Comparison of flux and resident concentrations in macroporous field soils

In many solute transport studies, either the flux or the resident concentra tion is used, In some cases, however, the transport parameters obtained fro m 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 perfor ming laboratory solute displacement experiments on a number of structured f ield soils, Breakthrough curves (BTCs) of flux and resident concentrations for a pulse injection of 10 g/L CaCl2 solution were monitored simultaneousl y at the bottom and middle of soil columns using an EC-meter and time domai n reflectometry (TDR) probes, respectively, Transport parameters were then obtained by fitting the convective lognormal transfer function (CLT) model to the observed ETC data and compared for different concentration modes, Fl ux concentrations predicted from the parameters of resident concentrations based on the CLT model were also compared with the observed ETC data. Compa rison of transport parameters between the flux and resident concentrations showed substantial differences caused by preferential movement of solute th rough 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 co ncentrations are not representative of solute transport in the soil macropo res but are primarily in the soil matrix region, and use of TDR for monitor ing resident concentrations in such soils becomes limited.