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Mr.Grant Fernelius <br /> Page 2 <br /> May 16,2012 <br /> Block E is an area of persistent high gas concentrations. Due to the pervasive nature of the gas <br /> concentrations outside of the influence of the current gas collection system,the MPCA recommends <br /> that the use category for Block E is restricted to open space. It is expected that landfill gas will be <br /> produced for a long period of time in the Block E area and that it is unwise to build structures on this <br /> parcel due to the risk of methane explosion. <br /> BLOCK F—A number of vapor monitoring points in the eastern portion of Block F have occasionally had <br /> exceedances of the action level for methane. Because the proposed use for the Block includes <br /> residential,additional vapor control mitigation measures should be taken in the fill along the eastern <br /> portion of Block F.If additional vapor controls are not proposed,the use of this Block will be restricted <br /> to commercial redevelopment. <br /> The MPCA requests a landfill gas mitigation plan for the Block F area fill.Additional vapor mitigation is <br /> needed in the Block F area to mitigate landfill gas that is beyond the influence of the present vapor <br /> mitigation system.As discussed previously with Barr staff,the MPCA requests that vertical passive vents <br /> be used to relieve the gas buildup in the fill adjacent to Block F.The construction details of vertical vents <br /> are included in Attachment B.The radius of influence of such vents is approximately 15 feet.It is <br /> recommended that a vertical vent be installed approximately every 30 feet along the southern edge of <br /> the dump in Block E and along the western edge of the fill that abuts Block F. <br /> The MPCA requests that a Response Action Plan(RAP)addendum be developed that outlines the gas <br /> mitigation work as outlined in this letter.The RAP addendum should be submitted to the MPCA for <br /> review and approval. <br /> The proposed frequency and methods of continued monitoring listed on Page 6 of the Report are <br /> hereby approved. <br /> This letter is subject to the disclaimers listed in Attachment A.If you have any questions about the issues <br /> discussed in this letter,please feel free to contact John Betcher at 651-757-2226 or me at 651-757-2465. <br /> Sincerely, <br /> • -,2-/-77 <br /> .Patrice Jense <br /> Project Leader <br /> Superfund and Voluntary Investigation and Cleanup Programs <br /> Remediation Division <br /> PJ:jmp <br /> Attachments <br /> cc: Dan Fetter,Barr Engineering <br />