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HISTORICAL RECORDS FOR LAKE SEDIMENT
SEDIMENT CORES
The success of pollution remediation programs is easily evaluated by comparing current with past levels of pollution. However, in many cases long - term records of pollution are lacking or of questionable validity. One method of obtaining an historical chronology of pollution is to examine sediment cores from receiving water bodies such as lakes, river deltas, and reservoirs. Fine - grained sediment (silt and clay) entrapped in the bottom of lakes and reservoirs preserve the historical nature and levels of many types of pollutants. Sediment cores can also be used to reconstruct lake pH conditions from planktonic material deposited with the sediment.In the absence of long time - series of pH measurements in lakes, it is difficult to reconstruct the historical record of acidification. Yet, the careful study of diatom assemblages in numerous lakes of various acidities, together with lake sediment dating (eg. with radioactive isotopes of 210Pb) has led to a new method of historical pH reconstruction based on key alkaliphilous and acidophilous diatom species. This method is now currently used in some countries such as the UK. In Lake Gardsjön, Sweden, an abrupt pH drop near 1960 marks the beginning of the acidification effect.
Studies of the sediments of Lake Ontario have shown that heavy metals such as copper (Cu) and lead (Pb) are associated with fine grain particulate matter. These particles are transported by currents and settle to the bottom sediments of the lake. Sediment cores taken from these areas show enrichment of heavy metals in the top 10 cm. It is significant however, that phosphorus control measures, especially the use of activated sludge and control of industrial sources, have had a significant and beneficial impact by reducing the discharge of metals. Metals have an affinity for sludge and are therefore removed as part of the treatment process. This can be seen in the surface layers of the sediment where copper and lead levels are declining.
Investigations of large lake water quality issues requires the integration of multi-disciplinary studies. Water temperature is a major driving force controlling the physics, chemistry and biology of the lake ecosystem. Temperature gradients can be observed in large lakes. For example, the Lake Ontario one metre surface temperature contours show the influence of wind and circulation on surface distribution.
UNEP has published a summary document on world lakes and reservoirs. The document highlights three major themes: the value of lakes and reservoirs, the deterioration of water resources and examples of prevention and restoration activities, and examples of prevention and restoration activities (Ref. 29).
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