Synchronous spectro-fluorometry and fluorescent dye-tracing (FDT) has changed our understanding of hydrogeologic systems, especially ground water systems affected with high concentrations of organic constituents. Synchronous spectro-fluorometry and FDT provide exceptional tools to accurately determine flow paths, seepage velocities, and permeability. They provide useful data to validate current ground water models and measure remedial system performance. Most dye-tracing tests indicate that ground water does not move as estimated by conventional hydrogeological investigations and models, but often deviates by refraction in the horizontal plane from the apparent direction of the hydraulic gradient. This refraction implies pronounced preferential flow paths, immobile contaminant ‘hot-spots’, and other heterogeneities in the subsurface. Utilization of FDT can have a significant impact on wellhead protection delineation, contaminant fate and transport modeling, and remedial system efficiency.