Octanol-water partition coefficients (K_ow) vs. pH for fluorescent dye tracers (fluorescein, eosin Y), and implications for hydrologic tracer tests

Abstract

Values of the octanol-water partition coefficient (K_ow) have been measured vs. pH for two fluorescent dyes (fluorescein, eosin Y) commonly used in hydrologic tracer tests. Values for both fluorescent dyes are similar, with slightly larger values for fluorescein vs. eosin Y at any given pH. Measured values of log K_ow show a negative correlation vs. pH, and range from 2.78 for fluorescein (pH 4.29) to -1.22 for eosin Y (pH 7.63). Values of log K_ow have been used to calculate example values of the organic matter-water partition coefficient (K_om) and the retardation coefficient (R), and suggest that fluorescein and eosin Y act as excellent hydrologic tracers at pH > 7, but become increasingly non-conservative at pH < 7. The results illustrate the importance of pH upon fluorescent dye speciation, and upon fluorescent dye adsorption behavior. Hence, measurement of pK_a and the variation of log K_ow vs. pH for other fluorescent dye tracers is recommended.