Irrigation water quality, ponds and roof runoff

There are many sites online advising against using untreated roof runoff from rain barrels for watering vegetable gardens. However, farmers are allowed to pull irrigation water from ponds full of all sorts of critters; I'd imagine that most ponds must be more contaminated then most roofs, at least from a bacterial stand point. (Let's ignore metals and other contaminants for now.)

Is this just a case of urban yuppie farmers not knowing anything about how real farms operate, or could there be some reason that roofs are more dangerous then ponds? Or are both too dangerous to use safely on vegetables to be eaten raw?

Wow, I've never heard that sort of advice. Almost all my water comes from rain that is collected off of my buildings' rooves. The water gets used for the household, the livestock, the plants, everything. We do treat the household catchment tank water with bleach, but not the livestock and irrigation tanks. The household greywater gets piped to the bananas. Almost all my neighbors are in catchment water systems too. Truthfully, I've never had a problem using the water.

Could there be contaminants? Sure, but not anything that isn't already in my gardens, such as bird poo, bugs, etc. Remember, bugs poo on the plants all the time. So do birds. Earthworm poor is everywhere too. Besides, I wash all my raw veggies before eating them, so what's the problem? And ya know, I've eaten peas and cherry tomatoes while working in my gardens and I have managed to survive to a ripe old age, so far.

I believe the warnings against roof runoff may have to do with the roof material itself. I recall reading something similar some time ago and I seem to remember it pertains to asphalt shingles. The petroleum products and other chemicals in some of them may leach nasties into the rain runoff. It seemed that steel roofs, clay tiles and natural roof materials were exempt from this. I think it would require a sample collection of water from an asphalt roof sent to a lab for analysis to really get factual data to determine whether it's safe or not.

I'm thinking about this again. I was planning to build a pond, with a set of waterfalls irrigating a series of wicking beds on the way down. However, there are raccoons in the area, and they are attracted to water. I know raccoon droppings can contain some really nasty parasites. So I'm not sure this is a good idea after all.