Mercury Pollution in Clear Lake, California: History and Environmental Impact

Located west of California’s Central Valley and about 120 miles north of San Francisco, Clear Lake is one of the largest natural freshwater lakes in the state. Geologists believe this body of water—which provides popular recreation areas for locals and important habitats for wildlife—could also be the oldest lake in North America.

Despite being touted as one of California’s top destinations for bass fishing (it's been nicknamed the “Bass Capital of the West”), the state’s Office of Environmental Health Hazard Association (OEHHA) has had an advisory on fish consumption since 1987. The reason? Mercury pollution.

History of Clear Lake

Back in the 1860s, the Sulphur Bank Mercury Mine began operations on the northeastern side of the lake, going on to leach mercury into the surrounding environment for almost a century. By the time the 150-acre mine site closed in 1957, it had produced 2 million cubic yards of mine waste on the property.

Today, a flooded open-pit mine measuring 23 acres long and 90 feet deep is located 750 feet from Clear Lake—and it's filled with a combination of contaminated mine waste and natural geothermal water that continues to seep mercury into the lakebed.

As a result, the Environmental Protection Agency (EPA) designated the property as an official Superfund site in 1991. The EPA Superfund program is responsible for cleaning up the nation’s most polluted land by responding to environmental disasters.

Mercury Contamination

The EPA doesn’t consider the mercury contamination to be high enough to ban swimming in Clear Lake specifically, though the pollution often results in algal and cyanobacteria blooms, making the water unsafe to swim in during mid to late summer. The presence of cyanobacteria is linked with high methylmercury concentrations in bodies of water.

The OEHHA fish advisory, last updated in 2018, sets specific limits on how many of which species people should eat depending on age. For example, women aged 18-49 years and children aged 1-17 years should limit their consumption of Clear Lake fish to one serving of Sacramento blackfish per week due to high levels of mercury found in the species. That same demographic should refrain from eating certain species entirely, such as the black bass.

The lake is also an important cultural site for California Indigenous peoples, specifically the Big Valley Band of Pomo Indians, whose ancestors inhabited the Clear Lake area over 11,800 years ago. Big Valley Rancheria, a territory of the Big Valley Band of Pomo Indians, has taken matters into their own hands when it comes to toxic cyanobacteria and mercury pollution in Clear Lake, and for good reason—the lake occupies a central role in their community livelihoods and many of their cultural ceremonies.

In 2015, Big Valley’s EPA department measured the mercury levels in different species of fish in different locations around the lake. Out of 33 tissue samples, 18 exceeded the California Waterboard limits for mercury contamination. While species like channel catfish and white crappie have a total maximum daily load limit of 0.19 milligrams of methylmercury per kilogram of tissue, at least two of the samples in Clear Lake exceeded as much as 1 milligram.

Methylmercury, the most toxic form of mercury, is formed when microscopic organisms in water and soil mix with inorganic mercury (formed naturally when mercury compounds combine with other elements like sulfur or oxygen).

What Are The Risks?

Mercury, the only metal that exists in liquid form, can be particularly dangerous when exposed to aquatic environments. And while mercury does accumulate in nature at low levels in soil and water, it becomes toxic when concentrations rise above naturally occurring conditions.

Mercury is absorbed easily into the food chain since the chemical can cross the biological membranes of exposed organisms and accumulate in animal tissues.

Tiny organisms are especially problematic as they are prey animals. Larger fish consume smaller fish that have been contaminated with mercury, and that bioaccumulation can then cause high levels of harmful mercury in the top predator fish that people eat. Methylmercury is concerning since our bodies have a less developed defense mechanism against it, so the toxin can cause a negative effect on the human nervous system.

In the 1990s, studies suggested that concentrations between 5 and 10 micrograms of methylmercury per one gram of tissue were enough to have sub-lethal or lethal effects on fish. We now know that this measurement was grossly overestimated and that as little as 0.3 micrograms in whole-body concentrations and 0.5 micrograms in muscle tissue concentrations compromise fish reproduction, embryonic development, change biochemical processes, and cause damage to cells and tissues.

Mercury is also absorbed by microalgae and aquatic plants, impacting photosynthesis by disrupting the genes involved in cell processes and energy metabolism.

Current Status

Clear Lake is the main source of water for at least 4,700 people living in the region. As recently as September 16, 2021, testing results in Clear Lake found the highest levels of cyanotoxin in recorded history, prompting the local public health authorities to alert those who receive their tap water from their own private intake into the lake to not drink the water. Over a week earlier, the testing site located on the lake’s lower arm organized by the Big Valley Band of Pomo Indians and the Robinson Rancheria EPA Department reported microcystin toxin levels at 160,377.50 micrograms per liter, the highest the labs had ever processed.

In June of 2021, the EPA updated the local community on the Clear Lake Superfund Site’s current status. The agency estimated that they were within four years of beginning the main clean-up project, which will be broken up into two phases: consolidation and capping.

Initially, the plan involves moving smaller piles of mining waste onto large piles to shrink the area that needs to be removed before installing a heavy cap to act as a barrier over the site. The cap will then be covered with clean soil so that plants can begin to grow and rehabilitate the area.