Superfund sites: http://yosemite.epa.gov/r9/sfund/r9sfdocw.nsf/vwsoalphabetic...
Brownfields: http://www.epa.gov/region9/cleanup/california.html
See the Google Map link below my piece. You can zoom out to the entire state of California and then into any neighborhood for any chemical over various time periods.
It isn't pretty.
> How can I learn more about where these sorts of toxic environments are located?
It depends. Some cities and counties have maps available online with this information. Also the EPA has maps of every site, http://www.epa.gov/superfund/sites/
(I'm inclined to believe him too. But I'm biased against the EPA anyway... I'm still mad at them for telling us it was safe to go back to lower Manhattan days after the towers fell.)