The Role of the Va- Dose Zone in the Natural Attenua- Tion of Gas Conden- Sate Hydrocarbons

Leaking underground concrete sumps used to store produced water have contaminated groundwater with gas condensate hydrocarbons at a gas production site near Ft. Lupton, Colorado. The source of contamination has been removed; however, a plume of hydrocarbon contamination extends some 46 m from the original source location. This is an agricultural area and the water table is rather shallow (about 1.4 m on average) and fluctuates seasonally over 0.3 to 0.6 m. Further, this site is in close proximity to a potential surface receptor, the South Platte River. Amoco Production Co. and the University of Tulsa have conducted extensive monitoring of the groundwater at this site for over 4 years in conjunction with microcosm studies to ascertain the rate and extent of intrinsic bioremediation of hydrocarbons at the site. Goechemical indicators of biodegradation of hydrocarbons by indigenous microorganisms include oxygen depletion, increased alkalinity, sulfate depletion, methane production, and Fe +2 production associated with hydrocarbon contamination. Sulfate-reducing bacteria and methanogens were also shown to be orders of magnitude higher in contaminated sediments compared to noncontaminated areas. Lastly, metabolites of hydrocarbons degradation, including metabolites characteristic of anaero-bic degradation of xylenes and toluene, have been found in contaminated groundwater. Clearly some purely anaerobic degradation of toluene and xylenes occurs at the site based on the characteristic metabolites of anaerobic degradation detected. However, the relative importance of this mechanism to the overall attenuation of these or other BTEX compounds is unknown. In addition the site still contains scattered free product floating on the water table; therefore, there is also a significant inventory of aliphatic hydrocarbons at the site, which have not been shown to degrade under anaerobic conditions. Since the plume has been observed to be stable and the total hydrocarbon inventory seems to be decreasing, some significant role for oxygen is suggested. Since this is a relatively shallow aquifer, the vadose zone may act as a significant source of oxygen for hydrocarbon degradation. In order to investigate the role of the vadose zone in the natural attenuation of hydrocarbons at this site, an extensive characterization of the vadose zone has been carried out as a function of depth. The vadose zone was sampled in 15-cm increments from the surface to the water table in both contaminated and uncontaminated areas of the site. Each 15-cm sample was composited and analyzed for individual C3+ hydrocarbons, hydrocarbon metabolites, methane, CO 2 , total organic carbon (TOC), …