Vapor Intrusion Pathway Investigation Report East Hennepin Avenue Site Minneapolis, Minnesota Prepared for General Mills, Inc. July 2015 4700 West 77th Street Minneapolis, MN 55435-4803 Phone: 952.832.2600 Fax: 952.832.2601 Vapor Intrusion Pathway Investigation Report July 2015 Contents Executive Summary ............................................................................................................................................................................. iv 1.0 Introduction ........................................................................................................................................................................... 1 1.1 Study Area Background ................................................................................................................................................ 1 1.2 Investigation Activities .................................................................................................................................................. 2 1.3 Report Organization ...................................................................................................................................................... 2 2.0 Background Information ................................................................................................................................................... 3 2.1 Response Action History .............................................................................................................................................. 3 2.1.1 Site History ................................................................................................................................................................... 3 2.1.2 Sub-Slab Soil Gas Sampling and Building Mitigation ................................................................................. 5 2.2 Study Area Overview ..................................................................................................................................................... 6 2.2.1 Site – 2010 East Hennepin Property .................................................................................................................. 7 2.2.2 Northeast Area ........................................................................................................................................................... 8 2.2.3 Central Area ...............................................................................................................................................................10 2.2.4 Southwest Area ........................................................................................................................................................11 2.3 Utilities ..............................................................................................................................................................................12 3.0 VI Pathway Investigation .................................................................................................................................................13 3.1 Investigation Activities ................................................................................................................................................13 3.2 Sampling Activities .......................................................................................................................................................14 3.2.1 Field Screening .........................................................................................................................................................14 3.2.2 Soil Samples ..............................................................................................................................................................16 3.2.3 Groundwater Samples ...........................................................................................................................................16 3.2.4 Soil Gas Samples ......................................................................................................................................................17 3.3 Analytical Quality Control Summary .....................................................................................................................18 3.4 Other Activities ..............................................................................................................................................................18 3.4.1 Investigation-Derived Waste Disposal ............................................................................................................18 3.4.2 Surveying ....................................................................................................................................................................19 3.5 Safety .................................................................................................................................................................................19 3.6 Additions and Deviations from Work Plan .........................................................................................................19 4.0 Physical Setting ...................................................................................................................................................................21 4.1 Surface Topography and Surface-Water Features ..........................................................................................21 i 4.2 Geology and Hydrogeology .....................................................................................................................................21 4.2.1 Glacial Deposits ........................................................................................................................................................21 4.2.2 Bedrock ........................................................................................................................................................................22 5.0 Discussion .............................................................................................................................................................................24 5.1 Geology and Hydrogeology .....................................................................................................................................24 5.2 Site – 2010 East Hennepin Avenue Property .....................................................................................................24 5.2.1 Buildings 10, 11, 12, and 14 ................................................................................................................................24 5.2.2 Western Exterior Storage Area ...........................................................................................................................25 5.2.3 Former Disposal Area ............................................................................................................................................26 5.3 Northeast Area ...............................................................................................................................................................28 5.4 Central Area ....................................................................................................................................................................29 5.5 Southwest Area ..............................................................................................................................................................31 6.0 Site Conceptual Model ....................................................................................................................................................32 6.1 Vapor Sources ................................................................................................................................................................32 6.2 Geology and Hydrogeology .....................................................................................................................................32 6.3 Spatial Distribution of Contaminants ...................................................................................................................32 6.4 Potential Receptors ......................................................................................................................................................33 6.5 Vapor Transport Mechanisms ..................................................................................................................................33 6.6 Vapor Mitigation ...........................................................................................................................................................33 7.0 VI Pathway Investigation Conclusions .......................................................................................................................34 8.0 Next Steps .............................................................................................................................................................................35 8.1 Additional Sentinel Network Monitoring ............................................................................................................35 8.2 Reporting .........................................................................................................................................................................35 9.0 References ............................................................................................................................................................................36