In recent years, over-exploitation of natural water sources by anthropogenic activities has led to negative environmental effects, and, consequently, to a growing need for developing new sources of water. Pressure on natural water sources can be relieved by using alternative sources for uses that do not necessarily require potable water. One of these alternative sources is onsite rooftop rainwater, which may be used for toilet flushing, garden irrigation, laundry, car washing, etc. Harvested rainwater is used not only in areas where water supply is limited by climate or infrastructure, but recently also in well-developed, water-ample regions. It is driven by an increasing water demand and rising awareness of the negative environmental impacts of rainwater runoff, such as soil erosion and non-point source pollution [1
]. Onsite rainwater harvesting also serves as a means for reducing urban flooding and for increasing water supply, with minimal costs for storage and use-dependent treatment.
Although rainwater is generally considered as non-polluted, or at least not significantly polluted, it may be acidic and/or contaminated by dirt, organic micropollutants, metals, pesticides, etc., which affect the quality of rainwater runoff [4
]. Forster [5
] suggested several factors that influence the quality of roof-harvested rainwater: roof material (chemical characteristics, roughness, surface coating, age, etc.); physical boundary conditions (size, slope, direction, and exposure); location (proximity to pollution sources); chemical properties and concentration of the considered substance (vapour pressure, water solubility, etc.); precipitation event characteristics (rainfall intensity and depth, wind characteristics, pollutant concentration in the rain); and, local meteorological factors (season, weather characteristics, length of antecedent dry period). Taffere et al. [6
], in a study performed in Mekelle (Ethiopia), described other important factors that are related to air pollution. They indicated a clear effect of source-specific contaminants from traffic and industrial areas, while residential areas were found to be relatively free from immediate major pollutant sources.
The physicochemical quality of roof runoff, as reported by many studies, is quite similar to potable water quality guidelines, with the notable exception of pH values (pH of rainwater is 4.5–6.5, increasing slightly once on the roof [4
]). However, wide variations in concentrations of ions, like lead, calcium, magnesium, sodium, potassium, chlorides, sulphates, and nitrates were observed [4
]. These variations were reported to be a result of differences in roofing materials, orientation and slope of roofs, air quality in the region, and precipitation characteristics [7
]. For example, Forester [5
] compared rainwater runoff from similar roofs in different seasons and at six locations in Bayreuth, Germany. He observed large variations in ammonium concentrations that were measured during the same rain event in different locations (1.8–12.6 mg/L). The highest concentration was measured on a roof adjacent to an agricultural field. At the same location, large variations were also observed between seasons. Forester [5
] further indicated that similar patterns were also observed for chlorides. Chang et al. [9
], who studied rainwater runoff quality of four different roofing materials in east Texas, found that pH, EC (electrical conductivity), and Zn were significantly affected by the types of roofing material. In addition, they reported that Al, Mn, Cu, Pb, Zn, and pH concentrations in roof runoff exceeded the national quality standards in at least 5% of the rainfall events.
The microbial quality of roof-harvested rainwater often exceeds microbial quality standards, probably due to pollution originating from the excreta of animals (birds, rodents, etc.) that have access to roofs [3
]. Evans et al. [12
] indicated that local weather patterns, such as environmental conditions and wind speeds/directions, can significantly influence the microbial profile and loads in roof runoff. These authors also claimed that potential microbial risks that are associated with rainwater harvesting systems could be predicted by analysing weather patterns. Other studies pointed out that microbial quality of roof-harvested rainwater is strongly influenced by season, length of antecedent dry period, animal activities in close proximity to the roof, characteristics of rainwater storage tanks, and geographical location [4
Roof material and its features may also have a significant impact on rainwater runoff quality. Studies investigating roof-harvested rainwater quality were conducted in Australia, Canada, Denmark, Germany, India, Japan, Spain, New Zealand, Thailand, and the United States [2
]. Most of these locations are in temperate climate regions, where dry periods between consecutive rain events are relatively short. However, semi-arid/Mediterranean climates have been scarcely studied in this context. These climates are generally characterised by two distinct seasons: a long, completely dry, summer, and a short rainy winter with a limited number of rain events. These significant differences in weather conditions (rain intensity and depth, rain distribution, dry periods between consecutive rain events, etc.) may impact the quality of roof-harvested rainwater.
This research studied the quality of roof-harvested rainwater collected from three types of roofs in an urban Mediterranean environment in northern Israel. The study analyses the physicochemical and microbiological characteristics of the roof-harvested rainwater and its heavy metal concentrations. The study further estimates the association between harvested rainwater quality and weather characteristics, roof type, and selected air pollutants.
4. Summary and Conclusions
The quality of roof-harvested rainwater in a Mediterranean climate (northern Israel), characterised by dry summers and erratic wet winters, was studied on three experimental roofs (concrete, tiles, and steel-sheets). Twenty three quality parameters were analysed, including physicochemical ones, metals, heavy metals, and faecal coliforms (as indicators of microbial quality). Thirteen of the analysed parameters are not mentioned in local potable water quality regulations, although some may have health and/or aesthetic effects. Concentrations of most parameters that do appear in the regulations were below their maximum allowable limit. Turbidity and faecal coliforms levels in the harvested water were above maximum allowed values. Concentrations of most metals and heavy metals were very low, and Cd was the only one that did not comply with the regulations. This means that harvested water should be used only for non-potable uses.
The effects of 12 environmental and air pollution factors on each of the 23 quality parameters were assessed by multivariate linear regression, to quantify their impact. Five of the 12 factors were found to affect most quality parameters. O3 and PM2.5–10, measured during the day preceding the rain event, affected most of the physicochemical parameters and heavy metal concentrations, as well as microbiological quality (17 parameters each). Wind speed and length of preceding dry period were shown to have an effect on 15 and 14 quality parameters, respectively. Roof type significantly affected 15 harvested rainwater quality parameters, due to its structural characteristics and composition.
Regression correlation coefficients of the MLR models were quite high for some of the quality parameters, indicating that the explanatory factors explained most of their variability. On the other hand, regression correlation coefficients for other quality parameters were quite low, indicating that other factors (not examined in this study) may affect their concentrations. This deserves further investigation.
The study demonstrated that the quality of roof-harvested rainwater is affected by environmental conditions and air pollution. Many of the affecting parameters are specific to the region studied and to the roof materials that are used. Hence, further studies of this type are expected to enhance the knowledge needed for designing onsite rainwater harvesting systems in various locations.