Pete Richerson and Scott Richerson
To reach the mine, turn right on Sulfur Bank Road. As you turn, notice the cinder cone on your left down Highway 20 that is being removed entirely for decorative aggregate. This is one of the youngest features of the Clear Lake Volcanics. In a mile and a half you will come to a triple fork in the road. The right fork leads to the Elem Colony. Currently there are no facilities open to the public at Elem, but a casino may reopen there in the future. The roundhouse dances that occur at Elem every year or so are religious rituals, but visitors are welcome. They are expected to make a small offering at the center-pole and then to observe quietly from the periphery. Take the middle road, which is the left of the two dirt roads. Stop on the high point of the road overlooking the Herman Pit with its pH 3 pond as the centerpiece.
The sulfur deposit at Sulphur Bank was first mined in 1865 using simple surface excavation techniques. John Veatch, the pioneer exploiter of borax in Lake County (see next stop) developed the mine at Sulphur Bank. 2 million pounds of pure sulfur were produced before increasing contamination with cinnabar made the production of sulfur uneconomical. In 1873, exploitation of the mercury ore began, using simple surface cuts. Later, shafts were driven into the deposit to find the seams of mercury deposited in fissures in the fractured andesite flow (medium density volcanic rock) and adjacent Franciscan Formation rocks that are the main formations at the site. You can often smell hydrogen sulfide (odor of rotten eggs) on the road above the mine and with binoculars you can see the vigorous gas springs that vent into the Herman Pit in the middle of the site.
Work in the mine was brutal. The active geothermal system made the temperature in the tunnels so hot that the Chinese and Indian workers could only work in 15 minute shifts before their body temperatures started to climb toward lethal levels. Sweat-bathed miners were hoisted up the shafts on an elevator, cooled by a blast from a firehose, and lowered back down for another stint. A hundred yards or so from the high point on the road you can see the remains of the headworks for one of the shafts. When these tunnels caved in, little effort was made to dig out trapped miners.
Notice the ruins of the retort up on the hill to the left, where cinnabar, the principal ore of mercury, was roasted to recover the pure liquid metal. Total production of mercury from the mine amounted to 4,500 tons or more of liquid mercury. About 60% of the total was extracted in the 19th century using shallow surface excavation and underground mining techniques. In 1927, the Bradley Mining Company brought in heavy excavating equipment and created an open pit mine. This episode of mining continued until 1944, when wartime parts shortages caused the mine to close. Some mining was conducted during the mid 1950s. 1957 was the final year of operation. The mine site now consists of about 120 acres of heavily disturbed waste rock, tailings piles, and denuded earth. Much of the waste material was bulldozed right into the lake, creating a new shoreline as much as 400 feet lakeward of the original.
In the 1970s, California Department of Fish and Game surveys found Clear Lake fish to be contaminated with levels of mercury slightly above official health warning levels. Subsequent surveys have found widespread mercury contamination in California waterways. Mercury was mined at many sites in the Coast Range, and in the 19th century, most mercury produced was used in the Sierra gold fields to amalgamate flour gold and silver.
Mercury is a dangerous contaminant in many parts of the world; international conferences on the problem draw thousands of participants. As with the DDD contamination, Clear Lake was a sentinel ecosystem for this important ecotoxicology problem. In the mid 1980s the California Department of Health Services issued warnings regarding the consumption of fish from Northern California Coast Range lakes. Levels of mercury in Clear Lake fish are below levels that would cause mercury poisoning in humans. Osprey are tolerating their not inconsiderable dose of mercury from Clear Lake fish without detectable effects. Still, mercury is a suspected carcinogen and may cause neurological damage in fetuses and children at low doses, so to be conservative, DHS recommends that adults limit their intake of fish from contaminated systems and that children and pregnant women refrain from eating them entirely. The only local reports of possible mercury poisoning in humans are from the Elem Pomo, who live very near to the mine and have traditionally consumed a lot of fish. In the past some Elem people suffered illnesses consistent with mercury poisoning.
Sulphur Bank was made an EPA Superfund site in 1990. In 1992, the EPA did an emergency project here to stop erosion from the waste rock piles along some 1200 feet of lake frontage. The graded and riprapped piles of dirt and rock you can see from the lake today are the results of that effort. Numerous contractors working for EPA have labored to understand the source and extent of mercury contamination in the lake since that time. About 100 tons of mercury have escaped from the mine and are buried in the sediments of Clear Lake. Most of this mass is now too deep to be a problem.
