Residue from a rocket fuel plant destroyed in an explosion nearly 17 years ago near Henderson, Nev., continues to pollute the lower Colorado River, whose waters irrigate much of the lettuce consumed in the United States.
Now the Food and Drug Administration has confirmed earlier studies showing perchlorate contamination from that plant and other sites around the nation is concentrating in lettuce and milk.
The agency's results, released this week on the FDA Web site from tests conducted in August, underscore earlier studies by the Environmental Working Group, university researchers and California journalists, but they are the first to document nationwide contamination of food.
The FDA reported finding perchlorate in 217 of 232 samples of milk and lettuce in 15 states. Most of the samples were taken in California, Nevada and Arizona. Nearly all the samples showed perchlorate levels higher than the 1 part per billion the federal Environmental Protection Agency has identified in a preliminary risk assessment as acceptable for drinking water.
That standard currently is under review by the National Academy of Sciences, which is expected to issue its evaluation in January.
Massachusetts has adopted the 1 ppb standard. California has set a preliminary safety standard of 6 ppb. Utah has no standard. State law says only that Utah's standard can't be more stringent than whatever the EPA adopts.
Perchlorate is the explosive component of rocket fuel. It is used to manufacture fireworks, gunpowder and highway flares. It also is used in tanning and leather finishing, rubber, paint and enamel production.
The lower Colorado River supplies most of Southern California's drinking water and irrigates over a million acres of farmland in California and Arizona, where much of the nation's winter produce is grown. Irrigation water used for alfalfa production is the most likely source of perchlorate contamination in milk.
FDA researchers say perchlorate at high doses disrupts thyroid gland functions. The biggest risks are to children and fetuses. Results include delayed development, mental retardation, hearing loss and motor skills impairment. Chronic lowering of thyroid hormones due to high perchlorate exposure may also result in thyroid tumors.
Even so, Bill Walker, EWG's West Coast vice president, said people shouldn't stop eating greens or drinking milk, because the foods' health benefits generally outweigh the perchlorate risk.
"The people we really think are getting the raw deal are the lettuce growers and dairymen," he said. "It's not their fault their products are contaminated with rocket fuels - or that their markets could be damaged by these findings."
The FDA emphasized that the data are exploratory and limited in scope, and that perchlorate levels don't necessarily translate to perchlorate exposure.
The Environmental Working Group, a research group with offices in California and Washington, D.C., has identified 162 sites in 36 states that use or manufacture perchlorate.
Twelve are in Utah.
The major pollution of the Colorado River started on May 4, 1988, when a series of explosions at the Kerr-McGee perchlorate manufacturing plant, PEPCON, at Henderson killed two employees and left a long-lasting legacy of perchlorate leaching into Lake Mead.
The Environmental Protection Agency estimates 500 pounds of perchlorate per day flows from the lake into the river.