Magnetic and electromagnetic survey to identify historic structures at the river mile redevelopment

Abstract

Background/Objectives: From the 1800s through the 1980s, the Central Platte Valley (CPV) of downtown Denver, Colorado housed various heavy industrial operations including railcar repair facilities, tar product processing, and a manufactured gas plant (MGP). In the early 1990s, a contiguous 60-acre parcel in the CPV near the confluence of two of Denver's major waterways; the South Platte River and Cherry Creek, was redeveloped as Elitch Gardens Theme and Water Park. Limited investigations performed at the time of initial site redevelopment identified impacts from historical industrial activity consisting predominantly of polycyclic aromatic hydrocarbons (PAHs). Remediation was limited and the property was levelled and capped with asphalt and concrete for property use as a theme park. In 2015, Revesco Properties LLC (Revesco) purchased the property. Revesco plans to again change the use of the property, including relocation of the amusement park and redevelopment of the site into a high-rise, multi-use urban mixed-use commercial-residential community that is more consistent with land use of the surrounding area. Approach/Activities: Ramboll's multi-step investigation approach for the Site and former Denver MGP included geophysical investigation techniques, conventional soil and groundwater sample collection, and PAH fingerprinting analyses. Based on previous project experience in the CPV, Ramboll anticipated the potential for building structural elements, foundations, and demolition debris to remain beneath the existing surface. Non-invasive geophysical investigation techniques were employed to identify the presence of underground storage tanks (USTs), ancillary pipes/pipelines, foundations, and vaults potentially present in the area of the former MGP. The geophysical investigation involved the use of frequency-domain and time-domain electromagnetics (FDEM and TDEM, respectively) and total-field magnetic (MAG) surveys. Most critically, the MAG survey identified the presence of a ferromagnetic anomaly in the approximate location of the former Relief Holder. Next, a total of 55 soil borings and 16 monitoring wells were completed at the site, located based on the existing data, geophysical results, and known historical site features. Based on the identification of the ferromagnetic anomaly, soil and groundwater samples were added to the site investigation program in the downgradient and bedrock downdip direction from this buried feature. Visual observation of dense non-aqueous phase liquid (DNAPL) and analytical data from these sample locations supported the initial conclusion that the material causing a ferromagnetic anomaly was likely acting as a continued source of impact to soil and groundwater. A subsequent phase of investigation included excavation of test pits to identify the source of the ferromagnetic anomaly. These test pits encountered sheets of approximately half-inch steel plate riveted together to comprise the base of the tank in the location of the historic relief holder. Conclusions: The results of the geophysical investigation and soil and groundwater assessment support the conclusion that features associated with the former Denver MGP were not entirely removed at the time the plant was decommissioned and have resulted in impacts to soil and groundwater downgradient of the former MGP. The combination of investigation techniques employed at this site allowed Ramboll to identify buried features associated with the former Denver MGP and locate soil and groundwater samples to characterize the associated impacts in a single mobilization.