Researchers have discovered that rice fields that are flooded generally contain more arsenic and less cadmium, while drier rice fields show the opposite pattern: less arsenic and more cadmium. This research could lead to improved policies that make baby food safer.
Rice and spinach are common foods in the diets of babies and young children, yet these foods can contain harmful metals and metalloids that may seriously affect health.
Heavy metals, including cadmium, lead, mercury, and arsenic, can hinder brain development in infants and young children.
In a recent study published in the journal Environmental Geochemistry and Health, scientists from the University of Delaware found that flooded rice fields have higher levels of arsenic but lower levels of cadmium. Conversely, when the fields are drier, arsenic levels decrease, and cadmium levels increase. Notably, even at lower water levels, cadmium concentrations remain below harmful health thresholds.
This research could influence strategies to lower the presence of these contaminants in foods consumed by infants and children. Earlier this year, the U.S. Food and Drug Administration (FDA) issued draft guidelines regarding permissible lead levels in baby food and is preparing to set new limits for arsenic, cadmium, and mercury under its Closer to Zero Action Plan.
Flooding rice fields
Unlike crops like corn, soybeans, and wheat that grow in less saturated soil, rice is cultivated in very wet, flooded conditions. In these conditions, the oxygen normally present in tiny soil pores is rapidly replaced by water, which shifts the microorganisms in the soil. These microorganisms change their respiration to utilize iron oxide minerals, giving the soil a rusty appearance.
“Arsenic tends to bind strongly to iron oxides,” explained Angelia Seyfferth, a soil biogeochemist and professor at UD, who co-authored the study. “When these organisms metabolize iron oxides, they transition from a solid form to a dissolved form, releasing arsenic into the water.”
Once arsenic is dissolved in water, it can be taken up by the rice roots and enter the grains.
The researchers conducted experiments in 18 small rice fields at the UD Newark Farm, testing various flooded and wet conditions.
“We aimed to identify an optimal irrigation method that would minimize arsenic and cadmium levels at the same time,” Matt Limmer, a research associate, said. “Unfortunately, we didn’t find such an approach in this soil.”
Upon harvesting and examining the rice for arsenic and cadmium levels, they discovered that more flooded conditions corresponded with increased arsenic and decreased cadmium. Conversely, drier conditions resulted in higher amounts of cadmium and lower amounts of arsenic.
“Despite the increased cadmium in drier conditions, the levels remained safe for human health,” noted Seyfferth.
In flooded fields where arsenic levels rose, the researchers also observed a process called methanogenesis, where soil organisms produce methane—a potent greenhouse gas. Excess water reduced sulfate in the soil, converting it to sulfide, which caused cadmium to precipitate out along with the sulfide.
When the soil was dried, arsenic and methane levels decreased. The sulfide was oxidized back to sulfate, enabling easier cadmium absorption by the plants.
“Drying the soil essentially slows down the microorganisms that metabolize iron oxides and arsenic,” Seyfferth stated. “Simultaneously, it increases cadmium levels as the sulfide turns into sulfate, allowing cadmium to be released more easily.”
Introducing oxygen into the soil during drying helps solidify the iron oxides again, which act like a filter, preventing arsenic from entering the water and reaching the plant roots.
This phenomenon—one metal increasing as the other decreases based on soil moisture—poses a challenging puzzle.
“The difficulty lies in determining the ideal moisture level to minimize both contaminants,” Seyfferth said. “There isn’t a universal solution suitable for all soils.”
With funding from a U.S. Department of Agriculture National Institute of Food and Agriculture grant, the researchers are exploring arsenic levels in rice through field studies in Arkansas, collaborating directly with farmers to develop effective irrigation management techniques.
The FDA may introduce fresh regulations for arsenic and cadmium levels in baby food by the end of the year, as part of its Closer to Zero Action Plan. The agency has been investigating the impacts of these substances, along with mercury and lead, on child health and exploring new technologies to reduce exposure.
“Our findings could contribute to shaping future policy,” Seyfferth expressed.
Members of the Seyfferth Lab, including Matt Limmer, Angelia Seyfferth, and graduate students Bekah Hanrahand and Frank Linam, harvest rice at the UD RICE Facility.
Engaging Farmers
The UD team found in a review published in the journal GeoHealth that producers are eager to implement measures to lower metal levels in their crops, but they need proper incentives, testing, and education to do so.
This was particularly evident in the spinach sector across five states, where the researchers engaged with producers, packers, processors, and marketers. Spinach can absorb cadmium and lead from soil.
“Understanding the feedback from stakeholders is crucial,” Seyfferth emphasized. “We must ensure that any suggested practices are manageable for farmers and do not impose excessive burdens on their existing operations to comply with food safety standards.”
The researchers investigated how plants interact with metals and metalloids, analyzing the movement of cadmium and lead in soils and their effects on leafy greens. They also devised strategies for farmers to reduce metal and metalloid levels in their produce.
“Many farmers operate on tight budgets, especially with crops like spinach,” Seyfferth added. “If regulations become too difficult to meet regarding cadmium or lead, they might opt to grow different crops.”
However, a notable issue with cadmium in spinach arises from the use of chlorinated water for disinfection, which can enhance cadmium uptake by the plants. “Our current methods for disinfecting irrigation water could be worsening the cadmium issue,” Seyfferth cautioned. “We should look for alternative disinfection methods that don’t involve chlorination.”
One potential approach to reduce toxic metals and metalloids in crops is by offering farmers subsidies to adopt specific strategies. For example, reducing soil acidity can lower cadmium levels in spinach, and washing spinach with lemon juice can remove up to 26% of lead, according to the researchers.
“There’s no one-size-fits-all solution,” Seyfferth highlighted. “Strategies must be tailored to specific sites.”
Limmer and Seyfferth agree that further research is needed to determine the best irrigation methods that minimize both arsenic and cadmium in rice.
“Similar studies should be conducted across various soils and conditions,” Limmer stated.
With the FDA poised to announce new regulations regarding allowable levels of arsenic, cadmium, and mercury in baby food, Seyfferth advocates for a reassessment of the last comprehensive survey, which was conducted in the 1980s. The FDA, U.S. Department of Agriculture, and U.S. Environmental Protection Agency had once examined paired soil and plant samples to assess metal and metalloid concentrations.
“Since that time, spinach cultivation has expanded to new regions, some with increased cadmium levels,” Seyfferth concluded. “It’s essential to initiate a new survey to reflect these changes.”
