NOTE: An earlier version of this story listed plutonium as one of the radioactive contaminants found in the area referenced in this story. According to a state official, further testing showed that plutonium was not present. Testing did show the presence of radium-226. All references to plutonium have been removed from this story. You can read more about the testing in this story.

Tests have uncovered radioactive contaminants in an open-air burn pit, already rife with chemical pollutants, at the Santa Susana Field Laboratory, according to state regulatory officials.

Low levels of radium-226 were discovered during testing this fall, said Norman Riley, the field lab project director for California's Department of Toxic Substances Control.

"These are very low levels of radionuclides, and certainly the discovery of radium is not that surprising," Riley said Monday. "It's fairly common to find radium in landfills. We don't know if we found all that there is to find, and it doesn't answer the question of where it came from."

One possibility for the source is old radio or instrument dials, or it might have been used in experiments. The Field Lab, which is currently owned by Boeing Co. and formerly owned by Rocketdyne, is a former rocket engine and nuclear test site in the hills south of Simi Valley.

Boeing officials notified the state in October about the discovery and, in an e-mail update about the burn pit, wrote: "The levels detected are low in comparison to radiation from a single chest X-ray."

In a letter to Riley, Thomas D. Gallacher of Boeing wrote: "As a precautionary measure, the locations where elevated levels were detected have been fenced and posted with caution signs... Boeing will expand the investigation at the Area 1 Burn Pit and, upon receiving appropriate regulatory approval, Boeing is prepared to expeditiously remove this localized contamination."

The pit became a source of controversy two years ago when nearly 200 pages of documents — many of them handwritten logs of what was destroyed in it — were made public. The logs detailed a stew of toxic chemicals that were incinerated there. Some of the toxics were trucked in from other Rocketdyne facilities, primarily its Canoga Park lab and another lab at the Santa Susana site where radioactive work was conducted.

"It's troubling they found radioactive contamination in an area where no radioactive materials were supposed to be used," said Dan Hirsch, co-founder of the nuclear watchdog group Committee to Bridge the Gap. "It reinforces the need for a full radioactive survey of the entire facility."

Napalm, potassium cyanide, dioxin and chromium were among the toxic chemicals burned in the pit. In 1971, two cans of mercury were destroyed, but in a column on the log marked "disposal method," there is a question mark.

The logs were given to the DTSC by Boeing Co., as the department prepared to launch a stopgap cleanup of the pit to keep the toxins from contaminating surface water running off the site. The project was halted with the revelation of the logs.

On Monday, Riley said the testing that uncovered the radionuclides was part of a prerequisite to remove contaminated soils. The burn pit is slated to undergo more testing.