ARCHIVED REPORTS XR0006546 - Laserfiche WebLink

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CLEARWATER C. R U L P I N C • fnae rang:ental Seroees (MW-2) These levels are consistent with a long term trend of contaminant level decrease following remediation at the site The samples collected from both sparge wells, screened deeper than the other monitoring wells, were also free of detectable contaminant concentrations, except MTBE at 5 8 pg/L in SW-2 The lack of contamination in these wells 1s noteworthy in that the results effectively delineate the vertical extent of residual dissolved-phase contamination on-site Samples from the on-site domestic well continue to be free of any petroleum contaminants Considering the well was not impacted by elevated contaminants levels prior to remediation, it appears that the well is not threatened by the minimal post-remediation contaminant levels now present in the soil and shallow groundwater beneath the site No other Fuel Oxygenates (TBA, ETBE, DIPE, TAME) or Lead Scavengers (1,2-DCA, EDB) were detected in any of the samples collected Recent and cumulative groundwater sample analytical data are summarized on Table 1, and are shown on graphically on Figure 4 Copies of the laboratory report and chain-of-custody form are attached Discussion As discussed earlier, approximately 99% of contamination in the vadose zone was removed by the SVE/AS remediation system that operated at the site from October 1998 to October 1999 The residual mass of TPHg in the vadose zone was estimated to be approximately 37 lbs Operation of the remediation system also had a dramatic impact on groundwater contaminant levels Pre-remediation levels of TPHg were as high as 150,000 pg/L in MW-2, and SPH was observed as sheen and / or droplets in purge water from wells MW-1 and MW-2 on several occasions prior to remediation Post-remediation levels of dissolved-phase contaminants have dropped steadily over time Water sample- from all wells except MW-1 and MW-2 have been non-detect for all hydrocarbons for several quarters of sampling Results from the most recent sampling event show that only low levels of dissolved-phase gasoline remain in samples from MW-1 (220 pg/L TPHg), and MW-2 (110 pg/L TPHg) Additionally, recent monitoring results indicate that benzene was only detected in one well, MW-1, at a concentration of 0 55 pg/L MTBE was the only Fuel Oxygenate detected and was only detected in two wells, MW-1 (150 pg/L) and SW-2 (5 8 pg/L) A trend of decreasing residual dissolved-phase contaminant concentrations has been observed at the site during and after remedial efforts TPHg, benzene, and MTBE concentrations for wells MW-1 and MW-2 were plotted against time since the start of remediation, and trend lines fitted to each data set The best fit lines shown on Figures 5a and 5b indicate an overall downward trend for TPHg, benzene, and MTBE at the site ZB107C GWMR 3Q00 4 August 3,2000