Arsenic

There is naturally occurring arsenic and that resulting from mankind's pollution.

Ocean Arsenic: A ‘Ticking Time Bomb’ Oil Could Make Worse

July 2, 2010 article from The Gulf Oil Monitor

http://www.gulfoilmonitor.com/?p=189

Ponds 'caused Bangladesh arsenic'

November 15, 2009 article from the BBC

http://news.bbc.co.uk/2/hi/south_asia/8361660.stm