Field Test of a Novel Microlaser-Based Probe for in situ Fluorescence Sensing of Soil Contamination

The use of lasers to induce fluorescence in environmental contaminants such as fuels offers the potential for real-time, in situ chemical characterization in a variety of environmental media. This paper describes the first reported use of a passively Q-switched, fiber-coupled microlaser in situ for fuel-contamination characterization in subsurface soil. A 266 nm microlaser-based probe mounted in a cone penetrometer was tested at an area contaminated by both aviation and heating fuels. By examining the spectral and temporal fluorescence characteristics as the probe was pushed into soil, we identified BTEX compounds, as well as moderate-weight and heavy aromatic hydrocarbons, and were able to map an underground contamination plume in real time. Specifically, BTEX compounds were identified by their unique fluorescence signatures (wavelengths less than approximately 315 nm and lifetimes less than 7 to 10 ns) and by comparison with extensive laboratory studies of BTEX and fuel-contaminated soils. These tests demonstrate that the microlaser-based probe offers the potential for in situ, real-time characterization of soils and groundwater in a compact, inexpensive package.