A recently published study by researchers at the U.S. Geological Survey (USGS) sheds light on how pesticide applications on land are impacting aquatic life in the nation’s rivers and streams.  The study compares pesticide concentrations across two decades to note trends in the temporal and geographic distribution.

The study, published in Environmental Science & Technology by USGS researchers Wesley Stone, Robert Gilliom, and Karen Ryberg, monitored pesticide concentrations in rivers and streams at 182 sites from 1992-2001 and 125 sites from 2002-2011.  Each of the sites were categorized as agricultural, urban, and mixed based on the dominant land use in the watershed using 2006 National Land Cover Data.  Agricultural watersheds could have no more than 10 percent urban land cover, and in watersheds where a small portion of the land contributed a majority of the water to a river or stream, the watershed classification was changed to reflect that portion’s land use.  You can read the full study from the USGS here.

Researchers used criteria outlined by the U.S. Environmental Protection Agency (EPA) to determine aquatic-life benchmarks.  For each pesticide, the EPA provides specific concentrations at which toxicity reaches acute and a chronic levels.  The EPA designates these benchmarks for fish, invertebrates, as well as nonvascular and vascular plants.  Concentrations are measured in micrograms per liter; for example, the acute aquatic-life benchmark for fish for atrazine is 2,650 micrograms per liter.

The study finds that, in both decades, pesticide concentrations exceeded aquatic-life benchmarks in the majority of “urban” and “agricultural” rivers and streams.  In fact, pesticide levels in 90 percent of urban streams exceeded one or more aquatic-life benchmarks in the last decade—a huge increase from just 53 percent the decade before.

In the first decade, 69 percent of agricultural rivers and streams had pesticide concentrations exceeding aquatic-life benchmarks.  In the second decade, this number dropped to 61 percent.   In mixed land use watersheds, concentrations increased slightly from 45 percent to 46 percent.

Among the most commonly detected herbicides in agricultural rivers were atrazine, deethylatrazine, and metolachlor, with metolachlor most frequently exceeding aquatic-life benchmarks.  In urban streams, the insecticide fipronil was a major culprit in the second decade, exceeding benchmarks in 70 percent of the urban water bodies monitored.  Fipronil was not measured in the first decade, but concentration of the insecticide in urban streams generally trended upward from 2001-2011.

After a peak in usage in the mid-1990s, pesticide applications began to rise again in the mid-2000s—a trend largely attributed by the authors to the rise in genetically modified crops.

With over half a billion pounds applied annually nationwide and more than 400 different pesticides used in agricultural settings during the study period, the researchers were only able to monitor for around half of all pesticides in use.

Click here to read the USGS’ press release on the pesticide study.