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Air Sparging Pilot Test Related Activities Report and Remedial Action Work Plan Former Countryside Market, Stockton, California April 30, 2015 of each phase of testing. Groundwater levels in the observation wells returned to background levels within approximately 20 to 30 minutes after air injection was ceased, and the measured displacements during testing were confirmed to not be the result of changes in barometric pressures. 5.5 Dissolved Oxygen Monitoring At injection flow rates greater than 5 scfm, DO concentrations increased significantly in OBS-1 (15 feet from AS-1 and 7.5 feet from AS-2). A DO concentration that increases to, or near, the DO saturation point (approximately 8.5 mg/L) indicates a direct influence from air sparging, and is an indication that the injected air has found a direct path to the observation well (i.e. bubbling in the well casing). Although a DO sensor calibration problem likely occurred sometime near the end of the first test (i.e., AS-1 testing), relative increases in DO levels on the order of 2 to 3 mg/L are also clearly evident from the AS-2 testing data. During AS testing, DO concentration increases did not occur in the more distant observation well, MW-5, located 28 and 42 feet from injection wells AS-1 and AS- 2, respectively. 5.6 Oxidation-Reduction Potential Monitoring During injection testing at AS-1, marked increases in ORP levels were observed in OBS-1, located 15 feet from the injection well. Additionally, modest increases in ORP levels were measured in observation well OBS-1 during AS-2 testing. ORP remained elevated for the time in between testing at AS-1 and AS-2, and remained elevated for the period of monitoring following testing. AS testing did not indicate influence to ORP in the more distant observation well, MW-5, 28 and 42 feet from injection well AS-1 and AS-2, respectively. 5.7 Soil, Soil Vapor and Groundwater Concentrations As detailed in Section 3.3, petroleum hydrocarbons were encountered in soil above the laboratory RL in OBS-1 and VW-1 between 40 and 60 feet bgs, and, as results show in Section 4.2.2, petroleum hydrocarbons generally decreased in observation wells from pre- to post-pilot AS test sampling. Results indicate petroleum hydrocarbon mass remains in the vadose zone and it does not appear AS activities significantly contribute to fugitive emissions. 5.7.1 Mass Calculation of TPHg in Soil As detailed in Section 3.3, TPHg and bulk density analytical soil results were used to calculate the approximate pounds of TPHg in the vadose zone near the former USTs. Inner and outer zones as shown on Figure 3 were identified to contain the majority of TPHg mass based on both current and historical analytical data. For calculation purposes the inner zone encompassed the former UST with a total area of 15,000 cubic feet (ft3) extending from 20 to 50 feet bgs with an average TPHg concentration of 4,333 mg/kg. The outer zone encompassed the area surrounding the former UST 5-2 The Source Group, Inc.