Hydraulic Conductivity Dynamics during Salt Leaching of a Sodic, Structured Subsoil. Issue 5 (29th August 2014) - Callaghan, Michael V.
Abstract : Leaching of salts in the presence of elevated soil sodicity can result in reduced hydraulic conductivity due to clay swelling or dispersion. This study examines the dynamics and mechanisms of hydraulic conductivity decrease induced during salt leaching of a structured, smectite‐bearing subsoil. Intact soil cores were initially equilibrated with aqueous solutions of elevated salinity and sodicity. Results of leaching with a solution of fixed sodicity demonstrated that, in this structured soil, salinity levels were successfully leached before reaching a stable hydraulic conductivity. This disequilibrium between decreases in hydraulic conductivity and permeameter effluent salinity appear to be the result of diffusion‐controlled swelling. Diffusion of salts out of low permeability soil aggregates may be slowed by diffuse double layer swelling trapping salts within the smallest pores. A higher degree of sensitivity to saline–sodic swelling effects was observed for the structured soil in this study compared to repacked soils of higher swelling clay content from other studies. Higher sensitivity of structured subsoils to saline–sodic swelling effects may be attributed to aggregate breakdown and plugging of macropores, heterogeneity in swelling‐clay distribution resulting in a locally higher degree of swelling, and/or increased internal swelling due to physical restriction caused by overburden loading. Potential irreversibility of pore network damage caused by soil structure breakdown motivates mitigating saline–sodic disruption of field hydraulic conductivity before it occurs.