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the current UST complex Based on the findings of the CPT investigation completed in <br /> September 2005, MTBE and TBA impact in this area extends to approximately 60 feet bgs, <br /> impacting multiple zones of saturated strata Subsurface petroleum hydrocarbon impact near the <br /> former UST complex and eastern fuel dispenser island is well documented based on site <br /> assessment activities in the early to mid 1990's Although compliance and site investigation soil <br /> samples were not chemically analyzed for MTBE and TBA, it is our understanding that MTBE <br /> laden gasoline would have been stored and distributed at the site prior to removal of the former <br /> USTs and fuel dispensers in March 1994 <br /> The relatively high TBA concentrations currently observed in groundwater in the eastern portion <br /> of the site appear to be related to the degradation of MTBE laden gasoline Based on our <br /> understanding of the site geology and the transport mechanisms of fuel oxygenates in the <br /> subsurface, a relatively long period of time would likely be necessary for MTBE and TBA to <br /> migrate to the observed depth of impact <br /> 7.3 Recommendation <br /> Given the findings of this investigation, and previous groundwater sampling events, the current <br /> well network does not allow for sufficient monitoring of the dissolved petroleum hydrocarbon <br /> impact Stratus recommends the installation of additional shallow screened groundwater <br /> monitoring wells in the eastern portion of the site, and east-southeast of the wells situated along <br /> Ryde Avenue, to further evaluate hydrocarbon concentrations in shallow groundwater <br /> • Stratus also recommends installation of additional groundwater monitoring wells to variable <br /> depths in the subsurface to enable repeated sampling and further characterization of impacted <br /> strata identified during this investigation The wells should be screened within water bearing soil <br /> horizons that could potentially represent pathways for lateral migration of the dissolved <br /> petroleum hydrocarbons <br /> K\USA Gasolme\0835\Reportslusa835-scm-assessment-report doc Page 16 5TRAW5 <br />