Contaminants almost always found in US streams

An investigation of 20 of the US's largest river basins has shown that streams in important agricultural or urban areas almost always contain complex mixtures of nutrients and pesticides.

Reporting on tests undertaken between 1992 and 1995, the US Geological Survey has published “Pesticides in Streams of the United States – Initial Results from the National Water-Quality Assessment Program”. The report summarises and interprets results from more than 2,200 water samples collected at 58 sites.

It represents the most comprehensive data to date on the widest range of pesticides found in US streams.

Each sample was tested for 46 pesticides and pesticide degradation products. Results show that most streams contained low-level mixtures of pesticides for much of the year, with higher levels occurring seasonally. An average of 21 different pesticides were detected at each site, with more than 95% of samples containing one or more pesticide.

Concentrations rarely exceeded US drinking water standards, but, in the case of several pesticides, the guidelines established for protection of aquatic life were often exceeded. According to the report’s lead author, Steven Larson: “The effects of this exposure are largely unknown because current procedures for evaluating the potential effects of pesticides on aquatic organisms generally do not account for the presence of mixtures of different compounds.”

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