Arsenic Release Dynamics of Paddy Field Soil During Groundwater Irrigation and Natural Flooding

Irrigation water in rice cultivation significantly affects the arsenic (As) mobilization in the paddy field soil. This research assessed the effect of rainwater (RW) and groundwater (GW) on arsenic (As) dissolution dynamics of paddy field soil. The up-flow column flooding experiments were conducted continuously for 80 d with simulated RW and GW to evaluate As dissolution phenomena in real field conditions. As dissolution from the soil was lower in the GW (309 µg/kg) irrigation than in the RW flooding conditions (1086 µg/kg). The redox potential (Eh) of the pore water decreased and pH increased with the irrigation time in both flooding conditions. With the flooding time, As and Fe dissolution increased and Mn dissolution decreased. Arsenic released in the soil pore water was due to the dissolution of Fe-As and Mn-As minerals as well as the microbial reduction of As. Fe-As dissolution ratios in the soil pore water were relatively low and estimated as 0.68 mol/mol and 4.9 mol/mol for RW and GW, respectively. As and Mn dissolution dominated in the initial phase (0-40 d), while the dissolution of As and Fe dominated in the second phase (40-80 d) of the flooding. Arsenic release was much lower in GW flooding than in RW flooding conditions. Presence of Ca, Mg, and Mn in the GW facilitated the reduction of As dissolution by precipitation of Ca-As and Mg-As and the oxidation of dissolved Mn in the soil pore water. The findings of this study help to investigate the As uptake by rice plants during monsoon flooding and groundwater flooding of natural paddy filed