When cinnabar breaks down under the right aquatic conditions it turns into methyl mercury, a highly toxic substance. Methyl mercury can be taken up in the tissues of fish, which when eaten in quantity, pose a threat to human health. Mercury is especially dangerous to pregnant women, as it can damage fetal development. Mercury is also suspected of retarding the cognitive development of children. According to the San Francisco Regional Water Quality Control Board, the Guadalupe River watershed is the most mercury contaminated river basin in the nation, based on samples of mercury in bedded sediments taken from the mouth of the river, where the Guadalupe enters the Bay.
Many people do eat fresh Bay-caught fish in spite of the warnings.
For more than one and a half centuries, humans have extracted mercury ore from South Bay mining works to make quicksilver, a form of mercury useful for extracting gold from its natural ores. Santa Clara County Quicksilver mines were once the 8th largest producers of mercury in the world. The Gilded City has made Santa Clara Basin the state s leading Mercury Hot Spot. More than a million pounds of released mercury line our Basin s streams and rivers. All three metals occur naturally in smaller or greater amounts in our Basin.