The mutagenicity of the chlorination by#x2010;products in water supplies of the Venetian region (Italy) and in humic acid solutions were analysed. The samples were concentrated with XAD#x2010;2 resin and tested for mutagenic activity onSalmonella typhimuriumTA98 and TA100 strains. Significant mutagenic activity on TA98 strain in the presence (raw water) or absence (chlorinated water) of metabolic activation was observed only for the Po river. All other raw and chlorinated water samples did not induce significant mutagenic effects under the test conditions. The absence of mutagenic activity in almost all the raw water samples suggests very low concentrations of genotoxic pollutants. However, the low values of natural organic carbon and the appropriate treatment conditions used in the local water plants could well explain the negative results obtained for chlorinated waters. In comparison with water samples, the extracts of chlorinated humic#x2010;rich solutions gave highly toxic and mutagenic effects on both theSalmonellastrains, with and without metabolic activation. The addition of H2O2at different times from the start of the chlorination greatly influenced the level of the observed mutagenic effects.
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