C. Haas, T. Staley, J. McCarter, J. Hazel, B. Friesen and M. Zebley

Abstract

A manufactured gas plant (MGP) operated near the downtown river corridor in Fort Collins, Colorado from 1905 to the 1920s. Downgradient of the site, a city landfill operated until the late 1960s and is now occupied by a community center, athletic fields, and a day care center. The Cache La Poudre River lies adjacent to the former landfill, approximately 700 feet from the MGP site. In 2002, the river was at a historic low, and a tar-like non-aqueous phase liquid (NAPL) was observed in riverbed sediments. Investigations conducted under the EPA Targeted Brownfields Program initially characterized environmental impacts, including potential sources of the NAPL. Further investigations revealed the presence of NAPL in bedrock fractures and alluvial sediment beneath the river, community center area, and adjacent athletic fields. It was also determined that fluctuations in river flow influence the appearance of NAPL and vertical flow mechanics in and around the river. With numerous stakeholders involved, remedy selection, design, and implementation were accelerated to accommodate river flow fluctuations, which limited available working time in the river. The final remedy included diverting a 900-foot stretch of river from January 2005 to April 2005, removing/disposing of NAPL-impacted sediment and bedrock, installing a subsurface vertical barrier wall with hydraulic controls and subsequent water treatment, and completing comprehensive site restoration. The project was completed within the scheduled time frame, with acceptance by stakeholders achieved through successful community outreach.

Key words: barrier wall, brownfields, community outreach, MGP, NAPL, restoration, risk management, river, sediment, stakeholders