Soil science offers way to remove arsenic from irrigation water


The 20th century’s Green Revolution is credited with saving hundreds of millions of people from starvation. Modern agricultural practices and improved crops arrived in places like Mexico, Africa, and Asia. Millions of wells were drilled. Millions more irrigation channels began moving well water to crops. But these creative solutions brought unintended problems. Among them is the fact that the water from many irrigation wells contains arsenic, a toxin.

Irrigation channel in rice field

Nobody knew at the time that arsenic existed in the much-needed well water, which people drink and use to irrigate fields. Arsenic, which is odorless and colorless, poisons slowly. Eventually it may cause skin diseases, cancer, and death. It also hurts how well crops—particularly rice—grow. But solutions are in the works. One young soil scientist is investigating a possible fix.

“What we’ve done is try to figure out what chemical and hydrological properties are influencing arsenic distributions in flowing irrigation water that is derived from contaminated wells,” explains Matt Polizzotto, an assistant professor of soil science at North Carolina State University. Using that information, the group then studies how to engineer irrigation channels on Bangladesh rice farms to remove arsenic.

The irrigation channels are U-shaped and built of packed dirt, and they can affect how much arsenic is in irrigation water in two ways. First, arsenic is removed by sticking to soil particles as it passes through the soil. Second, other non-toxic chemicals in the water react with air to form minerals. These minerals can then bond with arsenic, removing it from solution. In both instances the arsenic settles, while the water flows on to the crops in the fields. 

For example, there’s a lot of dissolved iron in the well water, Polizzotto explains. Once the iron is exposed to oxygen (when the well water comes to the surface), the iron oxidizes, forming iron oxide, or rust-like particles. Arsenic sticks to those rust particles very strongly, pulling the toxin out of solution. “So, if those particles can be settled or trapped within the channels,” Polizzotto says, “arsenic loading to rice fields can be minimized.”

While still early, the results are promising. In a paper published in the Nov.-Dec. 2013 issue of the Journal of Environmental Quality, Polizzotto and his colleagues found they could affect the amount of arsenic reaching crops by rebuilding the channels. Specifically, channels were rebuilt so the flow of water in them was slowed. This provided more time for the needed chemical reactions to occur that remove arsenic.

“That’s what we’re doing with these channels. We’re engineering them so the amount of time that water spends within a channel is extended,” Polizzotto says. “Alternatively, arsenic can also stick to soil and be pulled out of solution by adsorption. So what we do in that case is enhance the ability for the water to be in contact with soil, so the arsenic can accumulate in the soil, not the water.”

Doing “relevant science” like this motivates Polizzotto, he says. He hopes eventually he can offer practical solutions to farmers for preventing arsenic from reaching rice fields. But he cautions there’s still a lot of work ahead. More research is needed to understand both how to remove arsenic and determine where it goes.

“We don’t have a good handle on where the arsenic is ending up within these channels when it is removed,” Polizzotto says. “That’s an ongoing area of research. And it’s important to know [so we can] figure out how to manage the removed arsenic and determine the sustainability of an engineered channel.”

 



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