The following Tuesday a United States Geological Survey (USGS) employee took a boat into the Sacramento River at Freeport, a small town downstream from Sacramento, rinsed out a Teflon container three times and then filled it with local water.
The results were startling. On a typical dry season day, previous samples had shown that an average of seven ounces of mercury were washed into the river at Freeport, but on that day the samples indicated that over 70 pounds of mercury, some 160 times greater than the dry season average, had been washed into the river down into the San Francisco Bay Area.
Geologists and water quality scientists believe that a lot of this mercury came from sediment trapped behind old timber dams that failed during the floods.
The spread of mercury is one of the least understood and most toxic legacies of the Gold Rush. One gram of mercury poured into eighty million liters of water would be cause for concern under federal human health standards for drinking water, enough to contaminate a typical mid-western lake. Used to dissolve gold out of the ore, mercury is a deadly toxin that affects the kidney, the brain and the nervous system.
Perhaps the most famous example of mercury poisoning is the Mad Hatter of the novel Alice in Wonderland, who was modeled on hat workers. But far more worrisome are the tales of Chinese workers in the Gold Rush who worked with mercury in the San Jose area, and died "shaking toothless wrecks."
A century ago the Coast Ranges of California were the site of the world's second largest mercury mines. Over 100,000 tones of mercury were dug out of the mountains and then transported into the Sierra Nevada for miners in the Gold Rush to extract the yellow metal. Geologists today estimate that approximately 7,600 tones of this mercury were lost into rivers of the central Sierra Nevada alone.
Minute globules of the mercury today lurk hidden in the sediment behind dams in the Sierra Nevada, buried in mountain lakes and river beds, and in the estuary that stretches from Sacramento to San Francisco Bay. Already 250 million cubic meters of these mercury-laden sediments from the Gold Rush have filled the Bay. There is so much sediment that the level of the San Pablo bay has been raised by over three feet during the last century.
"To give you a sense of how much material this is, it would take ten quarry trucks a second dumping their loads for one year to equal this volume," explains Bruce Jaffe of the USGS whose team recently started to work out what the impact of this sediment load means for the Bay Area.
Every major flood brings hundreds of thousands of tones of new sediment into the estuary and the Bay. With it comes hundreds of pounds of mercury. In the presence of decaying organic matter that is often found in the tidal marshes, the mercury becomes methylated. This organic mercury accumulates in fish over time to one million times higher than those found in the surrounding waters.
USGS officials say that the mercury levels in the fish in the Bay have yet to reach danger thresholds but that is no guarantee that they won't do so in the future.
"There might be an awful lot of mercury caught up in the bottom of those dams, and if they fail we might have a sedimentation problem. We just don't know about the safety of these structures," says Rick Humphries, another geologist with the State Water Resources Control Board.
Meanwhile flood experts note that flooding in California has become frequent. In fact today the frequency of '100 year floods' has been steadily rising, a factor that designers of both old and new dams in the state did not anticipate.
For example, the Folsom River Dam on the American River was designed based on the flood record of 1905 through the 1940s. Professor Allen Bradley, a hydroclimatologist at the University of Iowa, says: "If the floods that we've seen in the last 50 years are more representative of (the) future, then the level of protection we have against floods is much less than we thought when we designed (the Folsom Dam)."
The American River is one of four of the most mercury-contaminated rivers in the state. Together with the other three such rivers - namely the Bear, the Feather and the Yuba - the American joins the Sacramento River to flow into the San Francisco Bay.
For now the dams on the Yuba, possibly the most contaminated of the four rivers, causes the mercury to be trapped in the Englebright reservoir. But there is new evidence that the streambed below the Englebright reservoir is starting to erode. This is allowing mercury trapped deep in the sediment to be released. Meanwhile, undammed rivers in this region like the Cosumnes, still bring mercury to the Bay Area.
Scientists are also beginning to suspect that a lot of the mercury may also be coming from the old mercury mines themselves that have never been cleaned up. The old mines stretch from Clear Lake, some three hours north of San Francisco, to New Idria in San Benito County South of Hollister.
For example, a University of California at Davis study shows that there is 100 tones of mercury trapped in Clear Lake. One ton of this mercury was believed to have been released into the Bay area via Cache Creek and the Yolo bypass in the wet 1995 winter alone. Another study that year showed that old mercury mines in the Mount Diablo mining area were releasing mercury in the Marsh Creek watershed in Contra Costa county.
One of the most polluted sites is the New Almaden mines just south of San Jose. This mine, which was worked by British, Chinese and Mexican miners beginning in 1863, produced a third of all the mercury in California. Also high up on the list of polluted sites further south is the New Idria mine in San Benito county which produced a sixth of the mercury in California.
North of the Bay area another major toxic site is located in Tomales Bay where a 20th century mercury mine on the Gambonini ranch has been leaking mercury from the waste rock. Water samples taken from Walker Creek in 1993 contained up to 2,500 times the mercury level considered hazardous to human health. Potential problems exist at other old mercury mines north and east of San Francisco such as Oat Hill and Oakville in Napa County, Mayacamas and Knoxville in the wine country and some 100 mines in the Sonoma region between Calistoga and Cloverdale.
In the far north mercury is also a worry for the Native Hupa and Karuk peoples in northwestern California. Robert Ulaberri, the environmental expert at the Hupa Valley reservation on the Trinity river, says that water testing near the Copper Bluff cinnabar mine on the reservation showed that the site was laden with heavy metals like arsenic, cadmium and lead.
"Copper Bluff was put on the Superfund National Priority List in 1982, the same year as Love Canal. Today it is still pouring out a toxic brew of heavy metals despite the fact that the Trinity was also named a wild and scenic river in March 1997," says Ulaberri.