ARCHIVED REPORTS XR0008945 - Laserfiche WebLink

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range at each site, with a geometric mean half-life for MTBE and TBA respectively, of 340 hrs and 90 hrs In 4 of 9 microcosms (3 of 3 from 30011 Crown Valley Parkway and 1 of 3 from San Juan Capistrano), an initial increase of TBA was observed, commensurate with a decrease in MTBE concentration In all of these cases, TBA concentrations later decreased Positive observation of MTBE and TBA degradation, and relatively short observed acclimation times in all of these microcosms, are indicative of a relatively significant indigenous biomass capable of degrading both MTBE and TBA at these sites in aerobic conditions 542 Anaerobic microcosms with methanol addition The anaerobic microcosms with added methanol were intended to evaluate degradation of MTBE and TBA in methanogenic conditions The added methanol, in excess, initially depletes the microcosm of available electron acceptors (oxygen, nitrate, sulfate, etc), allowing methanogenesis to proceed MTBE showed degradation in 8 of 9 of these microcosms, with, in the same 8 of 9 cases, an observed increase in TBA as the MTBE degraded Elevated methane in headspace was measured at the conclusion of the monitoring period for the same 8 of 9 microcosms, indicating methanogenic conditions TBA showed degradation in 1 of 9 of these microcosms, with an observed lag time of 145 days (in a microcosm test of 196 days duration) In the single microcosm for which MTBE did not show degradation (M-3d42, #18, 27101 Ortega Highway), neither TBA nor methanol was degraded, and methane was not detected to headspace vapor at the test conclusion Acclimation times for MTBE degradation in these microcosms ranged from 56 to 145 days Observed half-life, ranged from 87 to 777 hours This range is similar in magnitude to that observed for aerobic MTBE degradation in the same native soils (95 to 2965 hours) The sole observation of TBA degradation in this set of microcosms (MS3A, #18, 30011 Crown Valley Parkway) occurred between days 145 and 196 in a set of microcosms that were originally scheduled for a 150-day incubation period A longer incubation period may have increased the fraction of microcosms with observed TBA degradation Reported ancillary data for these three sites [personal communication by Seth Daugherty to George DeVaull, 23 July 2003, of data collected and measured by John Wilson of USEPA] showed dissolved methane levels in groundwater at these three sites as follows 17