No abstract

Research conducted on urban lakes in Poland has proven the degradation of the quality and utility of their waters. This is caused by the contamination of these lakes with sewage and wastewater from adjacent towns, urban run-off and excessive recreation. The most effective external source controls are rarely implemented due to high costs. However, in-lake activities are being conducted with application of different techniques. The removal of nutrient-rich hypolimnetic water by pipeline was the first restoration experiment designed and performed in Kortowskie Lake by Prof. Olszewski in the mid-sixties. It proved to be effective in retarding the process of lake degradation. Recently, aeration devices such as Ecoflox, Miniflox and Microflox have been installed in several urban lakes in order to improve its water quality. New devices, such as bio-hydro structures designed in Poland and installed in several water bodies, have given promising effects in water quality improvement. The long range assessment of restoration techniques and monitoring of lake water quality is conducted and supervised by the Institute of Environmental Protection.

Water exchange in lakes reflects on their trophic status and has implications on water quality. However, in contrary to common belief, beneficial effects of dilution or ‘flushing’, due to high hydraulic load, should not be expected in the case of polluted tributaries. Research conducted in Poland on several hundred lakes indicated, that 80% of lakes with intensive water renewal rates, (200%/y to 21400%/y) were characterized with bad water quality. Those lakes with low, (30% or less) water renewal rate per annum mostly had good or even very good water quality. Inferior water quality of lakes having low water renewal rate was usually associated with the presence of point sources of pollution. Contribution from the tributary sources would improve the lake water quality only if reduction of the loadings to the rivers is achieved through watershed pollution control.