Environmental scientists have tested a key US river for signs of a toxic waste spill from a botched Colorado mine clean-up that prompted a state of emergency in the country's southwest.
What started as an 11.4 million litre orange-hued plume last Wednesday in the swift-moving Animas River dissolved from view as it made its way down the slower San Juan River in New Mexico.
No longer easily visible, it was nevertheless flowing on into Utah and the Lake Powell reservoir in the direction of the Colorado River and Grand Canyon, leaving behind questions as to its long-term effect.
"It's so diluted, you can't really see it," Donna Spangler, a spokeswoman for the Utah Department of Environmental Quality, told AFP by telephone.
Intensive water testing is nevertheless underway for signs of such cancer-causing toxins as lead and arsenic, with results expected in a matter of days.
The spill prompted states of emergency to be declared in Colorado, New Mexico and the vast Navajo Nation reservation that straddles state lines.
Towns along the Animas and San Juan stopped drawing water from the two rivers, and kayakers and rafters were told to stay on land until Monday at the latest.
The spill, estimated to be 130km long, was a major embarrassment for the Environmental Protection Agency (EPA), the federal government agency charged with combating pollution.
It was supervising an attempt to halt a slow leak of toxic waste from the long-abandoned Gold King mine near the Colorado town of Durango when an earth-moving backhoe unleashed a deluge instead.