BIOAUGMENTATION FOR MTBE REMEDIATION - Bioaugmentation for Groundwater Remediation - page 287

The net effect of these cellular impediments is that MTBE degraders have low growth rates
(m max ), high substrate levels for removal (K s ), and relatively high threshold concentrations
(S min - the concentration below which adequate cell growth (Y g ) cannot be sustained). The K s
values that have been reported for single (pure) aerobic cultures of MTBE-degraders are
relatively high, at 45-50 mg/L for the PM1 and L108 strains (Hanson et al., 1999 ;M¨ller
et al., 2007 ), 80-130 mg/L for M. vaccae JOB 5 (Smith et al., 2003 ), and 175-350 mg/L for
Ps. putida G Pol (Smith and Hyman, 2004 ). It is evident that some naturally occurring microbes
which have been described (high K s , slow growth and low cell yield) may not be able to sustain
MTBE decay in plumes containing typical concentrations of 10-10,000 g/L, as these levels are

below the observed cell culture kinetic coefficients. These results suggest that intrinsic or
nutrient amended bioremediation may be limited in MTBE plumes.

m

10.3.2 MTBE and TBA Biodegradation in Microcosms

MTBE generally has been found to be readily biodegraded in microcosms containing
aquifer sediments and groundwater from contaminated sites, when incubated under aerobic
conditions. A review of available data indicates that MTBE concentrations ranging from 0.05
to 15 mg/L were at least 90% metabolized in 14-65 days, with initial lag times ranging from 5 to
30 days, and apparent first-order degradation rates ranging from 0.02 to 0.16/d (Salanitro et al.,
2000 ; Kane et al., 2001 ; Wilson et al., 2002 ; Magar et al., 2002 ; Schirmer et al., 2003 ; Zoeckler
et al., 2003 ). The fate of TBA is less consistent. In several microcosm studies, TBA did not
degrade, or was degraded slower than MTBE, whereas in other studies TBA was metabolized at
rates (0.12/d) comparable to MTBE (Schirmer et al., 2003 ).
Experimental evidence for the consistent and complete biodegradation of MTBE under
anoxic conditions (including nitrate-reducing [NR], iron-reducing [IR], sulfate-reducing [SR],
methanogenic conditions [MC], or mixed electron acceptor conditions [MX]) in aquifer sedi-
ment microcosms is not entirely convincing. Laboratory data have yielded a wide range of
results, including: (1) varying degrees of degradation (10-100% - IR, SR, MC) of 1.5-100 mg/L
ether in 130-600 days; (2) little or no degradation (NR, IR, SR, MC, MX) of < 5% of the ether
(50-75 mg/L) in 250-500 days; (3) very long lag intervals (few to several months) before
metabolism is observed; (4) inconsistent degradation among replicate microcosms or different
sediment site samples; or (5) TBA accumulating as a MTBE metabolite, with little or no
apparent TBA biodegradation (MC, MX) (see reviews by Schmidt et al., 2004 ;H¨ggblom
et al., 2007 ; Wilson et al., 2005 ). Also, no single or mixed cultures of anaerobic species have
been isolated and identified to verify anaerobic metabolism of MTBE.
Soil and groundwater microcosms used to evaluate the presence of indigenous MTBE-
degrading microorganisms often show no clear correlation between the presence or absence of
MTBE-degrading activity and current or historic exposure to MTBE or dissolved oxygen levels
(Salanitro et al., 2000 ; Kane et al., 2001 ; Lesser et al., 2010 ). Further, microcosm tests
performed with field core samples taken on one-meter (m) (3.3 feet [ft]) spacing suggest
there are large variations in degradation rates, lag times and completeness of MTBE metabo-
lism, and that there are large variations in the abundance of MTBE-degrading populations over
short distances.

10.3.3 Evaluating MTBE and TBA Biodegradation

MTBE and TBA degraders are difficult to study using conventional microbiological
methods (such as plate counts). Fortunately, several functional and molecular methods have
become available that do not require bacterial culturing, allowing effective monitoring of
biodegradation in the field (Table 10.1 ). Several oligonucleotide probes have been isolated