Rooftop runoff contamination

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Ancient cistern at El Jadida, Morocco. Source: Axel Rouvin

Lead author of this article is Dennis Lye

Rooftop runoff contamination is receiving increasing attention as environmental and health issues, particularly as human demand for marginal potable water is outstripping supply. Numerous rooftop runoff rainwater collection systems as of 2011 are being constructed around the world for non-potable uses such as landscape irrigation, toilet flushing and clothes laundering. Collection of rooftop water has been conducted since ancient times, especially in Mediterranean (Mediterranean Sea) areas where rainfall is sparse.

Countries such as Germany, Denmark, India, Japan, and Australia are leaders in the installation of systems for the collection of rainwater from rooftops with storage areas and distribution lines within individual households (Albrechtsen, 2002). The national legislative bodies in many of these countries are drafting legislation requiring all new construction to incorporate rainwater harvesting systems for the purposes of flushing toilets and external water uses. The purpose of this type of legislation is threefold: 1) to reduce demand for treated water, 2) to alleviate future expansions to existing water supply infrastructures, and 3) to collect and use rainwater instead of surcharging it directly into stormwater management systems.


Codes and regulations

Rooftopcisternsystem.jpg Rooftop rainwater collection system. @ Bord na Mona

Plumbing codes may allow for dual plumbing lines within a building for both potableand non-potable water but decisions concerning water quality are often left to municipal permit departments and plumbing inspectors. Because of a general lack of experience in this area, there is reluctance by local agencies to approve plans utilizing rainwater collection systems for buildings which are also served by potable community water systems.

Water intended for ingestion (potable water) requires much more stringent guidelines for the levels of allowable contaminationthan water intended for non-potable uses. This article reviews current scientific literature addressing the effects that rooftop collections have on 1) rainwater and its subsequent runoff (Surface runoff) quality, 2) the concerns of water quality issues, 3) perceived health risks associated with this source of water, and 4) the best uses for the variety of rooftop catchment systems now being employed worldwide.

A number of studies have indicated that rooftop runoff can be a major contributor of heavy metals to aquatic ecosystems. A report by Good (1993) concerning runoff from sawmill rooftops along the coast of Washington in the northwestern United States characterized the runoff water as exceeding ambient water quality (USEPA-1986) guidelines for copper, lead and zincin all samples tested. Profiles of pollutionlevels in runoff events from rooftops in Bayreuth, Germany, were described by in a series of reports by Forster (1996; 1998; 1999) as containing high concentrations of pollutants at the beginning of precipitationevents with subsequent steep decreases as runoff continues (a first flush effect). Forster’s studies also identified that pollution levels of runoff from different roofs within a small area were highly variable, dependent upon seasons, and also quite variable within single precipitation events. Parameters influencing rooftop runoff investigations as identified in his reports are listed in Table 1. Despite the fact that many chemicals accumulate in living tissues over time, there are no reports concerning health risks associated with human contact, inhalationor ingestionof rainwater runoff containing chemical pollutants.

Water quality issues

The chemical quality of water stored in roof-harvested rainwater cisterns and used for domestic consumption in Bermuda was recently assessed by Peters et al. (2008). They analyzed a suite of chemicals from 112 private residences and found that only 1% exceeded leadstandards and only 3% exceeded nitratestandards for a primary drinking water source. The elemental concentration profiles of sediments from cisterns was similar to the profiles of local soils. They suggested that local soil deposited on rooftop catchment areas was a significant source of cistern sediment. This also suggested that dry deposition of material to rooftops was the predominant source of contamination in collected rainwater and the type of roofing material present was not a major source of contaminants in their roof-harvested rainwater.

A recent report by Signor et al. (2007) addresses what is known about microbial contamination risks associated with rainfall-induced surface/ground runoff and is an excellent companion to this review concerning rooftop runoff. Their review documents evidence that untreated rainwater collection systems may constitute a risk for contamination of public drinking water systems if proper guidelines concerning back-siphonage, leakage, and incorrect installation with cross-connections are not designed into each system. This is especially pertinent to community systems worldwide that do not maintain disinfection residuals throughout the water system.

