This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: During periods of heavy rainfall, pipes known as storm overflows aim to prevent excess water from overflowing into the sewage system. But this is not always the case.
During intense rainfall, combined sewers (which carry both wastewater from homes and businesses and rainwater running off roads and buildings into drains) can exceed their capacity. To reduce the risk of sewage flooding homes, businesses and roads, storm overflow pipes can contain excess water. Then, when the time is right, these pipes can provide a controlled release point for excess water to flow into rivers and the sea.
These discharges are legally permitted under very specific conditions, mainly during very heavy rainfall when the sewer network would become overwhelmed. But public concern has grown over how frequently they are used and what effect they may have on the environment. To address these concerns, since January 2025, water companies in England have been required to publish near-real-time sewage discharge data via websites like Sewage Map or Waterwatch.
The aim was to improve transparency and allow the public, journalists, environmental organizations and researchers to better understand when sewage was entering rivers. For many people, particularly swimmers and other recreational water users, maps that show live discharges have become a way to decide if it's safe to enter the water. However, there is a problem: The data do not work in the way many people assume they do.
In December 2025, I began monitoring sewage discharges to investigate whether the information could help explain some of the microplastic pollution I was finding in the River Thames. I quickly realized that interpreting the data was far more complicated than expected. The issue was not only that the data changed over time, but that the information being provided did not answer the questions needed to understand environmental impact.
England's event duration monitoring (EDM) system records when a sewage overflow starts and stops operating. Essentially, it tells us how long a discharge appears to have lasted. But duration alone tells us very little about the pollution released.
Extract — continue reading at the source.
