A Multi-Tracer Field Leaching Experiment Reveals Macropore Structures and Its Influence On Solute Leaching Under Two Contrasting Land Uses.

Leaching of solutes below the root zone has been identified as a main source of potential groundwater pollution. In structured soils, preferential flow paths can have a significant influence on rapid leaching of solutes. Dye tracer experiments have been frequently used to map the spatial distribution of macropore structures. However, the relative influence of the macropore network on solute leaching under field conditions and its correlation with physical soil properties and land use has not been analyzed yet and requires innovative sampling techniques. The objectives of the present study were to map the macropore network and analyze the leaching behaviour of a conservative tracer under two contrasting land uses. Ponded infiltration experiments with Potassiumbromide (KBr) and Brilliant Blue (BB) were conducted on a silt loam soil in Lexington, KY. Two land use systems, grassland and cropland (wheat), were tested. At soil water content close to field capacity, a total of 30 mm multi-tracer solution was infiltrated on an area of 1.2 × 0.7 m with a ponding head of 20 mm. The concentrations of KBr and BB were 10 and 5 g L^-1, respectively. After 24 hours, 10 profile sections (width: 100 cm, depth: 70 cm) were excavated in steps of 5 cm and sampled. Dye stained areas were mapped based on digital image analysis. At every other profile section, the soil was sampled for soil water content at regular intervals along a 10 × 10 cm raster. Vane shear resistance was measured as a proxy for soil mechanical strength. X-ray fluorescence analysis was used to derive concentrations of Br and Si, the latter being used as proxy for  soil particle size distribution. By applying this new sampling scheme we were able to correlate physical and hydrological observations and reveal the relative influence of the macropore network on solute leaching.