Sep 13, 2014 3 min read

The development of safer pesticides and legal restrictions on their use have sharply reduced the risk to humans from pesticide-tainted rivers and streams, while the potential risk to aquatic life in urban waters has risen, according to a two-decade survey published Thursday.

The study, conducted by the United States Geological Survey and published in the journal Environmental Science & Technology, monitored scores of pesticides from 1992 to 2011 at more than 200 sampling points on rivers and streams. In both of the last two decades, researchers reported, they found insecticides and herbicides in virtually all of the waterways.

The results nevertheless documented a striking decline in dangers to humans from pesticide pollution. From 1992 to 2001, 17 percent of agricultural streams and 5 percent of other streams contained at least one pesticide whose average annual concentration was above the maximum contaminant level for drinking water. But in the second decade, from 2002 to 2011, the survey found dangerous pesticide concentrations in only one stream nationwide.

The decline occurred in part because manufacturers introduced new pesticides that are less toxic or require smaller applications than older compounds. Much of it was driven by regulatory actions that canceled or restricted the use of particularly hazardous pesticides like dieldrin and lindane.

“It’s very clear in the data that regulatory changes in use do affect what you see in the streams,” said Wes Stone, a hydrologist with the Geological Survey in Indianapolis and the lead researcher on the survey. “It’s showing what you would expect, and that’s good.” Stone and the study’s other two authors, Robert Gilliom and Karen Ryberg, conducted the research as part of the Geological Survey’s National Water-Quality Assessment Program.

The use of insecticides dropped about one-third in the 1990s, mostly because of changes forced by regulatory actions, and remained more or less constant during the first decade of the 2000s. The opposite was true of herbicides, whose use was steady during the 1990s but then rocketed as the weed killer glyphosate became popular on farms and in gardens.

While human-health hazards declined over 20 years, the share of streams whose pesticide levels posed a potential threat to aquatic life remained mostly steady: Between 60 and 70 percent of agricultural streams and roughly 45 percent of streams in mixed-use areas, registered levels above the benchmark for potential harm to aquatic life.

Urban streams – the survey monitored 30 – were the glaring exception. There, the proportion of streams with pesticide levels above the aquatic-life benchmark soared from 53 percent in the first decade to 90 percent in the second, even as other pesticides were phased out.

The culprits, researchers found, were two pesticides, fipronil and dichlorvos.

Of course, there’s still work to be done.

Recently, activists traveled from Minnesota to Washington during a Paddle to DC expedition to highlight the threat of proposed sulfide mines on the edge of the Boundary Waters Canoe Area Wildnerness in Minnesota.