Characterization of preferential solute transport in soil

The occurrence of preferential flow for rapid movement of solute through soil has been reported. However, there are not many practical methods to detect, quantify, and identify preferential flow. The characterization of preferential flow would improve our understanding of field water and solute behavior. A concept that separates water content into “mobile” and “immobile” domain has been successful in describing some forms of preferential solute transport. The mobile-immobile model (MIM) has three significant parameters- immobile water content (&thetas;_im), mass exchange coefficient (α), and dispersion coefficient (D _m). Although one can determine these parameters by applying inverse methods to effluent breakthrough curves (BTC), obtaining BTC in the field is not always practical.; In this study, a sequential tracer (ST) method to estimate &thetas; _im and α was tested in laboratory soil columns, comparing the estimated parameters from the ST method to the estimated parameters from the inverse BTC fitting method. The estimated parameters from the ST method were similar to the parameters from the effluent BTC.; Two time domain reflectometry (TDR) methods for determining MIM parameters were developed and evaluated by using carefully controlled laboratory experiments. For a shallow (0–2 cm) TDR probe method, the TDR determined parameters were similar to the parameters estimated from the ST method and from the observed effluent data. A simulation study predicting effluent BTCs using the TDR determined parameters demonstrated the feasibility of the shallow TDR probe for characterizing solute transport in soil cores. In all cases, the average coefficient of determination, r2, for the predicted BTCs was 0.99. For a vertical TDR probe method, there was not always full agreement between the TDR determined parameters and effluent determined parameters. However, predicted BTCs obtained from the TDR method were similar to the observed BTCs with the average r 2 being 0.94.; The ST method and the two TDR methods are promising methods for estimating MIM parameters. The TDR methods are relatively simple and reliable. Furthermore, TDR is portable and can be multiplexed so that one can install probes at multiple locations in field. However, the TDR methods should be further examined in situ.