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( Minnesota Pollution Control Agency 520lafayette Road North I St.Paul,Minnesota 55155-4194 1651.296-6300 800-657-3864 1651-282-5332 liv I www.pcastate.mn.us I Equal Opportunity Employer May 16,2012 Mr.Grant Fernelius City of New Brighton 803 Old Highway 8 NW New Brighton,MN 55112 RE: NW Quadrant—Commercial Redevelopment,Old Highway 8, New Brighton MPCA Project Number VP18560 Landfill Gas Monitoring Report Approval and Request for Additional Work Dear Mr. Fernelius: The Minnesota Pollution Control Agency(MPCA)staff In the Voluntary Investigation and Cleanup(VIC) program have reviewed the"Summary of 2010-2011 Landfill Gas Monitoring"report(the Report),dated February 24,2012,prepared by Barr Engineering(Barr)for the eastern portion(east of Old Highway 8) of the NW Quadrant at the intersection of 1-694 and I-35W in New Brighton(the Site). The document summarizes all the landfill gas data collected to date at the Site. It does not make recommendations regarding options to address the high gas areas that remain outside of the influence of the current vapor system.The areas impacted by elevated concentrations of landfill gas are primarily related to areas E and F as identified on Figure 1.On May 7,2012,MPCA VIC staff met with Peter Tiffany,Principal Engineer of our Closed Landfill Program staff,to review the gas data and discuss potential plans to address areas of high landfill gas concentrations.The review has resulted in the following comments and modifications. BLOCK F-A number of vapor monitoring points,outside the influence of the current gas control system in Block E,exceed the landfill gas action level and contain over 5%of the LEL for methane on a consistent basis.The dump materials that remain In BlockE are the thickest remaining deposit of dump materials on the Site and may be partially saturated.These are potential factors that contribute to the generation of high methane concentrations observed in Block E. It is assumed that landfill gas is being actively produced in Block E and that the higher gas pressures in that area cause landfill gas to impact areas outside the influence of the current gas collection system. The accumulation of landfill gas under Block E gas may also be the cause of the periodic exceedances observed along the east edge of Block F and may, if not addressed,impact the western corner of Block D. The MPCA requests a landfill gas mitigation plan for the Block E area.Additional vapor mitigation is needed in the Block E area to mitigate landfill gas that is beyond the influence of the present vapor mitigation system.As discussed previously with Barr staff,the MPCA requests that vertical passive vents be used to relieve the landfill gas buildup in Block E.The construction details of vertical vents are included in Attachment B.The radius of influence of such vents is approximately 15 feet.It is recommended that a vertical vent be installed approximately every 30 feet In the fill along the southern edge of the dump in Block E and in the fill along the western edge of the fill that abuts Block F.