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MTBE as a growth substrate and mineralize the compound (Park and Cowan, 1997; Mo et al.,
1997; Steffan et al., 1997). Biodegradation of MTBE has been observed to occur in a full-scale
biofilter operating at a gasoline soil vapor extraction site in Richmond, Calif. (Romstad et al., 1998),
and in a pilot-scale biofilter operating at the Los Angeles County Sanitation Districts Joint Water
Pollution Control Plant in Carson, Calif. (Eweis et al., 1997). A review of recent studies of biodeg-
radation and remediation of MTBE indicated that in situ biodegradation may be an effective
remediation alternative for soil and groundwater contaminated with MTBE (Zhang et al., 1998a).
Liquid-phase biological treatment in which MTBE was mineralized has been demonstrated at
laboratory scale at University of California, Davis (Reuter, 1998).
Phytoremediation has been described as a natural process carried out by plants and trees in
the cleaning up and stabilization of contaminated soils and groundwater. A huge number of studies
have demonstrated that plants have a role in the degradation of persistent organic contaminants such
as trichloroethylene (TCE), tetrachloroethylene (PCE), and polynuclear aromatic hydrocarbons
(Erickson et al., 1994; Davis et al., 1993; Newman et al., 1997; Zhang et al., 1997a; Zhang et al.,
1997b; Zhang et al., 1998c; Makepeace et al., 1996; Walton and Anderson, 1990; Ferro et al.,
1994; Narayanan et al., 1995; Schnoor et al., 1995).
Because the recognition of MTBE as an environmental problem is a relatively new area,
studying the feasibility of phytoremediation of MTBE has only recently been reported by a couple of
research groups (Zhang et al., 1998b; Newman et al., 1999). Using a six-channel experimental
system, Zhang et al. (1998b) investigated the fate of MTBE by monitoring MTBE concentration inthe groundwater flow and MTBE flux from the soil surface into the atmosphere. The comparison
between the results from the planted channels and the unplanted one indicated that vegetation
increases MTBE flux to the atmosphere and reduces the groundwater effluent flow rate, and that
MTBE is dissipated more quickly in planted channels than in the unplanted channel. In the study of
Newman et al. (1999), whole plants (hybrid poplars and eucalyptus) in mass balance chambers
were used to determine their ability to take up C14
–labeled MTBE from the soil. Hybrid poplars
were found to be able to incorporate 0.37% of the dosed MTBE into their tissues while transpiring
5.1%. Eucalyptus incorporated 0.4% of the dosed MTBE and transpired 16.52%.
In this study, with the same experimental system as used by Zhang et al. (1998b), we exam-
ined the effect of additional bacterial strains, which were capable of degrading MTBE in laboratory
cultures, on the fate of MTBE in soil channels under vegetation conditions.
MATERIALS AND METHODS
The experimental six-channel system has been schematically described in detail by Zhang et al.
(1998b and 1998c). Each channel is 110 cm long, 65 cm deep and 10 cm wide, packed with
sandy soil, and has influent and effluent ports on each end of the channel. Five of the six channels
(channels 1, 2, 3, 5, and 6) were planted with ten alfalfa plants (Medicago sativa) with 10 cm
between plants and channel 4 was unplanted. Channels 1 and 6 were air sparged through gas
distributors installed at the channel bottoms (Zhang et al., 1998c). Distilled water was supplied
through inlet water jugs to every channel at 1L/day to maintain a water table of 35 cm from channel
bottom up. Water exiting from each channel was collected and recorded daily using collecting
bottles. After entering the system, the water contained in the saturated zone (i.e., below the water