More than half a century after a partial nuclear meltdown near Los Angeles, a federal study has found ground radiation levels nearly 1,000 times higher than agreed-upon standards for mandatory cleanup.

The 1959 partial nuclear meltdown at the Santa Susana Field Laboratory ranks as the third worst nuclear accident ever, releasing up to 100 times more radioactive iodine into the atmosphere than at Three Mile Island.

In 2010, the U.S. Department of Energy, NASA and the state Office of Toxic Substances Control agreed to remediation measures at the site. As part of that effort, the U.S. Environmental Protection Agency has tested the soil around the reactor, collecting 475 samples.

Of these, the EPA reported that 75 exceeded “radiation trigger levels,” the point at which the state and federal agencies had agreed remediation for contamination by radioactive cesium, strontium and other materials would be necessary.

The standards for cleanup efforts at Santa Susana have changed over the years depending on the government agency involved and the contemplated future uses for the land, and through a series of agreements among regulatory agencies. The Department of Energy standard of 9.2 picocuries of cesium 137 per gram of soil, for instance, contemplated that the land might be used for suburban residential development.

The EPA goal for remediation at that land use was either .06 picocuries or a reduction to “background” – naturally occurring – levels. The current agreement between NASA, DOE and the state Office of Toxic Substances Control seeks to return Santa Susana to a cesium background level of 0.0207 picocuries. A picocurie is one-trillionth of a curie — a standard measure of radioactivity.

At one hotspot, the EPA found cesium 137 at 198 picocuries per gram. In other words, the intensity of cesium radiation at that particular location is almost 1,000 times the level that would trigger mandatory remediation.

Seventeen-year-old Devyn Gortner, who lives three miles from the former laboratory, founded Teens Against Toxins to campaign for a cleanup. She called the EPA findings “a relief and a curse at the same time.”

“What’s in my body? What am I going to be passing along to my kids? That’s such a scary thought to me,” she said. “But at the same time, to know that there is solid evidence means there will be a lot of pressure to get the parties that are responsible for the cleanup to actually clean it up.”

The accident in the rugged Simi hills between Los Angeles and Ventura County on July 14, 1959 remained largely unknown for 20 years, until a group of UCLA students discovered and publicized records of the release. Further research revealed additional radioactivity releases as well as contamination by carcinogenic dioxins and heavy metals from other experiments at Santa Susana.That led to decades of agitation by environmental activists and a series of failed cleanup efforts before the 2010 agreement.

Boeing Co. owns most of the 2,850-acre site, and, unlike NASA and DOE, has not signed on to the cleanup agreement with state regulators. Last April, a federal judge overturned a state law that would have made the company responsible to California officials for a cleanup. The state is appealing the ruling.

Kamara Sams, a company spokeswoman, said Boeing remains committed to cleaning its property under a 2007 consent order that would forbid contamination above levels typical of a residential suburban neighborhood. She noted that that standard is more stringent than would be normally required for the site’s future use as open space.

Daniel Hirsch, who led the UCLA students who first publicized the meltdown and went on to found the activist group Committee to Bridge the Gap, said Boeing has a poor record so far.

In a previous cleanup effort, DOE set a threshold of 9.2 picocuries of cesium 137 per gram of soil. Boeing was supposed to clean up any land that exceeded that level in the area around the former nuclear reactor site, Hirsch said.

But the recent EPA survey found several hotspots that were even above that level – the highest at 198 picocuries.

“We’re still finding stuff 22 times hotter than the lousy standard they used years ago,” Hirsch said.

Stephanie Jennings, a deputy federal project director with the DOE, said it’s not clear why the previous cleanup missed that spot. To try to head off such errors in the new effort, the agreement between the federal agencies and the state stipulates that the EPA will double-check all cleanup work and may require some sections to be cleaned again, Jennings said.

State Assemblywoman Julia Brownley (D-Santa Monica) said environmentalists will be watching closely as NASA undertakes an environmental review of the cleanup effort.

“Usually when you’re doing environmental assessments you identify what the problem is,” she said. “We know what the problem is, and we have an agreement. So we want the assessment to focus very narrowly on how to clean up to background levels.”

Nahal Mogharabi, an EPA spokeswoman, said additional soil sampling probably will conclude in July.

The federal agencies have agreed to complete their remediation efforts by 2017.