Breakthrough Curve of Bromide as Affected by Soil Structure in Saturated and Unsaturated Conditions

In this study، the effect of soil structure under saturated and unsaturated flow conditions on nonreactive bromide (BR) transport was investigated. The soil structure treatments consisted of undisturbed columns (prismatic and granular structures)، and disturbed columns (single- grain structure). A constant concentration (C0= 0. 005 M) of bromide was supplied on the surface of the columns in a steady-state flow condition. For the saturated flow condition، a flux equal to the highest saturated hydraulic conductivity (Ks) of the columns was applied on all of the columns. To create the unsaturated flow condition، a flux equal to the half of the lowest Ks of the columns was imposed on all of the columns. The leaching of the columns was followed for five pore volumes (5PV) and the bromide concentration of the effluent was measured at 0. 2PV intervals using bromide selective electrode. The breakthrough curve (BTC) of single- grain structure was sigmoidal (S-shaped) and similar to piston-capillry flow form. In contrast، BTCs of the granular and prismatic structures had a steep initial part and later gradual tailed part. The preferential pathways caused the early appearance of bromide in the leachate of columns of these two structures. Tailing of the BTCs might be due to dispersion and diffusion between mobile and immobile water fractions. In saturated condition، the bromide plume appeared earlier than that in the unsaturated condition because of domination of mass flow and rapid macroporous stream. The results demonstrated the importance of soil structure، preferential pathways، and flow conditions in solute and pollutant transport.