Mercury in Our Waters: The 10,000-Year Legacy of California’s Gold Rush

If you look closely in the waters of Deer Creek, near Nevada City, California, something strange may catch your eye; lying in globules amongst the gravel is quicksilver, or liquid elemental mercury. Carrie Monohan, head scientist for the Sierra Fund, lives next to Deer Creek, and became concerned about mercury contamination in the waterways when she pulled liquid mercury from the water in a turkey baster. “We’re not talking about parts per billion coming from a smokestack in China,” Monohan said, “We’re talking about liquid elemental mercury that we can see, and suck up, and get grams of, running through our streams and rivers.” 

Get a better understanding of the impact the mining activities at Malakoff Diggins State Historic Park had on the area. Roll your mouse over the image below to listen:

View of the pit at Malakoff Diggins State Historic Park as it looks today. | Wikimedia Commons/BenFranske/Creative Commons (CC BY-SA 3.0)
View of the pit at Malakoff Diggins State Historic Park as it looks today. | Wikimedia Commons/BenFranske (CC BY-SA 3.0)
Speaker 1: So we have over 500 tons of suspended sediment coming from this pit a year and at least 100 grams of mercury a year. This is an ongoing significant source of mercury to the watersheds of California. Parks is currently working … they got $8 million in general fund money to do an environmental impact assessment to weigh the alternatives of what could be done. I think the hard part is designing solutions. So here’s the sticky (unintelligible), currently, mining wouldn’t be allowed to leave the site looking like this today. They would have to slope it back and revegetate it. But we can’t do that now because those cliffs are in the National Register of Historic Places. That feeling you got when you came here and you were like “oh my gosh” and you totally started to understand the history, that’s not something that should be destroyed. It’s protected by (unintelligible) and it’s parks mandate to protect those cultural resources. So it’s very hard here to know how to fix the water quality problem when you have this amazing cultural resource which is actually the pit walls themselves. And so there’s a couple of different alternatives that would passive filter the water but not change the cliff walls. Speaker 2: So it’s like a scar? Speaker 1: It’s a huge scar. That’s why they call this mine-scarred land.

Mercury is everywhere in the waterways and ecosystems of the Sierra Nevada. The contamination is a living legacy of the California Gold Rush, which tore through the Sierras in the 1850s and reshaped the history of California. Though the Gold Rush ended over a century ago, thousands of abandoned mine sites and mine sediments, which were never properly reclaimed, have continued to leech mercury into the environment and will continue to do so for the next 10,000 years. The problem will only escalate with climate change, as increasingly intense flash floods and storm events and erosion following wildfires wash greater levels of mercury downstream and into aquatic ecosystems. These sources of mercury in the high sierras impact the entire California watershed and the ecosystems and communities that rely on it. “Here we have California, which is thought of as one of the most environmentally forward-thinking states,” said Monohan of the contamination, “yet this is maybe the longest neglected environmental problem in the world. And we’re not even talking about it yet, honestly.” 


Learn more about the impact of gold mining around the world on "Earth Focus | The Price of Gold in Peru."

On a clear March afternoon, Monohan and I stood at the lip of a giant ravine in the middle of Malakoff Diggins State Historic Park in California’s Nevada County. Before us, the rolling hills of the Sierra Nevada abruptly plummet 600 feet, giving a clear view across the land to the iron-red cliffs on the other side. “When I first came here, I was really struck with what happened to this place. The land looked messed up to me,” reflected Monohan as we drove up the winding roads leading to the outlook, “I knew something serious had taken place here to disturb so much.”  

Even to the untrained eye, it’s apparent that the ravine at Malakoff Diggins wasn’t created by any natural process; it gives the impression of having been gnawed out of the earth by some giant animal. In a way, that’s exactly what happened — the ecological ruin of Malakoff Diggins is an artifact of the Gold Rush era’s hydraulic mining, a process by which mining companies used highly pressurized water cannons to blast sediment away from the hillsides with a force of 5,000 pounds per square inch, scavenging ancient river deposits for gold. In many ways, Malakoff Diggins is the cradle of the Gold Rush; hydraulic mining was invented in nearby Nevada City, and soon, Malakoff Diggins became the largest hydraulic mine in the world. Over the next 44 years of the mine’s operation, over $3.5 million of gold was extracted by North Bloomfield Mining and Gravel Company from the pit.  

