A two-phased approach to the evaluation of natural attenuation

Natural Attenuation was considered as an alternative remedial strategy for groundwater contaminated with low levels (less than 100 micrograms per liter) of chlorinated and nonchlorinated volatile organic compounds (VOCs) at a former industrial site. Historically, the site had been strip-mined for phosphate ore, and later reclaimed, resulting in a highly disturbed groundwater system. Previous remedial actions, including the operation of a groundwater pump and treat system had been effective at reducing VOC levels to their present concentrations, but continued pumping will not provide additional beneficial remediation. A full-scale evaluation of natural attenuation, estimated to cost over $300,000, was considered too expensive and too risky to pursue without some assurance of a favorable outcome, so a two-phased approach to the evaluation was developed. Phase I involved examining historical data, and collecting a "snap-shot" round of groundwater samples for analysis of geochemical indicator parameters and dissolved gases. This preliminary, inexpensive ($30,000) evaluation clearly showed differences between samples collected from within the plume of contamination, and samples collected from up- or cross-gradient locations, suggesting that intrinsic bioremediation was degrading the VOC contaminants. The more rigorous, costly Phase II evaluation could then be implemented with some degree of confidence. The more detailed Phase II evaluation includes two rounds of data collection separated by a two-year period. Approximately 130 locations were targeted for groundwater sample collection to characterize three different water-bearing units in the extremely heterogeneous subsurface lithology. The change in contaminant mass between the two rounds will be used to project the general degradation rates for the various contaminants. Round I of Phase Il has been completed and several useful techniques were employed, including the extensive use of direct-push groundwater sampling technology combined with read-time formation conductivity logging, and rapid turnaround on-site analysis of the groundwater samples to allow in-the-field site evaluation and decision making. The data from Round 1 were interpreted qualitatively to identify the predominant electron-accepting processes. After Round 2 quantitative assessment will be conducted using techniques such as Theissen Polygon Method to establish the mass reduction in the contaminant plume.