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ARCADIS Ms. Amy Terrell January 29, 2014 result provided by the laboratory to an equivalent volume of LNAPL that was degraded to generate the CO2, resulting in an NSZD rate. Two rounds of CO2 flux field measurements were collected to confirm that the results were reproducible and to assess seasonality. CO2 traps were deployed at the four locations shown on Figure 2 during each of the sampling events. Two traps were deployed within the footprint of the LNAPL plume and two traps were deployed outside of the footprint of the plume for use in background correction. Upon retrieval and analysis, the CO2 flux data were "background" corrected as detailed below. During the first quarter sampling event, the traps were deployed on February 12, 2013 and recovered on February 26, 2013 and during the fourth quarter sampling event, the traps were deployed on November 4, 2013 and recovered on November 19, 2013. Samples were also collected during the third quarter of 2013; however, the samples were likely mislabeled and mishandled by the laboratory and the data were not used in this evaluation. Laboratory Background Correction Carbon-14 (14C) isotopes are typically found in natural CO2 soil respiration and not found in LNAPL degradation. Given the relatively short half-life of 14C, the proportion of 14C isotopes in CO2 evolved from petroleum hydrocarbons would be much lower than from soil respiration of other near-surface natural organic matter(Conrad et al., 1999). Therefore, to differentiate background natural CO2 soil respiration from CO2 flux associated with LNAPL degradation, the fraction of 14C isotopes in the captured CO2 was analyzed in the laboratory. Using this fraction, the laboratory conservatively corrected the NSZD rate by removing the natural CO2 soil respiration rate. Additional Background Correction Observation of 14C depleted CO2 in the CO2 traps placed at background locations outside of the footprint of the LNAPL plume can be attributed to either horizontal migration of CO2 in the vadose zone from areas within the footprint of the LNAPL plume and/or degradation of older organic matter in the soil. A conservative assumption was made for this assessment that 14C depleted CO2 in background samples (Station 3 and Station 4), was generated from biological degradation of relatively old organic matter and not due to horizontal migration of petroleum-derived CO2. Therefore, the average petroleum mass loss rates reported by the laboratory for the background samples were subtracted from those in the LNAPL footprint to determine the final LNAPL degradation rate. Page: " G\Projects\UPRR_RosevilleVRC000709_Tracy Mnt FacilAy\3_DELIVERABLES\2014.01 NSZD Rpt\Tracy ca-Maird Facility_NSZD Rpt(01-29-14).doc 4/9