DNAPL CHARACTERIZATION USING CONE PENETROMETER-BASED RAMAN SPECTROSCOPY

ROSSABI, J., B. D. RIHA, C. A. EDDY-DILEK, A. Lustig3 , W.K. HYDE , SRTC-Environmental Sciences & Technology Westinghouse Savannah River Site, Aiken SC

Cone penetrometer (CPT) based Raman spectroscopy was used to identify separate phase tetrachloroethylene (PCE) and trichloroethylene (TCE) contamination in the subsurface at two locations during field tests conducted at the U.S. Department of Energy’s (DOE) Savannah River Site. Clear characteristic Raman spectral peaks for PCE and TCE were observed at two sites and several depths during CPT deployment. Because of the uniqueness of Raman spectrum for a given compound, these data are compelling evidence of the presence of these compounds. The Raman spectral results correlated with high PCE and TCE concentrations in soil samples collected from the same subsurface zones confirming that the method is a viable dense non aqueous phase liquid (DNAPL) characterization technique. The Raman spectroscopic identification of PCE and TCE in these tests represents the first time that DNAPLs have been unequivocally located in the subsurface by an in situ technique.

Similar to CPT-based induced fluorescence techniques, the detection limit of the Raman spectroscopy is related to the probability of contaminant droplets appearing on the optical window in the path of the probe light. Based on data from this fieldwork, the Raman technique may require a threshold quantity of DNAPL to provide an adequate optical cross-section for spectroscopic response. The low aqueous solubilities of PCE and TCE and relatively weak optical intensity of the Raman signal precludes the detection of aqueous phase contaminants by this method making it selective for DNAPL contaminants only.