Spatial variability of water and bromide transport through variably saturated soil blocks

Water and solute transport pathways through soil are very complex. Soil properties affecting solute transport vary spatially and temporally within a soil profile and across landscape positions. The objective of this laboratory study was to evaluate water and bromide (Br) transport through 38- by 38- by 60-cm-deep undisturbed blocks of Cecil soil (Clayey, kaolinitic, thermic Typic Kanhapludults) collected from three contrasting landscape positions (interfluve, linear slope, and foot slope) in the Piedmont region of North Carolina. Two replicate soil blocks from each position were placed on a grid lysimeter-plate effluent collection system which facilitated collection of the effluent from 81 discrete cells under -2.5 kPa pressure. Each block was equilibrated for 5 d with a once daily application of 3.5 L of 0.005 M CaSO₄ solution by a water drop applicator at a rate of 14 mm h^-1. Four-hundred mL of KBr solution (4000 g Br m^-2) was uniformly sprayed onto the soil surface. Thereafter, 3.5 L of 0.005 M CaSO₄ solution was applied daily for the duration of each experiment (19-33 d). Effluent volume and Br concentration in the effluent were measured daily for each of the 81 4 by 4-cm cells. Cumulative water outflow and Br distribution plots, spatial distribution of cumulative effluent percent, frequency plots, and Br breakthrough curves (BTCs) showed that differences in preferential flow of water and Br occurred for soil blocks from different landscape positions. Differences in preferential flow of water and Br were attributed to soil horizon thickness, soil texture and structure, macroporosity, and slope gradient. Preferential flow of water and Br under variably saturated conditions was found to be highly variable within a given soil profile and that differences in the distribution and magnitude of preferential flow occurred across topographic positions.