in the 206 Pb/ 207 Pb ratio of
the lead deposited from the late 1920s onwards is attributed to car-
exhaust emissions of lead of lower 206 Pb/ 207 Pb ratio as a result of the use
of lead ores from Broken Hill, Australia, in the manufacture of alkyllead
additives for the UK market. On the basis of the published literature,
there appears to be reasonable confidence that, in the absence of post-
depositional mixing, sediments do, in general, preserve a record of the
deposition of lead to the sediments. 105-107 Many such records in Western
Europe and North America now show concentrations of lead declining
towards the sediment surface as a consequence of the withdrawal of
leaded petrol and also a reduction in other emissions. 105,108 Longer-term
palaeolimnological records, stretching back over several thousand years,
demonstrate the influence of lead mining and smelting in Greek-Roman
times and the later impact of the Mediaeval mining industry. 109
3.3.3.2 Perturbatory Processes in Lake Sediments. The extent to
which conditions for preservation of records are met depends upon
the characteristics of the specific individual systems and contaminants
under study. In reviewing the perturbation of historical pollution
records in aquatic sediments, Farmer 110 has suggested that in the case