Developing and testing a system for studying unsaturated solute transport on undisturbed soil blocks

Primary limitations associated with conducting laboratory solute transport studies are related to size and shape of undisturbed soil cores, effective water delivery system, effluent collection, and experimentation under unsaturated conditions. Compounding the problem of conducting solute transport studies is the occurrence of preferential flow. The objective of this study was to develop and evaluate the equipment and protocols for conducting unsaturated preferential flow experiments on large, 38 cm by 38 cm by 65 cm deep, undisturbed soil blocks that reduce some of these limitations. System components include a rigid steel framework to assist in collecting undisturbed soil blocks; an 81 cell grid lysimeter plate for partitioning effluent outflow, and applying the desired soil water pressure at the base of the soil block; a bi-directional water drop applicator capable of delivering a range of application rates continuously over the entire soil surface; and a network of soil water monitoring devices for measuring volumetric soil water content and matrix potential. The soil block collected for this experiment was a Cecil sandy loam (Clayey, kaolinitic, thermic, Typic Kanhapludult) from the eastern United States Piedmont region. To evaluate the experimental system, potassium bromide (KBr) was applied to the soil surface (40 kg Br ha^-1) and displaced through the soil block. Seventy percent of the cumulative effluent volume and 75% of the cumulative bromide (Br) mass were conducted through 50% of the basal area of the soil block. Cells that conducted high volumes of effluent (> 1000 mL) also transported considerable amounts of Br, r² = 0.99. The experimental control offered by the equipment and procedures described in this study allow for detailed study of mechanisms controlling preferential flow of water and chemicals through soils under various boundary conditions.