From roof to tap: Emerging public health risks in harvested rainwater.

As pressure on urban water resources grows, roof-harvested rainwater is increasingly used for drinking and domestic purposes. However, changing roof infrastructure and expanding vulnerable populations may be introducing under-recognised public health risks. This study assessed the microbiological and physicochemical quality of domestic roof-harvested rainwater across Adelaide and the Adelaide Hills, South Australia. One hundred rainwater tanks were sampled, and 97 households completed a survey on rainwater use, system characteristics, wildlife presence and rooftop solar infrastructure. Most households used rainwater for drinking (67%) and gardening (76%), and 62% had rooftop solar panels. Quantitative PCR detected enteric and opportunistic pathogens in a proportion of tanks, including Campylobacter jejuni (13%), Pseudomonas aeruginosa (12%), Salmonella spp. (10%) and Shigella spp. (3%), with pathogens also present in tanks used for drinking. Between 1 and 4% of tanks exceeded Australian Drinking Water Guideline health values for cadmium, iron, manganese, lead and arsenic, while 1-11% exceeded aesthetic values for zinc, manganese, aluminium and sodium. In unadjusted analyses, elevated metal concentrations were associated with solar panel presence (p = 0.004) and tank type (p = 0.004). However, multivariable analysis adjusting for tank material showed that households with solar panels had lower odds of elevated metal concentrations (aOR = 0.265, p = 0.026), while metal tank construction was associated with increased odds of elevated metal concentrations (p = 0.031). These findings indicate that domestic rainwater systems can act as reservoirs of microbial and chemical hazards with potential implications for human health. As climate change and water insecurity increase reliance on alternative water sources, understanding and managing these risks will become increasingly important. Further research is needed to examine how evolving roof infrastructure affects contamination pathways and emerging risks, including antimicrobial resistance, within domestic rainwater systems.