Pakistan is among five countries that account for almost 90 percent of farmland across the globe irrigated by sewage water, a study shows.
China, India, Iran and Mexico are the other four countries where major portion of clean water from rivers and canals is consumed in urban areas and water is contaminated with industrial and household waste by the time it irrigates downstream cropland.
The study shows that wastewater agriculture is common in at least 60 countries and is around 50 percent more widespread than previously thought.
These findings have been published in Environmental Research Letters Journal July 2017 edition. Rather than relying only on case studies like previous such studies, the study conducted by a team led by Anne Louise Thebo, a researcher based at the University of California Berkeley, is based on Geographical Information System (GIS) data gathered from across the globe. It puts the number of citizens exposed to health hazards from agricultural products grown with wastewater in the five countries at around 855 million.
The lead author of the study has stated that reuse of urban wastewater is understandable given the combination of increasing water pollution and declining freshwater availability in many developing countries. Thebo has urged governments in affected countries to invest in wastewater treatment to ensure provision of healthy agricultural products.
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However, the study also acknowledges that shortage of clean water may not be the only reason why wastewater irrigation goes on across the globe.
Water contaminated with pathogen-rich sewage is also rich in nitrates and phosphates. This reduces the need of additional spending on fertilizers. Farmers prefer this water for irrigation purposes. Even water contaminated with human faeces is good for yield. In fact, an earlier study had found that farmers in Pakistan are willing to pay more for wastewater than canal water because the former’s nutrients reduced their expenditure on fertilizers.
The bad news is that vegetables and fruits grown with this water contain heavy metals, pathogenic bacteria and worms – all hazardous for human health.
“Wastewater is rich in nutrients and it boosts yield but it also contains traces of heavy metal which is harmful for human health as well as soil used for agricultural purposes,” says Rafiul Haq, an ecologist and former consultant with International Union for Conservation Nature Pakistan chapter.
In 2013, a study conducted in Sialkot and Wazirabad districts had found higher levels of heavy metal in crops irrigated by sewage water compared to those supplied water from tubewells.
The study conducted by Environmental Biology and Ecotoxicology Laboratory at the Quaid-e-Azam University compared coriander, okra, onion, garlic, capsicum, carrot, brinjal, spinach, mint, radish, tomato and wheat crops irrigated by both contaminated water from Nullah Aik and Palkhu streams and tubewell water.
It found metals like cadmium, lead and chromium in affected crops, especially in leafy vegetables, far in excess of the permissible limits.
“Cadmium causes kidney and lung damage. It also weakens bones causing frequent fractures in both children and adults,” says Dr Zafar Ahmed Fatmi, a professor of Community Health Sciences at Aga Khan University, Karachi.
Speaking to MIT Technology Review Pakistan, he says lead consumption can stunt brain development among children and can also cause blood disorders. Nickel, another metal frequently found in wastewater, has systemic immunologic, neurologic, reproductive and developmental effects on human body, he adds.
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However, he says, risk assessment of a heavy metal needs to be carefully calculated based on the cumulative dose of that metal consumed over a period of time.
There are sewage treatment plants in Sindh and Punjab province but Dr Ghulam Murtaza, a senior research officer at Pakistan Council for Research in Water Resources (PCRWR), says most of them are not functional and need repairs.
Meanwhile, farmlands irrigated with sewage water also provide breeding grounds for mosquitoes varieties including those carrying dengue virus. A research study led by International Water Management Institute, Lahore, found vectors of Anopheles, Culex and Aedes species of mosquitoes in wastewater samples collected from farms in southern districts of Punjab.
Stricter sanitation policies were recommended in the study to deal with this public health menace.