During heavy downpours, combined sewer systems are not always able to handle the volume of rainwater and wastewater. The excess water is then discharged via overflows into ditches, canals and rivers. In the process, bacteria from wastewater also enter surface waters. These bacteria can ultimately reach beaches and swimming areas.
To gain a better understanding of these processes, the researchers developed an integrated ‘source-to-sea’ model. The model links sewerage systems, inland waters and coastal waters into a single continuous chain and shows how microbial contamination spreads through the water system.
Reliability tested
The model was applied to the Rijnland-Katwijk area. The study shows that it can reproduce bacterial dynamics on the beach with 89 per cent accuracy. Even extreme pollution events, which pose the greatest risk to swimmers, are predicted accurately.
The results show that the highest concentrations of E. coli are mainly caused by nearby sewer overflows following heavy rainfall. These short-lived events can have a major impact on water quality at popular swimming spots.
"The model also maps pollution peaks at downstream beaches, enabling the identification of hotspots and areas where pollution control measures can have the greatest impact."
- Hao Wang, water quality modelling
Why is forecasting important?
Climate change is increasing the likelihood of heavy downpours. As a result, sewer overflows will occur more frequently, leading to a greater risk of microbial contamination of bathing water. The new model helps public authorities and water managers to:
- better protect public health by issuing earlier warnings about contaminated bathing water;
- target investments in sewerage and water infrastructure more effectively;
- gain a clearer picture of the impacts of climate change on water quality.
Contribution to climate-resilient water management
A key advantage of this approach is that it looks not only at the source of the pollution, but at the entire system. This enables managers to better understand where pollution originates, how it spreads and which measures are most effective.
The model also offers possibilities for real-time predictions of water quality and for simulating future climate scenarios. In doing so, it supports water managers in developing climate-resilient solutions for urban and coastal areas.
The results of the research have been published in ‘Modelling peak microbial pollution events caused by combined sewer overflows in a source-to-sea system’ in the journal Environmental Pollution.