Natural organic matter (NOM) is a ubiquitous fluorophore in freshwater systems.  NOM is derived from the decomposition and recycling of vegetative and animal material in the watershed.  In dissolved or colloid form, NOM is highly fluorescent and exhibits broadly overlapping spectra that can obscure spectroscopic detection of tracer dyes at very low (< 0.01 ppm) concentrations.  A regional survey of natural background fluorescence in karst aquifers revealed significant seasonal and spatial variations in the spectral properties of groundwater.  

The fluorescent properties of NOM provide important information about catchment areas and modes of recharge affecting a given spring.  Variations in peak wavelength and intensity indicate seasonal or storm-related changes in the composition of NOM.  These properties can change rapidly over the course of a dye tracer test, influencing dye selection, tracer recovery, and analytical sensitivity.  Our study suggests that fluorescence varies predictably in E. TN karst aquifers, when examined on a quarterly to bi-monthly basis.  

A multi-seasonal approach to the characterization of background fluorescence is essential to the timing and design of low concentration dye tests.  An understanding of the correlation between NOM fluorescence and local hydrochemistry, geology, and land use adds another dimension to typical watershed and aquifer studies.  This presentation will review field and laboratory techniques designed to enhance analytical sensitivity and fluorescent signature separation between NOM and tracer dyes.