Can blue-green infrastructure mitigate waterborne infection risks through recreational activities in densely urbanized waterways?

Combined sewer overflows (CSOs) release pathogens into urban recreational water bodies and pose a threat to water quality, ecosystems, and public health. This risk is expected to increase with climate change, as more frequent and intense rainfall events are likely to exacerbate the number of overflows. Exposure to contaminants from CSOs can cause waterborne diseases, underscoring the need for effective stormwater management strategies. Blue-green infrastructure (BGI) offers a sustainable solution to mitigate the adverse impacts of CSOs while enhancing urban resilience through multiple co-benefits. This study combines hydrologic modeling with quantitative microbial risk assessment (QMRA) to assess the potential of BGI implementation strategies ranging from 0% to 50% of converted impervious surfaces, to mitigate the impacts of climate change on the microbiological quality and safety of urban rivers used for recreation downstream of CSOs. A strategy involving increased storage capacity by 28 000 m³ was also considered to compare its performance in terms of risk reduction with BGI implementation. The approach was applied to an Austrian urban river catchment frequently used for recreational activities such as swimming, wading, and playing. Three planning horizons were analyzed – baseline (C20), near-term future (NTF) and long-term future (LTF). Results show that BGI reduces the probability of infection across all seasons, with the highest benefit observed in summer when recreational water use peaks. For Cryptosporidium, the 95th percentile infection risk in a worst-case scenario (i.e., children swimming in the river) is reduced, when adding 50% of BGI, by 0.4 log₁₀ for the C20 period, 0.5 log₁₀ for the near-term future, and 0.6 log₁₀ for the long-term future, demonstrating the potential of BGI to improve the safety of recreational waters under changing climate.