A clear consensus on the quality and health risks associated with rooftop collection systems depends on the use of the water and the maintenance of the systems. Previous reviews have reported that properly designed, constructed, and maintained systems involving roof-harvested and tank-stored rainwater were of acceptable quality for drinking and cooking purposes (Dillaha and Zolan, 1985; Heyworth et al., 2006).

However, there have also been reports of rainwater systems not meetingstandards for consumption or even non-potable contact. Simmons et al. (2001) investigated one-hundred and twenty-five domestic rooftop rainwater systems in four rural Auckland districts. Samples from cold water faucets were analyzed for chemical and microbiological contaminants. Their studies suggested that rooftop rainwater was of relatively poor quality. Potential microbial pathogens such as Salmonella, Aeromonasand Cryptosporidiumwere identified in some of the rooftop collected rainwater.

In 2002, Lye reviewed the common occurrence of various pathogenic microorganisms reported in scientific studies sampling rainwater systems worldwide. The microbial risks associated with rainwater from rooftop collection could be attributed to diseases ranging from bacterial diarrhea and bacterial pneumonia to tissue helminth infestations when untreated rainwater was consumed.

From 1978 to 2006, there had only been six documented disease outbreaks associated with rainwater consumption worldwide (Heyworth et al., 2006). Three new outbreaks (two involving bacterialgastroenteritis and one involving bacterial pneumonia) outbreaks of have been reported in the short period since 2006.

Fewtrell and Kay (2007) recently reviewed the microbial quality of rainwater supplies in developed countries. Table 2 lists a selection of pathogens reported to possibly be present in collected and stored rainwater. They suggest that harvested rainwater supplies vary widely in terms of microbial quality and are unlikely to consistently meet standards currently set by developed countries.

A study of 102 households in Bermuda by Levesque et al. (2008) revealed a high frequency of fecal contamination of household tank rainwater. Their survey suggested that most preventive measures against water contamination appeared to be inefficient. As a result of their study, the Ministry of Health and Family Services in Bermuda has advised the public to incorporate multiple barriers to avoid contamination and infection from collected rainwater (Bermuda Department of Communication and Information 2004).


Guidelines for rooftop runoff collection systems are being developed throughout the world. Research studies such as those reviewed in this report have identified the parameters listed in Table 3 as particularly important for optimum performance of rainwater collection systems.

Risks of rooftop runoff contamination appear to be limited to those rainwater systems that do not have proper design, proper materials, proper treatment procedures, or adequate disinfection procedures. Policies concerning the best designs, materials, and most effective maintenance routines of rooftop collected rainwater systems need to be established for minimizing contamination of these water sources.

Large scale integration of rainwater catchment systems will require governmental and legal considerations. Such systems would represent a transfer of responsibility from local water authorities to property owners. Occupants of buildings that contain rainwater collection systems would assume legal liabilities for ownership, operation and maintenance of such systems.

Even when identical local codes have been followed, individual systems are still unique in some aspects. Well designed catchment systems will have to address fit-for-purpose water qualities. If design regulations do eventually become implemented they will be useful for the physical parameters of a system, but may not address the actual performance of each system. Maintenance requirements must be achievable and ensure the long-term success of rainwater systems integration into urban water systems.


This article has been reviewed in accordance with the U.S. EPA's peer and administrative review policies and approved for publication. Mention of policies, trade names, or commercial products does not constitute endorsement or recommendation for use by the U.S. EPA. The details of this review were published in the journal Science of the Total Environment, 407 (2009): 5429-5434.

