It's been more than half a century since an experimental nuclear reactor at the Santa Susana Field Laboratory near Los Angeles suffered a partial nuclear meltdown, spewing radiation over a period of weeks.

Private industry and government agencies followed that with repeated chemical spills, accidental releases of more radioactive material and the open-air burning of poisonous chemicals and gases at the former site use for the development and testing of nuclear reactors, rockets, missiles and munitions.

Both NASA and the federal Department of Energy are behind on their legal agreements to clean up all traces of the pollution they’d caused at the mountaintop laboratory about 30 miles northwest of downtown Los Angeles.

The Boeing Company, which inherited more than three-quarters of the laboratory grounds when it acquired Aerojet Rocketdyne in 1996, waged a successful lawsuit to overturn similarly rigorous cleanup requirements for its share of the land.

Instead, it has been preparing its cleanup under the terms of a 2007 consent order in which the state Department of Toxic Substances Control allows the company to write its own risk assessment.

Boeing has completed numerous studies on where the contamination is and how to clean it up, in accordance with DTSC requirements. It's filed scores of reports.

The 2007 order stipulates that Boeing must complete remediation of contaminated soil and related cleanup tasks by the end of next month. Dave Dassler, the company's site closure director, estimates that might require digging out and hauling away up to 400,000 cubic yards of dirt, enough to fill the Rose Bowl. The digging hasn't even started.

Boeing Creates Easement to Set Aside Land as Open Space

Many public health and environmental activists say that Boeing's characterization of the threat posed by the contamination at Santa Susana is far too permissive for a severely contaminated site that is only half a mile away and steeply uphill from residential neighborhoods.

But a recent Boeing legal maneuver may clear the way for the company to negotiate terms that are even less rigorous.

Company officials have long declared that Boeing will preserve its property as open space to protect wildlife and preserve pre-Columbian historic sites in the hills adjacent to former test and accident sites.

Now, Boeing has made that commitment legally binding by granting the Pennsylvania-based North American Land Trust a conservation easement that declares the company's property permanent open space.

For years, activists have predicted that Santa Susana, which is surrounded by suburbs, might be turned into a housing development. Now the company's agreement with the land trust bars residential, commercial, industrial or agricultural development, North American president Stephen Johnson said.

Clark Stevens, executive officer of the Resource Conservation District of the Santa Monica Mountains, said the agreement protects a vital travel corridor for wildlife, such as deer and mountain lions, between the Los Padres National Forest to the north and the Santa Monica Mountains.

Stevens, who has worked with conservation easements for 15 years, said Boeing stands to receive a substantial tax write-off for voiding its property's development value. He said the public will benefit as well, because the building ban is legally inviolable.

"It rides with the land," he said. "It doesn't matter who the owner is in the future. They will be restricted by the conservation easement. My organization never felt this land had a secure open space future until now, because Boeing saying they would keep it open space was just a policy that could change. This easement changes that. It has teeth."

With a firm development prohibition in place, it's appropriate to reconsider cleanup standards, Stevens said.

"Now that we know we have protected open space, we should take seriously the fact that no one will ever live there and not overdo the cleanup," he said. "My organization does habitat restoration. You can't restore land that has been scraped to bare rock."

Like other Boeing officials, company spokeswoman Megan Hilfer has argued that Boeing is committed to making its property clean enough for people to live there.

Now, with the easement in place, the cleanup standard Boeing will apply "remains to be seen," Hilfer said in a telephone interview.

"We look forward to working with the DTSC to determine the appropriate cleanup actions for Boeing's portion of Santa Susana," she said.

Dan Hirsch, president of the nuclear watchdog group Committee to Bridge the Gap and director of the Program on Environmental and Nuclear Policy at UC Santa Cruz, fears the conservation easement will undermine a thorough cleanup at the field laboratory. By declaring its land "open space," Boeing glosses over the fact that its contamination is very close to residential neighborhoods, Hirsch said.

"The problem is that the people living nearby aren’t open space. They are real people who for the rest of their lives will face the risk of exposure to the toxic contamination leaking off that site," he said.

The Simi Hills, where the laboratory is located, lie between the suburban communities of Chatsworth and Simi Valley. Residential neighborhoods extend to within half a mile of the laboratory gates, and a new housing development is rising just north of the portion of Santa Susana where the experimental reactor once operated.

At a public meeting this month, the  project team leader for Santa Susana at the state Department of Toxic Substances Control, Mark Malinowski, said the department only recently received a copy of Boeing's easement agreement and hasn't determined how it might affect the cleanup.

