A biological monitoring program (1992-93) was undertaken with the aim of testing the toxic effect of the Lambro, one of the most polluted rivers in Northern Italy. The filtered river samples were tested with S. capricornutum in a 96h exposure growth inhibition assay and with a photobacterial inhibition assay with the LUMISTox System. The unfiltered samples were also tested with LUMISTox, in order to evaluate the role played by the suspended and colloidal material in the water toxicity. The river samples were passed through a series of columns filled with Carbopack B, XAD-2 and C-18 respectively to concentrate organic compound for chemical analysis and enriched with EDTA to complex metals. The Carbopack B procedure proved to be the most efficient among the tested extraction techniques. The de-toxificant effect of the sample treatments was evaluated in terms of percentage increase of the cell density by the algal assay, while the toxicity of the extracted organic compounds was evaluated by LUMISTox System. The comparison of algal assay with chemical analysis results pointed out that the toxicity of the Lambro waters was mainly related to pesticide contamination.

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