Santa Clara County has 23 active Superfund sites, more than any other county in the United States. All of them were designated as such in the mid to late 1980s, and most were contaminated by toxic chemicals involved in making computer parts. Completely cleaning up these chemicals may be impossible.
-- Tatiana Schlossberg, The Atlantic
The article focuses on the semiconductor industry, but Silicon Valley also has roots in the aerospace and defense industry. Santa Clara County has 133 contaminated ground water sites and, to add insult-to-injury, the south-end of the valley has naturally occurring asbestos.