BIOAUGMENTATION FOR MTBE REMEDIATION - Bioaugmentation for Groundwater Remediation - page 300

1. Delineation is critical : The downgradient edge of the source zone is often poorly
understood, and careful characterization should be conducted before barrier imple-
mentation.
2. Sufficient oxygen delivery is a common limitation : A stable, robust zone of oxygena-
tion is required for barriers to stay effective. Pure oxygen delivers higher equilibrium
dissolved oxygen levels (about 40 mg/L for pure oxygen gas versus 8 mg/L for air) and
may offset the spatially non-uniform nature of gas distributions in aquifers. Anoxic
field conditions tend to be associated with low MTBE degradation rates.
3. Biostimulation is typically sufficient, though a time lag may be experienced :In
many instances, native microbial populations contain the necessary degraders, and
increased dissolved oxygen levels result in increased biodegradation rates. This
increase in activity may be sufficient to achieve the desired concentration reductions.
If it occurs, the success of biostimulation (e.g., as measured by reduced MTBE
concentrations) may not be immediately evident. A lag time of 6-12 months before
degradation rates are equivalent to bioaugmented plots has been observed in some field
studies. Given the age of current MTBE plumes, and the fact that few new MTBE
plumes are likely to arise, this timeframe is likely to be acceptable if a receptor is not
immediately at risk.
4. Bioaugmentation cultures can survive and be active in situ : Supplementing indige-

nous communities with high mass loads of MTBE-degrading cultures may result in
enhanced degradation rates with little to no lag time, if coupled with a stable oxygen
delivery system. However, it is important to note that several side-by-side field
applications have reported better results from biostimulation than bioaugmentation.
5. Typical cocontaminants must be considered : Other aerobically biodegradable fuel-
related chemicals (e.g., BTEX) may affect system performance, depending on culture
composition. MTBE was reported to be effectively degraded in a full-scale demonstra-
tion treating a mixed MTBE/BTEX/TBA plume (Salanitro et al., 2000 ) and in some
microcosms (Kane et al., 2001 ). Raynal and Pruden ( 2008 ) reported that culture
composition is a key factor in determining the success of MTBE-degraders in the
presence of BTEX, with more diverse populations being capable of degrading both
MTBE and BTEX, while the MTBE-degrading potential of populations dominated by
PM1-like strains was severely retarded in the presence of soluble BTEX. Deeb et al.
( 2001 ) observed severe inhibition of MTBE degradation in consortia dominated by
PM1-like strains in the presence of ethylbenzene, m -xylene and p -xylene (no degrada-
tion in 4 months), strong inhibition in the presence of o -xylene (degradation after a lag
period of 2 months), and slight inhibition in the presence of benzene and toluene
(degradation after a lag period of several hours). In the presence of alkanes (hexane,
isopentane), MTBE-cometabolizers may dominate.
6. Effective treatment typically requires 6-12 months : Based on experience, the effects
of oxygen addition on dissolved oxygen concentrations in the target treatment zone
generally occur over a few weeks to a few months. Corresponding increases in
biodegradation activity and concentration reductions in the target treatment zone
might not be observed for a few months, but are generally observed within 6-12
months (if there is MTBE biodegradation occurring). MTBE degraders are generally
regarded to be slow-growing low-yield bacteria, so slow response times in some
settings are to be expected. As mentioned above, an 8 month period was necessary to
achieve significant activity in one of the biostimulated biobarrier pilot test plots at
NBVC, while another pilot test plot nearby showed significant activity after 3 months.