## 4 Discussion
### Source Zone Test Cell (EVO + Hydrogen Gas Infusion)  
Results from samples collected at four wells (the extraction and injection wells EW-3 and IW-3D and two monitoring wells located in-between the injection and extraction wells, SMW-3S and SMW-3D) were used to assess the expected performance of full-scale enhanced bioremediation at the source zone. The extraction well used for recirculation at the source zone was EW-3. Approximately 0.9 m (3 feet) from EW-3 and in line with nested injection well IW-3S/D is the nested monitoring well SMW-3S/D. The shallow monitoring well SMW-3S was screened in the higher permeability material in the upper portion of the shallow aquifer at between approximately 1.5 m and 5.5 m (5 feet and 18 feet) bgs. The deep monitoring well SMW-3D was screened in the deeper low-permeability unit between approximately 6.1 m and 7.6 m (20 feet and 25 feet) bgs. Injection well IW-3D, which is located approximately 3.1 m (10 feet) southeast of the nested injection well IW-2S/D and 5.2 m (17 feet) north-northeast of extraction well EW-3, was screened in the low-permeability unit (similar to SMW-3D) and was used as an additional monitoring point. Monitoring at each of these sites is discussed below. EW-3 Test Cell Extraction Well: After their initial increase, cVOC levels declined over the month before the midpoint EVO addition. During this period a steady rate of reductive dechorlioration of PCE to DCE was observed, including the production of vinyl chloride. A trend analysis of PCE and TCE time-series datasets from the EW-3 samples showed evidence of significant decline during this month-long period. The trends in TOC levels at EW-3 matched approximately what was observed at the EVO Test Cell extraction well HSE-14. SMW-3S Shallow Monitoring Well: The trends observed at SMW-3S closely resemble those observed from analysis of results from samples collected from EW-3. An M-K trend analysis of the time-series dataset showed evidence of significant decline in PCE and TCE as a result of enhanced reductive dichlorination. IW-3D Test Cell Injection Well: Similar to the EW-3 observations, total cVOC levels steadily declined up to the second dose of EVO after they peaked following the initial injection of EVO. During this month-long period a steady and rapid rate of reductive dechorlioration of PCE to DCE, including the production of vinyl chloride, was observed. The M-K trend analyses of the time-series datasets of the total cVOC, PCE, and TCE concentrations showed evidence of a statistically significant decline. Prior to the second dose of EVO, vinyl chloride production peaked and began to drop. The trend in VC levels matched the peak and drop in TOC levels. As a result of the second dose of EVO, total dissolved chlorinated VOC concentrations slightly increased before steadily decreasing. The concentrations of DCE and VC tracked the rise and fall of TOC concentration. By the end of test, PCE and TCE were not detectable. SMW-3D Deep Monitoring Well: Unlike the results from the shallow monitoring well SMW-3S and the test cell extraction well EW-3, the M-K trend analysis of the time-series datasets showed no evidence of a significant change in total cVOC concentrations or its components (PCE/TCE/DCE) at SMW-3D prior to the administration of the second dose of EVO. The majority (>99%) of the cVOC mass remained as PCE and the total concentration remained relatively unchanged compared with baseline sampling results. After the additional dose of EVO, changes in the distribution of chlorinated ethenes began to appear and TOC levels began to increase slightly. At the end of the test the majority (>91%) of the cVOC mass at SMW-3D remained as PCE. VC was not detected, and TOC levels, although above background levels, were low at approximately 5 mg/L. Summary: Recent studies have shown that DNAPL source longevity may be reduced by as much as an order of magnitude (e.g., from hundreds of years to decades) by ERD [28; 29].