Shrivastva, Naresh Gopal
Lakes are suffering from different stress factors and need to be restored using different approaches. The eutrophication remains as the main water quality management problem for inland waters: both lakes and reservoirs. The way to curb the degradation is to stop the nutrient sources and to accelerate the restoration with help of in-lake technologies. Especially lakes with a long retention time need (eco-) technological help to decrease the nutrient content in the free water.
The microbial and other organic matter from sewage and other autochthonous biomasses causes oxygen depletion, which has many adverse effects. In less developed countries big reservoirs function as sewage treatment plants. Natural aeration solves problems only partly and many pollutants tend to accumulate in the sediments.
The acidification by acid rain and by pyrite oxidation has to be controlled by acid neutralizing technologies.
Addition of alkaline chemicals is useful only for soft waters, and technologies for (microbial) alkalinization of very acidic hard water mining lakes are in development. The corrective measures differ from those in use for eutrophication control.
The contamination of lakes with hazardous substances from industry and agriculture require different restoration technologies, including subhydric isolation and storage, addition of nutrients for better self-purification or anaerobic technologies, to eliminate the pollutant nitrate by microbial denitrification. The retention time is an important parameter for modelers and limnologists that allows them to decide, which technologies – hydromechanical, chemical or biological – have to be applied alone or in combination to cope best with each specific problem. The technologies have to be economical and ecological safe.
Keywords: lake degradation | lake restoration | restoration technologies | residence time | recovery
Introduction
Lakes, both natural and man-made suffer from urban, industrial, agricultural and other impacts. As a result, many aquatic ecosystems have become severely degraded and need to be restored. In many countries, large sums of money now being spent to restore such lakes (Allan 1997). The last three decades good experience and expertise has increased worldwide. But the high investments, that are needed demand for a more scientific and sustainable ecosystem management, i.e. to restore the degraded lake to a level that can be permanently sustained through protection and conservation. The water-quality target should be in accordance with quality of natural waters, and without the stresses, that cause degradation, i.e. with a good ecosystem health, long-term stability and sustainability. Such conditions prevailed in the pre-industrial time. In the drainage basins of the lakes the organic matter production has now dramatically increased because of the intensive use of nutrients (N, P) by the agriculture in the lake catchments areas, the industrialization and the growth of the population density. A good orientation about the pre-industrial status may be gained with help of paleolimnological investigations. Deeper layers contain fossils of more sensitive indicator organisms and the thickness of the yearly sediment layer is growing. The time needed to achieve the restoration target is longer than one would expect on basis of some existing models: it depends first of all from residence time (RT) and is included in most mode is applied (Imboden & Gaechter 1978).
Water quality management of lakes and reservoirs was first oriented to control eutrophication and the factors causing increase of nutrient load (Table 1). Many solutions are now available to control eutrophication by minimizing the nutrient inflows. But often organic loads, acidity, salinity or contamination with hazardous substances need to be controlled. The amendment of the conditions may be very different, in some cases contrary to the technologies developed to control eutrophication.