Field tests of a DNAPL characterization system using cone penetrometer-based Raman spectroscopy

Cone penetrometer test (CPT) based Raman spectroscopy was used to identify separate phase tetrachloroethylene (PCE) and trichloroethylene (TCE) contam ination in the subsurface at two locations during field tests conducted at the U.S. Department of Energy's (DOE) Savannah River site. Clear characteri stic Raman spectral peaks for PCE and TCE were observed at two sites and se veral depths during CPT deployment. Because of the uniqueness of a Raman sp ectrum for a given compound, these data are compelling evidence of the pres ence of the two compounds. The Raman spectral results correlated with high PCE and TCE concentrations in soil samples collected from the same subsurfa ce zones, confirming that the method is a viable dense nonaqueous phase liq uid (DNAPL) characterization technique. The Raman spectroscopic identificat ion of PCE and TCE in these tests represents the first time that DNAPLs hav e been unequivocally located in the subsurface by an in situ technique. 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 re quire a threshold quantity of DNAPL to provide an adequate optical cross se ction for spectroscopic response. The low aqueous solubility of PCE and TCE and relatively weak optical intensity of the Raman signal precludes the de tection of aqueous phase contaminants by this method, making it selective f or DNAPL contaminants only.