Subsurface screening of petroleum hydrocarbons in soils via laser-induced fluorometry over optical fibers with a cone penetrometer system

A novel field screening method is described that couples a fiber optic based chemical sensor system to a truck mounted cone penetrometer. The system provides the capability for real- time, in situ measurement of petroleum hydrocarbon contamination and soil type to depths of 50 m. The technique uses a hydraulic ram in a truck with a 20 ton reaction mass to push an instrumented probe into the ground. Fluorescence is excited through a sapphire window in the probe by 337 nm light from a pulsed nitrogen laser. The excitation pulse is transmitted down the probe over a 100 m silica clad silica optical fiber. The resulting fluorescence from aromatic hydrocarbons in the soil is returned to the surface over a second fiber, dispersed with a spectrograph, and quantified with an intensified linear photodiode array. Field test data is presented that demonstrates how the system can be used for rapid three-dimensional delineation of a POL (Petroleum-Oil-Lubricant) contaminant plume at a hazardous waste site. Fluorescent fingerprints from 14 samples of 9 fuel types are used to show how spectral differences can be used for identifying contaminant sources. The effects of volatilization of different fuel types on the measured fluorescent signal are discussed.