Preferential transport of bromide in undisturbed cores under different irrigation methods

Bromide (Br-) column leaching studies were conducted to estimate solute velocities and magnitude of preferential flow (ratio of solute to piston flow velocities) under pending and high intensity sprinkler methods. Twenty undisturbed Portneuf silt loam soil cores (250 mm x 300 mm) were collected at spatially distributed locations in a 0.81-ha corn field. The samples were divided into two sets of 10, and Br- leaching studies were conducted under pending and high intensity sprinkler methods. Analysis of the breakthrough curves (BTCs) indicated that preferential flow occurred under both methods in all of the samples. The peak and median solute velocities were 29-311% and 20-60%, respectively, larger than corresponding piston flow velocities. Though the solute velocities were larger under the pending method due to larger input flux values, the average magnitude of the preferential flow was similar between the two methods. The mobile-immobile model predicted the BTCs better than the convection-dispersion model, with an average mobile percentage of 75 to 80. For the Portneuf silt-loam soil and given test conditions, the assumption of peak solute arrival to a given depth based on piston flow is not valid because of large differences between the peak and piston flow velocities. The large variabilities measured for peak solute velocity under both methods indicate the importance of repeated field measurements for determination of both magnitude and level of uncertainty.