TOWARDS MEASURING AND INTERPRETING TRACE LIGHT HYDROCARBONS IN SURFACE WATER AND GROUNDWATER

Light hydrocarbons have many different environmental sources, sinks, and transport pathways. The occurrence and lifecycle of many of these compounds in aquatic systems are not well understood due to the limitations of available analytical methods. Contemporary methods typically target a small number of compounds at concentrations relevant to water quality consideration, in the range of μmole (10^-6)-nmole (10^-9) per liter. This sort of approach is very narrow, considering the biologic, geologic, atmospheric, anthropogenic, chemical, and physical processes influencing light hydrocarbons in a given aquatic system. To further elucidate the aquatic lifecycles of these compounds, a method using a purge-and-trap gas chromatograph with an atomic emission and flame ionization detectors (GC-AED/FID) is being developed to quantify and identify C₁-C₆₊ hydrocarbons in the range of μmole (10^-6) to better than pmole (10^-12) per liter in a single sample. By employing a method with over 6 orders of magnitude of dynamic range and high selectivity, samples of uncertain origin can be analyzed with a single method with a high certainty of obtaining useful results. The high selectivity of the system allows trace dissolved compounds, including biogenics, isomers, alkenes, and alkynes to be quantified. Additionally, the high sensitivity allows resolution of mixing and dilution processes that are undetectable with other approaches. These aspects of the method make it possible to more completely study the various processes at work, with the ultimate goal of developing a framework for interpretation that will allow greater understanding of hydrologic systems by using light hydrocarbons as forensic indicators, as fingerprints of specific areas and activities, and as tracers of mixing and transport. The method has been employed on several USGS research projects and is helping USGS scientists to better understand groundwater and stream flow transport processes with respect to dissolved hydrocarbons not possible with previous analytical methods.