By Ian Simpson

WASHINGTON (Reuters) - The proportion of urban streams in the United States with potentially worrisome levels of pesticides for aquatic life has surged to 90 percent, a two-decade government study said on Thursday.

Some of the more than 500 million pounds (220 million kg) of pesticides used yearly in the United States are concentrated at levels that pose a concern for fish and water-dwelling insects, the U.S. Geological Survey report on pesticides from 1992 to 2011 said. The levels seldom topped human health standards.

"We're at the stage of saying, 'OK, these (levels) compared to a benchmark indicate more evaluation need to be done,'" said Wesley Stone, a Geological Survey hydrologist and the study's lead author.

Ninety percent of urban streams had one or more pesticides exceeding an Environmental Protection Agency aquatic-life standard from 2002 to 2011. The proportion was 53 percent in the 1992-2001 decade.

Stone said the increased use in cities of the insecticides fipronil and dichlorvos was the main reason more urban streams were showing pesticide levels topping EPA benchmarks.

For streams and rivers draining agricultural land, the proportion topping the EPA standard fell to 61 percent in the 2002-2011 period from 69 percent in the previous decade.

The proportion of streams in mixed land use areas with one or more pesticides topping benchmarks was 46 percent in the 2002-2011 period, almost unchanged from the earlier period.

The potential for damage to aquatic life was likely underestimated since lack of resources meant researchers could monitor fewer than half of the more than 400 pesticides used agriculturally each year, the study said.

The report also only looked at pesticides that dissolved in water. The concentrations of individual pesticides have shifted since 1992 because of regulatory and market changes and the launch of new products, it said.

The report comes amid heightened concern by some environmental and consumer groups about contamination of drinking water by agricultural chemicals.

On Wednesday, a coalition of environmental health groups sued the EPA, asking a federal court to require a ban on chlorpyrifos, a pesticide that can interfere with brain development. The suit claims that people are exposed to it through drinking water and other means.


(Additional reporting by Carey Gillam in Kansas City, Missouri; Editing by Mohammad Zargham)