Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2005
Publication Date: 6/28/2005
Citation: Kung, K.S., Hanke, M., Helling, C.S., Kladivko, E.J., Gish, T.J., Steenhuis, T.S., Jaynes, D.B. 2005. Quantifying pore size spectrum of macropore-type preferential pathways. Soil Science Society of America Journal, 69:1196-1208. Interpretive Summary: Rapid preferential transport of dissolved chemicals through soils to groundwater can cause water contamination. However, directly measuring this in the field, and describing this process mathematically, are very difficult. In a continuation of related studies, chemical tracers that do not absorb to soil were leached through the soil profile by applying irrigation water at several intensities. The recovery of those tracers was monitored in the drain tile effluent water beneath the field plot. Mathematical evaluation of these chemical recovery-versus-time curves has given insight into this highly complex process of chemical transport through soil. The results have impact on basic soil physics theories of water and chemical movement in soils. These will therefore be relevant to many environmental and other problems that relate to movement of water and dissolved chemicals through soil.
Technical Abstract: Some structural pores associated with macropore-type preferential flow can greatly accelerate chemical transport in unsaturated soils, thereby potentially causing groundwater contamination. Classical deterministic models to predict chemical transport depend on using the pore lumping approach embedded in Darcy's conductivity parameter instead of quantifying the actual soil pore spectrum. We contend, however, that quantifying the field-scale pore spectrum of preferential pathways, i.e., without lumping the contributions of individual pores, is the appropriate method for simulating convective chemical transport through macropore-type preferential pathways. In this study, we used an improved tile drain monitoring protocol to measure the breakthrough patterns of conservative tracers. The tails of these patterns suggested that the impact of preferential pathways on contaminant transport can be conceptualized as that occurring through cylindrical capillary tubes. We then proposed a function with sharp "cut-off" points on both sides of the distribution to represent the pore spectrum of these tubes. Finally, we used the measured tracer breakthrough curves as data sources to find the parameters of the proposed function. Our results, based on the best fitting, showed that the preferential pathways are naturally clustered into domains; the smaller stagnant pathways could become active when infiltration rate increases.