Irrigation Promotes Arsenic Mobilization via Goethite: Insight from the Perspective of the Solid–Liquid Interface Interaction Process

Dramatic changes in the farmland soil–groundwater environment caused by irrigation affects the geochemical behavior of arsenic (As). However, the mechanism of As mobilization during soil–groundwater interactions remains unclear. This study explored the effects of phosphate (PO43−), fulvic acid (FA), and oxic/anoxic conditions on As mobilization through batch and column experiments. The results indicated that a saline–alkali environment and the involvement of PO43−/FA suppressed the adsorption capacity of goethite for As(III) in the water environment and that PO43− had a primary effect. An increase in the PO43−/FA concentration further increased its inhibition. Notably, oxic/anoxic conditions did not affect the adsorption capacity of goethite for TAs in the presence of high concentrations of PO43−/FA. In the soil environment, periodic irrigation led to regular fluctuations in the As content in the soil pore water. The addition of PO43− to irrigation water resulted in a higher content of As in the pore water in the short term. In contrast, the addition of FA greatly increased the long-term mobility of As. This study highlighted that irrigation amplifies As mobility in soil–groundwater systems, particularly in saline–alkali environments, and PO43−/FA addition exacerbates this effect, posing risks to agricultural safety. The systematic management of irrigation practices is recommended to mitigate these risks.