Tables US EPA. 1986. Quality Criteria for Water. EPA 440/5-86-001 (Rooftop runoff contamination) {| width="744" border="1" | colspan="2" | Table 1. Factors that influence the water quality of roof runoff (Forster, 1996) Roof Material Chemical Characteristics, Roughness, Surface Coating, Age Catchment Parameters Size, Inclination, Exposure Precipitation Events Intensity, Wind, Duration Local Weather Season, Antecedent Dry Periods As well as chemical properties of pollutants. (Rooftop runoff contamination) Table 2. A selection of pathogens common to rainwater supplies in developed countries (Fewtrell and Kay, 2007, modified). Case fatality rate Pathogen Infection Transmission per 100,000 cases Campylobacter spp. gastroenteritis oral 5 Escherichia coli O157:H7 gastroenteritis oral 8.3 Legionella pneumophila Legionnaire’s Pontiac Fever inhalation inhalation 10,000 zero Mycobacterium avium complex respiratory gastroenteritis inhalation oral only in immuno- compromised Salmonella spp. gastroenteritis oral 41 Cryptosporidium spp. gastroenteritis oral 22 Giardia spp. gastroenteritis oral 1 (Rooftop runoff contamination) Table 3. Parameters important for optimum performance of rainwater collection systems. Proper design/sizing of all parts of the system Use of most appropriate materials in construction Exposed metal surfaces contribute heavy metals to the runoff contamination Coating of metal surface rooftop materials minimizes leaching Deposition of material from local environments is the predominant source of contamination for rooftop runoff The type of impervious roofing material used (ceramic tile, clay tile, asphalt) is secondary to the material deposited on roof catchments as a source of contamination Proper treatment/disinfection materials and procedures Regular schedules of maintenance for existing systems Intended use of the water Periodic testing of water quality Education and certification of individuals associated with the governing, approval, and usage of these types of systems See also Nonpoint source (Rooftop runoff contamination) ==Further reading== * Albrechtsen H-J. Microbiological investigations of rainwater and graywater collected for toilet flushing. Water Science and Technology 2002;46(6-7):311-316. * Bermuda Department of Communication and Information. 2004. Press release. Drinking your tank water - what you should know. Hamilton, Bermuda. * Dillaha TA, Zolan WJ. Rainwater catchment water quality in Micronesia. Water Research 1985;19:741-746. * Fewtrell L, Kay D. Microbial quality of rainwater supplies in developed countries: a review. Urban Water Journal 2007;4(4):253-260. * Forster J. Patterns of roof runoff contamination and their potential implications on practice and regulation of treatment and local infiltration. Water Science and Technology 1996;33(6):39-48. * Forster J. The influence of location and season on the concentrations of macroions and organic trace pollutants in roof runoff. Water Science Technology 1998;38(10):83-99. * Forster J. Variability of roof runoff quality. Water Science Technology 1999;39(5):137-144. * Good JC. Roof runoff as a diffuse source of metals and aquatic toxicity in storm water. Water Science Technology 1993;28(3-5):317-321. * Heyworth JS, Glonek G, Maynard EJ, Baghurst PA, Finlay-Jones J. Consumption of untreated tank rainwater and gastroenteritis among young children in South Australia. International Journal of Epidemiology 2006;35:1051-1058. * Levesque B, Pereg D, Watkinson E, Maguire JS, Bissonnette L, Gingras S, Rouja P, Bergeron MG, Dewailly E. Assessment of microbiological quality of drinking water from household tanks in Bermuda. Canadian Journal of Microbiology 2008;54:495-500. * Lye DJ. Health risks associated with consumption of untreated water from household roof catchment systems. Journal of American Water Resources Association 2002;38:1301-1306. * Peters AJ, Weidner KL, Howley CL. The chemical water quality in roof harvested water cisterns in Bermuda. Journal of Water Supply: Research and Technology – AQUA 2008;57(3):153-163. * Signor RS, Ashbolt NJ, Roser DJ. Microbial risk implications of rainfall-induced runoff events entering a reservoir used as a drinking-water source. Journal of Water Supply: Research and Technology – AQUA 2007;56(8):515-531. * Simmons G, Hope V, Lewis G, Whitmore J, Gao W. Contamination of potable roof-collected rainwater in Auckland, New Zealand. Water Research 2001;35(6):1518-1524. |}


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