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Page 2 of 2 City Project 10-17, Northwest Quadrant East Side Groundwater and Vapor Monitoring Wells Explanation: Explanation: Prior to the dump closure project, Old Miller Dump contained approximately 800,000 cubic yards of waste that covered about 23 acres, or about 60 percent of the redevelopment area. There were also residual petroleum impacts to soil and groundwater at the two LEAK sites outside the dump area. Almost half of the dump materials are still present below the shallow groundwater table, and the dump contains a variety of contaminants which have resulted in low level impacts to the shallow groundwater. The most serious environmental concern is the generation of landfill gas (LFG) associated with the decomposition of garbage in the dump. The LFG includes very high levels of methane and other VOCs that pose a serious risk of subsurface migration towards existing and future buildings. The extremely combustible methane in LFG poses an explosion risk if it were allowed to migrate into structures and mixed with oxygen in the air. The planned response actions included excavating the two petroleum release sites and the western twenty percent of the dump materials. Most of those materials were consolidated on the eastern portion of the dump area and a four foot clean soil cover was established over the consolidated dump materials and petroleum impacted soil. The reconfigured dump is approximately 18 acres in area. The majority of the final dump area is public space involving a park, ponds, and roads. An area -wide LFG collection liner system was constructed over the consolidated waste at the base of the four foot clean cover. The LFG collection system intercepts and collects LFG that is being generated in the waste, and safely vents it to the atmosphere (passively). Originally, MPCA requested a more expensive active venting system involving blowers, fares, etc. Later, it was decided that a passive collection system would be appropriate along with perimeter LFG monitoring. The effectiveness of the system will be verified with perimeter soil gas monitoring to ensure that LFG is not migrating away from the dump through the soil towards buildings. Additionally, all future buildings will have a sub -slab venting system as an added protection. The MPCA also required groundwater monitoring around the closed dump as part of agreements reached for consolidating dump materials at the Site, which avoided the prohibitive cost for removing all of the dump materials to an offsite landfill. Shallow groundwater impacts at the site will be addressed through one year of monitoring at several new monitoring wells installed as a part of City Project 10-17. Reports and excerpts from two attached reports prepared by Barr Engineering discuss what is being tested and where the wells will generally be located. Costs and Funding: This project may be funded from a variety of funding sources including tax increment and the Municipal Development Fund. *GrMffel E. Director of Publ c orks Attachments Contingency Plan for Landfill Gas Control Appendix B: Groundwater Monitoring Plan 40