A new industry-backed study finds that concentrations of many harmful substances, including PFAS compounds, are decreasing, even as mercury levels continue to rise.
A long-term national monitoring program suggests that increased policy controls and regulations are beginning to permanently reduce the presence of chemicals in the UK’s water environment.
New data from the Chemical Inspection Program (CIP), co-ordinated by UK Water Industries Research (UKWIR), shows that concentrations of most high-priority pollutants measured in wastewater are falling. The results cover a four-and-a-half year monitoring period from 2020 to 2025 across England and Wales.
The analysis found a downward trend in 19 of the 20 substances examined, including some chemicals with long-standing environmental concerns, such as PFAS compounds and heavy metals. The findings provide new evidence that restrictions on industrial chemicals and changes in product manufacturing can significantly reduce pollution entering rivers and coastal waters.
Researchers involved in the program say the results demonstrate the importance of tracking pollutants over time and addressing their sources.
Jenni Hughes, Strategic Program Manager at UKWIR, commented: “Our data shows that the water industry is not a source of these chemicals and that with strict source controls, the environment can recover without the need for intervention by carbon-intensive and costly utility companies.”
“This evidence-based approach provides a blueprint for how the UK can tackle emerging compounds without neglecting end-of-pipe solutions that impose significant costs on claimants.”
Long-term monitoring of England and Wales
CIP was established in 2010 as a joint initiative between water companies, regulators and government agencies including the Environment Agency, Defra and Natural Resources Wales.
The program, currently in its fourth stage, focuses on identifying hazardous substances entering wastewater systems and understanding how those chemicals behave in treatment processes and the wider environment.
Our latest trends monitoring project looked at data collected over four years from sites across England and Wales. By applying consistent monitoring methods, scientists were able to compare changes in concentrations of substances considered to be of high regulatory concern.
The analysis found that many chemicals previously detected in wastewater are occurring less frequently or at lower levels. The results suggest that environmental regulations, including banning and phasing out products, are having a measurable effect.
Mark Craig, head of long-term asset strategy at Severn Trent Water and co-lead of wastewater research within UKWIR, said the value of the program was in producing reliable evidence on environmental trends.
He explained that consistent monitoring allows researchers and regulators to check whether measures designed to reduce chemical pollution are actually working.
PFAS and other pollutants show decreasing trends
Among the substances that have shown significant declines are tributyltin (TBT) and perfluorooctane sulfonate (PFOS), both of which have been widely scrutinized for their environmental persistence and toxicity.
TBT has historically been used in antifouling paints applied to ship hulls. This compound has become notorious for its harmful effects on marine life, especially as an endocrine disruptor.
Since the international ban introduced in 2008, environmental concentrations have decreased significantly. Monitoring results suggest that its presence in wastewater discharges continues to decrease due to the long-term ban.
PFOS, a member of the PFAS group commonly referred to as forever chemicals, also showed decreased levels in treated wastewater. These chemicals were previously used in products such as antifouling textiles and firefighting foam.
Although concentrations are decreasing, PFOS remains a concern in some catchments. Monitoring shows that water levels in certain rivers still exceed the environmental standards used in the European water regulatory framework.
Scientists believe that part of the reason for the continued presence of chemicals is their extreme persistence. PFOS can persist in soil and groundwater for decades and can slowly leach into water systems from historical sources such as landfills.
Mercury emerges as the main exception
While most chemicals analyzed in this study showed a downward trend, only dissolved mercury increased in concentration during the monitoring period.
Mercury contamination can arise from several sources. Natural geological processes release some mercury into the environment, but industrial activities such as fossil fuel combustion, mining, and waste incineration also contribute to emissions.
Elevated mercury concentrations highlight the complexity of chemical contamination in water systems, whose sources may extend far beyond the wastewater infrastructure itself.
Researchers say identifying these trends is essential for planning future environmental interventions. Monitoring allows regulators and water companies to detect which substances may cause violations of environmental standards and which river catchments are most at risk.
Evidence to guide future water policy
This monitoring program aims not only to track current pollution levels, but also to support evidence-based decision-making in the water sector.
By identifying chemicals most likely to cause future regulatory non-compliance, the data can help direct investigation and remediation efforts for specific locations. This targeted approach could allow water companies and regulators to prioritize investments where they have the greatest environmental impact.
The framework is designed to remain flexible as new contaminants emerge and scientific understanding evolves. Substances can be added or removed from the watch list depending on environmental risk assessment and regulatory priorities.
As the program moves into its next phase into the 2030s, researchers expect the scope of chemical investigations to continue to expand.
For policymakers and environmental scientists, the latest results offer cautious optimism. Chemicals in the UK’s water environment remain a significant challenge, but data suggests that tailored regulation, monitoring and improved source control measures can gradually reduce the presence of the most persistent pollutants.
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