Hydraulic mining has always been known for its destructive nature; one contemporary account of hydraulic mining by L. P. Brockett described it as such:  

Hydraulic mining washed away 1.5 billion cubic yards of sediment from the Sierras in the duration of the Gold Rush. The greatest environmental legacy of Malakoff Diggins, however, isn’t the ravine or the gold that was taken from it; it’s not even visible to the naked eye. Unseen in the sediment at the bottom of the pit, tiny particles of mercury have attached themselves to fine silts and clays, which slowly erode and become suspended in the water during storms. This mercury was once used by miners to amalgamate gold from the sediments that were being washed down from the face of the cliff. Today, every year from Malakoff Diggins alone, 100 grams of mercury will flow from this pit and into the watershed of the Yuba river. 

The pit at Malakoff Diggins during heyday of hydraulic mining. | Wikimedia Commons/Watkins Photo-Bancroft Neg. #8111/Creative Commons/Public Domain
The pit at Malakoff Diggins in 1890. | Wikimedia Commons/Watkins Photo-Bancroft Neg. #8111/Public Domain

The typical romantic image of the Gold Rush is that of the rugged individualist, an egalitarian landscape in which anyone with a gold pan could strike it rich. This story is a feel-good one and like in many other Gold Rush-era nostalgia towns, tourists visiting Malakoff Diggins can reenact the myth by gold panning in Humbug Creek or milling around the nearby historic town of North Bloomfield, complete with an old-timey general store.  

Underneath the rosy tint, however, the history of the Gold Rush is bloody, with a towering human and ecological cost. The original sin of the of the Gold Rush is the foundational sin of all of America: the way in which the land where the gold was mined was brutally stolen through the genocide of Indigenous peoples. In the first 25 years of the gold rush, from 1848 to 1870, 80% of the Indigenous communities of California, some 120,000 people, were killed. Mass migration of miners during the Gold Rush fueled the genocide in the Sierras; those not killed from diseases brought by the flood of gold miners were systematically chased off their lands, massacred and enslaved. This genocide was carried out through extrajudicial as well as state-sanctioned violence; the California state government encouraged the killings, paying bounties for the scalps and heads of Indigenous people. As the Gold Rush unfolded against the backdrop of genocide, gold extraction quickly became more technologically intensive. Soon, mining technologies became more advanced and required more capital; individual mining claims were bought up and consolidated by larger corporations like North Bloomfield. As the hunger for gold grew, investment flowed into new technologies for hard rock and hydraulic mining to extract additional gold. Key for this intensification process? Vast quantities of mercury. 

Listen to how scientists measure water quality at Hiller tunnel. Roll your mouse over the image below to listen:

iller tunnel, pictured here, is where water and slurry discharged from Malakoff Diggins during the days of hydraulic mining.  | Alexandria Herr
Hiller tunnel, pictured here, is where water and slurry discharged from Malakoff Diggins during the days of hydraulic mining. | Alexandria Herr
Speaker 1: So here is Hiller Tunnel, and what made this a really great study site is that the pit on the other side of this hill all discharges here. I’m going to take you up high and show you the whole pit. 6,000 feet long and 1,000 feet wide and 600 to 1000 feet deep in places and that whole bathtub-sized pit over there discharges out here. So if you want to measure what the water quality impact of that pit is, you take a sample here. It’s really lucky that this was set up this way so we could do a good job measuring it.


About 200 million years ago, during the Mesozoic era, a shift in the tectonic plates pushed the sea floor beneath the American continent, creating liquid hot magma. This magma rose to the surface of the continental crust, over time cooling and solidifying into the granites of the Sierra Nevada. During this process, the magma came into contact with water, which dissolved minerals in the magma, including quartz, silver and gold. The cracks in granitic rocks were thus impregnated with gold-laced quartz veins. As the sierras were uplifted by even more tectonic upheaval, these deposits were exposed and eventually weathered and carried away by ancient rivers. These two sources of gold; the hard-rock quartz veins and the ancient river gravels, were what 19th century miners were searching for. 

After the gold on the surface had been panned away, miners needed a way to access the gold that lay in these ancient deposits. Mercury was the key. Mercury has been used in gold and silver mines since at least the time of the Roman Empire; gold dissolves in mercury like salt in water, preventing gold from washing away in the sediment. In hydraulic mines, the power-washed sediment was drained into tunnels and run through mercury-coated sluice boxes. In hard-rock mines, mercury was added to the pulverized ore to amalgamate the gold particles. Without the addition of mercury, the Gold Rush might never have gotten much farther than individual prospectors picking lumps of gold out of the riverbeds.   

Imagine how gold was excracted from Malakoff Diggins State Historic Park in the heyday of hydraulic mining. Roll your mouse over the image below to listen:

The pit at Malakoff Diggins in 1871, the heyday of hydraulic mining. | Wikimedia Commons/National Gallery of Art Patrons' Permanent Fund/Public Domain
The pit at Malakoff Diggins in 1871, the heyday of hydraulic mining. | Wikimedia Commons/National Gallery of Art Patrons' Permanent Fund/Public Domain
Speaker 1: This is what it looked like in the heyday when it was in operation. So they had big monitors, seven of them. Seven high-powered water cannons pulverized the hillsides to release the gold, creating the enormous pit before you, yes. And that water was all conveyed into these trough-like settings where they then could add the mercury and collect. And whatever they missed on this first level would then, water would go towards that tunnel and they would have a mercury (unintelligible) in the tunnel. These were super powerful that they were known to kill people even at a distance of 200 feet.

