Submitted to: Journal of Contaminant Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2001
Publication Date: 10/3/2004
Citation: Lee, J., Noborio, K., Horton, R., Jaynes, D.B. 2004. Characterization of preferential flow in undisturbed, structured soil columns using a vertical TDR probe. Journal of Contaminant Hydrology. 51(3-4)131-144. Interpretive Summary: Scientists have a wide array of computer models for predicting the fate and transport of agrochemicals applied to soil. Before these models can be accurately applied to specific situations, the parameters required for the model need to be measured or estimated for the soil of interest. One group of models, that is very useful for estimating rapid movement of water and chemicals through undisturbed natural soil, has been little used outside of the laboratory because of the difficulty in measuring the required model parameters. This research developed and validated a new methodology for measuring these parameters that is easy, quick, and adaptable for use in the field. We show that the new method gives model parameter estimates that are as reliable as the more traditional, but time consuming measurement methods. Now that the new model has been validated, scientists will be able to use the method with confidence in estimating parameter values for a wide range of soils and locations. The method will allow more accurate modeling of water and chemical transport through soils and better estimates of potential ground water contamination by agrochemicals.
Technical Abstract: Previous studies have shown that time domain reflectometry (TDR) can be used to characterize solute transport in soil. However, few of these studies have been conducted in undisturbed, structured soils with preferential flow properties. In this study, we tested a TDR method under controlled laboratory experiments using 20-cm-long by 12-cm-diam. undisturbed, structured soil columns. We used a vertically installed TDR probe and a short pulse of tracer application to obtain residual mass (RM) breakthrough curves (BTCs). The RM BTCs obtained from TDR were used to estimate mobile/immobile (MIM) parameters that were compared to the parameter estimates from effluent data. A conventional inverse curve fitting method (CXTFIT) was used to estimate parameters. The TDR- determined parameters were then used to generate predicted effluent BTCs for comparison with observed effluent BTCs for the same soil columns. Overall, the RM BTCs obtained from TDR were similar to the RM BTCs obtained from effluent data. The TDR-determined parameters corresponded well to the parameters obtained from the effluent data, although they were not within the 95% confidence intervals. Correlation coefficients between the parameters obtained from the TDR and from effluent data for the immobile water fraction (Tim), mass exchange coefficient (alpha), and dispersion coefficient (Dm) were 0.95, 0.95, and 0.99, respectively. The TDR-predicted effluent BTCs also were similar to the observed effluent BTCs having an average coefficient of determination of 0.94. The vertical TDR probe method is simple, minimally-destructive, and provides representative preferential flow properties that enabled the characterization of solute transport in soil.