Hydraulic gradient, aging, and water quality effects on hydraulic conductivity of a vertisol

Aging, manifested by an increase with time of the aggregate mechanical stre ngth, has been reported recently to affect the response of a soil to water quality. However, experimental evidence demonstrating its effect on the hyd raulic conductivity is lacking, We present the results of aging, hydraulic gradient, and water quality effects on the saturated hydraulic conductivity of disturbed smectitic soil samples (A horizon, Chromic Haploxerert), The effects of two hydraulic gradients (2.9 and 12) and two leaching durations (3-4 and 20 h) vc cre tested. Soil consecutively leached with 0.5, 0.05, 0. 01, and 0.003 M Cl- solutions (Na adsorption ratio, SAR = 10) or 0.5, 0.01 M Cl- solutions and deionized water (SAR = 0), The saturated hydraulic cond uctivity of the soil decreased with an increase in soil exchangeable Na per centage and a decrease in total electrolyte concentration. Also, the relati ve hydraulic conductivity of the soil decreased from 1.1 to 0.94 when subje cted to a high-hydraulic gradient treatment, and subsequently increased to 2.32 under prolonged leaching with 0.01 M Cl- solution of SAR = 0. This dec rease was attributed to the compression of clay particles at the column out let. The subsequent increase in hydraulic conductivity was ascribed to thix otropic stabilization of the clay fabric and cohesive bond development. The effects of hydraulic gradient and aging were less pronounced in soil in eq uilibrium with solution of SAR = 10 because the higher sodicity increased t he repulsion forces between clay particles, thus reducing clay-to-clay cont act, Some practical consequences were postulated concerning aging effect un der field conditions.