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The 30-mile (50-kilometer) taxi ride from the airport to the Rio Centro convention center offers visitors a cityscape that could double as a to-do list of issues to be tackled.

To the side of the highway, you can often see children playing in the dirty canals that run below unfinished brick buildings clinging to vertiginously steep hillsides. That’s the shantytown of Manguinhos. When it rains, homes like those run the risk of collapsing. Last year, more than 900 people died when torrential rains caused mudslides and tore down similarly fragile housing in the mountains north of the state.

The taxi slows down in a sea of red brake lights. The topographical features that make Rio so pretty to visitors _ the mountains, lakes and rivers _ also make it difficult to lay down a straight road. Add to that a 40 percent hike in the number of cars over the last decade, and you’ve got an idling, polluting transit mess.

“It’s much worse now,” said Mauricio Pinto Gama, a taxi driver who has been working Rio’s streets for 14 years. “Before, you knew what the rush hour was. Now you don’t. It can be bad at any time. You never know how long it’ll take.”

That traffic is one reason the level of Rio’s air pollution index is triple what the World Health Organization considers acceptable.

In preparation for the World Cup and the Olympics, Rio is laying down new thoroughfares connecting the far ends of town. But the transportation along these corridors will be by buses _ newer, cleaner ones, but still, fossil-fuel burning hulks.

At the convention center, the contrast of extremes that mark this city repeats itself. Rio Centro is set against a stunning backdrop of soaring granite mountains topped by lush forests. It is surrounded by marshland that sparkles with interconnected coastal lakes. Nearby is a vast beach whose white sands go on for miles.

On closer inspection, this landscape also reveals how much it has been abused. Trash and sewage accumulated over the years choke channels that let water flow between the lakes and the ocean. Pigs root through islands of clotted refuse just hundreds of yards (meters) from where world leaders will nail down shared goals on topics like ocean acidity and biodiversity.

A heritage of poor infrastructure dating back to when Brazil was a Portuguese colony, coupled with untrammeled development in recent decades and negligible monitoring, has left many of the communities surrounding the convention center without a connection to wastewater treatment centers. That is as true for the expensive high-rises in gated communities that have sprouted on the marshy fields as for the low-income squatter communities housing the maids, doormen and landscapers who work there.

Occasionally, cyanobacteria blooms, coating the lakes with a vivid green film that produces toxins that can sicken even in small doses, said the biologist Azevedo.

“Twenty years ago, these lakes were nurseries, places where fish reproduced,” said Azevedo. “Now you have places with zero oxygen, with a pH of 0 or 1, very acidic.”

The pollution spread via channels, including one that travels right through the heart of the convention center, where the visitors will sun themselves on grass beside waste water.

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Follow Juliana Barbassa on Twitter: (at)jbarbassa