Site History

A privately-owned and operated dump operated on the Property during the 1950s and 1960s.  Mixed municipal wastes typical of that time period were likely placed on the Property during dump operations.  Interviews indicated that the wastes were frequently burned. Some soil was placed as cover over the dump following cessation of operations. The dump was conveyed to the City of Excelsior with stipulation that it be used as a park. A 2007 Phase I ESA identified the presence of the former dump, and subsequent investigation and remediation activities were performed. 

Initial investigation activities 2008 identified potentially flammable subsurface soil vapors, primarily methane, as the primary environmental concern at the site.  To address potential offsite soil gas migration of methane, along with less-significant observations of volatile organic compounds, a perimeter passive soil vapor venting system was constructed in 2009.  Perimeter vapor monitoring was performed to document that the system was preventing offsite migration.  A supplemental addition to the vapor venting system was constructed in 2013 to address an area where the vapor venting system was initially completed at a shallower depth due to the presence of utilities. Perimeter vapor monitoring was discontinued following successful documentation of system effectiveness in 2015.

Soil investigation and remediation consisted of several rounds of soil borings, hand augers, test trenches and surface sample collection and analysis.  Soil samples collected from depths less than four feet below ground surface and soil samples collected from depths deeper than four feet contained constituents at concentrations in excess of applicable criteria for volatile organic compounds (“VOCs”), metals, semi-volatile organic compounds (“SVOCs”), and polychlorinated dibenzo dioxins and furans (“dioxins/furans”). Remedial actions were completed in 2011 to place a 4-foot thick or greater soil cover over the impacted soils in undeveloped areas and a 2-foot thick or greater soil cover under areas that were subsequently paved.  A concrete slab was constructed near the parking lot that was underlain by permeable material and vent pipes to facilitate passive sub-slab venting, in consideration of the potential future construction of a small non-occupied structure such as a garden shed on the slab. Two feet of clean fill was placed before the slab was constructed. 

Groundwater investigation activities includes installation of four onsite groundwater monitoring wells, including one nested pair, and one offsite groundwater monitoring well.  Due to the proximity of the City of Excelsior municipal supply wells on the City property on the opposite side (north) of Oak Street, groundwater/drinking water samples were also collected from the municipal wells.  Surface water samples were also collected from Studder pond, located adjacent to the park. Water samples were analyzed for VOCs, SVOCs, dissolved metals and dioxins/furans.  Onsite groundwater samples collected from MW201 contained concentrations of a suite of SVOCs [benzo(a)pyrene equivalents] that exceeded United States Environmental Protection Agency (“EPA”) Maximum Contaminant Limits (“MCLs”) prior to 2011. Following redevelopment of MW201 to reduce turbidity, all site groundwater concentrations were below criteria. Surface water samples were collected twice in 2010, with analysis of VOCs, SVOCs, and dissolved metals.  Concentrations of all analyzed parameters were below applicable surface water criteria in both sampling events.  Municipal well data is omitted from this Environmental Covenant, since the data was not collected on the Property. Groundwater monitoring wells were properly abandoned in 2012, following MPCA concurrence that groundwater monitoring activities could be discontinued. 

The four-foot thick soil cover in unpaved areas is a barrier to direct contact of elevated concentrations of contaminants of concern (“COCs”) in soils associated with dump materials, and represents an engineered barrier to the Restricted Area.  Similarly, the passive vapor venting system represents an engineered barrier to lateral migration of sub-slab soil vapor.  No active maintenance is required for either engineered feature, but annual inspections will document continued soil separation and passive venting system operation. 

MPCA approvals of investigations occurred in several phases, reflecting the iterative nature of investigations.  The Vapor Response Action Plan Implementation Report was approved by MPCA in February 2010.  The Soil Cover Response Action Plan Implementation Report was approved by MPCA in 2012. Groundwater monitoring discontinuation was provided by MPCA in 2012. Vapor monitoring was discontinued following a 2015 email from MPCA.