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Evaluation of Natural Attenuation_ 7500 West Eleventh Street, Tracy, Cs1_ Page 32 f BTEXb,,= 0.451blday The rate of loading of dissolved BTEX into the groundwater as it passes through the source area, which, for a stable plume, is equal to the rate at which natural attenuation is removing BTEX contamination from the site, as given by Equation 2, is: f j BTEXioi= 0.05 + 0.45 = 0.53 lb/day r- l Although, due to the large number of approximations and simplifying assumptions ' necessary for practical computation of mass balance, SJC cautions that unwarranted reliance should not be placed on the results of such calculations, an estimated natural attenuation rate for BTEX of 0.53 lbs/day is substantial. For example, it is greater than the rate of 0.50 lb/day that might be achieved by a successful pump and treat system based on an array of five recovery wells distributed across the source zone on section line C-C, with each well being pumped at a rate of 10 gal/min. Natural attenuation can be r=1 implemented at a small fraction of the cost of such a system. In fact, remediation by natural attenuation is a superior choice of remediation technology for the Navarra Site because, without consideration of its large cost savings, it is technically superior to methods based on pump and treat technology that are notoriously ineffective in the complex depositional and hydrogeological environment of an alluvial fan such as that in the subsurface beneath the 7500 West Eleventh Street site. 7.2 Secondary Evidence for Natural Attenuation As has been demonstrated above,there is strong primary evidence that natural attenuation is in progress at the 7500 West Eleventh Street site. According to ASTM Standard E1943-98, such evidence without additional support is sufficient for a conclusion that F:;► natural attenuation will adequately remediate the site without resorting to other corrective q Y g action (American Society for Testing and Materials 2004). However, to ensure an abundance of conservatism in this evaluation, as has been discussed in Section 4.0 above, S7C performed field tests and laboratory analyses to assess secondary lines of evidence for natural attenuation. The principal secondary lines of evidence for natural attenuation at the subject site are based on geochemical indicators that demonstrate that natural biodegradation of analytes of concern is in progress in the subsurface beneath the site. Biodegradation is the process 11 by which naturally-occurring subsurface micro-organisms biodegrade contaminants and completely degrading biocarbons to carbon dioxide and water. Micro-organisms transform organic molecules such as the BTEX compounds and obtain carbon and energy from those sources for survival, growth, and reproduction. Initially, the microbes attack those hydrocarbons through a series of enzyme-catalyzed oxidative- reduction reactions. The resulting metabolic intermediaries are then either diverted to biomass-producing pathways or completely oxidized to carbon dioxide and water. The sic i