Marshall Landfill sits on 80 acres in northern Boulder County, Colorado. It accepted municipal waste, sewage sludge, and unknown potentially hazardous wastes between 1965 and 1974. The adjacent Boulder Landfill operated on another 80 acres until January 1992. EPA added Marshall Landfill to the National Priorities List (NPL) in 1983. The NPL is the federal list of the most serious hazardous waste sites in the country.
Contamination reached both surface water and shallow groundwater. Sources include saturated refuse areas, waste disposal trenches, and two unlined leachate lagoons. EPA identified 16 contaminants of concern across the site. Those listed include 1,1-dichloroethene, benzene, cadmium, chromium, iron, lead, mercury, and phenol. The one main cleanup area, called Operable Unit Sitewide-1, covers the entire site. The cleanup remedy, selected in 1986, included groundwater pumping, air stripping, carbon adsorption, and bioremediation. A groundwater collection and treatment system ran from 1993 until EPA placed it on standby in November 2004, after water consistently met discharge requirements. Physical construction of the cleanup is now complete across the whole site.
The 2021 five-year review found the remedy currently protects human health and the environment. Human exposure is considered under control, with no unacceptable exposure pathways identified for contaminated shallow groundwater near the site. However, groundwater migration status remains uncertain due to insufficient data. Work also continues to evaluate contamination exceedances east of the site boundary and to map the extent of 1,4-dioxane in groundwater to the north. The next five-year review is expected between August and October 2026. Following the Marshall Fire in September 2025, site assessments found no concerns about landfill impacts. Groundwater wells and the soil cap were undamaged, and the decommissioned treatment building was demolished in December 2025. In February 2025, Colorado recorded environmental covenants restricting land use and notifying future owners of the site's Superfund status. Annual groundwater monitoring continues.
The site has been put to productive use while cleanup protections remain in place. A 500-kilowatt community solar project was completed adjacent to the site in 2013. Additional solar opportunities are being assessed for former landfill areas. Zoning restrictions prevent residential and other incompatible uses, and an intergovernmental agreement limits development along the U.S. 36 corridor near the site.
Community members can stay involved through public meetings and site documents. EPA and the State of Colorado held a public meeting on March 5, 2026, to discuss site status and the upcoming five-year review. Meeting materials and a site reuse assessment are available to the public. Site records can be viewed at the Boulder Public Library's main branch on Arapahoe Avenue or requested from the EPA Superfund Records Center in Denver at 303-312-7273.
