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• 2001) TBA has been identified in some cases as an intermediate metabolite of MTBE biodegradation (Deeb, et al , 2000, Rittmann, 2003) Several hydrocarbon degraders have been identified that can transform MtBE to TBA fortuitously during their growth phase (in co-metabolism) using propane or cyclohexane as growth substrate (Steffan, et al , 1997) In many other cases, degradation is observed with either TBA or MTBE as a sole-carbon source In aerobic conditions, we note that almost all known MtBE degraders can also use TBA as their sole-carbon source for growth, although the individual organism may have different relative rates of MtBE and TBA degradation In direct microbial metabolism of a chemical, part of the chemical is converted to an increase in biomass In a number of studies of biomass growth on either MTBE or TBA in aerobic conditions, reported maximum growth rate and yield for both TBA and MTBE is significantly less than that for a number of other readily biodegradable chemicals This can lead to relatively long acclimation times before degradation of MTBE or TBA is observed, either in laboratory microcosms or field observations Observed reduction of the TBA or MTBE substrate will occur only after a critical active biomass concentration is reached Observed acclimation times and degradation rates for TBA and MTBE may be different, depending on initial active biomass concentrations in the soil In some cases, TBA can be a 0 temporary intermediate product, particularly during the initial acclimation period of the biodegradation process No routine measurements are available to directly measure active biomass levels in soil For TBA and MTBE degradation, the active biomass is a small fraction of the total biomass in soil 5.2. Laboratory Microcosm Experiments As noted previously, three Shell remediation project sites in Orange County, California were selected for laboratory microcosm experiments These sites include 30011 Crown Valley Parkway, Laguna Niguel 16969 Brookhurst, Fountain Valley 27101 Ortega Highway, San Juan Capistrano Saturated soil cores were collected from three locations within selected regions of the groundwater plume at each site, along with site groundwater, for microcosm analysis The spatial location of these samples is indicated in the figures of Appendix 1 For three soil samples from each site, a test series of six microcosms were prepared, each in duplicate, as indicated in Table 5 1 13