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table) was examined as groundwater in this study.
Two bacterial strains capable of degrading MTBE were introduced into four of the six chan-
nels before the MTBE feeding. We obtained the strains from Dr. Kulpa’s group at the University of
Nortre Dame. The strains were identified as Rhodococcus (#33) and Arthrobacter (#41) (Mo et
al., 1996 and 1997). Each of the two bacterial strains was mixed into four liters of distilled water to
make a 1.5 g cells/L suspension. Two liters of the strain #33 suspension were added into channel 1
such that one liter was uniformly spread on the top and the other was injected through the inlet port
of the channel. The other two liters were added into channel 2 similarly. The same method was
used to inoculate the suspension with strain #41 into channels 5 and 6.
Ten days after the strain inoculation, a solution of 0.84 mM MTBE was continuously fed into
each channel at 1 L/day until a stable MTBE concentration level in the groundwater was estab-
lished; then the feeding was switched back to distilled water. MTBE solution (0.84 mM) was fed
for 83 days before the influent was switched back to distilled water for 96 days. The experimental
conditions and treatments in each channel are summarized in Table 1.Groundwater effluent concentrations and soil gas fluxes were monitored from the beginning of
MTBE feeding until no MTBE was detected from either the effluent groundwater streams or the
gas-collecting containers placed at the channel soil surface. The methods for sampling and analyzing
MTBE were described in Zhang et al. (1997b, 1998b, and 1998c). By estimating the amount of
MTBE added into each soil channel and the amount of MTBE recovered from the groundwater
effluent and the soil surface, we employed the mass balance approach to investigate the fate of
MTBE during the entire testing period. The alfalfa plants were harvested monthly by cutting the
whole plants from approximately 6 cm above soil surface. The biomass production was recorded
as the dry weight of the harvested part.
To look into the MTBE concentration in plant water, the growing plants were cut and the parts
in between the harvesting point and 8-10 cm above were sealed into bottles so as to measure the
headspace concentrations for MTBE. The amount of water in fresh plants was obtained by the
difference between the weight of fresh plants and that of plants oven dried (800
C) for 24 hours. We
converted the headspace concentration into plant water concentration by assuming equilibrium
between the gas phase and the plant water and neglecting MTBE adsorption to plant biomass in
making the mass balance on MTBE that is required for accurate headspace analysis (Zhang et al.,
1998c). All ten plants in each channel were sampled under steady state conditions and distributed
into six bottles.
RESULTS AND DISCUSSION
The groundwater effluent concentrations of MTBE are plotted versus the channel effluent
water volume in Figures 1, 2, and 3. Figures 1 and 2 each show the results for two vegetated
channels, which had the same bacterial strain added except that one was aerated and the other was
not. Figure 3 presents the results of the two channels without any bacteria added; however, channel
3 was vegetated while channel 4 was not. All of the five planted channels demonstrated similar
profiles for the effluent MTBE concentration as a function of effluent water volume, whereas the
unplanted channel had a different tendency.
After around three weeks, the groundwater effluent concentrations reached a relatively
stable level. We assume that the system was under steady state from this point on through the day
of feed switching (i.e., day 83 since the MTBE feeding). To analyze the standard deviations of