API PUBL 4668-1998

$20.15

Delineation and Characterization of the Borden MTBE Plume: An Evaluation of Eight Years of Natural Attenuation Processes

Published By	Publication Date	Number of Pages
API	1998	82

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Description

In 1988, a natural gradient tracer test was performed in the shallow sand aquifer at Canadian Forces Base (CFB) Borden. This study investigated the fate of a methyl-tertiary-butylether (MTBE) plume introduced into the Borden aquifer in order to quantify the status of this contaminant in shallow, aerobic settings. Solutions of groundwater mixed with oxygenated gasoline were injected below the water table along with chloride (Cl^–), a conservative tracer. The migration of benzene, toluene, ethylbenzene, the xylenes (BTEX); MTBE; and Cl^– was monitored in detail for about 16 months. The mass of BTEX compounds in the plume diminished significantly with time due to intrinsic aerobic biodegradation. MTBE, on the other hand, was not measurably attenuated. In 1995, additional exploratory sampling of the Cl^– and MTBE plumes found both at lower concentrations. The MTBE/Cl^– ratio was more than two orders of magnitude lower than that of the injection solution and earlier sampling events suggesting some mass loss of MTBE may have occurred. In 1995-96, a comprehensive groundwater sampling program was undertaken to define the mass of MTBE still present in the aquifer. Since the plume had migrated into an unmonitored section of the Borden aquifer, numerical modeling and geostatistical methods were applied to find an optimal sampling grid. A drive-point profiling system was then used to obtain groundwater samples. In the 1995-96 sampling rounds, MTBE concentrations measured were more than an order of magnitude lower than expected based on the modeling that considered dispersion and diffusion as the only attenuation processes. A mass balance for the remaining MTBE mass in the aquifer eight years after injection was performed using the geostatistical software packages GEOSOFT^TM and GMS^TM. Although the possibility exists that part of the MTBE plume was missed, the extensive sampling in a well-characterized aquifer, with the location of MTBE where it was anticipated, suggests otherwise. Only about 3 percent of the initial MTBE mass was found and it is hypothesized that biodegradation played an important role in the attenuation of the MTBE within the Borden aquifer. Nevertheless, additional lines of evidence of biodegradation, such as laboratory batch and column experiments, are necessary to confirm this possibility. Studies are underway, but no confirming laboratory evidence has been found to date. Thus, while there is confidence that MTBE mass has been lost, biodegradation cannot yet be confirmed as the process.