Dynamics of cadmium and arsenic at the capillary fringe of paddy soils: A microcosm study based on high-resolution porewater analysis
Yuang Guo, Sha Zhang, Williamson Gustave, Hao Liu, Yu-Jia Cai, Yufei Wei, Zheng Chen
Abstract
Arsenic (As) and cadmium (Cd) are prevalent in paddy soils, posing serious threats to food safety and public health. The bioavailability of As and Cd is sensitive to moisture-driven changes in soil pH and Eh; however, the sensitivity is barely described at the critical dry-wet interface. Here, tempo-spatial changes of soluble As and Cd were captured by In-situ Porewater Iterative (IPI) samplers at the capillary fringe that extended from saturated to unsaturated moisture gradient in the millimeter scale (60 mm profile in depth) through two episodic dry-wet cycles (55 days in total). It is found that the As and Cd has less significant fluctuation in second cycle compared to the initial dry-wet cycle. The study also revealed at the capillary fringe profile (from 20 mm above to 40 mm below), the As concentrations increased from 4.6 ug L-1 in unsaturated soils to 13.5 ug L-1 in saturated soils, while Cd decreased from 3.3 to 0.2 ug L-1. This observed correlation was aligned with the vertical changes in soil Eh (+287 to +381 mV) and pH (3.42 to 6.07). The study found a distinct zone characterized by low As and low Cd concentrations, typically situated approximately 10 to 30 mm beneath the capillary fringe. Upon further analysis, it was determined that soil with an Eh of 249 mV and a pH of 4.3 potentially serves as an optimal environment for decreasing arsenic and cadmium levels in pore water. These findings suggest that it is feasible to reduce As and Cd concentration in the soil by implementing appropriate depth-controlled water management techniques.