Environmental impact of two successive chemical treatments in a small shallow eutrophied lake: Part I. Case of aluminium sulphate

This paper deals with the efficiency and effects of addition of aluminium sulphate on soft water quality of a shallow eutrophic lake. Almost all the controlled variables improved with treatment, especially nutrient concentrations such as soluble reactive phosphorus (SRP) and transparency. However, aluminium sulphate was not added in sufficient quantity to reduce the total phosphorus content, SRP concentration was significantly reduced in the short term. Moreover, external loading of phosphorus was high and not taken into account by the in-lake treatments. Finally, resuspension of sediment (polymictic lake) removed the alum hydroxide layer on the sediment surface, which reduced treatment effectiveness. No significant pH decrease was noted following alum addition. According to bibliographical toxicological data, monomeric aluminium content does not show any toxic effect on aquatic fauna and flora. In spite of low SRP in the water column, the treatment did not prevent appearance of Microcystis sp. colony (>10 colony per ml) approximately 30 days after alum application.