The Thames river basin has been identified as having the worst water quality in the UK, according to the latest Great UK WaterBlitz report from the environmental charity Earthwatch UK.

The nationwide survey, carried out by hundreds of volunteers, tested the health of rivers and streams across the country and findings for south west London make for "grim reading" believes Green Party's Councillor Chas Warlow.

According to the report, 81% of water samples taken in the Thames basin showed unacceptable levels of nutrient pollution, with every sample containing evidence of sewage discharge and agricultural run-off.

The basin, which includes the River Thames and its tributaries running through Richmond, Twickenham and Kew, was also found to contain worrying levels of chemicals including pharmaceuticals, antibiotics, illicit drugs and pesticides.

Of the 20 chemicals analysed, 12 were present at concentrations posing risks to aquatic life.

Antibiotics were found at levels that could contribute to antimicrobial resistance in the UK's freshwater systems.

Meanwhile, illicit drugs such as ketamine were also detected, though researchers noted these pose less of a direct risk to wildlife.

One of the most concerning findings was the high concentration of imidacloprid, a common insecticide used in tick and flea treatments for pets.

The chemical poses a "moderate to high risk" to aquatic invertebrates, which are vital for maintaining healthy river ecosystems.

Earthwatch UK's report calls on the government to strengthen regulations around pet treatments containing such chemicals and to discourage their blanket use.

Researchers attribute the Thames basin's poor results to two major factors: extensive agricultural land use, and chronic underperformance at Thames Water's treatment plants.

Samples revealed the Thames river basin contains worrying levels of chemicals (credit: Nub News).

More than half of the company's 351 sewage works reportedly lack sufficient capacity to treat current sewage volumes, with many of the worst-performing sites located upstream of London.

South Richmond's Cllr Warlow who participated in the WaterBlitz survey said the findings were "deeply troubling".

"I was genuinely shocked when I saw the high levels of pollutants in the samples I took," he said.

"Unfortunately, my results were not exceptions – they matched others throughout the Thames basin.

"Clean water is a public good, but private corporations owned by overseas investors have no incentive to act in our interest.

"The government must step up, renationalise our water companies, and clean up our waterways."

Campaigner Ian McNuff, from the Save Our Lands and River group, which opposes Thames Water's proposed Teddington Direct River Abstraction (TDRA) scheme, said the findings reinforced the need for stronger environmental accountability.

McNuff commented: "These results reflect just how badly we've treated our natural water bodies.

"Water companies should be environmental guardians, not polluters. We shouldn't have to fight them to take responsibility while they continue paying out huge dividends."

The Thames basin is home to millions of people and one of the country's most biodiverse river systems.

Local groups, including Friends of the River Crane Environment and Thames 21, have long warned that pollution and sewage discharges threaten both wildlife and public health.

The full Great UK WaterBlitz Autumn 2025 Report is available to read at earthwatch.org.uk and residents can explore results on an interactive map at freshwaterwatch.org.

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