Contribution of rooftop rainwater harvesting to climate adaptation in the city of Hannover: Water quality and health issues of rainwater storage in cisterns and ponds.

Rooftop rainwater harvesting systems and blue-green infrastructure are becoming important resilience alternatives for urban climate adaptation. This study sheds light on the largely unreported physicochemical and microbiological quality of private roof-harvested rainwater (RHRW). We aimed to identify the physicochemical and microbiological characteristics of RHRW, explore potential correlations between them and assess probable health risks associated with recreational interactions of children with the water. RHRW was collected from cisterns and ponds located in an inner courtyard in Hanover, Germany. Physicochemical parameters were measured on site and samples were collected once a month in two campaigns in 2020 and 2021. Escherichia coli concentrations ranged from 1 × 10° to 24.1 × 10 MPN/100 mL, Enterococci from 1 × 10° to 19.7 × 10 MPN/100 mL, Salmonella from 1 × 10 to 39 × 10 CFU/100 mL and Pseudomonas aeruginosa from 1 × 10° to 3 × 10 MPN/100 mL. Correlation analysis indicated potential relationships between bacteria, oxygen, and water temperature. The results of the health risk assessment indicated a potential risk of gastrointestinal illnesses due to exposure to Enterococci and Salmonella spp. present in the cisterns and ponds, highlighting the need for appropriate regulations and guidelines for RHRW aimed for non-potable uses. Blue-green infrastructure, when effectively managed and maintained, can offer benefits both by enhancing urban climate resilience and promoting citizens well-being.