The thousands of mine shafts that pockmark the Sierra Nevada and testify to California's Gold Rush riches have also left a legacy of toxic contamination in some of the state's popular recreation areas, according to a new study.

Soil tests on a handful of trails near mine mouths in the foothills have revealed extremely high levels of lead, arsenic and asbestos, said researchers at the Sierra Fund, a small environmental advocacy group.

The naturally occurring minerals were pounded to dust generations ago and carted to the surface, where they are now stirred up and inhaled by hikers, off-roaders, bikers and horseback riders.

The Gold Rush "not only brought wealth and hundreds of thousands of people to California, it also brought mining machines that ripped down sides of mountains and tunneled thousands of feet into rock, leaving behind arsenic and lead," said Elizabeth Martin, chief executive of the nonprofit group. "This is the longest neglected environmental problem in California."

While their analysis was limited to 80 samples from 11 trails and recreation spots in the Foresthill, Downieville and Nevada City areas, the group says California's 47,000 abandoned mines pose significant threats to public safety, particularly in the dry summer months when families flock to the foothills.

"A lot of people are aware that their kids can fall into a hole at an old mine. But they don't know that asbestos fibers can lodge into their lungs or lead can be absorbed into their skin," Martin said in an interview Tuesday.

For that reason, the group said additional testing must be done in select areas where historic mine waste intersects with well-trafficked trails. The fund is also pushing for warning signs and, in some cases, restricting access to public trails with high levels of hazardous minerals.

Federal attention

So many mines are scattered across different landscapes and under various ownership that it is unclear who would coordinate such an effort and how - not just in California but across the country.

With development and recreational activity encroaching on remote areas, the problem has gained the attention of federal authorities.

A federal audit in 2008 charged the Bureau of Land Management, which controls a number of abandoned mining sites in California, Nevada, Arizona and other states, with endangering public health by failing to clean up arsenic, lead and mercury near the shafts or to erect barriers around them. In the Rand Mining District outside Los Angeles, the inspector general of the Interior Department found piles of toxic mine waste in residential areas as well as biking trails awash in arsenic particles.

That same year, the agency closed about half of the Clear Creek Management Area, a 31,000-acre off-road vehicle paradise in the hills outside of Hollister, after a U.S. Environmental Protection Agency report said waste from the defunct asbestos mining in the area posed a serious risk to those who work, camp, hunt and ride on the property.

Above safe levels

In the Sierra Fund's study, researchers reported levels of arsenic, asbestos and lead well above levels deemed safe for human exposure. At the Foresthill Off-Highway Vehicle area near the Marall Chrome Mine pit, science director Carrie Monohan said lead levels were nearly 18 times the state and federal standards, and 40 percent of the soil samples showed asbestos contamination.

Asbestos, a mineral that shows up in nature as bundles of tiny, twisty fibers, is of significant concern because it can embed in throat and lung tissue, causing cancer and other respiratory diseases.

Difficult to clean up

Because of the diffuse nature of the hazardous minerals in surface soils, they are almost impossible to clean up, according to David Christy, spokesman with the Bureau of Land Management's Central California division.

"The technology is a challenge," he said. "The approach to cleaning up mines is steam cleaning them and cementing over them, and that costs a lot of money."

Under President Obama's federal stimulus plan, California received about $20 million of the $73 million set aside for cleaning and maintaining abandoned mines nationwide. Some estimates peg the number of abandoned mines in the United States at 500,000 and the amount needed to detoxify them in the billions.