No Peril to L.A. Water Seen : Radioactive Carbon Found in Mono Lake

Unusually high, but apparently not unhealthful, amounts of radioactive carbon have been discovered in Mono Lake, not far from the source of about 20% of Los Angeles’ drinking water, Columbia University scientists said Thursday.

In an article in the April issue of the scientific journal Earth and Planetary Science Letters, geochemist Wallace S. Broecker said his data suggests that the pollution was caused by “clandestine disposal of (nuclear) waste” in the lake in the 1950s and again in the 1970s.

Broecker stressed that the radioactive pollutants were found only in the brackish lake, not in the freshwater streams that feed the lake and are diverted to Los Angeles for drinking water.

“This in no way endangers the Los Angeles water supply,” said Columbia spokesman Henry Fuhrmann.

A spokesman for the Los Angeles Department of Water and Power concurred. He said Los Angeles drinking water is routinely sampled for radioactivity as well as myriad other pollutants and is well within federal guidelines.

Broecker, of Columbia’s Lamont-Doherty Geological Observatory, discounted the possibility that the radiocarbon could have been introduced to the lake by open-air testing of nuclear bombs. Similar lakes in nearby western Nevada do not show the same high radiocarbon readings, he said.

Instead, the evidence points to surreptitious disposal into the lake of spent nuclear-medicine tracers or, less likely, the remnants of a nuclear research reactor. Broecker said he is even investigating the possibility that the lake may have been used by the Navy for testing a small underwater nuclear bomb in 1957, although he said this appears to be the least likely explanation.

More will be learned about the source of the radiocarbon once researchers collect the results of further tests, which Broecker expects may begin within a week. Scientists now will look for other radioactive materials.

“If there was a nuclear bomb blast in the lake, or if nuclear fuel was dumped there, there should be other radioactive materials in the lake--such as strontium 90 or cesium 137--and in the sediment,” he said.

Broecker’s team did not look for these materials earlier because they found the high radiocarbon levels by accident. The scientists originally went to the exotic lake 180 miles east of San Francisco to study how carbon dioxide gas is exchanged between the lake and the atmosphere. This study was done by tracking the movement of easily traceable radiocarbon created by open-air nuclear bomb tests in the 1950s and early 1960s.

‘Readings Way Too High’

“Everything fit together well (with readings at the ocean and other lakes), except the radiocarbon readings at Mono Lake were way too high,” he said.

Thorough analysis of the data indicated that the readings were due to something peculiar at Mono Lake, something not explained by radioactive fallout. “I’ve been scratching my head over it for years,” Broecker said.

The radiocarbon levels in the lake were plotted over time by measuring the amount of radioactive material deposited on the lake’s shoreline.