Feb. 12—The public will have an additional month to weigh in on a federal report assessing the possible impacts of the latest proposed measures for cleaning up a toxic chromium plume beneath Los Alamos National Laboratory.

The U.S. Energy Department issued the 115-page environmental assessment in November, then offered a 60-day period for public comment that was set to end Monday but now will go to March 13.

Officials say the assessment is required under the National Environmental Policy Act.

Federal waste cleanup managers have proposed four broad options to deal with the 1.5-mile long hexavalent chromium plume that has lingered deep underground in Los Alamos canyons for more than a half-century, with remediation efforts barely denting it so far.

The managers also assessed the possible impacts each option might have on the environment, wildlife, water systems and cultural resources. This report was issued as federal officials and state regulators work to resolve their differences on the best way to tackle the sprawling plume.

In an email, New Mexico Environment Department spokesman Matt Maez wrote the lab notified the agency about the draft analysis in mid-December.

“NMED is compiling comments from the appropriate bureaus,” Maez wrote.

Residents, community advocates and conservationists have expressed concerns about the slow progress, saying the contaminants pose a health and environmental threat.

Hexavalent chromium is a known carcinogen that, when ingested in drinking water, can harm the liver, kidneys, reproductive systems and, some research suggests, cause stomach cancer if consumed over a long period.

Between 1957 and 1972, lab workers dumped water from an old power plant’s cooling towers into Sandia Canyon — water that had been funneled through steel pipes laced with hexavalent chromium to prevent corrosion.

From there, the water traveled several miles to Mortandad Canyon and pooled about 1,000 feet underground in a huge plume the lab discovered in 2005.

Since 2018, crews have extracted tainted water, treated it and injected it back into the plume to dilute the pollution.

Officials recommend stepping up treatments, which would require adding up to 15 injection wells, 15 monitoring wells and 30 piezometers that measure groundwater pressure.

A 10,000-square-foot treatment plant, access roads, undergound pipes and well pads also might be installed.

The new infrastructure and equipment would accommodate three of the options, which call for “mass removal.”

One option would sharply boost the amount of water treated and put back into the aquifer. One would involve injecting chemicals or biological agents into the plume to nullify the contaminants. And another would dispose of about 10% of the contaminated water by spraying or irrigating a designated chunk of land.

Some of the treated water could be discharged into the nearby canyons.

The fourth option is to let the less-polluted water naturally dissipate and shrink over time while monitoring it.

The analysis mostly describes the effects on land, water and ecosystems as temporary, with the main ones coming from crews and trucks while the new infrastructure and devices are installed.

The overall plan is to apply “adaptive site management” — a term for making adjustments as conditions change or new information becomes available.

Scott Kovac, Nuclear Watch New Mexico’s operations director, said the proposed actions seem broad, lacking important details on what actually would be done.

Also, it would make more sense to have the Environment Department sign off on a plan of action — because the agency has final say — before going through the NEPA process.

“They’re doing it backward,” Kovac said.

But Stephanie Gallagher, a spokeswoman at DOE’s Environmental Management field office, wrote in an email that the assessment was required for this project and is being done separately from the agencies deciding on a final remedy for the plume.

State and federal officials are wrangling over the best approach to take.

State regulators ordered the federal agency a year ago to stop the operation, saying it was pushing the contaminants toward San Ildefonso Pueblo and deeper into the aquifer.

Federal officials insist the pump-and-treat method was reducing the hexavalent chromium and creating a “hydraulic barrier” to keep it from spreading to the pueblo. But the agency complied with the state’s order.

To help resolve the dispute, state and federal officials are working together to select an independent panel of experts to review how the cleanup has been conducted, including data collected on-site and the computer modeling used.