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ARCADIS Ms. Amy Terrell <br /> January 29, 2014 <br /> Quantifying NSZD Rates <br /> Also as described in the Response to Comments (ARCADIS 2012), volatilized <br /> LNAPL constituents are degraded by aerobic soil microorganisms. Methane gas <br /> produced by methanogenic organisms in the saturated zone is also consumed in the <br /> vadose zone by methanotrophic organisms that respire oxygen. <br /> Low oxygen, elevated carbon dioxide, and elevated methane within the vadose zone <br /> (as compared to atmospheric gas) are a characteristic signature of ongoing NSZD. <br /> Methane uptake coupled with aerobic attenuation of volatilized petroleum <br /> constituents within the vadose zone are typically the predominant contributors to <br /> overall NSZD. <br /> Scope of Work Conducted <br /> LNAPL Removal from Wells <br /> To address the RWQCB's request to physically remove LNAPL from the source area <br /> at the Site, LNAPL was physically bailed from the six wells with measurable LNAPL <br /> (MW-06A, MW-08A, MW-11A, TW-01, TW-02, and TW-04; Figure 2) quarterly, <br /> concurrent with the routine groundwater monitoring events. <br /> A new disposable bailer was used to remove LNAPL from each well and LNAPL <br /> removal continued until only sheen was detected in each well. The intent was to <br /> maximize the amount of product recovered and minimize the amount of water <br /> removed. <br /> Natural Source Zone Depletion Sampling <br /> NSZD rates can be estimated by measuring CO2 fluxes from the subsurface <br /> associated with hydrocarbon degradation (Sihota et al. 2011). CO2 flux <br /> measurements were collected during this NSZD assessment. <br /> CO2 flux was measured by employing traps filled with CO2 adsorbents that convert <br /> gaseous CO2 into solid phase carbonate salts and water. Each trap includes an <br /> upper and lower chamber to separate CO2 entering the traps from the subsurface <br /> and atmospheric CO2 entering the traps from above. The mass of CO2 adsorbed over <br /> the deployment period was measured in an analytical laboratory (by E-Flux, LLC, a <br /> Colorado State University Venture) by acidifying the solid adsorbent and measuring <br /> the mass of CO2 gas evolved. ARCADIS converted the detected CO2 gas mass <br /> Page: <br /> G.Wrojects\UPRR Roseville\RC000709_Tracy Mnt Facilily\3DELIVERABLEST014.01 NSZD Rpffracy ca-Maint Fadlity_NSZD Rpt(01-29-14).doc 3/9 <br />