Environmental controls on rice grain arsenic contamination in Cambodia: Insights from hydrologic conditions and stable isotopes

Rice is a food source vital to the global population, providing approximately 20% of the calories consumed worldwide. However, the unique environmental conditions that rice grows in, paired with its physiological properties, enables elevated levels of arsenic (As) to concentrate within the grain. A known toxin, inorganic As contamination poses a significant health risk to those for whom rice is a dietary staple, such as the people of Cambodia, who get upwards of 60% of their calories from rice. The mobilization of As from soil and its subsequent uptake into the grain is strongly influenced by the redox conditions in which the crop grows in, a factor heavily tied to the rice paddy environment (e.g., soil moisture, soil composition). To garner a more complete understanding of the connections between environmental conditions and rice grain As concentrations, we have sampled over 200 rice paddies across a steep gradient of total annual precipitation within Cambodia, with many of these fields sampled across multiple growing seasons. At each field we have collected and analyzed soil and rice grain for elemental composition, stable isotopes (C, N, O), and soil properties (pH, texture, salinity, color). Furthermore, we have linked our field and lab measurements to remotely sensed data on soil moisture and flooding. Here we find that (1) rice grain As levels vary dramatically across small spatial scales (< 1 km) though nearly always exceed FDA guidelines, and (2) within a given paddy rice grain As levels vary between different harvests, with variation largely controlled by flooding conditions. Furthermore, we find that flooding/inundation conditions during rice growing are reflected in the rice grain stable carbon isotopes - thus these isotopes serve as a proxy for environmental conditions under which rice is grown and shed light on conditions conducive to As uptake.