FLOW VELOCITY EFFECTS ON THE RETARDATION OF FD-AND-C-BLUE-NO-1 FOOD DYE IN SOIL

Dyes are often used as tracers of water and solute now in soil. Howeve r, there has been little published research on the effects of now velo city 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 n ow paths in soils, and has attractive properties suggesting it will be used increasingly in the future. The objective of this study was to c haracterize the effects of now velocity on the adsorption characterist ics of the FD&C Blue no. 1. We performed a series of laboratory experi ments on the characterization of the equilibrium adsorption isotherm a s well as the effect of now velocity on adsorption characteristics of the FD&C Blue no. 1 in three horizons of Verndale sandy loam (coarse-l oamy over sandy, mixed, frigid Udic Argiboroll). Batch equilibration ( 24 h) measurements resulted in Linearized Freundlich distribution coef ficients (K-d) of 4.33, 23.95, and 4.59 L kg(-1) for the Ap, Bt, and C horizons, respectively. Neither organic C nor clay content difference s accounted for the variation between horizons. Leaching experiments w ith a 1 g L-1 dye solution and repacked soil columns containing either Ap, Bt, or C horizon material showed that the dye was retarded relati ve to the wetting front, and that the extent of retardation was invers ely related to the time period during which dye solution was applied. Therefore, this dye (and perhaps others) should not be used to make in ferences about the movement of water or other solutes until the effect of the application rate is evaluated for the now conditions in the sy stem(s) of interest.