BBC News | By: Matt McGrath | 5 July, 2017:
The global use of untreated waste water from cities to irrigate crops is much more widespread than previously estimated, says a new report.
According to this updated assessment, nearly 30 million hectares are now using untreated water within 40km of an urban centre
Abstract From Report:
A global, spatially-explicit assessment of irrigated croplands influenced by urban wastewater flows:
By: A L Thebo, P Drechsel, E F Lambin and K L Nelson
Published 5 July 2017 • © 2017 IOP Publishing Ltd – Environmental Research Letters, Volume 12, Number 7:
Abstract:
When urban areas expand without concomitant increases in wastewater treatment capacity, vast quantities of wastewater are released to surface waters with little or no treatment.
Downstream of many urban areas are large areas of irrigated croplands reliant on these same surface water sources.
Case studies document the widespread use of untreated wastewater in irrigated agriculture, but due to the practical and political challenges of conducting a true census of this practice, its global extent is not well known except where reuse has been planned.
This study used GIS-based modeling methods to develop the first spatially-explicit estimate of the global extent of irrigated croplands influenced by urban wastewater flows, including indirect wastewater use.
These croplands were further classified by their likelihood of using poor quality water based on the spatial proximity of croplands to urban areas, urban wastewater return flow ratios, and proportion of wastewater treated.
This study found that 65% (35.9 Mha) of downstream irrigated croplands were located in catchments with high levels of dependence on urban wastewater flows.
These same catchments were home to 1.37 billion urban residents. Of these croplands, 29.3 Mha were located in countries with low levels of wastewater treatment and home to 885 million urban residents.
These figures provide insight into the key role that water reuse plays in meeting the water and food needs of people around the world, and the need to invest in wastewater treatment to protect public health.
Some 800 million people, including farmers, vendors and consumers are said to be exposed to serious health risks.
China, India, Pakistan, Mexico and Iran account for most of the treated land.
The huge populations in big cities across the developing world make very attractive markets for farmers.
The lack of refrigeration and transport means that crops need to be grown close to these consumers. Being close to cities also provides a key element for the crops – plentiful amounts of nutrient-rich waste water.
“Some might call it sewage, but it’s mostly domestic waste water, although it can contain industrial effluent,” says study co-author Dr Pay Drechsel, from the International Water Management Institute (IWMI).
“But in our report we’ve not just looked at waste water, we’ve also looked at the waste water that enters the rivers and streams, where it gets diluted to some extent but it is still tremendously dangerous when it comes to farmers’ fields.”
According to the study, some 65% of all irrigated land areas are within 40km downstream of urban areas. A significant amount of that land is in countries with very limited waste water treatment, exposing 885 million urban consumers, farmers and food vendors to health risks, according to the authors.
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