I have read for years that one quart of used motor oil can contaminate a million gallons of fresh water. I do not know what level of contamination that is, though. I once saw a used oil filter half under water in the drainage culvert for the creek behind my house. A lot of people are pigs, plain and simple. There's probably a bunch of used oil seeping down out of older landfills. As recently as the late 80's we used to just put our used oil into milk jugs and throw it in the trash. That is actually what the Dallas EPA field office told me to do when I called them during my college years. I found a quick lube center that was willing to take my used oil, but used oil filter collection only started happening around the early to mid 90's. So, all that oil from big V-8's that was changed every 3,000 miles is still out there, waiting.
I guess some of you understand now why I get a bit miffed about people changing their oil TOO often.