Hydraulic conductivity and clay dispersion as affected by application sequence of saline and simulated rain water

Summary

Saturated hydraulic conductivity HC, and degree of clay dispersion DD, were determined for a sandy loam and a clay loam soil with waters of different combinations of sodium adsorption ratios SAR (5, 15, 30 and 45 mmol^1/2l^−1/2) and total electrolyte concentration TEC (15, 30, 60 and 90 me l⁻¹) followed by distilled water to simulate rainwater. Increase in SAR and decrease in TEC of leaching water increased DD and decreased HC of soils. The HC values were more highly correlated with SAR than TEC. The critical ratio of TEC/SAR of water below which the relative HC is less than the ‘hreshold’ value (i.e. ≦ 0.75) was 3.82 and 2.01 for clay loam and sandy loam, respectively taking the HC of initial soil with good quality water (SAR = 0.5, EC = 0.3dS m⁻¹) as the reference. Drastic reductions in conductivity were observed even at SAR = 5 (60–83%) when saline water was displaced by rainwater, sensitivity being greater for the sandy loam than for the clay loam; recovery was negligible when the saline water was again applied. Data of EC and clay content of the effluent on application of distilled water suggested that clay dispersion, its movement and lodgement into conducting pores, may be the major cause of HC reduction in sandy loam, whereas in clay loam, surface sealing is the major cause. With distilled water application HC values were governed by SAR rather than TEC of initial water used. The study thus suggests that existing water quality criterion may underestimate the real soil permeability hazards from saline-sodic waters during rainfall infiltration in monsoon season.