The linkage between near-saturated hydraulic conductivity and tritium leaching

Macropore flow in structured soils is an important process in relation to the transport of water, contaminants and nutrients in the soil. A close relation exists between hydraulic conductivity K(h) near saturation and the potential of macropore flow. At the same time, tracer breakthrough experiments in the laboratory can determine the degree of macropore flow. In this study, we aim to investigate a direct link between tracer breakthrough characteristics and soil hydraulic properties (SHPs) of structured soils, which may explain spatial variation of solute transport in soils. We used SHPs and tracer breakthrough characteristics for 71 undisturbed topsoil columns (19 cm height, 20 cm diameter) from Denmark. We defined K10 (near-saturated hydraulic conductivity) as K(h) at a matric potential (h) of −10 cm and used logarithmic transformation, logK10. On the same soil columns, we calculated the 5%, 25%, and 50 % arrival times (AT) as the percentage of the cumulative relative mass of tracer percolating through the bottom of the soil column. The regression analyses proved significant positive relation between logK10 and 5% AT, 25% AT, and 50 % AT. The saturated hydraulic conductivity appeared to be less critical for the shape of the tracer breakthrough curves. In addition, the 5% AT, 25% AT, and 5 0%AT correlated with soil pF values at 1.7, 2.0, and 2.5 (volumetric water content at h equal to −100 cm, −300 cm, and −500 cm, respectively) showing significant negative correlations.  Linking SHPs with tracer breakthrough characteristics on large intact columns thus proves highly useful for characterizing soil macropore functions.