Rainwater Harvesting from Bitumen Roof for Managed Aquifer Recharge: First Flush Characterization and the Organic Carbon Challenge

Urbanization critically reduces groundwater recharge and elevates flood risks, making rainwater harvesting from rooftops (RWHR) for managed aquifer recharge (MAR) an essential strategy. While the technical implementation of MAR is well-established, our understanding of RWHR water quality and its subsequent impact on the hydrochemical status of groundwater remains ambiguous. A primary concern is the ‘first flush’ – the initial, often contaminated runoff after dry periods. This study defines the first flush and evaluates roof rainwater quality from bitumen-sheet covered flat roofs. In this research, we continuously monitored roof rainwater from three bitumen roofs in Israel, sampled groundwater, and conducted controlled rain simulator experiments on sun-exposed and shaded bitumen samples, analyzing the release of major ions, total organic carbon (TOC), total nitrogen (TN), trace elements (TE), and electrical conductivity (EC). The results show strong EC-inorganic solute correlations, enabling effective first flush identification. The first flush defined as the initial 10 mm of cumulative rainfall, and beyond this threshold, most pollutant concentrations significantly decrease, making ‘subsequent flow’ water suitable for MAR. Critically, TOC behaves differently; despite substantial reduction in subsequent flow, its concentrations remain two orders of magnitude higher than typical groundwater concentrations. Rain simulator tests confirmed high TOC release from sun-exposed bitumen, with low biodegradability, indicating TOC as a persistent concern.