WASHINGTON, D.C. — A new study tracking rivers and streams throughout the United States for 20 years concludes risks to people from pesticide use has declined, but the risk to aquatic life has soared in urban areas.

Researchers for the U.S. Geological Survey (USGS) were unsure of the reasons behind increases in urban areas, noting they can’t track every type of pesticide and herbicide found in waterways, nor can they account for chemicals not generally connected with agriculture uses.

“Getting good data to explain the causal mechanism of this can be very difficult in urban environments,” said Karen Ryberg, a USGS scientist who co-authored the study with Wes Stone and Robert Gilliom. “We have pretty good data for agricultural uses, but they don’t have the same for urban areas” because sales of household products are not as closely tracked.

The study, published in the journal Environmental Science & Technology, monitored dozens of pesticides from 1992-2011 at more than 200 sampling points.

In both of the last two decades, researchers reported, they found insecticides and herbicides in virtually all of the waterways.

The study, conducted the research as part of USGS’ National Water-Quality Assessment Program, can be found online at www.usgs.gov

Results documented a significant decline in dangers to humans from pesticide pollution.

From 1992-2001, 17 percent of agricultural streams and 5 percent of other streams contained at least one pesticide whose average yearly concentration was above the maximum contaminant level for drinking water. But from 2002-11, the survey found dangerous pesticide concentrations in only one stream nationwide.

The decline occurred in part because new, less toxic pesticides have been developed, along with those that require less in fields to be effective, Stone said. Regulatory actions also banned or restricted how some of the more hazardous pesticides such as dieldrin and lindane have been used.

“It’s very clear in the data that regulatory changes in use do affect what you see in the streams,” said Stone, a hydrologist based in Indianapolis and the study’s lead researcher. “It’s showing what you would expect, and that’s good.”

The proportion of streams with one or more pesticides that exceeded an aquatic-life risk benchmark was similar between the two decades for streams and rivers draining agricultural and mixed-land use areas, but it was much greater during 2002-11 for streams in urban areas.

That’s where the aquatic-life benchmark increased from about 53 percent in the first decade to 90 percent in the second, even as other pesticides were phased out.

The main chemicals found were two pesticides, fipronil and dichlorvos. Fipronil is used in many products from flea collars to roach killers and was not included in the first decade’s surveys, but emerged in the second as an alternative to other pesticides whose uses were being restricted.

It was found to exceed potentially toxic levels for aquatic life in 70 percent of streams in the second decade. Byproducts of fipronil’s natural decay – longer-lived and more toxic than the insecticide itself – also were widely detected in urban streams, researchers noted.

“Levels of diazinon, one of the most frequently detected insecticides during the 1990s, decreased from about 1997 through 2011 due to reduced agricultural use and the U.S. Environmental Protection Agency’s regulatory phase-out of urban uses,” Stone said.