I had always assumed that when Brazilians told me not to swim in the sea the day after heavy rain they were being fussy. But there’s good reason.
Millions of the people who live in Rio de Janeiro are not connected to a proper sewage system. So even at the best of times raw sewage drains into the sea or the huge Guanabara Bay with Sugarloaf Mountain at its narrow mouth.
And when there are tropical downpours much of the rubbish which builds up in the poorer communities is washed out into the water - especially into the Bay. In places it stinks.
The source of the problem is mainly the favelas or shanty towns which have built up. But while they may be the source of the problem, they are not its cause. Instead, that is due to the decades of neglect of these poor communities by successive governments and city authorities.
While the privileged have been content to let poor people flock to Rio to provide cheap labour, those workers have been left to cobble together their own communities without the basic amenities most people would take for granted: proper water supplies, sewage services and refuse collection.
In my report a marine biologist warns that after heavy rain Guanabara Bay may not be safe to swim in. And yet this will be the location for the Olympic sailing and windsurfing events. Olympic hopefuls we spoke to, who are already out familiarising themselves with the winds and currents of the bay, were not impressed.
One told ITV News the water quality was a “disgrace.” Another said one of his colleagues had come out in a red rash after capsizing. He added that he was also worried about debris becoming entangled in his boat’s rudder, slowing him down at a critical point in an Olympic race.
Off Copacabana beach, where marathon swimmers and triathletes will compete, fecal coliform bacteria sometimes spike at 16 times the Brazilian government’s “satisfactory” level. One professor of environmental engineering in America warned that athletes should make sure their jabs are all up to date.
Millions of pounds have been spent trying to clean up the bay in the last two decades. It has not been money well spent. At least five new sewage plants don’t run at capacity because they have not been properly connected to collection pipes - one has not treated any sewage at all.
The city authorities promise that pollution going into Guanabara Bay will be cut by 80% before the Rio Olympics in two and a half years. One mechanism will be to build treatment plants over the rivers which feed the bay to filter out rubbish and human waste which will then be taken to land-fill sites.
The fear is that such stop-gap measures won’t continue long after the Olympics. The scale of the task is enormous. And once the international spotlight moves away from the bright sails skimming past Sugarloaf Mountain across the newly-pristine waters of the Bay ... will the pollution will once again be allowed to flow into it?