Characteristics of Rhodamine WT and Fluorescein as Adsorbing Ground‐Water Tracers

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Abstract
Fluorescent dyes are being used as adsorbing ground‐water tracers for conducting solute transport studies for adsorbing organic chemicals (e.g., pesticides). In this research, the ability of two fluorescent dyes (rhodamine WT and fluorescein) to mimic the adsorptive behavior of two herbicides (atrazine and alachlor) with alluvial aquifer sands was evaluated. Laboratory studies (batch and column) indicated the following order of increasing adsorption: fluorescein, atrazine, alachlor, and rhodamine WT. Thus, the use of fluorescein and rhodamine WT as adsorbing ground‐water tracers was observed to delimit the appearance of the atrazine and alachlor (fluorescein appeared before and rhodamine WT after the herbicides). Several adsorption characteristics of the fluorescent dyes differed from those commonly observed for pesticides. The levels of adsorption for the dyes were several orders of magnitude greater than predicted from empirical relationships based on Kowand foc. The presence of divalent cations was observed to increase the level of rhodamine WT adsorption. The rhodamine WT breakthrough curves were not of the conventional sigmoidal shape but instead leveled off at a C/CO value of 0.5 for a number of pore volumes prior to increasing again towards a C/CO value of 1.0. These characteristics indicate that the adsorptive mechanisms for the fluorescent dyes differed from the adsorptive mechanisms for most pesticides. These variations in fundamental adsorptive mechanisms raise concerns as to the transferability of the results of this research (fluorescein and rhodamine WT delimiting the appearance of atrazine and alachlor) to other subsurface media.