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CALI, COLOMBIA - JUNE 4: A group of environmental authorities in Cali have carried out research on the pollution levels of the rivers that cross the city. Footage shows the river of Cali dyed in red as a 'tracer test' is being conducted, which involves the dissolution of a chemical called rhodamine WT in the surface waters of the river. The substance is harmless and not toxic for the environment, says the research leader Javier Holguin. "It will allow us to understand the transport mechanisms of dissolved substances in rivers," he tells. "So these transport mechanisms are important because they are going to allow us to understand precisely when wastewater without treatment or with treatment is invested in our rivers and to understand what is the treatment for those contaminants." The procedure leads the water to take on a red hue for a few hours. The objective is to establish the real conditions of the tributaries to define what their future may be in the face of possible catastrophes in an average period of ten years. (Footage by Juan Sebastian Arias Rodriguez /Anadolu Agency via Getty Images)