Rooftop runoff contamination
[The 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 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
Plumbing codes may allow for dual plumbing lines within a building for both potable and 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 contamination than 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 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 zinc in all samples tested. Profiles of pollution levels 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 precipitation events 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, inhalation or ingestion of 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 lead standards and only 3% exceeded nitrate standards 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 meeting standards 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, Aeromonas and Cryptosporidium were 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 bacterial gastroenteritis 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.
As well as chemical properties of pollutants.
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- Bermuda Department of Communication and Information. 2004. Press release. Drinking your tank water - what you should know. Hamilton, Bermuda.
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- 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.
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