Mercury levels are on the rise in California, largely due to gold mining in the 19th century. Now, the same mercury used to extract gold decades ago, has been detected along the lower Yuba/Feather River system in the state’s Central Valley, which could pose a threat to nearby wildlife.

The bulk of mercury used for gold recovery in California was derived from mercury deposits in the Coast Range on the west side of California’s Central Valley. Between 1850 and 1981, the state’s total mercury production exceeded 220,000,000 lb (pounds), reaching its production climax in the late 1870s. Most of the mercury was exported to western states; however, around 12 percent was used to extract gold in the sunshine state.

Mercury stays in dry river sediment for thousands of years, and can become a threat when subjected to dangerous weather. When flooding occurs, for instance, it can spark a process known as methylation, which converts inorganic mercury into toxic monomethylmercury. READ MORE