Influence of Atmospheric Deposition and Roof Materials on Harvested Rainwater Quality

Rainwater harvesting has been made mandatory in some of the metropolitan cities in India, such as New Delhi, Ahmedabad, Bengaluru, Hyderabad, and Chennai, to meet the pressing demand for water. This study assesses the quality of wet precipitation and harvested rainwater samples in an urban area, located in Bengaluru city, by collecting and analyzing the samples for hydrogen ion concentration (pH), total dissolved solids (TDS), conductivity, cations, anions, and metals. Harvested rainwater samples were collected from roofs built of reinforced cement concrete (RCC), galvanized iron (GI), asbestos cement (AC), and galvalume (GLV) sheets. The wet precipitation samples reported a mean pH of 5.44—slightly acidic—a positive relationship between conductivity and TDS; bicarbonate and calcium were the predominant anion and cation in the ionic composition with descending pattern HCO3−>Ca2+>SO42−>NO3−>Mg2+ and presence of metals in particulate matter 10 μm (PM10). The concentration of harvested rainwater samples from roofs built of different materials was comparatively higher than the wet precipitation. The pH of the harvested rainwater from AC, GI, and GLV roofs was less than 6.5 (below drinking water standards). It is recommended to divert the first flush and avoid the impurities from the roof surface. Rainwater harvested from the RCC roof sample reported comparatively low metals and high anion and cation concentrations than roofs built of other materials, except for zinc and iron. Roofs built of materials such as GI and AC are prone to surface leaching action, resulting in further increase in metal concentrations in the harvested rainwater. Flat roofs, such as RCC, with coarse surfaces retain more dry deposition compared with inclined roofs, with smooth surfaces, such as AC, GLV, and GI. The harvested rainwater can be used by the consumer for groundwater recharge or storage for residential use, such as washing and bathing, by providing pretreatment after diverting the first flush. Treatment should focus on achieving drinking water standards, especially toward pH, metals, especially iron, in the harvested rainwater. Rainwater harvesting near industrial areas requires detailed study and assessment regarding the types of industries while designing harvesting systems.