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L w Second Semiannuai2090 G oundwaterMonitoring Report `,r► <br /> Former Texaco Service Station <br /> Chevron Site ID 309075 <br /> November 22, 2011 <br /> 5.1.5 Site Conceptual Model Updates <br /> There are no site conceptual model (SCM) updates. Historical analytical soil data, groundwater <br /> gradient and flow direction data, and soil boring and well construction details are presented as <br /> Tables 2, 3 and 4, respectively. Information regarding the collection of soil samples associated <br /> with the possible removal of the underground storage tanks (UST) and analytical data for the <br /> samples collected from monitoring well MW-1 is not available. Pursuant to SJCEHD letter, <br /> dated November 30, 2010, soil samples were not collected from MW-1 during well installation. <br /> 5.1.6 Trends <br /> V <br /> TPH-MO and TPH-DRO, without silica gel, concentration ranges of groundwater samples <br /> collected during the first semiannual event of 2011 are generally consistent with the <br /> concentration ranges detected during recent monitoring and sampling events. The TPH-MO and <br /> TPH-DRO <br /> r. With the exception of MW-1 and the groundwater sample collected on December 1, 2010 from <br /> MW-6D, TPH-GRO, BTEX, MTBE, TBA, DIPE, ETBE, TAME, 1,2-DCA, EDB, ethanol and <br /> methanol have been non detect since sampling began in 2006. Additionally, TPH-GRO <br /> concentrations in MW-1 have decreased from 4,700 pg/L to 500 pg/L. <br /> 5.1.7 Data Gaps <br /> Information documenting the fate of five former USTs associated with the former Texaco service <br /> station is not available. A subsurface investigation using electromagnetic technology and ground <br /> penetration radar, conducted in June 2008, concluded definitive UST signatures do not exist at <br /> the site. <br /> 5.2 Conclusions and Recommendations <br /> The TPH-MO and TPH-DRO results, without and with silica gel cleanup, indicate the organic <br /> compounds being measured in groundwater beneath the site are polar compounds, and are not <br /> dissolved hydrocarbons. <br /> Additionally, data gathered during recent semiannual monitoring and sampling events indicate <br /> the plume is stable. More strongly reducing conditions in groundwater in the vicinity of MW-1 <br /> may indicate petroleum hydrocarbon COCs are providing a source of organic carbon that <br /> supports microbial activity. This microbial activity likely is resulting in natural attenuation of <br /> petroleum hydrocarbon COCs in groundwater in the vicinity of MW-1. <br /> ARCADIS recommends discontinuing monitoring and sampling the wells associated with this <br /> site. Additionally, ARCADIS recommends this site be considered for no further action required <br /> (NFAR) status. ARCADIS will submit a NFAR during the first quarter 2012. <br /> V <br /> V <br /> g:\aproject\env\chevron\309075\6 deliverables\periodicmonitoring\2011\1sa1IUemppdf\text\3090751sa11 gmr fin 11222011.doc 8 <br />