Introduction.

El Niño phenomenon causes changes in the water quality of surface water sources due to temperature increase and flow reduction.

Objective.

To analyze the effect of el Niño phenomenon occurring between the second half of 2015 and the first quarter of 2016 on the water quality of the Magdalena River, municipality of Purificación (Tolima). The water analysis is based on the characterization of its turbidity, apparent color, pH, temperature, total hardness, phosphates, nitrates, nitrites, chlorides, iron and mesophilic microorganisms; to determine its behavior before and during said natural event characterized by loss of rainfall and its consequent reduction of river flow, considered the one with the greatest impact ever recorded.

Materials and methods.

Raw water samples were taken during 2015 and the first quarter of 2016 at the site of the urban aqueduct of the municipality of Purificación - Tolima.

Results.

Comparatively, during the months of occurrence of El Niño phenomenon in relation to the first half of 2015, water turbidity decreased and the concentration of other indicators increased, especially phosphates, nitrites and iron, which, as dissolved substances, may pass the conventional treatment process and be present in drinking water in concentrations higher than those established by current regulations. An increase in mesophilic microorganisms was also evidenced.

Conclusion.

During the occurrence of el Niño phenomenon, the Magdalena river presented changes in its water quality with an increase in some contaminants that can pass the treatment process and imply risk to human health.