ESTIMATING SOLUTE TRANSPORT IN UNDISTURBED SOIL COLUMNS USING TIME-DOMAIN REFLECTOMETRY

Time-domain reflectometry (TDR) was used to monitor solute breakthroug h curves (BTC's) in 30 saturated undisturbed soil columns collected al ong a 35-m-long transect in the field. The BTC's were obtained by rela ting the bulk soil electrical conductivity, EC(a), to the relative con centration of a KCl solute pulse applied to the soil surface. Values o f EC(a) were estimated by measuring the soil's impedance to an electro magnetic wave generated by a cable tester. Parallel two-rod TDR probes inserted horizontally at a depth of 10 cm were used to monitor the so il's impedance during transport of the KCl solute pulse. Calculated ex perimental time moments indicated that the BTC data were very variable in time and space. This variability was attributed in part to the rel atively small volume of soil sampled with the TDR probes, and in part to the natural heterogeneity of the sandy loam soil. The observed BTC' s were classified into three groups. One group showed bell-shaped curv es consistent with the classical convection-dispersion equation (CDE). A second group was characterized by early breakthrough and long taili ng. The BTC's in this group could be described by a mobile-immobile tr ansport model (MIM). A third group of BTC's showed irregular shapes wi th several peaks. Time moments were used to compare the estimated (fro m the moments), fitted (CDE and MIM) and independently measured pore-w ater velocities. The disparities between the observed and fitted veloc ities suggest that for structured soil several TDR probes may be neces sary in order to obtain reasonable estimates of column-scale solute tr ansport behaviour.