Study Shows Multiple Aeration Events During Rice Growing Reduces Arsenic Without Increasing Cadmium
A multi-year study of rice grown in commercial Arkansas production fields found that repeated mid-season soil aeration events significantly reduced arsenic concentrations in rice grain without increasing cadmium levels, providing field-scale evidence that water management can help mitigate arsenic accumulation in rice.
The study, published in Agricultural and Environmental Letters, was conducted by researchers at the University of Delaware, the University of Arkansas, the University of Arizona, and the U.S. Department of Agriculture’s Agricultural Research Service (USDA-ARS).
The researchers analyzed 412 polished white rice samples collected from 60 rice fields across Arkansas between 2018 and 2024, representing 103 field-years of production. The study evaluated grain arsenic and cadmium concentrations under three water management approaches: conventional flooding, a single aeration event, and multiple aeration events.
Multiple Aeration Events Significantly Reduce Arsenic in Rice
Results showed that only multiple aeration events, in which fields dried with the water table falling below the soil surface more than once during the growing season, significantly reduced grain arsenic concentrations. Compared with conventionally flooded fields, multiple aerations reduced median total arsenic concentrations by 39 percent and median inorganic arsenic concentrations by 31 percent.
Although all samples remained below the Codex Alimentarius maximum level of 0.2 milligrams per kilogram (mg/kg) for inorganic arsenic in rice, approximately 30 percent of samples exceeded the U.S. Food and Drug Administration's action level of 0.1 mg/kg for inorganic arsenic in infant rice cereals.
According to the study, when flooded rice paddies are dried, oxygen can directly oxidize ferrous iron into ferric oxides, which strongly adsorb inorganic arsenic.
Cadmium Remained Low Despite Aeration
Cadmium concentrations remained low across all water management systems and were not significantly affected by aeration. No samples approached the Codex limit of 0.4 mg/kg for cadmium in rice, suggesting the elevated grain cadmium from aerobic rice cultivation may not be a concern in the Arkansas growing region.
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Soil Characteristics Affecting Arsenic, Cadmium in Rice
The study also identified soil characteristics associated with lower arsenic concentrations. Higher soil pH and greater clay content were both negatively correlated with grain arsenic levels. For cadmium levels, soil pH and organic carbon were negatively correlated, while higher clay content was positively correlated.
Predicting Future Arsenic Levels
Researchers noted relatively low variability in arsenic concentrations within individual fields, with an average relative standard deviation of 20 percent. Year-to-year variability within the same field was somewhat higher but remained modest, suggesting previous measurements may help predict future grain arsenic levels.
The authors concluded that water management is an effective tool for reducing arsenic accumulation in rice under commercial production conditions, but emphasized that multiple aeration events were necessary to achieve meaningful reductions. They also noted that concerns about increased cadmium uptake under drier conditions were not observed in the Arkansas fields studied.
