The presence of the herbicide glyphosate in rivers, historically attributed to agriculture, may actually comes from a source closer to home, say scientists.

Glyphosate herbicides are some of the most widely used weed control chemicals, and until now, their use in farming was considered to be the main reason for presence of the compound in bodies of water worldwide. However, data shows that European glyphosate reduction measures have not led to any noticeable reduction in its presence in the environment.

A research team at the University of Tübingen in Germany has now found that most glyphosate that ends up in European rivers may actually be caused by household detergents - and chemical changes that take place at the wastewater treatment plant.

Professor Carolin Huhn and her colleagues analysed a wealth of data around glyphosate concentrations in rivers from authorities in France, Germany, Sweden, Italy, Luxembourg, UK, Netherlands and USA. Their hypothesis was that certain aminopolyphosphonates, used for example in laundry detergents, are converted to glyphosate in the sewage sludge of wastewater treatment plants.

"In soil and water, glyphosate is partly converted to aminomethylphosphonic acid - or AMPA. Both substances can be mobilised by precipitation and find their way into rivers," says Professor Huhn.

The starting point for the study was the observation that glyphosate concentrations in European rivers show a strong seasonality with high concentrations in summer and low concentrations in winter. Were agriculture the dominant source, the researchers say, increased concentrations would be expected in spring and autumn, the main application seasons, as well as after rain events.

"A survey of literature data shows that glyphosate is found in all the wastewater treatment plants examined in Europe, and its concentration was remarkably constant throughout the year," says Huhn.

The researchers say that the study showed that the main source of glyphosate and AMPA in rivers must come from wastewater from sewage treatment plants, and that there must be a source other than herbicides.

"The detailed analysis of the millions of individual values shows us that municipal wastewater plays a role, and some of the findings of the study also show us that we have to locate a source for glyphosate close to households," says Huhn. "However, everything we cannot explain about the data disappears if we assume that we are not really looking at glyphosate which was previously used as a herbicide, but instead at glyphosate that has formed from a larger substance."

Laboratory tests have confirmed that glyphosate formation in wastewater treatment plants comes from this laundry additive, and the results may explain why it has not been possible to reduce glyphosate pollution in rivers in Europe through herbicide reduction strategies. The new glyphosate source still requires verification in independent studies, say the researchers, but add that the authorities need to identify ways to monitor and regulate aminopolyphosphonates, or improve elimination rates in wastewater treatment processes.

The situation in the US was found to be different from that in the EU, with concentration patterns of glyphosate in US river water closely following those of other herbicides, indicating a dominant agricultural input. In contrast to Europe, aminopolyphosphonates are rarely used in the USA in laundry detergents.

"We are aware that our findings may have far-reaching implications, including for industry and policymakers but also for how surface water monitoring can be improved," says Huhn.

The study has now been published in the journal Water Research.