Hydraulic Gradient, Aging, and Water Quality Effects on Hydraulic Conductivity of a Vertisol

Abstract

AbstractAging, manifested by an increase with time of the aggregate mechanical strength, has been reported recently to affect the response of a soil to water quality. However, experimental evidence demonstrating its effect on the hydraulic 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) were 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 conductivity of the soil decreased with an increase in soil exchangeable Na percentage and a decrease in total electrolyte concentration. Also, the relative hydraulic conductivity of the soil decreased from 1.1 to 0.94 when subjected 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 decrease was attributed to the compression of clay particles at the column outlet. The subsequent increase in hydraulic conductivity was ascribed to thixotropic stabilization of the clay fabric and cohesive bond development. The effects of hydraulic gradient and aging were less pronounced in soil in equilibrium with solution of SAR = 10 because the higher sodicity increased the repulsion forces between clay particles, thus reducing clay‐to‐clay contact. Some practical consequences were postulated concerning aging effect under field conditions.