The Odra, Poland's second longest river, has been polluted by a still unidentified toxin that has killed thousands of fish in one of Poland's worst environmental disasters in recent years.
Minister Anna Moskwa said on Tuesday that three hypotheses were being taken into account in connection with the mass fish die-off in the river.
The first hypothesis speaks about the possibility of chemical contamination of the River Odra by an industrial plant or an illegal discharge of industrial waste.
"The Chief Inspectorate of Environmental Protection (GIOS) has analysed the samples. No toxic substances were found," Moskwa told reporters.
The second hypothesis speaks about natural causes of the disaster, namely, high temperature, a low water level and the growth of pollutant concentration.
According to Moskwa, the third hypothesis speaks about the possibility of a large discharge of industrial wastewater with chlorine which could have potentially stirred pollutants in bottom sediments.
The minister also said that no pesticides were found in dead fish, and that radioactive isotopes were below the legal level. (PAP)