‘Good lord, what a smell’: can Brazil’s biggest city save a vital source of water from sewage, bacteria and organised crime?

In a small motorboat laden with water-monitoring equipment, biologist Marta Marcondes and community activist Wesley Silvestre Rosa cross Billings reservoir on the far southern edge of São Paulo. Bright white herons glide over the water, which is flanked by thick dark green clusters of Brazil’s Atlantic forest, as the boat heads towards one of the more polluted parts of the reservoir.

“We see where sewage is entering, we see what has been deforested and how that has affected the water quality of the reservoir,” Marcondes says.

Marcondes and Rosa are dedicated to the upkeep of Billings, which at 127 sq km (49 sq miles) is Brazil’s largest urban reservoir by surface area and volume and a vital water source for the almost 22 million people who live in São Paulo’s metropolitan area. It also generates energy via a hydroelectric dam and plays a crucial role in flood control and irrigation; it provides a cooling effect during periods of extreme heat and people use its cleaner parts for recreation and fishing.

Despite its importance, large areas of Billings are polluted: contaminated with household and industrial waste, pharmaceutical residues, microplastics and fecal matter. Urban planners blame neglect by local authorities, flawed water management policies and uncontrolled urban expansion.

As the boat reaches a heavily polluted part of Billings called Grota Funda, Marcondes observes bubbles rising from the water, which she identifies as fermenting bacteria. Donning rubber gloves, she lowers a metallic collection device into the water, empties its dark contents into a bucket before taking a sample in a plastic tube.

“Good lord, what a smell,” she says. “You could die if you drank this.”

Marcondes, who analyses water samples at the lab she runs at the nearby Municipal University of São Caetano do Sul, is also the project coordinator for the local NGO Water Pollutant Index. She notes that water quality and the reservoir’s storage capacity have deteriorated over the past 10 years.

“This problem has been dragging on for decades, and if we don’t do something about it now, we risk having a collapsed system,” she says.

In January, residents blamed São Paulo’s water utility, Sabesp, for dumping waste into the reservoir and the company was later fined by environmental authorities. Sabesp says: “The recorded incident was caused by the irregular entry of rainwater into the sewage network and the carrying of garbage, a situation intensified by the rains, which caused a hydraulic overload of the system.”

As the boat heads toward the Pedreira pumping station, which connects Billings to the Pinheiros River, the water thickens and turns green. Billings, which marked its 100th anniversary last year, was built to power the growing industrial base of São Paulo, South America’s richest city, via the Henry Borden hydroelectric plant that captures energy from water cascading over the Serra do Mar mountain range. Nabil Bonduki,** **a city council member with the Workers’ party and veteran urban planner, says the redirection of polluted water from the Pinheiros and Tietê rivers to supply the plant has turned Billings into an environmental sacrifice zone.

Roughly 1.5 million people live around Billings, many in favelas or other irregular housing, up from 110,000 in 1970, after which rural migrants increasingly flocked to Brazil’s capitals in search of industrial jobs. But the pollution contributes to health problems.

“According to the São Paulo city climate plan, our region is one of the most susceptible places to climate change in the city,” says Rosa, who lives in Jardim Apurá, a densely populated, lower-income neighbourhood on the edge of the reservoir.

In 2018, authorities completed construction of Residencial Espanha, a development of nearly 4,000 public housing units for lower-income families in Jardim Apurá, but access to housing remains a critical issue in the region.

Wanderley da Silva, 46, lives in the Favela da Fumaça on the edge of Billings. His makeshift wooden home, which has a corrugated plastic roof, floods up to his knees during heavy rains. “All of a sudden it’s really hot, and then it pours down,” he says. “Everyone knows why, after humankind destroys nature, then comes the payback.”

Bonduki says Billings serves as a stark warning of what São Paulo’s other reservoirs could become, especially Guarapiranga, in the southern zone.

“Billings is deeply compromised, but it is not a lost battle,” he says.

Bonduki blames insufficient inspections from local authorities for the reservoir’s continued degradation. “It’s a political issue. It’s about having a public authority that wants to do something. These days, we have satellites that can detect deforestation in real time.”

Illegal deforestation along the reservoir’s banks, mostly to clear land for clandestine construction, increases sediment levels in the reservoir and reduces its water storage and flood-control capacity, Marcondes says.

Almost all of Billings’ 700km (435 miles) of shoreline is technically protected under local environmental laws. Yet with a booming demand for real estate in São Paulo, powerful local actors seek to circumvent these rules for profit. Advertisements offering plots of land and properties for sale proliferate in the region and across online social media groups.

In a statement, São Paulo’s public prosecutor’s office for housing and urban planning blamed “the emergence and growth of illegal land subdivisions in the water catchment area of the southern region of the municipality of São Paulo”, and mentioned a civil inquiry “aimed at investigating the structure, planning, and possible deficiencies of state and municipal bodies”.

State legislation specifically prohibits heavy construction and dense urbanisation around the Billings reservoir. Yet using a drone from one of its polluted shorelines, it is possible to observe pockets of construction in cleared patches of Atlantic forest.

The structures are solid and professionally built, unlike the precarious dwellings of the Favela da Fumaça. Bonduki refers to them as “clandestine allotments”, speculative constructions often illegally built for future profits.

The Guardian spoke to sources, who asked not to be named, who cited collusion between local land barons, dominant political networks in the region and organised crime groups, enabled by corrupt lawyers and inspectors.

Those who challenge entrenched power in the region face a threat of violence. Last year, authorities discovered bodies buried next to the reservoir in Buffalo Park bearing signs of execution, and in 2022 a Billings activist, Adolfo “Ferrugem” Duarte, was killed and his body found in the reservoir.

In a statement, São Paulo city hall acknowledged environmental crimes had happened around the reservoir, such as “deforestation of native vegetation”, “the disposal of solid waste, mainly construction waste and human waste” and “the clandestine sale of land plots, non-compliance with embargos and the illegal subdivision of land”.

It says that “in partnership with the state government, [it] operates an integrated water defence operation (OIDA) focused on protecting water sources such as Billings and Guarapiranga”. These operations “focus on inspection, fines, and seizures of materials and machinery, as well as the dismantling of unfinished or uninhabited constructions, based on court orders”. In 2026, the note concludes, about 20 operations have already been carried out.

The challenges facing Billings reservoir are becoming more urgent as São Paulo experiences mounting water shortages due to the climate crisis. In 2015, the Billings reservoir became part of São Paulo’s drought response, as authorities used it to help supply the city during the worst water crisis in its history. As a new crisis approaches, with climate-induced drought already depleting the city’s reservoirs ahead of the dry season, the NGO Institute of Water and Sanitation has warned that without planning, resilience is impossible.

Now, Billings is set to play a larger role during times of scarcity, with a new infrastructure project by Sabesp. In periods of crisis, clean water will be drawn from it to help supply the city.

To protect remaining green space around the reservoir, local people campaigned to create Buffalo Park, a home to 101 species of wildlife where local people can plant seeds. Matthew Richmond, a lecturer in political geography* *at Newcastle University and Alameda Institute affiliate says: “Environmental activists on São Paulo’s peripheries are fighting to salvage the green spaces that survived, in the face of continued state neglect and unmet housing demand, which drives new land occupations.”

Rosa says local people have been blamed unfairly. “We suffer from environmental racism,” he says. “They blame us for the pollution, but we, the poor, black and peripheral people, keep our green spaces clean and alive.”