Immobile Water Content and Mass Exchange Coefficient of a Field Soil

Determining the preferential flow characteristics of a soil is important because agrichemicals can contaminate groundwater via preferential flow pathways. A model that predicts solute transport due to preferential flow is the mobile‐immobile solute transport model, which partitions the total water content (θ, m 3 m −3 ) into a mobile fraction (θ m ) and an immobile fraction (θ im ). Recently, an in situ method was proposed for determining the mobile‐immobile model parameters of θ im and mass exchange coefficient (α) between the fractions by using a tension infiltrometer to apply a series of four fluorobenzoate tracers. The objective of this study was to test the in situ technique at 47 sites along a transect in a ridge‐till corn ( Zea mays L.) field of Nicollet soil (fine‐loamy, mixed, mesic Aquic Hapludoll). The immobile fraction (θ im /θ) ranged from 0.394 to 0.952 with a median of 0.622. The mass exchange coefficient ranged from 0.000237 to 0.00481 min −1 with a median of 0.00123 min −1 . These values are similar in magnitude and range to values reported by other investigators, and they follow the same relationships. The values of θ im /θ and α along the transect indicated no obvious spatial trends or spatial correlations. Significant linear correlations did exist between α and soil water flux, α and θ im , and θ and θ im .