Toxic “forever chemicals” that are hazardous to humans and unborn children have been found polluting the Colne.

Industrial, agricultural, pharmaceutical and sewage waste were detected in the river’s water and sediment in an extensive analysis.

More than half of the hundreds of chemical pollutants found, including toxic “forever chemicals” which do not break down in the environment such as PFAS and PCBs, are classed as hazardous to humans.

Of these, 25 are known or suspected to cause cancer and 17 can be harmful to unborn children or damage fertility, researchers said.

Samples taken between 2019 and 2022 by members of the Colne Valley Fisheries Consultative from nine locations between Watford and the Colne’s source revealed the scale of the pollution that scientists said is typical of many urban rivers.

Affinity Water analysed samples and found 267 chemicals, including persistent organic pollutants, heavy metals and dioxins in the sediment.

While the levels were found to be low, experts are worried about the cumulative effect over time and the possibility of persistent, toxic substances like PFAS and PCBs seeping into aquifers used for drinking water.

Professor Andrew Johnson of the UK Centre for Ecology and Hydrology said “it is the dose that makes the poison” and the presence of toxic chemicals in low quantities is not necessarily harmful.

“There are many thousands of chemicals in rivers, the question is whether they are there in sufficient quantities to harm wildlife,” he said.

Watford Observer: Rediscovering the River Colne project volunteersRediscovering the River Colne project volunteers (Image: Watford Borough Council)

“The answer is generally not but some are higher risk than others.”

Affinity Water and Thames Water said they routinely monitor for at least 47 different compounds of PFAS.

Drinking Water Inspectorate guidelines say water companies should consult with health professionals and monitor PFAS levels in drinking water if they find 0.01 micrograms per litre (ug/L) and reduce concentrations “as soon as is practicable” for more than 0.1 ug/L.

Thames Water said it has five sites with PFAS levels higher than 0.01 ug/L but none above 0.1 ug/L.

A spokesperson from Affinity Water said: “The vast majority of the water sources that we use have untraceable concentrations of less than 50 ng/l (0.5 ug/L).

“We continually monitor all of our source waters on a risk-based frequency and drinking water is supplied to customers well within the standards set by the DWI.”