I looked it up. I saw 1 gallon of used motor oil will ruin 1 million gallons of fresh water. I saw 1 pint will create a oil slick an acre long. I saw 1 gallon of motor oil will ruin 2 million gallons of water.
Yet no site explained how the water would be ruined. Ruined for drinking? Ruined for fish? Ruined for plants? Explain ruined.
A lot of things, both natural and man-made can "ruin" water and in much smaller quantities.