This is an article from the Discovery Channel website. The author argues that regulating concentrations of toxins released into nature isn't good enough to keep our water pure and our people healthy. This approach leads too often to the practice of simply diluting a known toxin to a "safe" level before releasing it. It doesn't disappear, it just is diluted to a hard-to-detect concentration.
The problem is that many experiments to determine "safe" concentrations of different toxins look mainly at acute effects of contact with the substance tested. There aren't many experiments to determine the effects of repeated low-level dosages of toxins, or the effects of exposure to a combination of low levels of various toxins.
That said, the article's claim that pollutants never go away isn't quite right either. Granted, for elements like mercury or lead that are inherently toxic, there is no way to break them down further to less-toxic forms. But for many of the most nasty toxins, like organic pesticides, the problem isn't the elements composing the substance, but rather their particular arrangement. In cases like this, over time the offending molecule will indeed break down to less harmful or totally harmless molecules.