Every drop is sacred. Every drop is great.

Q:

I have a relatively new roof and the usual granular material in runoff is obvious. That settles out, of course, but it makes me wonder about other things added after the rain lands. Are chemicals from roofing carried into rain barrels collecting rain water? I would appreciate any information you can give me.

Thanks.

Jim

A:

Hello Jim,

Humans have been harvesting rainwater for 4000 years to irrigate crops and cool their homes and up until a few generations ago municipal water supplies were largely unheard of, leaving many folks dependant on well or rainwater to meet their daily needs. Rainwater is the only source of water for many people in the world today, including many rural areas in places like Arizona, Texas and Australia. The quality of rainwater for lawn and garden irrigation is unsurpassed; it has a natural temperature that doesn’t shock plants and, best of all, it’s free of chlorine, a chemical added to our water that inhibits plant growth. The myriad benefits associated with rainwater harvesting have, to some extent, overshadowed the contamination concern that you raised. Because we are not using this water for drinking, there hasn’t been much attention paid to this issue here in Calgary. Luckily though, with a few exceptions, contaminants from our roofs do not appear to pose a great risk to our yards or even our veggie gardens. Let me explain.

The quality of harvested rainwater is a function of the type of roofing material, climatic conditions, and the surrounding environment. The rainwater harvesting system involves a catchment surface - typically the roof - the gutters, downspouts and the storage tank or tanks (i.e. rain barrels). Roofing materials vary widely, as does the composition of the materials from different manufactures. Some cedar shakes are pressure impregnated with fire retardants and preservatives to extend decay time, while asphalt composite shingles are impregnated with asphalt, also known as bitumen – a sticky, black and viscous substance found in most crude petroleums. Asphalt is insoluble in water and thus rainwater from a roof with asphalt shingles is not a major concern when it is used for non-potable uses. Painted metal roofs, particularly those containing lead are also suspect. It is important to note that Michigan State University’s Rain Harvesting Guide, similar to many other rainwater harvesting guides from around the world, states that any roofing material is acceptable for collecting water for non-potable (non-drinking) uses.

For people considering replacing their shingles, it would be wise to choose materials that are non-toxic and inert (non-leaching). Something like factory-enameled galvanized steel with a backed-enamel, lead-free finish would be best for rainwater harvesting. However, for folks looking to use the water for non-potable purposes most materials will suffice. Obviously some materials are better than others for water harvesting purposes; products impregnated with flame retardants, wood preservatives and moss inhibitors are probably not the best, especially after a new product is installed, as this is the period when leaching of these undesirable products is highest. It would be wise to talk to individual manufactures about their products and the chemicals, if any, that they use in their products. Rain gutters should also be free of paint or solder containing lead.

Like any large surface, including gardens, roofs may accumulate such things as animal and bird feces, mosses and lichens, and dust and particulates from urban pollution. Cleaning the gutters periodically and placing a screen either in the gutter at the point where water flows down the downspout or before the water enters the barrel will reduce the amount of debris getting into the rain barrel, thus improving the water quality significantly. Again, the collected rainwater is being used in yards and in gardens, not for drinking, so people should not worry about a few bird droppings getting into the water.

For anyone overly concerned about debris from the roof getting into their rainwater, a “first-flush diverter” can be fashioned to capture the initial rainfall to “cleanse” the roof of any debris that may have collected since the last rain event. Several schematic diagrams and accompanying text is available in The Texas Manual on Rainwater Harvesting. Note that these “first-flush diverters” are common on potable water systems but almost certainly overkill for non-potable uses.

In the majority of cases collecting rainwater for lawn and garden watering is hugely beneficial. Given that a typical 2000 square foot house here in Calgary can capture up to 52,000 litres of water each year, there is little reason to rely on treated tap water for watering in our yards and gardens. Continuing this 4000 year old tradition, especially in a water stressed region like Calgary, is not only empowering but will allow us to continue to live a high quality of life, even as the city continues to grow.

Quenched,

Ashley

Ashley Lubyk, BSc. in Environmental Science, formerly a Calgary Materials Exchange Program Assistant at Clean Calgary Association. Please send your question concerning Water, Waste, or Recycling to Ashley.