|Natural Attenuation of MTBE in the Subsurface Under Methanogenic Conditions (EPA/600/R-00/006) January 2000
At many fuel-spill sites, the spread of contamination from benzene, toluene, ethylbenzene, and the xylenes (BTEX compounds) is limited by natural biodegradation of the petroleum hydrocarbons in the ground water. There is uncertainty about whether methyl tertiary butyl ether (MTBE) from fuel spills will follow the same pattern as the petroleumderived hydrocarbons, or whether MTBE is biologically recalcitrant in ground water. If MTBE does not biodegrade in ground water, then dilution and dispersion are the only mechanisms available to attenuate MTBE. As a consequence, plumes of MTBE could expand farther than plumes of benzene or the BTEX compounds in the absence of biodegradation.
This case study was conducted at the former Fuel Farm Site at the U.S. Coast Guard Support Center at Elizabeth City, North Carolina. The geochemistry of the site is typical of sites where natural biodegradation limits the spread of BTEX compounds. The plume is undergoing extensive anaerobic oxidation of petroleum hydrocarbons, as well as fermentation of hydrocarbons to methane. The hydrocarbon metabolism through sulfate and iron oxidation is approximately equivalent to the hydrocarbon metabolism through methanogenesis. The amount of hydrocarbonmetabolized through anaerobic pathways is about ten times the amount degraded with molecular oxygen.
Two laboratory studies report the biotransformation of MTBE in aquifer material under methanogenic conditions. Neither study included an evaluation of the field-scale performance of natural attenuation. This case study is intended to answer the following questions:
The apparent first order rate of removal of MTBE in the field was a sensitive function of ground water seepage velocity. The rate of removal was calculated for an upper boundary on velocity, an average velocity, and a lower boundary on velocity. The rate was 5.0 per year at the upper boundary, 2.7 per year at the average velocity, and 2.2 per year at the lower boundary. Methane was considered to be a conservative tracer of ground water flow at the site. The apparent rate of removal of methane was taken as an estimate of attenuation along the flow path due to dilution and dispersion. The apparent first order rate of removal of methane at the average estimate of seepage velocity was 0.50 +/- 0.65 per year.
Biodegradation was evaluated in laboratory microcosms that were constructed with material from the contaminated portion of the aquifer. After 490 days of incubation, the average concentration of MTBE remaining in six replicates of a treatment that was supplemented with BTEX compounds was 81 micrograms per liter (μg/L), compared with 5,680 μg/L at the beginning of incubation. The average concentration remaining in the control treatment after 490 days was 1,470 μg/L, compared with 3,330 μg/L at the beginning of incubation. MTBE was also removed in microcosms that were not supplemented with alkylbenzenes. After 490 days of incubation, the concentration of MTBE in all six of the replicate microcosms that were sampled was below 40 μg/L, compared with 3,110 μg/L at the beginning of incubation. Removal of MTBE in the microcosms did not require the presence of BTEX compounds. The removal of MTBE did not begin until the removal of the BTEX compounds was complete.
The first order rate of removal of MTBE in microcosms supplemented with alkylbenzenes was 3.02 per year +/ - 0.52 per year at 95 percent confidence. Removal in the corresponding controls was 0.39 +/- 0.19 per year at 95 percent confidence. The removal in the microcosms without added alkylbenzenes was 3.5 per year +/- 0.65 per year at 95 percent confidence. Removal in the corresponding controls was 0.30 per year +/- 0.14 per year at 95 percent confidence. The rate of removal of MTBE in the laboratory studies can explain the apparent attenuation of MTBE at field scale.
The rate of natural attenuation of the source area was evaluated by comparing that flux to the total mass of MTBE in the source area. The mass transfer of MTBE from the source light nonaqueous phase liquid to the ground water moving underneath was estimated by calculating the flux of MTBE moving away from the source, then dividing the flux into the quantity of MTBE remaining. The flux of MTBE away from the source area in 1996 was 2.76 kilograms (kg) per year. The lower boundary on the total quantity of MTBE in the source area was 46 kg. If the rate of transfer of MTBE to ground water is proportional to the amount of MTBE in the source, the instantaneous rate of transfer is 0.06 per year. The average concentration at the most contaminated location in the transect is 1200 μg/L. At this rate of attenuation of the source, it would require at least sixty years for the concentration to reach 30 μg/L.
Tertiary butyl alcohol (TBA) has been documented as a transformation product of MTBE in a number of studies. At the Old Fuel Farm Site, there is no evidence of accumulation of TBA in the ground water plume as a whole. With two exceptions, the concentration of TBA in ground water downgradient of the source area was less than 200 μg/L. Ground water from a location immediately downgradient of the source area had a higher concentration of TBA, near 2000 μg/L. In this sample, there was a corresponding reduction in the concentration of MTBE. At this location the TBA was probably produced from transformation of MTBE
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