Water and solute movement in soil as influenced by macropore characteristics: 1. Macropore continuity

In most contaminant transport modeling studies, only the macropores that are visible at the soil surface are considered. Furthermore, it is assumed that these macropores are straight and continuous throughout the soil profile. Little is known on the importance of other types of macropore continuity and tortuosity on preferential movement of contaminants through soils. This paper describes the results of a laboratory study dealing with macropore continuity effects on breakthrough curves (BTCs) and solute distribution in a Forman loam (fine-loamy mixed Udic Haploborolls) soil. BTCs were obtained under a constant hydraulic head of 0.08 m from a 2-D column (slab) containing artificial macropores. The input solution contained 1190 mg l−1 KBr, 10 mg l−1 Rhodamine WT, and 100 mg l−1 FD&C Blue #1. The continuity types studied were: macropore open at the soil surface–open at the bottom of the column (O–O), open–closed (O–C), closed–open (C–O), and closed–closed (C–C). A treatment without macropore served as a control. As expected, the solution in the O–O treatment reached the bottom of the macropore about 100 times faster by bypassing most of the soil matrices. As a result, the breakthrough time for O–O treatments was much faster than any other continuity treatments. Both the O–O and O–C type macropores favored earlier breakthrough, smaller apparent retardation coefficient (R′), deeper center of mass, and higher anisotropy in tracer concentrations in the horizontal direction than the C–O, C–C, and the Control treatment. The C–C macropore was favored in deeper penetration of tracer only when another macropore was present nearby. The importance of macropore continuity increased with an increase in the adsorption coefficient of the tracers.