INSITU MEASUREMENT OF THE EFFECTIVE TRANSPORT VOLUME FOR SOLUTE MOVING THROUGH SOIL

A simple field method for measuring the soil's effectively mobile water fraction during near-saturated flow is presented. Initial charging of the immobile fraction theta(im) is achieved by first wetting the soil with a disk permeameter until steady conditions prevail. The disk is then removed and rapidly replaced, but now filled with a tracer solution. Subsequently after a period of infiltration with tracer (0.1 M KBr), the soil underneath is sampled. The ratio of the measured to applied concentration, c*/c(m), in the samples will be the fraction of the soil's water that is effectively mobile. We assume that the antecedent tracer concentration is zero, and that the mobile fraction, theta(m), is at concentration c(m). Disk permeameters, set at the slightly unsaturated potential head psi(o) = -20 mm, wetted Manawatu fine sandy loam to a water content of theta(o) = 0.414 m3 m-3. From analysis of the tracer concentrations measured under the disk, we deduced theta(m) to be just 0.203. This semimobility was in accord with the observed depthwise penetration of tracer, as well as other measures of mobility previously found by others from longer term leaching studies carried out nearby.