In 1917, the U.S. Color and Chemical Company, later named New England Aniline Works, Inc., opened an industrial facility that manufactured textile dyes and dye intermediates off Megunko Road in Ashland, Massachusetts. Nyanza, Inc. acquired the facility in 1965 as the company expanded its operations and became one of the largest dye manufacturers in the United States. Over the years, more than 45,000 tons of sludges, spent solvents, and other industrial wastes were buried on a 35-acre parcel next to the facility. Solid wastes were disposed of on Magunco Hill, and a large volume of partially treated wastewater was released into a system of unlined settling lagoons that overflowed into wetlands along Chemical Brook and Trolley Brook, two small tributaries of the Sudbury River. Liquid wastes were also stored in an underground storage tank that frequently leaked called “The Vault”. In the early 1970s, the Massachusetts Department of Environmental Protection (MassDEP) found elevated mercury levels in the Sudbury River and ordered the facility to stop discharging wastewater into the wetlands. Nyanza went bankrupt and closed all their facilities in 1978.

Several hydrogeologic and environmental studies documented widespread contamination caused by the dye facility in the early 1980s. The Environmental Protection Agency (EPA) listed the Nyanza Chemical Waste Dump on the National Priorities List (NPL) of Superfund sites in December 1982. Between 1990 and 1992, the sludge deposits and contaminated soils and sediments were consolidated around Magunco Hill and capped with an impermeable cap. A diversion system for surface and groundwater was installed on the upgradient side of the hill, excavated areas were backfilled, and native vegetation was planted in the disturbed wetlands. Between 1999 and 2001, EPA excavated mercury-contaminated sediments from four adjacent wetland areas and transported them to an on-site landfill for disposal. In 2007, vapor mitigation systems were installed in nearby homes to remove volatile organic compounds (VOCs). Two systems were installed in 2013 to extract non-aqueous phase liquids (DNAPLs) from the contaminated groundwater plume. Advisory signs were posted at access points along the Sudbury River to warn the public against the consumption of mercury-contaminated fish. Remedial activities are ongoing, and MassDEP conducts operations and maintenance (O&M) activities for the remedies already implemented.

In 1998, the Commonwealth of Massachusetts, National Oceanic and Atmospheric Administration (NOAA), and U.S. Fish and Wildlife Service (USFWS) recovered approximately $3 million in a settlement with the potentially responsible parties (PRPs) to compensate for injuries to natural resources. $230,769 was allocated to the Commonwealth for the groundwater injury. The $2.8 million allocated to the State and federal Trustees for habitat restoration was placed in an interest-bearing account that increased to approximately $3.9 million. The funding was used to complete 13 projects including controlling invasive species, restoring coldwater fish habitat, assessing migratory fish passage, promoting river conservation through education, restoring riparian grasslands, creating a wildlife preserve, conserving habitat through land acquisition, improving public access for outdoor recreation, and restoring and protecting habitat along the Sudbury River for neotropical migratory birds as well as their wintering habitat in Belize.