Biomedical Engineering Reference
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(Winter Oxyfuel Program, implemented in 1992) and oxygen concentrations of at least 2% w/v
fuel in cities with the worst ground-level smog (Year-round Reformulated Gasoline Program,
implemented in 1995) (Franklin et al.,
2000
).
Soon after MTBE's introduction into motor fuels, it began appearing in groundwater wells
downstream of subsurface gasoline storage tanks and rapidly became a widespread groundwa-
ter contaminant (Deeb et al.,
2003
). MTBE was the third most frequently detected volatile
organic compound (VOC) in samples from aquifers studied by the National Water-Quality
Assessment Program of the United States Geological Survey (Zogorski et al.,
2006
)in
1993-2002. MTBE is persistent in groundwater due to its high water solubility (48,000 milli-
grams per liter [mg/L]), low sorption to aquifer sediments (log K
oc
[soil organic carbon/water
partition coefficient]
1.1), and slow biodegradation rates (0.001-0.1/day [d]) (Wilson et al.,
2000
; Kane et al.,
2001
; Zoeckler et al.,
2003
; Schirmer et al.,
2003
). Although typically found
with petroleum hydrocarbons, MTBE is sufficiently mobile and recalcitrant in the subsurface
that it can move well ahead of benzene and related petroleum constituents (Squillace et al.,
1997
; Landmeyer et al.,
1998
). The distinctive, solvent-like odor of MTBE is detectable in very
low concentrations (10s of micrograms per liter [
ΒΌ
g/L]) in water, rendering MTBE migration in
groundwater a significant aesthetic risk to surficial and deeper potable water supplies.
Although MTBE and TBA are biodegradable under some conditions, the loss rates are
relatively slow compared to the volatile petroleum hydrocarbons. Further, because they are
highly water soluble and poorly sorbed, they can move almost as fast as the groundwater. As a
result, MTBE and TBA can persist after the petroleum constituents are removed, and MTBE
plumes can be much larger than the petroleum plumes. The apparent mass flux losses of MTBE
due to dispersion, dilution and biodegradation in aquifers are much slower than those for most
petroleum compounds, with reported first-order decay rate estimates ranging from 0.1 to 0.001/d
(aerobic) to 0.007/d (anaerobic) (Borden et al.,
1997
; Schirmer et al.,
2003
; Wilson et al.,
2000
).
It is also important to note that the field anaerobic decay rate represented the transformation of
MTBE to TBA only. Assuming a conservative aquifer decay rate of 0.001/d and MTBE-only
plume concentrations of 100 to
m
10,000
m
g/L, Wilson (
2003
) calculated that it would require
10-25 years to decrease ether levels to
g/L solely by natural attenuation at most sites.
MTBE plumes, therefore, are likely to persist in aquifers for at least 10 years after fuel spills
occur. Consequently, treatment measures, such as enhancing biodegradation, appeared likely to
be necessary at many MTBE/TBA sites, and bioaugmentation was originally viewed as a
potentially important component of an effective
in situ
bioremediation treatment approach.
MTBE has a federal drinking water advisory level of 20
20
m
m
g/L, and several states have
enforceable cleanup levels that range from 13-202,000
g/L (ITRC,
2005
). Statewide phaseouts
of MTBE were instituted in order to decrease the likelihood of drinking water impacts starting
in 2000 (USEPA,
2004
). These phaseouts resulted in a sharp decline in MTBE production in the
United States after 2003 (Figure
10.1
), and an apparent stabilization in the prevalence of MTBE
in groundwater. However, there are still numerous MTBE/TBA plumes, and these continue to
be difficult to remediate (ITRC,
2005
).
m
10.3 SCIENTIFIC BASIS FOR BIOAUGMENTATION
OF MTBE AND TBA
10.3.1 MTBE Degrading Bacteria
Since the early 1990s, several MTBE-degrading bacterial cultures have been isolated or
enriched from subsoils, sediments and biosolids. Some organisms use MTBE as the sole carbon
and energy source (e.g.,
Rhodococcus aetherivorans
SC100,
Methylibium petroleiphilum
PM1,
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