It was then an accident of earth history that the same geologic processes that formed the gold veins in the granitic rocks of the Sierras also formed deposits of Cinnabar, or mercury ore, along the California coast. In fact, the fifth largest deposit of cinnabar in the world is located just outside of San Jose. The cinnabar ore was known to the Ohlone people as mohetka (red earth) and was a valued as an item for trade, otherwise crushed and mixed with water to create bright red paints. In 1845, Mexican Cavalry officer and mineralogist Andres Castillero recognized the cinnabar ore as a source of coveted quicksilver and started what would become the New Almaden mercury mine. Over the course of its operation, New Almaden mine would produce $70,000,000 in mercury; more wealth than pulled from any gold mine in the Sierras.  

As the Gold Rush boomed, a second, but equally important mercury rush unfolded in parallel. By 1851, California produced over half of the world’s mercury, and by 1890, mercury mining was the number two industry in value. Over the course of the Gold Rush, 26 million pounds of mercury were imported from the Coast Range mercury mines to the gold mines of the Sierra Nevada. Of this mercury, 10-30% slipped away into the rivers and ecosystems of the Sierras. 


Gold and mercury miners at the time of the Gold Rush would have known about the potential for mercury poisoning. In 1865, J. Ross Browne, a visitor to the New Almaden mercury mines, published a firsthand account of mercury poisoning among mine workers in Harper’s Magazine: 

A monitor that would have been used in hydraulic mining to blast pressurized water at the side of the cliffs at sites like Malakoff Diggins. | Alexandria Herr
A monitor that would have been used in hydraulic mining to blast pressurized water at the side of the cliffs at sites like Malakoff Diggins. | Alexandria Herr

The roman god Mercury was the god of financial gain and commerce. He was also known as the god of trickery. Much like the god, mercury the element is tricky, capable of infiltrating ecosystems and the body in insidious ways. Mercury enters aquatic systems in an inorganic form, bound to mining sediments. Once it enters streams, lakes and rivers, iron and sulfur-reducing bacteria can convert mercury to its organic form, known as methylmercury.  Methylmercury is highly toxic, capable of passing the blood-brain barrier, and also very easily absorbed into the tissue of living organisms. Once mercury is methylated, it makes its way slowly up the food chain; first through filter-feeding zooplankton, then to larger and larger fish. At each rung on the food chain, the methylated mercury accumulates in the tissue of the organisms that consume it. As you move up the food chain, the concentration of mercury in organisms increases tenfold through each link in the chain in a process known as biomagnification.  

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The sources of mercury contamination stemming from the Gold Rush are manifold. There are thousands of abandoned gold and mercury mine sites throughout the Sierras, such as Malakoff Diggins, and each of them are ongoing sources of mercury to the waters and ecosystems of California. The contamination isn’t limited to the ecosystems of the Sierras, either; during big flood events, these sediments can be carried from the Sierras all the way to the lowlands of the Central Valley. Michael Bliss Singer, deputy director of the Water Research Institute at Cardiff University and a researcher at UC Santa Barbara, studied these sediment transport processes and the contamination of Central Valley ecosystems. 

“We can’t just think of this problem as: there was a mining legacy back in the 1800s and we just have to manage the problem of what was left from that time. It’s an ongoing process that we have to be aware of,” Singer said. “When you think of the amounts of contaminated sediment and the amounts of mercury that are out there, it’s pretty scary. Everything we tested, every creature, had methylmercury in it. It’s invisible. That’s the part that’s challenging. Even when it’s in the creatures, you can’t see it.” 

Envision how a scientist measures mercury in the water at Malakoff Diggins State Historic Park. Roll your mouse over the image below to listen:

Carrie Monohan, head scientist for the Sierra Fund, checks a gauge that monitors water quality in Malakoff Diggins State Historic Park. This gauge measures turbidity, which can be used as an approximation for mercury content. | Alexandria Herr
Carrie Monohan, head scientist for the Sierra Fund, checks a gauge that monitors water quality in Malakoff Diggins State Historic Park. This gauge measures turbidity, which can be used as an approximation for mercury content. | Alexandria Herr
Speaker 1: So, I don’t know if you can hear but this is our gauge and then we have two meters in the water. So, there’s one meter there to tell us how much water is in the stream and then that black type over there is how cloudy the water is. We’re down here, we’re not, we don’t see the same orange, right? So the two streams kind of come close a little bit, but during storm events, you see a lot of it. Speaker 2: So this is where you’re measuring? Speaker 1: This is where we’re measuring and we take measurements right at the tunnel too and here we measure continuously.

