1) Ecofreaks exaggerate almost everything.
2) Lake Erie was very badly polluted, and dredging is still problematic. Buried layers of silt from the "pollution years" are still contain the various poisons that were a problem then. Lake Erie is far from unique in this regard.
3) See above ... the zebra mussels are a mixed blessing, and I would recommend against introducing them as a pollution control measure. They arrived in the Great Lakes by accident.
I suppose the lesson is to think through your pollution control measures BEFORE implementing them, whether it's physical pollution or moral pollution that you wish to control. Still, it does no good to deny that pollution is pollution.
I guess in that case were talking about "pollution absolutes". And in absolute terms anything short of pristine is polluted.