Macropore Transport of a Surface-Applied Bromide Tracer: Model Evaluation and Refinement
- L. R. Ahuja ,
- K. E. Johnsen and
- G. C. Heathman
Rapid transport of surface-applied agrichemicals to groundwater through macropores is a major problem that requires a better understanding and quantification. A simple model of macropore flow and transport of a surface-applied Br− tracer under rainfall was evaluated and refined by testing against soil-column data reported earlier. The experiments consisted of eight treatment combinations, each in duplicate, of the following conditions: soil initially air dry vs. soil initially wetted by rainfall; a 10-mm layer of dry aggregates on the surface vs. no aggregates; and a 3-mm artificial macropore made along the column's vertical axis vs. no macropore. A solution of SrBr2 was atomized over the surface, followed by application of simulated rainfall. Evaluation of the model indicated that: (i) once the Green-Ampt infiltration parameters were calibrated on control columns, water flow into other columns was simulated correctly; however, in order to match the macropore bottom outflow, the lateral absorption of water from macropore to soil matrix had to be adjusted for compaction of the macropore wall and variable water pressure around the pore circumference under small macropore flow rates; (ii) for chemical mass in macropore outflow, better results were obtained if, in the wetted portion of the soil profile, the water flow down the macropore mixed with ≈0.5 mm of soil around the walls and its soil solution; (iii) microporosity of surface aggregates determined from control columns resulted in fairly good simulations of the increase of the chemical in macropore flow caused by aggregates.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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