This study examines the impact of saturated-unsaturated flow conditions, different pore-water velocities, and vermicompost application on nitrate and colloid transport in soil. Twenty-four columns were filled with a sandy loam soil or the soil amended with vermicompost. Three sets of miscible-displacement experiments were conducted, varying the saturation and mean pore-water velocity. A non-reactive tracer test using nitrate was first conducted for each column, after which the colloid transport test was performed under the same conditions. The one-dimensional advection-dispersion equation was employed to model nitrate transport, while the coupled advection-dispersion equation with two-site kinetic model was applied to simulate colloid transport. Dispersivities determined from the nitrate tracer tests were very similar for all conditions for the two media, which is consistent with the experiment conditions (homogeneously packed columns and small changes in water saturation). The addition of the vermicompost caused a slight increase in dispersivity compared to the unamended soil. Employing nitrate dispersivities to model the colloid transport slightly reduced the accuracy of estimated breakthrough curves, which resulted in small changes in the fitted colloid transport coefficients. The experimental results revealed that colloid retention was greater under unsaturated conditions. Conversely, adding vermicompost to the soil reduced colloid retention. The findings of this study provide some recommendations for agricultural researchers and policy-makers to manage soil quality and control water resources pollution by understanding the critical role of hydraulic conditions (flow rate, water saturation) on transport of nitrate and colloids in soil.
