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2.5 CALCULATION OF RADIUS OF INFLUENCE Because the LFG extraction wells will be installed in native soil, they must be treated as soil vapor extraction wells: Therefore, equations 1 and 2 are more appropriate than equations 3 and 4. If the wells had been installed in waste, equations 3 and 4 would be appropriate. A computer simulation using equations 1 and 2 is shown on Figure 5. The simulation was performed for the purpose of evaluating the radius of influence (ROI) of the perimeter system. Well and probe construction information was obtained from BAS drawings. Operational data for extraction wells F075 and F076 was used to determine the soil permeability. Extraction wells F075 and F076 are installed approximately 100 feet apart with the LFG monitoring probe GP -9 located between the wells. To have influence on GP - 9 the ROI of the two wells must be greater than 50 feet. It has been reported that the vacuum readings in the probe are intermittent and minimal. BAS assumed a negligible vacuum of 0.01 inches water column was applied to the probe by the adjacent extraction wells. This negligible vacuum was insufficient to bring GP -9 into compliance. The site geology does not lend itself to significant amounts of vertical air flow. Subsurface soils generally consist of 10 to 15 feet of clayey silt or silty clay. This material would have an air permeability of 1 x 10x4 darcies. Underlying the silty clay is 16 to 20 feet of silty to clayey sand. This material has an air permeability on the order of one darcy. Below the sand is another clay layer. This clay layer is very stiff and over consolidated. The air permeability of this material would be less than or equal to the upper silty clay layer. Based on the site geology, the only conductive layer for LFG to travel in is the silty/clayey sand. Simulation #1 yielded an air permeability of 3.7 darcies for the silty/clayey sand layer, which generally agrees with what would be expected for this material. Currently 26.5 inches of water column vacuum is applied to the two extraction wells F075 and F076. The flow rate from these wells was not provided and was assumed for this analysis. In addition, a value of 0.01 inches water column vacuum was assigned to probe GP -9. Based on these assumptions a soil permeability value was obtained. A simulation was performed using the soil permeability value and a vacuum of 25 inches water column on the two extraction wells F075 and F076. This simulation yielded an ROI of approximately 23 feet. The soil permeability of the silty/clayey sand layer remained approximately one darcy for this simulation. 2.6 CONCLUSION The simulation performed for this evaluation indicated that an ROI of approximately 23 feet can be achieved by the LFG extraction wells with a vacuum of 25 inches of water column. Subsurface conditions show the presence of a confining silty/clayey sand layer that may be the conduit for LFG to migrate west to the property boundary. This silty/clayey sand layer will also act as a confining layer for the vacuum applied by the extraction wells, allowing the ROI to potentially expand beyond the 23 feet obtained in the simulation. 8 Installation Work Plan to Remediate Landfill Gas Migration J:\Allied\Forward\Workplan\Forward Evaluation - rev4.doc:9/15/2004