MASON VALLEY

Managing the Carson River; How did the river become so polluted?

Amy Alonzo
Reno Gazette Journal
Waterfowl are seen along the Carson River just to the east of Carson City on Aug. 6, 2020.

The Carson River is one of the region’s most beautiful waterways. It’s also one of the most polluted.

Millions of pounds of mercury were imported to the Comstock region in the 1880s for mining purposes, and more than a decade later, much of that mercury is spread throughout the Carson River watershed, raising mercury levels in the soil and wildlife to toxic levels. The river was placed on the Environmental Protection Agency’s Superfund Site in 1990, where it was listed on the National Priorities List.

In early fall, the EPA will issue a proposed plan considering ways to continue monitoring the mercury-contaminated soil to prevent human exposure and environmental releases from the river basin.

Legacy site, area of major pollution

In the 1850s, miners discovered large deposits of gold and silver in the Comstock Lode near Virginia City.

Mill operators imported mercury from the San Francisco area to improve the efficiency of extracting ore, mixing mercury and water into a slurry to help liberate the silver and gold from the nearly 240 stamp mills throughout Carson City, Washoe Valley, Virginia City and Dayton.

A sketch of the Douglass Mill, located on River Street in Dayton.

Not all the mercury that was used for mining was recaptured, and between 1850 and 1890, more than 14 million pounds of mercury were released into the Carson River area.

The mercury entered a 130-mile stretch of the Carson River between the Mexican Dam in Carson City to the Carson Sink, the terminus of the river, south of Fallon. Over the years, flooding and irrigation continued to spread the contamination.

Now, properties that are former mill sites, drainage areas, the Carson River channel and areas within the 100-year floodplain have high concentrations of mercury, as well as arsenic and lead.

Primary concern is protecting the kids

The Carson River Superfund Site is unique in its contamination.

“There is no responsible party for this site,” Bureau Chief for NDEP’s Bureau of Corrective Actions Jeff Collins said. This makes management of the site tricky.

The EPA mandated several small cleanups during the 1990s, primarily in residential Dayton.

According to an NDEP fact sheet, “The remainder of the site has not undergone cleanup and due to the size and the scope of the area impacted, most likely never will.”

But, that doesn’t mean the site isn’t being managed.

“We have to manage the pathways of concern to make sure there aren’t health risks,” said Rebecca Bodnar, supervisor for the Superfund branch of NDEP’s Bureau of Corrective Actions. “We also want to make sure the site doesn’t move elsewhere. We’re managing for human health.”

The Carson River is seen just to the east of Carson City on Aug. 6, 2020.

Department of Environmental Protection Engineer Dave Friedman said mercury levels are monitored at the Weeks bridge in Silver Springs and Lahontan Dam. The Lahontan Dam was built in 1915, and Friedman said it has acted as a reservoir for much of the mercury-contaminated sediment flowing down the Carson River.

In the 1970s, the U.S. Geological Survey reported high levels of mercury in the Carson River, and subsequent studies found elevated mercury levels in the soil, water and wildlife. The Nevada State Health Division recommends not eating fish from the river between the Mexican Dam and Lahontan Dam.

EPA studies have found no direct evidence of increased mercury levels in people residing near the Carson River. But, if someone were to eat large amounts of contaminated fish or soil, there could be health risks.

From endangered species to thriving: Lahontan cutthroat trout doing well at Summit Lake

One population NDEP worries about is children under the age of 6. The agency requires developers to clear all soil of mercury to a depth of two feet so that children playing in soil, who might possibly eat it, aren’t at risk.

“That’s the reason the two-foot level was arrived on,” Friedman said. “Our primary concern is protecting kids.”

“When developers come to us and work with us to make sure the soil is at least clean to a depth of two feet, that’s kind of the goal. We want to make sure there’s a two-foot clean layer before we put homes on those sites,” Collins said. “If they show us there’s no mercury, arsenic or lead that pose a risk to folks, they can further develop those properties … All we can do is manage residential development and other soil development throughout this area.”

Managing a legacy site

“When people hear the word ‘Superfund,’ it has such a bad connotation. But it was such a pivotal point in our history,” Bodnar said of the Carson River area’s history. Silver mined in the Comstock helped fund the Union during the Civil War, as well as the construction of San Francisco and Sacramento, she said. “This is a historic site that shaped our country the way it is and now we’re left with a legacy. This is really a legacy site and is part of the heritage of Nevada.”

The Carson River is seen just to the east of Carson City on Aug. 6, 2020.

Public input on the continued management of the site will be the focus of the EPA’s fall meeting.

“You’re not really going to bring it under control and have any sort of cleanup,” Friedman said, adding that management looks more like smaller projects such as bank stabilization along the river to prevent erosion and other “modest engineering efforts.”

The EPA will not decide on a monitoring plan until the public has had opportunity to provide public comment. Due to coronavirus restrictions, the EPA is considering hosting virtual public meetings. NDEP said it will notify people within the Carson River basin of the meetings once times and dates are set. 

For more information about Carson River pollution, contact EPA project managers Andy Bain (bain.andrew@epa.gov) at (415) 972-3167 or Sarah Watson (watson.sarah@epa.gov) at (415) 972-3032. You can also contact Dave Friedman (dfriedman@ndep.nv.gov) with NDEP at (775) 687-9385.

Amy Alonzo covers Mason Valley and rural Nevada. Reach her at aalonzo@gannett.com or (775) 741-8588. Here's how you can support ongoing coverage and local journalism