1 gallon of water weighs 3.8 Kg.
The EPA standard for mercury in drinking water is no more than 2 parts per billion.
Each CFL contains about 5 milligrams of Mercury.
So, that is 5 milligrams diluted into 2.5 billion miligrams of water, or 2.5 million grams, or 2,500 Kg of water.
2,500 Kg of water is about 660 gallons.
When measuring parts per...remember Avagadro. He has his own number. ;)