A new report from environmental NGO Earthwatch Europe suggests that the Thames river basin district has the worst water quality in the UK, with 81% of measurements showing unacceptable levels of nutrient pollution, despite the Thames Tideway Tunnel becoming fully operational in February of this year.
The report - Great UK WaterBlitz Autumn 2025 Report - creates a comprehensive citizen-led snapshot of water quality across the UK based on a survey of 3,430 freshwater sites by 5,708 citizen scientists between 19-22 September 2025. Of the 3,430 surveys, 2,856 were collected in England; 200 were collected in Scotland; 208 measurements were taken in Wales; and 166 datapoints were gathered in Northern Ireland.
The findings show:
- England: 66% of its nutrient pollution is classified as unacceptable, while 34% is acceptable.
- Scotland: 32% is unacceptable, and 68% is acceptable.
- Wales: 27% is unacceptable, and 73% is acceptable.
- Northern Ireland: 27% is unacceptable, and 73% is acceptable.
The county of Northumberland has the best water quality, with 94% of measurements indicating good ecological status, while Cambridgeshire has the worst water quality in the UK, with 91% of measurements indicating poor ecological status.
The counties of Tyrone in Northern Ireland, Mid Glamorgan in Wales, and the four counties of Argyll and Bute, Ayrshire and Arran, City of Aberdeen and Dunbartonshire in Scotland have the best water quality, with 100% of measurements indicating low levels of pollution.
The data shows that every freshwater sample has additional chemical contamination evidencing pollution from sewage discharges and agricultural runoff.
Earthwatch says that of the 20 chemicals assessed, 12 of them are present at concentrations that pose risks to aquatic life. In addition, antibiotics are found at concentrations that suggest we consideration should be given to risk of antimicrobial resistance developing in our freshwaters.
Illicit drugs such as ketamine are also prevalent in waterways, although they pose limited risk to aquatic life.
The insecticide imidacloprid is found at high concentrations in some samples from across the UK; posing moderate to high risk to aquatics invertebrates which are vitally important for ecological health
EarthWatch says that as it continues to collect more data over repeated WaterBlitzes, the findings of previous WaterBlitzes are confirmed as a trend. Over four WaterBlitzes, England consistently has the worst water quality in the UK; this autumn 66% of sites tested had unacceptable levels of nutrient pollution.
Taking the average across all four WaterBlitzes, 34% of measurements in Wales and Northern Ireland, and 36% of measurements taken in Scotland indicate poor water quality. In comparison, 71% of measurements in England indicate poor water quality.
Click here to download the full Great UK WaterBlitz Autumn 2025 Report
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