But Malinowski questioned whether the laboratory endangers its neighbors. He said a detailed survey of a property next to the laboratory's most active radiological site didn't find any contaminants that would pose a public threat. DTSC spokesman Russ Edmondson said the same holds true for the rest of the land around the laboratory.

That drew a terse response from Denise Duffield, associate director of Physicians for Social Responsibility in Los Angeles.

"So how do they know better than the National Academy of Sciences?" she asked. "The academy says there is no safe exposure level for radioactivity."

How Rigorous Should the Safety Standards Be?

The Los Angeles Regional Water Quality Control Board has repeatedly called attention to poisons and radioactive material in runoff from the laboratory. In 2007, for instance, it fined Boeing  $471,000  for releasing wastewater with elevated levels of chromium, dioxin, lead and mercury. Tests in 2009 found elevated levels of cesium-137, copper and lead. In 2010, the board fined Boeing again after the laboratory's runoff contained levels of radioactive materials, dioxins, mercury and other contaminants that the board considered excessive. This time, the fine was for $500,000.

In an interview, Malinowski stood by his claim.

"The numbers that the water board looks at for their discharge limits are much, much lower than human health standards are," he said. "They are extremely low numbers."

The water board isn't the only government agency with stricter requirements than those the DTSC is applying at Santa Susana. Records show the department is holding Boeing to much weaker standards than the federal government’ s general guidelines as well. One example: The state has accepted a threshold for strontium 90, a carcinogen, that is more than 1,000 times more lenient than the EPA cleanup standard.

Before it can clean up a toxic site, a pollution control agency has to identify where the contamination is and how dangerous it is. To do that, it chooses from a range of possible “risk-based screening levels” that can be used to create a pollution map.

The federal EPA's standard for determining how much pollution must be removed from a polluted site is to achieve a contaminant level low enough that the pollution can be expected to cause no more than one cancer per million people over a 30-year period.

Regulators may set different screening levels depending on the land’s expected use, but residential scenarios are strict, because that’s where people spend the most time and face the greatest risk from pollutants.

For instance, the EPA considers the possibility that residents will eat fruit and vegetables from backyard gardens. It’s not that the EPA is trying to promote gardening. It’s the agency’s way of acknowledging that often, ingesting contaminants is more dangerous than merely passing by them or touching them.

Boeing has argued that a residential cleanup isn't necessary at Santa Susana because nobody is going to live there. The company has promised that it would clean the land to a residential standard anyway, to be absolutely sure that the environment is protected, as well as the health of people living nearby or visiting the site.

Some residential scenarios are more rigorous than others. DTSC scientists who assesses the health risks of pollution recommended including a strict residential standard in setting the screening levels for Santa Susana. But in 2013, the department accepted a Boeing method that skipped the strictest benchmarks.

One result was a 2015 risk assessment in which Boeing discreetly contradicted itself. It started out by claiming the pollution really wasn’t that bad. But buried in an appendix was an acknowledgment that if Boeing were to factor in the possibility of people eating fruit and vegetables grown on the site, it would have to admit to a cancer risk as high as three in 10.

An assessment for another portion of the laboratory grounds acknowledged that if Boeing were to apply the strict standard there, it would have to admit the pollution was severe enough that for every 10 hypothetical people living there, nine would get cancer.

The revelations infuriated elected officials, who pointed out that real people live very close to these sites. The DTSC ordered Boeing to change its assessments to include gardens. But Boeing didn't change its screening methods.

In response to repeated inquiries, department public affairs officials offered no explanation as to why the DTSC allowed Boeing to continue to employ a screening method the department had expressly forbidden.

However, in a March 24 email, Boeing spokeswoman Megan Hilfer defended the company's ongoing use of the less rigorous standard.

"When considering a risk-based approach, future land use is a critical consideration to ensure the property is adequately cleaned for that end use, while also protecting against adverse cleanup impacts to natural and cultural resources," the email states.

"This is why including a garden exposure pathway in that assessment for Santa Susana, as some have suggested, makes no sense: the property will be legally-restricted open space where no produce of any kind will ever be grown for consumption on-site."

Such arguments anger John Detwiler, who lives just downhill from the laboratory.

"Don't tell me no pollution comes down every time it rains, every time the ground shakes, when the wind blows, we're all victims," he said. "Including me. Including my wife. Including her daughter. We're all victims."

Note: Reporting for this series of stories received financial support from the Fund for Investigative Journalism.