Comparison of flux and resident concentrations in macroporous field soils

In many solute transport studies, either the flux or the resident concentration is used. In some cases, however, the transport parameters obtained from different concentration modes may not be identical, especially for soils having preferential pathways. In this study we investigated differences in the transport parameters between flux and resident concentrations by performing laboratory solute displacement experiments on a number of structured field soils. Breakthrough curves (BTCs) of flux and resident concentrations for a pulse injection of 10 g/L CaCl₂ solution were monitored simultaneously at the bottom and middle of soil columns using an EC-meter and time domain reflectometry (TDR) probes, respectively. Transport parameters were then obtained by fitting the convective log-normal transfer function (CLT) model to the observed BTC data and compared for different concentration modes. Flux concentrations predicted from the parameters of resident concentrations based on the CLT model were also compared with the observed BTC data. Comparison of transport parameters between the flux and resident concentrations showed substantial differences caused by preferential movement of solute through soil macropores. The predicted flux concentration BTCs also differed greatly from the observed BTCs in peak and travel time. This suggests that for structured soils having preferential flow, the TDR-measured resident concentrations are not representative of solute transport in the soil macropores but are primarily in the soil matrix region, and use of TDR for monitoring resident concentrations in such soils becomes limited.