In many countries, urban populations are expanding fast. Vital infrastructure, including wastewater treatment plants, is failing to keep pace with population growth; more than 80% of wastewater produced globally enters the environment without treatment. This wastewater contains high concentrations of waterborne pathogens that cause diarrhoeal diseases, which can be particularly dangerous for children and people with compromised immune systems.

For farmers, although it is dangerous, untreated wastewater also has benefits. It provides a reliable source of water for irrigating crops, and it has a higher concentration of nutrients than other water sources.

Previous case studies have shown how bad the problem is in some areas. For example, in Ghana around 2000 farmers grow cash crops close to the five biggest cities, using wastewater for irrigation. Many of those crops are consumed as street food by around 800,000 people, but studies have shown that the crops are often teeming with E. coli bacteria. So, what is the global extent of this problem?

To find out, Anne Thebo, then at the University of California at Berkeley, US, and colleagues used GIS-based modelling to assess how widespread wastewater irrigation was across a much broader scale than has been analysed before.

The team classified croplands according to their likelihood of using poor-quality water based on their spatial proximity to urban areas, contribution of wastewater to local surface water supplies, and the amount of wastewater treatment in the region. In addition, the scientists assessed indirect or de facto wastewater use, where wastewater enters rivers and streams and is used downstream.

The study, published in Environmental Research Letters (ERL), found that 65% of all irrigated croplands within 40 km downstream from urban areas – amounting to more than 35 million hectares – were in catchments with high levels of dependence on urban wastewater flows. These same catchments were home to 1.37 billion residents.

Five countries in particular – China, India, Pakistan, Mexico and Iran – accounted for the majority (86%) of the irrigated croplands reliant on the reuse of largely untreated wastewater. Thebo and her colleagues estimate that worldwide some 885 million urban residents are exposed to serious health risks as a result of routine irrigation with highly polluted water.

"There are a lot of factors at play in determining vulnerability, but water scarcity coupled with a lack of local infrastructure for collecting and treating wastewater definitely appears to be a driver in many regions," said Pay Drechsel, a co-author from the International Water Management Institute (IWMI) in Sri Lanka.

Globally, wastewater is the only source of water that is increasing in volume with population growth and water consumption. And it must be recognised that the high nutrient content of the water also makes it desirable for irrigating crops.

"To feed the growing population, wastewater irrigation is going to play an important role, but we need to improve safety," said Drechsel. Ideally all such water would be processed in wastewater treatment plants, but in practice this won't be feasible anytime soon.

That said, there are less sophisticated interim measures to improve food safety, such as encouraging cooking of vegetables and washing salad greens with a sanitizer at home. The new study reveals how widespread the problem is, but also identifies the regions where wastewater irrigation presents the biggest risk, and highlights where interventions could have the largest impacts on food safety.

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