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these concentration measurements, the concentration data from day 39 through day 83 were
chosen. The standard deviations for measurements of the channels ranged from 0.14 mM to 0.27
mM and are given in the captions of Figures 1-3. The relative measurement error for groundwater
effluent concentration was within 18~34% of the respective mean values.
The concentration data were used to estimate the amounts of MTBE recovered from the
groundwater effluents of channels through integrating the groundwater effluent concentration over
the effluent water volume. The subsequent results are listed in Table 2 as from groundwater inunplanted channel as groundwater flow due to the lack of plant transpiration. Consequently, more
MTBE was recovered from the groundwater flow of channel 4 (unvegetated) than those of the
vegetated channels.
The soil gas fluxes of MTBE are presented in Figures 4, 5, and 6 as a function of days from
the beginning of MTBE feeding. Gas fluxes for the four inoculated and planted channels are shown
in Figures 4 and 5, while gas fluxes for the non-inoculated channel 3 (planted) and channel 4
(unplanted) are in Figure 6. From Figures 4 and 5, we observe that the soil surface gas fluxes
reached steady state on around day 60, which is one month later than the groundwater effluent
concentrations. Therefore, the flux data of day 67 through day 114 were chosen for standard
deviation analysis and the results are indicted in the captions of Figures 4-6. The relative measure-
ment error for gas fluxes was within 27~38% of the respective mean values. The air sparged
channels 1 and 6 had slightly lower gas flux than channels 2 and 5, which were not air-sparged.
Among all the six channels, channel 3 had the highest fluxes and channel 4 had the lowest fluxes
most of the time.
By integrating over time the MTBE flux data in Figures 4-6, we can have the amounts of
MTBE recovered from the channel soil surfaces. Figure 7 shows the accumulated amount recov-
ered as a function of time from the beginning of MTBE feeding. Channel 3, vegetated but non-
inoculated, had the highest loss of MTBE, whereas the non-vegetated channel 4 had the lowest loss
of MTBE into the atmosphere. The four vegetated channels with introduced bacteria had less
MTBE loss from the soil surface than channel 3, which was vegetated but without any introduced
bacteria. The total integrated amount of MTBE from soil surface for the entire testing period was
put into Table 2 as from gas in terms of millimoles recovered.
The mass balance results for MTBE are summarized in Table 2, in which the Added amount
of MTBE was obtained by summing the volume of the MTBE solution added every day and then
multiplying by the influent concentration (i.e., 0.84 mM). The values for from groundwater and
from gas are the amounts recovered from the groundwater effluent and the surface soil gas, respec-
tively. More MTBE was recovered in the groundwater for the unplanted channel 4, while its soil
gas recovery was lower than for the planted channels. For the four channels with additional bacte-
ria, the two aerated channels had lower soil gas recovery than the two without aeration. Air
sparging appeared to reduce the amount of MTBE released into the atmosphere from the soil. This
may be because the aeration improved the aerobic biodegradation of MTBE in the vadose zone by
supplying oxygen, or it may be due to the method of measuring the gas flux. The convective flow of
gas caused by air sparging may go around the containers because of pressure difference.
The total recoveries from the groundwater effluent and from the soil surface are presented as
Recovered/Added in Table 2. The value greater than 1.0 for channel 4 reflects experimental errors概括