EVALUATION OF THE INFLUENCE OF BACKGROUND FLUORESCENCE ON LOW CONCENTRATION DYE TRACER TESTS IN KARST AQUIFERS OF EAST TENNESSEE

Fluorescent tracer dyes are inexpensive and direct tools for identifying surface-to-subsurface connections and flowpaths, especially in karst and fractured rock aquifers. Source water protection areas for community drinking water supplies and endangered species habitats are sensitive settings that could benefit from tracer delineation studies, but where highly concentrated dye injections are usually deemed unacceptable. The widespread use of low concentration dye injections is effectively inhibited by the presence of naturally fluorescing substances consisting of humic and fulvic acids, but also including polynuclear aromatic compounds in fossil fuels and soils, terpenes, sterols, proteins, long-chain alcohols and chlorophyll. These natural fluorophores exhibit broadly overlapping spectra that obscure analytical differentiation of specific dyes at very low concentrations. Solutions of complex organic acids are difficult to differentiate analytically, however, seasonal and spatial trends in the composition of fluorophore cocktails are evident in dissolved organic carbon (DOC) fluctuations with time, temperature, precipitation, land use, soil depth and source. Depending on these factors, the molecular weight, fluorescent intensity and wavelength of natural fluorophores should vary rather predictably. As a basis for injecting and detecting tracer dyes at very low concentrations, research is underway to systematically characterize trends in spatial and temporal variations of DOC and background fluorescence in public groundwater sources in karst aquifers in East Tennessee. Data collection will focus on the hydrochemical characterization of each source, experimentation with sample preparation techniques for suppressing background fluorescence, and alternate methods of introducing and detecting tracer dyes at low influent concentrations. The purpose of this research is to produce information that facilitates greater application of dye tracing for groundwater basin delineation, characterization and protection.