Once mercury makes its way into fish, it’s not a far leap to humans. As mentioned before, mercury is a neurotoxin that can cross the blood-brain barrier, so the strange behavior of the Mad Hatter in “Alice in Wonderland” may have been modeled from symptoms of the neurological damage of mercury contamination (a nod to the fact that hatmakers in the 1800s used mercury to treat felt). At high levels, mercury exposure can cause birth defects and impaired neurological development. At low levels, the symptoms can be much more diffuse: concentration and memory problems, fatigue and headaches. In children, it can cause permanent learning disabilities and difficulty in school, much like lead exposure. Low level mercury contamination is insidious. The subtlety of the symptoms means that unless a doctor is specifically aware of mercury contamination in a community, individuals may never be tested for high mercury levels. 

Today, a major exposure pathway for humans to mercury is fish consumption; California’s Indigenous communities, for whom fishing in traditional water bodies is a cultural and food security priority, are particularly impacted. The California Indigenous Environmental Alliance was specifically formed in part to address the ongoing problem of mercury exposure in Californian tribal communities. Since its founding, C.I.E.A. has worked to educate communities about the health risks of mercury, particularly for pregnant mothers. Surveys conducted by C.I.E.A. indicate that tribes have reduced rates of fish consumption because of fear of mercury contamination, but reducing exposure by reducing fish consumption isn’t the goal. Sherri Norris, executive director of C.I.E.A. says that while it’s important to reduce mercury exposure, “ultimately, we are trying to get to a place where tribal members can eat traditional foods at traditional rates. We don’t want to lock in repressed consumption.”  

For Indigenous populations, the contamination of traditional foods and exposure of tribal members to mercury is an ongoing violence. Even 160 years later, the Gold Rush is still exacting a toll on the Indigenous communities of California. Ensuring access to traditional diets is both a matter of cultural preservation and food sovereignty for Indigenous populations; telling tribes simply not to eat the fish isn’t a desirable outcome for C.I.E.A. “The main public health strategy for dealing with mercury can’t be posting fish consumption advisories,” Monohan said. “That’s not justice. It doesn’t undo the harm.”  


Water collecting at the bottom of the pit in Malakoff Diggins State Park. Every year, 100 grams of mercury flow from this pit into the watershed of the Yuba river. | Alexandria Herr 
Water collecting at the bottom of the pit in Malakoff Diggins State Park. Every year, 100 grams of mercury flow from this pit into the watershed of the Yuba river. | Alexandria Herr 

Part of the reason the environmental impacts of the mercury and gold mining from the Gold Rush have been so lasting is that the Gold Rush tore through California before any kind of environmental protections existed at the state or federal levels. Today, legislation like California’s Surface Mining and Reclamation Act dictates how mine sites must be remediated. In the 1800s, there were no regulations to speak of, so California is still paying the price more than 150 years later. The scope of the problem is massive, and the question of where the money will come from to remediate sites is often a sticky legal battle. In October 2019, the Sierra Fund released their Headwater Mercury Source Reduction Strategy, which lays out strategies for tackling mercury contamination, including the development of inventories of contaminated hydraulic mine features and a plan to implement pilot projects to evaluate plans to reclaim abandoned mine sites and remediate contaminated reservoirs. The report represents a step forward in the ongoing battle to get the problem of mercury contamination the attention it deserves. Bit by bit, organizations like C.E.I.A. and the Sierra Fund are continuing the long, hard battle of healing the injustices of the past.  

Monohan has been working on mercury contamination for 10 years; her kids have grown up in the Sierras, living and playing next to the same waterways where Monohan once pulled quicksilver from the water in a turkey baster. “We are still suffering. That is our point,” Monohan told me as we drove back down the bumpy road leading out of Malakoff Diggins State Park. “The legacy impacts of the Gold Rush are still being felt by the communities and ecosystems of the Sierra Nevada, and the people who benefited from that are long gone.”  

In many ways, the work of the Sierra Fund goes deeper than the contamination itself; for Monohan, it’s a healing project, both for the land and for the various communities that rely on it. “People have a role in nature. Tending the land and stewarding the land is what we’re here to do and learn from,” Monohan said. “We want the Sierra Nevada to provide resources for generations to come.”  

Top Image: Water collecting at the bottom of the pit in Malakoff Diggins State Park. Every year, 100 grams of mercury flow from this pit into the watershed of the Yuba river. | Alexandria Herr

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