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<br /> Calculated estimates of this depletion show that water-washing of gasoline will deplete
<br /> TBA from gasoline faster than MTBE In volatilization, MTBE will deplete from
<br /> gasoline faster than TBA It is, however, difficult to model a reasonable scenario that
<br /> matches site observed measurements (high TBA, low MTBE, low BTEX) based solely on
<br /> the physical mechanisms of dissolution, partitioning, mixing, and dilution A
<br /> transformation loss of MTBE, with intermediate generation of TBA, is a possible
<br /> explanation Biological transformation, as a possible source of site measured TBA in
<br /> groundwater, is discussed in Section 5 of this report
<br /> 5. Biological Transformation of MtBE to TBA in the
<br /> groundwater environment
<br /> Task 3 of the work plan included investigation of the possibility of biological
<br /> transformation of TBA and MTBE, and potential identification of TBA as an
<br /> intermediate degradation product of MTBE The plan included microcosm studies using
<br /> samples of the native soil and groundwater, under aerobic and anaerobic conditions A
<br /> revision to the initial work plan has been to extend the period of study from 60 days to
<br /> approximately 150 days for the anaerobic microcosms
<br /> 5.1. Review of Potential Biological Degradation Mechanisms for
<br /> MTBE and TBA
<br /> Natural attenuation (ASTM E1943-98, Wiedemeier et al 1995, 1997) of chemicals in
<br /> groundwater may be indicated by multiple lines of evidence including
<br /> • Phime stability direct measurement that concentrations in the groundwater plume are
<br /> stable or decreasing in time and extent,
<br /> • Geochemical indicators monitoring of electron acceptors (sulfate, nitrate, oxygen),
<br /> mineralization products (methane, carbon dioxide), or intermediate metabolites
<br /> (chemical-specific) as evidence of biological activity, or
<br /> • Biological indicators demonstration that the activity is due to biological activity with
<br /> laboratory microcosm studies of site media samples
<br /> Laboratory microcosm analyses conducted as part of this work plan are intended to
<br /> determine biological activity under a range of environmental conditions (aerobic,
<br /> anaerobic) for the sites under study, and in particular, to identify if TBA may be an
<br /> intermediate metabolite of MTBE
<br /> Biodegradation of MTBE has been reported in the literature for aerobic conditions
<br /> (Bradley et al , 1999a, 1999b, Landmeyer et al , 2001, Mo et al , 1997, Salanitro et at ,
<br /> 1994) and described in summary reviews (Deeb, et al , 2000) MTBE degradation has
<br /> been reported in anaerobic conditions (Kolhatkar et al , 2002) and in specific reducing
<br /> conditions including nitrate-reducing (Bradley et al , 2001a), sulfate-reducing (Bradley et
<br /> al , 200 lb), iron-reducing (Finneran and Lovley, 2001), and methanogemc (Wilson, et al ,
<br /> 2000) conditions TBA degradation has been reported in aerobic conditions (Bradley et
<br /> al , 1999a, 1999b) and anaerobic conditions (Bradley et al , 2002, Finneran and Lovley,
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