Abstract
A massive fish death was reported in the Atoyac river, presuming as its cause the discharge of a very toxic chemical pollutant. The analysis of water, sediment and tissue samples, showed a low concentration of dissolved oxygen and high levels of ammonium and nitrite in the river the day of the incident, likely to be the direct causes of the death of the fish. By gas chromatography/mass spectrometry, persistent contaminants weren't detected, instead high concentrations of anthropic organic micropollutants were detected, which indicates a discharge of municipal waste to the riverbed, the indirect cause of the fish killing. The concentration of coprostanol and epicoprostanol indicates the presence of mammal's feces, including those of ruminants, in the discharge. The obtained results show the usefulness of the organic micropollutants levels determination to clear up the causes of such incidents.