Pollution in rainwater harvesting: A challenge for sustainability and resilience of urban agriculture

Water plays a pivotal role in urban agriculture. Although rainwater harvesting (RWH) is viewed as a sustainable route for crop irrigation at an urban setting, rainwater contaminants challenge the RWH practices due to potential threats to the health of crops, soil, and human (i.e., farm workers and food consumers). This frontier review article first recognized principal pollutants of irrigation concern in harvested rainwater based on literature data and crop irrigation demands. Major traditional pollutants, likely exceeding irrigation quality criteria, include particles, some toxic metals (e.g., Cd, Cu, and Zn), certain synthetic organic chemicals (e.g., agrochemicals and polycyclic aromatic hydrocarbons), and waterborne pathogens. Recent concerns have also been directed toward contaminants of emerging concern, such as per-and poly-fluoroalkyl substances, though the information regarding their occurrence and impacts on urban farming remains limited. This study subsequently identified challenges, knowledge gaps, and research needs for RWH irrigation at urban farms. Five aspects are highlighted, including the role of roof, nutrient management, development of RWH treatment tailored for urban crop irrigation, water quality monitoring, and the interactions between water, crops, and soil. The information and perspectives offer a basis to explore technical strategies of RWH irrigation for supporting sustainable and resilient urban agriculture.