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' 24 June 2005 AGE-NC Project No 95-0103 Page 11 of 12 ' • Based on the limits of the known impacted ground water and current laboratory analytical results of ground water samples, approximately 0 778 kilograms (1 7 lbs) of benzene and 7 78 kg (17 15lbs) of TPH-g are estimated to be dissolved in ground water(Appendix D) ' Based on the mass of dissolved hydrocarbons removed over approximately 1430 days and the mass estimated to remain dissolved, the mass flux from the adsorbed contamination within the saturated zone is sufficient to Increase the dissolved contamination over the efforts of the ground water extraction to removed contamination from the subsurface • Natural attenuation parameters detected from the site Indicate low to moderate mass 1 reduction, utilizing electron transfer and possible bio-attenuation A comparison of data collected near the core of the BTEX plume and in background locations off-site Indicated an expressed assimilative capacity (EAC) for the ground water at the site was estimated to be ' 9 044 mgll (Appendix D) However,only empirical field data(analytical data) can be relied upon to reflect contaminant degradation and mass reduction • As with all estimates of mass loss, EAC does not represent actual subsurface reactions ' Additional analytical data from the site, including a trend of the EAC and other bio-degradation parameters would better support the use of natural attenuation for the site 5.0. RECOMMENDATIONS ' Based the following implication TPH-g and BTEX concentrations that have remained stable in the extraction well samples over the remediation period,the increased water table level has re-mobilized absorbed hydrocarbons from the soil media into the ground water(MW-4) and trend analyses that Indicate ground water extraction is capturing and removing dissolved petroleum hydrocarbons,AGE recommends ' Continued quarterlyground water monitoring at'the site as required by California Code of Regulations and as approved by the EHD The natural attenuation parameter values should be included in the next two monitoring events in order to establish the static water conditions at the site without active ground water remediation ' An active remediation system on the site may be beneficial to removed the higher dissolved concentrations,including the operation of the ground water extraction and treatment system with the installation of additional ground water recovery wells near wells VE-1, EW-3 and VE-6 However, the static water conditions required to stimulate and evaluate the natural attenuation activity on the site preclude ground water extraction Advanced GeoEnvnronmental,Inc I 1