California Water At Risk From Abandoned Sierra Nevada Mines

"There are 47,000 abandoned mine lands in California that have been inventoried and over half of those are in the Sierra Nevada," said Keegan. "And of those, there are 3,000 that are known to contain chemical hazards, including mercury."
Keegan said there are probably more mines in the Sierra Nevada with chemical hazards that haven't been assessed.
http://www.capradio.org/articles/2014/12/08/california-water-at-risk-from-abandoned-sierra-nevada-mines/