A recent University of Delaware study investigated the effect of different irrigation management methods on the presence of toxic heavy metals cadmium and arsenic in rice crops. The results suggested that, although certain approaches can reduce the levels of either cadmium or arsenic, irrigation management may not be able to simultaneously mitigate both of the chemicals in rice.

At the University of Delaware’s RICE Facility, irrigation management was studied in 18 experimental paddy enclosures. Hybrid rice was cultivated over a period of two years, with irrigation treatments designed to simulate different soil redox conditions. Treatments included four versions of Alternate Wetting and Drying (AWD) with different depth of drainage and frequency of dry drowns, along with flooded and nonflooded controls. AWD drainage depth varied by allowing the water table to drop to either 15 or 30 centimeters (cm) below the surface before reflooding. Regarding frequency, some paddies experienced longer intervals before the next dry down (low frequency AWD), while others had shorter intervals (high frequency AWD).

Over the study, high-frequency AWD paddies experienced 9–13 dry downs per year, while low-frequency AWD paddies had 5–8. The experiment concluded after 100–105 days, with harvest occurring at 111–112 days after transplant.

Irrigation management significantly influenced arsenic and cadmium accumulation in rice, with varying impacts on arsenic speciation. Under more flooded conditions, rice absorbed higher levels of arsenic, due to ferric iron-reducing conditions, which increased soil water arsenic concentrations. However, arsenic decreased under drier irrigation management—especially organic arsenic—while inorganic arsenic required more aerobic conditions for a significant reduction.

The type of AWD most frequently recommended to prevent yield losses showed no significant difference in arsenic levels compared to flooded conditions, suggesting that recommended irrigation practices may not significantly reduce inorganic arsenic in rice.

In contrast, cadmium accumulation was higher under drier, more aerobic conditions. The study found a negative correlation between manganese in soil water and cadmium in rice, as manganese oxides can adsorb cadmium and limit its uptake. Manganese also competes with cadmium for uptake in rice roots, further influencing levels of the chemical contaminant.

Optimizing irrigation management to minimize both arsenic and cadmium remains challenging due to the tradeoff between their accumulations. Severe dry-downs might reduce both arsenic and cadmium but could risk yield loss. Additionally, soil-specific factors such as ferric iron reduction, soil pH, sulfur availability, and organic matter content also play critical roles in determining the levels of arsenic and cadmium in rice.

The researchers proposed a hazard index combining both arsenic and cadmium for better risk assessment in rice cultivation.