Modeling the Transport of Inorganic Arsenic Species through Field Soils: Irrigation and Soil Structure Effect

Dissolved arsenic (As) may appear at the tile line level through preferential flow (PF), leading to contamination of shallow water bodies. Limited work on the movement of As forms in field soils urged the need for more research. The PF of arsenate (As(V)) and arsenite (As(III)) compared to chloride (Cl) at constant flow under saturated (10 mm), slightly unsaturated (-10 mm), and unsaturated (-40 mm) pressure heads was evaluated in replicated large field columns varying in subsoil structure. A solute containing As(V), As(III), and Cl was pulsed until the Cl concentration ratio in the drainage samples reached maxima and flushed with solute-free irrigation. HYDRUS-1D software version 4.15 was utilized to fit the breakthroughs of As(V) and As(III) in the dual-porosity physical non-equilibrium model (DP-PNE). The Langmuir equation was used to fit the As(V) and As(III) sorption isotherms, and blue dye staining was used for the marking of flow paths. Dye leaching was observed up to 50 cm or deeper in the soils. Under saturated conditions (+10 mm), Kotli, Guliana, and Mansehra soils showed chemical non-equilibrium (CNE) for As(V) and As(III); however, the extent of CNE was less under unsaturated conditions (-40 mm). These results implied that these well-structured soils had enough large macropores, which cause PF, but at the same time, they were also small enough to retain water and leach solutes under unsaturated conditions (-40 mm). It is concluded that irrigation of contaminated water or dumping solid waste on well-aggregated soil may exhibit PF of dissolved As during and after rains, and additionally As(III), which is more toxic and mobile under reduced conditions, has equal or greater potential for movement.