The U.S. Environmental Protection Agency plans to cleanup a polluted 400-acre South Jersey superfund site that emits “harmful vapors” the agency says entered residential homes or commercial structures.

The EPA cleanup of the Cinnaminson Groundwater Contamination site, polluted by two former landfills, is spread over four “units,” or areas of the site where contamination originated. The latest proposal focuses on the third unit, which is in the site’s northwestern corner and the source of the vapors.

The remediation plan calls for digging up soil and treating groundwater, some of which may be flowing into the nearby Delaware River.

The EPA says that a risk assessment shows that treatments it is proposing “are necessary to protect human health or the environment from actual or threatened releases of hazardous substances into the environment.”

The EPA first discovered contaminated groundwater in the 1980s while plans were underway to close the landfills bordered by Union Landing Road, U.S. Route 130, River Road, and Taylors Lane.

The Superfund site is surrounded by heavy industry, business parks, and also homes where contaminants either may have spread or threaten to do so. The groundwater has been found to contain trichloroethene, cis-1, 2-dichloroethene, benzene and arsenic.

However, residents and businesses get their drinking water from private and municipal water companies, and don’t drink the contaminated groundwater. Since 2009, the EPA has tested 73 homes or businesses and installed 13 mitigation systems that protect occupants from harmful vapors and limit any public health threat.

The EPA has opened the plan to public comments on unit 3 and will host a public hearing Aug. 10.

“This proposed cleanup plan builds off previous actions and is an important step in fully addressing groundwater contamination at the Cinnaminson site,” EPA Regional Administrator Lisa F. Garcia said in a statement. “EPA will also continue to investigate and address instances of vapors entering residential homes or commercial structures.”

The proposal calls for contaminated soil to be dug up and replaced with clean fill. The EPA will inject chemicals into the contaminated groundwater, a process it says will break down the contamination over time.

The agency will also monitor for vapors using special system similar to those used to detect radon in homes. It will monitor groundwater for years to make sure the cleanup is working.

Despite the name, the Cinnaminson site also spills into neighboring Delran. It consists of two former landfills in the northwest and southeast areas of the property and contains some homes and industrial properties.

The Delaware River lies just to the northwest and U.S. Route 130 to the southeast. Two small creeks — Pompeston and Swede’s Run — receive runoff from the site during storms and flow into the Delaware River. The site was placed on the Superfund National Priorities List in 1986.

The landfills were capped in 1987, and cleanups are underway, or finished, for the first and second units of the plan. Plans for the fourth unit were announced in July.

The site has a complicated history with multiple owners.

The landfill in Cinnaminson was owned by Lockhart Construction Co. and also operated as a sand and gravel mining pit. But, during the 1950s, municipal solid waste was dumped into old mining pits, while mining continued on other parts of the property. Municipal waste was then routed to those other pits when they were closed. As a result, two landfills were created.

SLI, a Waste Management subsidiary, bought the property in 1970 and the New Jersey Department of Environmental Protection allowed it to continue landfill operations, dumping about 240,000 tons of waste a year of various forms. In 1993, SCH bought the land from SLI.