CORRESPONDENCE_2014-2015 - Laserfiche WebLink

Home Browse Search

Phase 2 LFG Extraction Well Installation If monitoring results following the completion of Phase 1 indicate that the continued presence of VOCs in the vadose zone or continued impacts to groundwater remain, the County intends to install up to eight (8) additional gas extraction wells in Module "I" (up to a total of twelve (12) wells). These wells will be installed similarly to the wells in Phase 1; however the final Phase 2 well design will depend on the results of the ROI testing. Conceptual locations of Phase 2 wells are shown in Appendix B, Figure 2. Water Board Question: Will the proposed system contain the landfill gas within Module "I"? Evidence? County Response: Following startup and tuning of the new wells, it is our professional opinion that LFG entrapped under the impermeable liner in the vicinity of MW -3 will have a high probability of being collected, and the impact of that entrapment removed. Based on previous hydrogeological response time between surface correction actions to LFG migration of within four (4) to five (5) years, it is our professional opinion that the detection of VOCs in groundwater will be eliminated by 2020. This is an estimate and the actual duration may vary depending on in-situ conditions. In addition, five (5) new vadose zone multi -probe soil gas monitoring wells will be installed around Module "I" to assess the reduction of LFG in the vadose zone and the performance of the GCCS. The description of this monitoring system is presented below in the Landfill Gas Monitoring Outside of Module "I" section of this document and as shown in Appendix B, Figure 3. Water Board Question: What criteria (spacing? Gas concentration? Distance from other extraction wells?) was used to select the location of the landfill gas extraction wells? County Response: The wells proposed in Phase 1 are sited to immediately target gas recovery under the over -liner and in the vicinity of groundwater well MW -3 where VOCs have been intermittently detected in groundwater. For the design of future Phase 2 wells, the County will determine where additional wells should be placed by performing field ROI testing in Module "I" to empirically obtain the actual ROI at each existing well. To estimate the actual ROI of the existing wells, the County will conduct a vacuum radius of influence study at Module "I". The following procedures will be conducted in the field. 1. Vacuum to each well will be turned off for a period of 24 hours, one (1) well at a time for testing. During the 24-hour period, the field vacuum will be maintained consistent with the field vacuum prior to turning off the selected well thereby ensuring that the vacuum applied during the test at all other wells is consistent with the vacuum applied during normal operation. Foothill Sanitary Landfill 7 Department of Public Works, Solid Waste Division Revised Engineering Feasibility Study County of San Joaquin — December 19, 2014