My first blog post, bear with me…..
It has been proclaimed in a DIY rainwater system video that even asphalt roof runoff can be considered potable water. This is erroneous, and can be grossly misleading to followers of well-intentioned gurus in spite of their qualifications and successes.
Potable water is water fit for human consumption, normally from treated and managed municipal systems, or from approved (tested) ground or surface water sources. Harvested rainwater cannot be considered potable unless tested and treated to meet or exceed drinking water guidelines(1). While taste, appearance or wishful thinking may seem to indicate that collected water is safe, it cannot be assumed.
Asphalt shingles, which comprise a large portion of potential rainwater collection surface area, are reported to leach various chemicals although not normally in large quantities. However, in areas of wet, humid climates (west coast, southeastern US), algae-fighting granules of ceramic-covered copper are often added to the coloured granule mix during the manufacturing process. Leaching action releases the copper, which helps prevent algae from forming on the roof areas. Not all shingles will have these granules, only certain brands or styles, check with the manufacturer to be sure.
Other roofing materials have their own lists of contaminants, from heavy metals to organic matter to possible pathogens. Add to that dust, air-borne chemicals, and animal droppings, and the potential for collecting a soup of unknowns becomes real. Filters can help keep the big stuff out of cisterns and remove a lot of the smaller suspended or absorbed materials post-storage and before point-of-use, and additional UV treatment or other sanitation may be required, but purity and potability cannot be proclaimed, only tested.
This post is not intended to dampen (sorry) rainwater harvesting efforts, but rather to point out that what you hear or see may not always be the reality or even legal (approvable). In teaching rainwater harvesting and management best practices, a tremendous amount of information from codes, regulations, standards, guidelines and research has been looked at. Where rainwater is used for gardening purposes, the soil can act as a purifier through filtration and microbial activity. When used to meet potable demands, a sincere effort to meet or exceed water quality guidelines is essential, since it is public safety that is their first priority.
Task Force of Canadian Council of Resource & Envir. Ministers Guidelines for Canadian Drinking Water Quality, 1996. Ammend. Health Canada (2006); Canadian or B.C. Health Act Safe Drinking Water Regulation BC Reg 230/92, & 390 Sch 120, 2001.