The Mason City Coal Gasification Plant operated in downtown Mason City, Iowa, from 1900 to 1951, producing gas for local lighting and heating. After demolition in 1952, contamination surfaced in 1984 when city workers found oily sludges during sewer line work. EPA placed the site on the National Priorities List in 1994. The site has been divided into two operable units, one covering site evaluation work and one covering complete cleanup.
The site holds 50 contaminants of concern spread across soil, groundwater, sediment, and surface water. Key contaminants include polycyclic aromatic hydrocarbons (PAHs) such as benzo[a]pyrene and naphthalene, volatile organic compounds including benzene, toluene, ethylbenzene, xylene, and vinyl chloride, and metals such as arsenic and lead. Dense coal-tar liquid, known as DNAPL, is also present underground.
Between 1986 and 1996, workers excavated roughly 21,000 tons of contaminated soil and sent it for off-site thermal treatment. The final cleanup plan, selected in September 2000, relies on monitored natural attenuation to reduce groundwater contamination over time. Full remedial action ran from January 2003 through April 2008. Recovery systems continue pulling dense coal-tar liquid from the ground. The site is fenced and closed to the public. Nearby residents and businesses use municipal water, not groundwater. An environmental covenant filed with Cerro Gordo County in 2008 and 2009 bans groundwater wells, blocks residential development, and requires vapor intrusion protections in any new buildings. Alliant Energy Corporation currently uses part of the property for a power substation and equipment storage.
Annual groundwater monitoring has continued since 2003. Results show contaminant levels are generally stable or declining in shallow zones where coal-tar liquid is absent. Some deeper zones show increasing benzene and other contaminants at certain locations. EPA completed its Fourth Five-Year Review in April 2023 and determined that the remedy is currently protective of human health and the environment. Human exposure to contaminated groundwater is under control, and groundwater migration is under control. The next Five-Year Review is scheduled for April through June 2028. The site has not yet been deleted from the National Priorities List.
Community members can join the site mailing list to receive updates and learn about future participation opportunities. No public meetings or comment periods are currently scheduled. Free technical help is available through EPA's Technical Assistance Services for Communities (TASC) program, which provides scientists and engineers to explain site science and EPA decisions. Qualified community groups can also apply for Technical Assistance Grants (TAGs) of up to $50,000 to hire independent advisors. Site records are available at the Mason City Public Library at 225 2nd Street SE in Mason City, Iowa.
