Flow velocity effects on the retardation of FD&C Blue no. 1 food dye in soil

Dyes are often used as tracers of water and solute flow in soil. However, there has been little published research on the effects of flow velocity on the relative movement of dye and wetting fronts. Recently, the FD&C Blue no. 1 dye has been used as a tracer of preferential water flow paths in soils, and has attractive properties suggesting it will be used increasingly in the future. The objective of this study was to characterize the effects of flow velocity on the adsorption characteristics of the FD&C Blue no. 1. We performed a series of laboratory experiments on the characterization of the equilibrium adsorption isotherm as well as the effect of flow velocity on adsorption characteristics of the FD&C Blue no. I in three horizons of Verndale sandy loam (coarse-loamy over sandy, mixed, frigid Udic Argiboroll). Batch equilibration (24 h) measurements resulted in linearized Freundlich distribution coefficients ([K.sub.d]) of 4.33, 23.95, and 4.59 L [kg.sup.-1] for the Ap, Bt, and C horizons, respectively. Neither organic C nor clay content differences accounted for the variation between horizons. Leaching experiments with a 1 g [L.sup.-1] dye solution and repacked soil columns containing either Ap, Bt, or C horizon material showed that the dye was retarded relative to the wetting front, and that the extent of retardation was inversely related to the time period during which dye solution was applied. Therefore, this dye (and perhaps others) should not be used to make inferences about the movement of water or other solutes until the effect of the application rate is evaluated for the flow conditions in the system(s) of interest.