Controlling solute transport processes in soils by using dual-porosity characteristics of natural soils

Soils are notorious for their heterogeneity, and macropores conduct solutions by bypassing the surrounding soils, sometimes wasting the applied fertilizer or remediation chemicals. It would be beneficial in agriculture or environmental engineering fields if solute transport in soils were controlled with relatively simple techniques. In this study, the solute transport process was controlled using dual-characteristics of the soil pore system. Specifically convection and dispersion were controlled by changing the structure-dependent flow regime. Soil samples with/without artificial small macropores (diameter = 1 mm) and undisturbed soil samples were prepared, and solute transport experiments were conducted, in which a variety of breakthrough curves (BTC) was obtained by changing flow rate (from 1 to 0.1 of saturated conductivity) and saturation (saturation to −3 kPa). The results for the artificial macropore system showed that completely different BTCs were obtained with small suction differences, namely saturation and −3 kPa. At saturation, the BTC showed a bi-modal distribution typical for soils with macropores. At a slightly unsaturated condition of −3 kPa, however, the BTC showed a normal distribution quite similar to that of a repacked soil column. The results for undisturbed soil showed that the BTC gradually transitioned from a bi-modal to normal distribution, with the suction changing from saturation to only −3 kPa. These results suggest that effective use of fertilizer or remediation chemicals is possible with a relatively simple and inexpensive technique, even when macropore networks are present.