Silicon Valley has lots of Superfund sites but it's not all about the silicon, and the nastiest stuff is older.

The Superfund site in my neighborhood is the former ag supply business. Thy sold pesticides to the local farmers, and were not careful about cleanup. There is a thick layer of clay over the old ground and houses on top now. Disturb it at your peril.

On the other side of town where a semiconductor plant used to be, they have a little fenced off well and a pump spraying water in the air. The contaminants evaporate out and blow away. Eventually the groundwater will be clean again.