From the Minneapolis Star Tribune Fixit column of 04/04/06:
"...You can't drink the water collected, nor should you use it to water vegetable gardens. It's likely to be contaminated with chemicals and bacteria. But you can use it to water flower gardens and lawns or to wash lawn furniture, cars, etc...."
From an Environmental Toxicologist with the Minnesota Department of Health (April 2006):
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Rainwater washing off of roofs has been studied to determine the load of contaminants picked up from roofing material. Some rainwater collection systems, intended for drinking water, discard a first "flush" of water off the roof in order to make sure that organic material such as bird droppings do not contaminate collection tanks. The water is then treated for drinking.
But the contaminants that you could be worried about are the heavy metals and polyaromatic hydrocarbons from asphalt shingles and other contaminants that may deposit onto roofs from air. It appears that contaminants that rainwater washes off of shingles may be a significant source of surface water contamination. The contaminants that are washing off of roofs include zinc, lead, chromium, arsenic, and polyaromatic hydrocarbons. It is similar to what you might collect off of a parking lot.
It is possible to find data on the amount (concentrations) of chemicals in rainwater from asphalt roofs. However, I was not able to find information on whether or not the levels were high enough to accumulate in garden plants intended for consumption.
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From another website: "When NOT to use a rain barrel for watering: If you have certain kinds of roofing material you shouldn't use rain barrels for watering plants. If your roof is made of wood shingles or shakes that have been treated with any chemical (usually chromated copper arsenate-CCA) to make them resistant to rot and moss, lichen and algae growth, don't water your plants from a rain barrel. Water collected from copper roofs or copper gutters also should not be used. Zinc (galvanized metal) anti-moss strips-usually mounted at the roof peak-also produce toxic chemicals you don't want in your garden. Don't use rain barrels if you have these strips (you may want to remove them), or if you have had your roof treated with moss-, lichen or algae-killing chemicals within the last several years. Note that nowadays there are asphalt shingles on the market which have zinc particles imbedded in the surface. Check your shingle specifications if you have recently re-roofed.
In addition, general practice is to avoid watering vegetables and other edible plants, such as herbs you plan to use in cooking, with rain barrel water collected from asphalt-shingle roofs. These kinds of roofs may leach various complex hydrocarbon compounds, so most people avoid using water from asphalt-shingle roofs or flat tar roofs on plants meant for human consumption. To date there is no definitive research on the amounts and types of hydrocarbon compounds which may leach from such roofs, though it is common practice to use water collected from asphalt-shingle roofs for watering ornamental plants and shrubs. Enameled steel and glazed tile roofs generate little or no contamination and rainwater harvested from them is commonly used to water vegetables."
From a University of Minnesota Horticultural Specialist (April 2006):
[paraphrase] The advice of not using rain barrel water in vegetable gardens is sound and precautionary. Such water should not be used for drinking or for vegetables. Until a detailed study can be conducted, the best advice at this time is to use the water for ornamental landscape plants/lawns.
From a Physician with the California Public Health Service (March 2009):
1) In California (and probably across the nation), rooftops are often sites for raccoon latrines. Raccoons leave feces on rooftops, usually where valleys form, or alongside the intersection of walls and roofs. The danger is the Raccoon roundworm (Baylisascaris procyonis), which is a common intestinal parasite in the raccoon. The roundworm eggs are found in the raccoon feces and the eggs develop in the feces -- often surviving for over a year in dried raccoon feces. These roundworm eggs can be found in roof runoff water; an internet search on "raccoon latrines" will give several references.
2) Many shingles are now made with a mild algicide and/or fungicide. Usually this is a copper compound, but may be a more complex chemical.
