Humans threaten health of San Pedro River

Dutch Nagle, task president for the Friends of the San Pedro River, stands at a pond near the San Pedro. The river runs north from its headwaters in northern Mexico to its confluence with the Gila River near Winkelman, Arizona.
Dutch Nagle, task president for the Friends of the San Pedro River, stands at a pond near the San Pedro. The river runs north from its headwaters in northern Mexico to its confluence with the Gila River near Winkelman, Arizona.

Water swirls and gurgles at the foot of a towering cottonwood, its reflective leaves shimmering in the wind. The thirsty trees line the banks of the San Pedro River, which flows undammed from its headwaters in northern Mexico to its confluence with the Gila River near Winkelman, Arizona.

Shallow and serene for most of the year, the stream can quickly morph into a surging torrent during summer thunderstorms. This year, higher-than-average rainfall caused the San Pedro to flood several times.

Along with precious water, the summer storms brought some unsavory intruders. The bacteria, E. coli, was flushed from livestock fields littered with feces, and a mining spill in Mexico dumped toxic contaminants into the floodplain that feeds the river.

And, even with the influx of billions of gallons of water during a given storm, man-made wells continue to withdraw more groundwater than is annually deposited by rainfall. The growing water deficit caused by over pumping is the biggest environmental threat facing the San Pedro, said Tricia Gerrodette, president of the Huachuca Audubon Society.

“The biggest concern is that [the San Pedro] will die like so many other rivers in the state,” she said.

Losing the San Pedro would not be water under the bridge. In addition to supplying roughly 80,000 people with potable water, the river is also home to nearly 45 percent of the 900 bird species in North America, according to the Center for Biological Diversity.

Mining mishaps

The San Pedro River originates in a drainage basin near Cananea, Sonora, roughly 20 miles south of the U.S.—Mexico border. The city’s major economic driver: copper mining.

The largest mine in the area is Buenavista del Cobre. On Sept. 18, heavy rains caused a holding pond containing processed rock material to overflow, spilling water that likely contained heavy metals such as arsenic and lead into the headwaters of the San Pedro.

After Mexican authorities issued a binational alert for the possible contamination, the Arizona Department of Environmental Quality tested the river at two locations near the border for both metals and pH levels.

The initial tests showed no significant changes in either pH or metals, according to Mark Shaffer, media relations director for the ADEQ.

But one negative test does not prove that contaminants never got into the river. Follow-up tests of the surface water as well as the soil are necessary, said Sandy Bahr, director of the Sierra Club’s Grand Canyon Chapter.

“Metals will sink down into the bed of the river and that’s where they get picked up by fish and other creatures,” Bahr said.

The spill came on the heels off a much larger breach at the same mine that, in August, leaked more than 10 million gallons of mining acid into a pair of rivers that flow south toward the state capital of Hermosillo. Grupo Mexico, the company that owns the mine, was fined $3 million dollars for the incident, which it failed to report for 24 hours.

As opposed to the first leak, the September spill had little downstream effects, due in part to the conditions that led to the spillover, said Dutch Nagle, task president for the Friends of the San Pedro River, a non-profit organization dedicated to the conservation of the river.

“The [second] spill happened during major flooding, which means that anything bad that got into the water would have been swept away rapidly and would have covered a huge area,” Nagle said. “So we’re quite hopeful that even if some did escape, it would probably have little impact on us.”

Bacteria boom

As the San Pedro snakes its way through southern Arizona, it passes through the grazing areas of cattle and other livestock. During storms, feces from both domesticated and wild animals gets washed into the stream, causing an explosion in E. coli levels.

Bahr and Gerrodette are both members of the Water Sentinels, a group that monitors the water quality of the San Pedro River, among others.

“For three years now, we’ve noticed that the first rains of the monsoons bring a big load of E. coli,” Gerrodette said. “It just goes off the charts.”

E. coli is not restricted to the surface water, either. On Aug. 9, the Sierra Vista Herald reported that up to 2,000 residents could be at risk after a groundwater well was found to have elevated levels of the bacterium.

Within a week, the E. coli levels in the well were back to normal, though the incident spurred many Sierra Vista residents to buy bottled water to avoid drinking tainted tap water.

A growing deficit

While pollution from mining and animal waste pose serious health threats to the San Pedro, conservationists usually focus on what’s coming out of the river more than what’s going in.

“The single biggest concern for the San Pedro is declining amounts of water in the river [caused by] over pumping of the regional aquifer,” Gerrodette said.

Currently, the communities along the San Pedro pump about a billion gallons more water from the aquifer than is recharged by rainfall. If over pumping continues, Nagle said, the water table that supports the San Pedro will continue to drop. A decline of just a few feet could turn the river into another dusty wash is southern Arizona.

If the river were to dry out, it would be catastrophic for the complex ecosystem it supports. The cottonwoods, for instance, have shallow roots that rely on a large supply of water everyday, Nagle said.

“If the water table goes down, even 10 feet, all of the cottonwoods die,” Nagle said. “And if they go, the whole riparian area goes.”

Solving the problem of liquid overdraft involves placing restrictions on both public and private water use, Gerrodette said. She suggests that the city of Sierra Vista impose restrictions on outdoor planting and watering, and replace grass facilities with artificial turf.

“The river, in general, is healthy,” Gerrodette said. “The fight is about making sure there’s enough water left in the ground so that the San Pedro River continues to be a healthy ecosystem.”

Mark Armao is a reporter for Arizona Sonora News, a service from the School of Journalism at the University of Arizona. Reach him at

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