Is your roof safe for rainwater harvesting?


Photo courtesy of Sandawg at Flickr.com.

In general, water from a rainbarrel is not fit for human consumption. Even though it’s not safe to drink, rainwater is good for watering the yard, orchards, and gardens. Non-potable water works for all sorts of things where food-grade water would be a waste, and using it eliminates the use of chemicals and electricity that have a large impact on the environment. All of the following have to be mined, refined, transported, activated, and mixed with water to make it safe for humans:

chlorine and ammonia (which combines to make chloramine) or ozone to disinfect the water; lime and iron sulfate to remove suspended solids in the water and for corrosion control; activated carbon to control offensive tastes and odors; and fluoride to help prevent tooth decay.

Plants don’t need bottled water! They will thrive with rainwater. On the other hand, people and pets can easily get sick from rainwater due to bacteria, parasites, or chemicals. Even trace amounts of certain chemicals can build up over time, so its important to ensure that your water is safe before using it on fruit trees or vegetable gardens. Some rainwater is dangerous even to plants that are used for landscaping!

If you live in an area where the air quality is bad, then there is a good chance that dangerous soot will collect on your roof. Areas downwind of coal power plants and brick furnaces are particularly at risk. Many factories emit fly ash, which is a toxic mix of silica, carbon, and lime. Fly ash often has significant amounts of alkali, sulfur, mercury, iron, and other particulates. When it rains, these particulates can wash down the drainage spout and into your rain barrel. If the soot in your area contains water soluble chemicals, they will contaminate the water.


Photo courtesy of selmasiddiqui at Flickr.com.

One way to gauge how much soot there is around your home is to leave a white sheet out overnight on a clothesline. If the sheet is stained with dust and grime, then there’s a good chance that the same types of dust and grime will accumulate on your roof. Even if your sheet has some grime, that isn’t the end of the world. Most dirt and grit is harmless, you just might want to test the water to make sure.

If your roof is clean, there may still be unseen dangers. Some asphalt roofing shingles contain trace amounts of zinc. These shingles can produce some dangerous water soluble compounds. If you use this water in your vegetable garden, the compounds may be absorbed by your plants. Some veggies do a good job of filtering contaminants out of their water, but others incorporate environmental poisons into their fruit and leaves.

The expensive (and most comprehensive) way to test your water is to take a water sample from your rain barrel to the local department in charge of environmental monitoring or contact the local community college and see if they offer water testing. There are also professional services that charge a hefty fee for water testing. A less expensive way to check is to talk with whoever put the roofing on your house. If the shingles were zinc free, they should know.

If you prefer an alternative method, you can also test water runoff by putting fish in your rain barrel. After the barrel has been thoroughly rinsed out (to remove any plastic chemicals) put fish in and see if they survive. If you choose to do that, you’ll need to make sure that the fish get enough oxygen and food. Many cities offer mosquito fish to control mosquitoes in abandoned swimming pools, and I would recommend getting a few of those. Just leave the lid off on your rain barrel so mosquitoes can get to the water and that will solve the food and air supply issues at the same time (while also eliminating mosquitoes in your yard)!


Photo courtesy of addicted Eyes at Flickr.com.