Complex NAPL Site Characterization Using Fluorescence Part 3: Detection Capabilities for Specific Excitation Sources

Abstract

Non-aqueous phase liquid (NAPL) contaminants comprised of polycyclic aromatic hy drocarbons (PAHs) can be detected using fluorescence spectroscopic methods. Dense non-aqueous phase liquid (DNAPL) contaminant source zones can be delineated us ing commercially available cone penetrometer (CPT)devices by detecting commingled oils fuels, and naturally occurring organic materials entrained by DNAPLs and carried to depths below the water table. It has been demonstrated that commercially avail able CPT based fluorescence detection systems can be ranked based on how effectively their excitation source wavelengths induce fluorescence using excitation emission ma trices (EEMs). Several neat NAPLs and dilutions with selected DNAPLs were analyzed for specific fluorescence characteristics to determine the optimal excitation source for site characterization efforts. A comprehensive spectral library and corresponding opti mization matrix were generated for complex petroleum mixtures. Based onfield results documenting successful indirect CPT fluorescence detection of a DNAPL source zone, aviation and dieselfuels were selected from this library, diluted with chlorinated sol vents, and evaluated for fluorescence characteristics. Dilution of these complex NAPL mixtures led to changes in the corresponding EEMs. The optimal excitation source for aviationfuel remained relatively constantf or each dilution. However, sensitivityf or each of the commercially available CPT excitation sources was strongly dependent on diesel concentration, whereby higher energy (lowerwavelength) sources yielded improved sen-sitivityfor lower concentrations. Since field concentrations can be highly variable, these observations support the need for multiple wavelength excitation sources for optimal detection capabilities, particularly when diesel fuel ispresent.