Characterization of DNAPL using Fluorescence Techniques

Dense non aqueous phase liquid (DNAPL) contaminants, comprised of chlorinated aliphatic compounds, are a major source of groundwater contamination at the Savannah River Site (SRS). To successfitlly remediate a site contaminated by DNAPLs, it is imperative that the slowly dissolving, non-aqueous phase source be fbund and removed. There are few technologies that can successfully and consistently detect DNAPLs in the subsurface either directly or by inferred measurements. Because of the use of chlorinated solvents to remove petroleum-based cutting oils and lubricants at SRS (and other manufacturing sites) in degreasing operations, waste solvents may contain small amounts of the oils and lubricants. This mixture will fluoresce when excited by light of wavelengths capable of being transmitted over optical fiber. Samples of DNAPL fiom the A/M area of SRS were analyzed to assess the possibilities of contaminant detection by fluorescence spectroscopy. The DNAPL sample exhibited a strong, distinct fluorescent spectrum when exposed to an appropriate excitation wavelength. A cone penetrometer-based, laser induced fluorescent system may be capable of providing direct detection of DNAPLs in the subsurhce based on these results. Introduction Dense non aqueous phase liquid (DNAPL) contaminants are a major source of groundwater contamination at the Savannah River Site (SRS). These chlorinated organic contaminants are located in discrete pockets of the subsurface, both above and below the water table and continuously contaminate clean groundwater through dissolution (Looney et al., 1992). To successfully remediate a site contaminated by DNAPLs, it is imperative that the slowly dissolving, non-aqueous phase be found and removed. There are few technologies that can successfully and consistently detect DNAPLs in the subsurface either directly or by inferred measurements. Environmental scientists have had much success locating light non aqueous phase liquid (LNAPL) plumes at depth using cone penetrometer-based, fiber optic laser induced fluorescent techniques. The LNAPL compounds are normally petroleum-based, containing aromatic components, and will fluoresce when excited by wavelengths in the high ultraviolet to low visible range. These wavelengths can be transmitted through fiber optics over the distances required for many subsurface investigations (10 to 200 feet). DNAPLs are normally chlorinated aliphatic compounds that will only fluoresce when exposed to very low ultraviolet wavelengths. These low wavelengths cannot be transmitted through optical fiber as required by the cone penetrometer-based systems. Fluorescent techniques have been used to locate petroleum by the oil industry for many years and have been suggested as a technique for locating DNAPL either dxectly or through the identification of fluorescent co-contaminants (Cohen and Mercer, pp 9-40,1993), however, this method has not been widely applied to date. Some of the reasons for the modest use of fluorescence for DNAPL detection are: