By Bryanna Poczatek

Rainwater can be a sustainable source of water for many applications. But water treatment professionals need to consider how to test for and treat a variety of contaminants that may have been picked up, as the rain falls through the sky or when it is collected and stored. Microbiologicals, metals and organics are among the big concerns that need to be addressed, especially for potable applications.

Rainwater catchment (also known as rainwater harvesting) is the deliberate containment of rain or other natural precipitated water with the intention of controlled discharge or beneficial use in potable or non-potable applications.1 Potable uses of rainwater include drinking, cooking, handwashing, dishwashing, bathing or any other purpose that could result in the ingestion of water or water touching the skin. Non-potable applications for rainwater include watering lawns and gardens, washing clothes and flushing toilets. The most widely used form of rainwater catchment is roof-based catchment, where rainwater is collected as it falls on the roof of a home.2

Rainwater catchment has been used as a source of water for thousands of years, but in recent years there has been a renewed interest in the practice. Reasons for this include a greater awareness and concern about water quality issues, the rising costs of water from centralized treatment and private wells, and the cost efficiencies associated with rainwater harvesting.3 Rainwater harvesting can also be an economic solution for areas where drinking water is scarce. Rainwater, however, is at risk for picking up contaminants. The risk of microbiological contamination in rainwater has been well-studied and a standard has been developed to address these concerns.4 But research has shown that in addition to microbiological contaminants, other contaminants may be present as well. Rainwater can be a sustainable source of high-quality water, but before harvesting, it is important to consider all contaminants that might be present. Water intended for potable applications will require a higher level of treatment than rainwater intended for non-potable applications.

Contaminants that might be present in rainwater include microbiologicals (parasites, bacteria, viruses), metals, organic chemicals and debris. These vary in the pathway by which they are picked up by rainwater. The main pathways are atmospheric deposition, leaching and weathering of roof materials, fecal contamination and leaching of materials that occurs via the storage and conveyance system. Other factors that impact the presence of contaminants in rainwater include weather conditions, materials used in catchment systems, land-use practices and roof characteristics such as materials, design, age and condition. For example, urban/industrial areas tend to have higher heavy-metal contamination due to fossil-fuel combustion and other emissions, while catchment systems in agricultural areas tend to have higher concentrations of pesticide residues, fertilizers and other organic chemicals. Microbial and fecal contamination tend to be higher in rural catchment systems.2,5,6,7,8,9

Microbiological contaminants are the most well-studied and can include both non-pathogenic and pathogenic organisms. Non-pathogenic organisms do not cause disease or illness but they can reduce the aesthetic quality of water. Pathogenic organisms cause disease and can contaminate rainwater if the collection or storage of the rainwater has been contaminated by fecal material. Pathogens than have been found in harvested rainwater include Giardia Lamblia, Cryptosporidium parvum, Toxoplasma gondii, Campylobacter spp., Salmonella spp., E. coli, Shigella spp., Pseudomonas spp. and Hantavirus spp.4,8,10

Chemical contaminants can also be found in rainwater. Rainwater can become contaminated by absorbing airborne chemicals (especially in urban areas); however, most of the chemicals in rainwater are picked up during collection and storage. Volatile organic chemicals (VOCs) can be introduced to rainwater when raindrops fall through air containing gasoline or solvent vapors, or when the water encounters materials containing plastics, glues, solvents, gasoline, greases and oils. This typically occurs when the materials used to construct the catchment and storage system were not designed for use with drinking water. Synthetic organic chemicals (SOCs) are chemicals found in pesticides, herbicides and other man-made products. SOCs can be introduced when dust and leaves enter a catchment system or when the rainwater is collected near an agricultural area where application of these chemicals occurs.4,8

Metals such as lead, arsenic and copper can also be picked up by rainwater. Metal oxides which may be airborne in industrial areas may be absorbed by rainwater, but more commonly metals are introduced when rainwater comes into contact with lead solder, iron and copper pipe and brass fittings. These metals can be picked up from metallic roofs, catchment systems and storage tanks. Rainwater is slightly acidic and as a result, tends to be aggressive and corrosive toward these materials.4,8,11

Debris commonly found in rainwater includes leaves, twigs, dust, dirt, bird and animal droppings, insects and other visible material. Debris is usually picked up in rainwater as the rainwater is collected on the roof, gutters, etc. Often debris only reduces the aesthetic quality of water, but debris can leach chemical contaminants like herbicides and pesticides (leaves and dust) as well as parasites, bacteria and viruses (bird and animal droppings).4

When designing a rainwater catchment system, one should consider using materials and components that have been tested for that application. NSF testing protocol P151 establishes testing guidelines for products like roofing materials and coatings to ensure they do not add contaminants into the water at levels that exceed US EPA health guidelines.12 Using products in your rainwater catchment system that have been tested according to this protocol can help ensure a higher quality of rainwater.

Rainwater can be a sustainable source of water for many applications, but before harvesting and using rainwater, especially for potable applications, water treatment professionals should consider testing for microbiologicals, metals and organics and implement additional treatment as needed. It is also important to carefully select materials for the catchment system that have been tested for that purpose. Please note: It is important to understand that local regulatory rules for rainwater catchment, and the subsequent use of rainwater as a drinking water source must be followed despite any contrary guidance offered within this document.


  1. Water Quality Association. (2018). WQA Glossary of Terms. Retrieved from
  2. Texas Commission on Environmental Quality (TCEQ). (2007). Harvesting, Storing, and Treating Rainwater for Domestic Indoor Use. Austin, TX.
  3. American Rainwater Catchment Systems Association (ARCSA). (2012). Rainwater Harvesting: The Forgotten Resource [Brochure]. Tempe, AZ.
  4. American Society of Plumbing Engineers. (2013). ARCSA/ASPE/ANSI 63-2013: Rainwater Catchment Systems Standard. Retrieved from
  5. Radaideh, J., Alzboon, K., & Al-harahsheh, A. (2009). Quality Assessment of Harvested Rainwater for Domestic Uses. Jordan Journal of Earth and Environmental Sciences,2(1).
  6. Ahmed, W., Sidhu, J. P., & Toze, S. (2012). An Attempt to Identify the Likely Sources of Escherichia coli Harboring Toxin Genes in Rainwater Tanks. Environmental Science & Technology,46(9), 5193-5197. doi:10.1021/es300292y
  7. Huston, R., Chan, Y., Chapman, H., Gardner, T., & Shaw, G. (2012). Source apportionment of heavy metals and ionic contaminants in rainwater tanks in a subtropical urban area in Australia. Water Research,46(4), 1121-1132. doi:10.1016/j.watres.2011.12.008
  8. Gwenzi, W., Dunjana, N., Pisa, C., Tauro, T., & Nyamadzawo, G. (2015). Water quality and public health risks associated with roof rainwater harvesting systems for potable supply: Review and perspectives. Sustainability of Water Quality and Ecology,6, 107-118. doi:10.1016/j.swaqe.2015.01.006
  9. Bae, S., Maestre, J. P., Kinney, K. A., & Kirisits, M. J. (2019). An examination of the microbial community and occurrence of potential human pathogens in rainwater harvested from different roofing materials. Water Research,159, 406-413. doi:10.1016/j.watres.2019.05.029
  10. Lye, D. J. (2002). Health Risks Associated With Consumption Of Untreated Water From Household Roof Catchment Systems. Journal of the American Water Resources Association,38(5), 1301-1306. doi:10.1111/j.1752-1688.2002.tb04349.x
  11. World Health Organization. (2011). Guidelines for Drinking Water Quality, 4th Edition. Retrieved from
  12. NSF International. (2019). Rainwater Collection. Retrieved from

About the author
Bryanna Poczatek is Technical Affairs Coordinator at the Water Quality Association (WQA), which she joined in 2016 as an Associate Project Leader in the Product Certification Department. In early 2017, Poczatek transferred to the Technical Affairs department and works on various technical projects. She represents the industry as a voting member of the Water Equipment & Policy Research Center Industry Advisory Board, funded through the National Science Foundation (WEP IUCRC). Poczatek also participates in numerous industry committees and task forces through NSF International and other organizations. Prior to joining WQA, she worked as a Project Coordinator for Syngenta Crop Protection LLC and as a Research Assistant in the Biochemistry, Biophysics and Molecular Biology Department at Iowa State University. Poczatek holds a Bachelor’s Degree in biology from Iowa State University.

About the organization
WQA is a not-for-profit international trade association representing the residential, commercial and industrial water treatment industry. WQA is a resource and information source, a voice for the industry, an educator of professionals and a laboratory for product testing.


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