In addition to whatever mercury flowed into the lake before 1992 from erosion, considerable rain-water and geothermal inflows seep into and through the mine waste, become quite acid, dissolve mercury, and discharge into the lake. Because the toe of the waste rock pile is underwater, the main discharges are below the lake surface and hard to see and measure. UC Davis field workers sampling lake bottom mud discovered masses of a suspicious white flocculant material near the mine face after the rainy 1994-5 winter, which turned out to be comprised mainly of a clay mineral precipitated from acid mine drainage. Sometimes, white billows of this material can be seen at the lake surface right along the base of the waste rock piles.
Practically all sulfide ore mines are sources of acid mine drainage. During open pit operations especially, large volumes of sulfide rich minerals, mainly iron sulfide, are disturbed and exposed to leaching by rainwater and any surface or ground waters that penetrate the waste rock heaps. When sulfide is exposed to air or dissolved oxygen in water, bacteria oxidize the sulfide to sulfate, creating large amounts of acid in the process. The result is the not-so-dilute solution of sulfuric acid you see in Herman Pit. In the winter and spring the waste rock piles west of Herman Pit show a pinto pattern of green and brown (straw and brown in summer). The brown patches are surface lenses of sulfide rich waste that form soils too acid for any plants to grow.
Under acid oxidizing conditions, mercury is about as soluble as table salt. Since the floc found in the lake contains significant amounts of mercury, somewhere beneath the waste rock piles, air is dissolving into the acid water in sufficient quantity to extract mercury. The EPA's task is to understand how this underground system is plumbed and to try to find a way to stop the production of acid mine drainage. The trouble is that the miners ripped the relatively compact natural sulfide deposit to pieces, jumbled uneconomic minerals together with miscellaneous overburden rock, and piled the resulting poisonous mélange 30 or 40 feet deep over the tunnel-laced rock of the former underground mine. Finding out how water and air flow through the resulting mess is a very difficult task. Remediating acid mine drainage is also not easy. The Roman-era tin mines of Cornwall are said to still be sources of acid discharge.
The trouble with the mercury in the sediments is not the inorganic mercury compounds themselves, which are harmless in the concentrations in which they are found in the lake. Rather, bacteria living in the sediments convert a small portion of the mercury to methyl mercury. We don't know why bacteria form this compound, but in the anaerobic sediments a half an inch or so below the surface, they do produce it. The concentrations of methyl mercury in the water (levels of about a part per trillion) and sediments (levels of a few parts per billion) are not obviously alarming. As sanitary engineers say, "the answer to pollution is dilution." Usually this is so, and these levels are dilute indeed. However, quite unusually for a heavy metal, methyl mercury bioaccumulates in food chains just like chlorinated hydrocarbon pesticides. The compound has a high affinity for the sulfur-bearing amino acids in protein and is excreted very inefficiently from the body.
The UC Davis team studying the mercury problem in Clear Lake for EPA has documented a classic case of food chain accumulation of mercury. Small invertebrates near the base of the food chain have methyl mercury levels around a few hundredths of a part per million. Small fish run around a tenth of a part per million. Big, long-lived predatory fish-the bass and catfish that appeal to most consumers-run about 1 part per million. Osprey run around 2 parts per million. The levels of methyl mercury in shellfish eaten by the Japanese people who were poisoned in the 1950s at Minamata Bay had mercury concentrations around 10 parts per million, so it is probably impossible for an adult to contract mercury poisoning from eating Clear Lake fish. The DHS warning reflects, as it should, a safety factor. The objective of the EPA cleanup is to reduce influx of inorganic mercury into the lake so that natural burial of the contaminated sediments eventually reduces the supply of mercury for the methylation process and brings fish methyl mercury concentrations back into the safe range. UC Davis investigators estimate that five to ten years should be sufficient to dramatically reduce the inorganic mercury available at the sediment surface, once the ongoing supply is stopped.
Cores taken by a team of UC Davis investigators document the increase in sedimentation rate that earlier investigators thought mainly responsible for the post-1925 increase in sediment and phosphorus supply that supposedly made the lake more susceptible to large bluegreen blooms. Sediments after 1927 are drier and contain less nitrogen than pre-1927 layers, as if the organic-rich sediments were being diluted by inorganic erosion products transported by winter storm flows. Sedimentation rates after 1927 are about 1 cm per year versus about 1 mm per year over the preceding 10,000 years in the USGS core. However, other hypotheses as to what causes the bluegreen blooms are on the table. The acid mine drainage carries enough sulfate into the lake to at least double the sulfate entering the lake. Sulfate in turn may have triggered changes in lake chemistry, ironically by making the lake more basic (alkaline).
Geology of Putah-Cache
Gone to Gold
Upper Cache Creek
The structure and design of the Putah and Cache website is copyright © 2001 University of California.
The material on this page is copyright © 2001 Pete Richerson and Scott Richerson.