Overview of Outfall Safari
One of the major threats to water quality in urban rivers is misconnected pipes. These send pollution into rivers via the surface water drainage system and compromise the biodiversity and amenity value of our waterways. In partnership with Thames Water, the Environment Agency (EA), fellow NGO’s and communities in Greater London, the Zoological Society of London (ZSL) has developed the Outfall Safari citizen science method for locating, assessing the impact of, and reporting on these polluted surface water outfalls (PSWOs).
In dry weather, trained volunteers walk the riverbanks with a mobile app that allows them to geolocate, photograph and assess outfalls for evidence of pollution. The assessment gives outfalls a score between zero and 20. The higher the score, the greater the visible impact of the outfall – more points indicate more pollution. Data are sent directly to a database for analysis and reporting to the regulator and water company.
Suitability for monitoring purpose
| Monitoring purpose category | Likely tier(s) What’s this? | Suitability |
| Engagement: Education and raising awareness | 0-1 | ✓ |
| Surveillance: Ecosystem health screening | 1 (2) | ✓ |
| Investigation: Helping to target further action | 2 (3) | ✓ |
| Evaluation: Assessing the impact of actions | 2 (3) | ✓ |
Data system: No central data system currently (though CaSTCo is developing one in 2025). Templates are provided for local coordinating groups to set up survey recording forms in ESRI Survey123 and Epicollect for local sharing.
Cost: Significant. Guide and data collection templates are available free of charge. The main costs are time for local coordinating groups to plan and run the scheme, train and coordinate volunteers, interpret and share data and engage with regulators and water companies.
Ease of use: There is a manual and volunteer guide and training resources on the website, and the data capture form templates include guidance for volunteers.
Quality control: Local coordinators responsible for quality control
Coverage / scale: The scheme is available nationally but has only been implemented in selected catchments and regions, including selected catchments in the Thames, Trent and Mersey basins and the south coast of England, as well as selected catchments in Scotland.
Health & safety: The guidance manual covers the health and safety aspects of the survey. Different groups take different approaches to guidance regarding entering the water versus only surveying from the bankside.
Method sponsor/ owner / primary user: Zoological Society of London
Weblink: https://catchmentbasedapproach.org/learn/outfall-safari-guide/
CaSTCo Investment: CaSTCo is reviewing and investing in this method to optimise data quality, ease of use and cost, including rolling out training and investing in data systems.