Safety of Rooftop/Rain Barrel Collected Water
As rain barrels increase in popularity, questions about their use have arisen . . . Below is some information gathered from various credible sources (I neglected to ask permission to post their names):-
From the Minneapolis Star Tribune Fixit column of 04/04/06:
From an Environmental Toxicologist with the Minnesota Department of Health (April 2006):
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.
I believe that warnings not to use roof-top collected rainwater for vegetable gardens are taking a precautionary approach. I do not know if the calculations have been made that would determine the extent to which these substances are accumulating in plants. Those calculations would need to be made before the MDH could tell you whether you could safely use the water for vegetable gardens.
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 an urban rainwater collector and rainwater system designerwho works for the Council on the Environment of New York City (April 2006):
From someone at the Minnesota Pollution Control Agency (April 2006):
Anyway, I think I would tell this person that a rain barrel is an excellent idea for the reasons that are listed in the Fixit column, but they should take the normal precautions in cleaning their veggies before eating them. You might also advise them to allow the spring rains to flush off the roof before setting up the barrel.
I personally have two rain barrels and have no compunctions about using them for watering any plant, veggie or not. I did find (especially when we were heating with wood) that the first few flushes of rainwater from the roof in early spring had some soot and I wasn't comfortable using it for watering plants.
From someone at the Pesticide and Fertilizer Management Division of the Minnesota Department of Agriculture (April 2006):
From a University of Minnesota Horticultural Specialist (April 2006):
Additional comment: I do not think there would be enough zinc or other metals in the collected rainwater to be toxic to plants. This is because any of these substances that may be present would be diluted substantially with the rainwater. The main concern with the collected rainwater is if applied to edible plants, there could be negative effects to humans if those plants are ingested. This would be due to accumulation of metals in the plant tissue over time. More importantly, if there are bacteria (E. coli) in the water and then sprayed on edible plant parts, this could also cause human sickness if the plants are eaten.
More from an Environmental Toxicologist with the Minnesota Department of Health (April 2006):
From the University of Connecticut: "Based on monitoring at a site with asphalt shingles in CT, we found very low (mostly ND) concentrations of Cu,Pb, and Zn in runoff from the roof. The roof did not have lead flashing, though. We did not test for mercury"
From Snohomish County government, Washington: "Galvanized or copper flashing on asphalt shingle roofs should be a concern also. Here in the Pacific Northwest, where moss grows on nearly everything that's not moving, it's somewhat common to place a galvanized ridge cap on gable-roofed houses with cedar shake roofs. Minute amounts of zinc from the cap wash down the roof surface in rain and prevent moss from growing on the shakes. Without the caps moss grows thick on shake roofs here. I assume that if runoff from a 3-inch wide galvanized flashing is toxic enough to kill moss on an entire roof, it could be affecting other plants as well."
From Volusia County government, Florida: "The adhesive for shingles is now including parts washer solvents. I understand a waste company in Florida picks up the parts washer fluid, ships it elsewhere and then uses it as part of the adhesive for shingles. I am unsure if the company removes the heavy metals, therefore, I would not use the rain water from the shingled roof-but that is my personal opinion."
I hope that this is helpful. The consensus from web sites and these interactions seems to be that unless the roof is designed with materials and methods intended for rainwater collection, there is a possibility that toxic substances will end up in the water. The point made about the rainwater being toxic enough to kill moss and mildew suggests that it may actually be toxic to garden vegetables if collected water is a primary source of water for a garden.
From a Physician with the California Public Health Service (March 2009):
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.
Filter The Water
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