Pesticide levels in rivers and streams across the United States are down, potentially reducing risks to human beings, but the dangers to wildlife remain, according to a new report.

The United States Geological Study carried out an investigation into pesticide pollution in American waterways that has lasted 20 years, from 1992 to 2011. Roughly 500 million pounds of chemicals are used each year in the United States to reduce damage done to crops by insects, increasing harvest yields. Over 400 varieties of insecticides are currently in use throughout the country.   

Some of the pesticides used by American farmers run off into the environment, potentially posing hazards to wildlife. Such levels in rivers and streams remained steady for most the 20 years of the study for waterways draining agricultural and mixed-use areas. However, water coming from urban regions showed increased concentrations of the toxins between the years 2002 and 2011.

During those years, the percentage of waterways testing positive for such concentrations were up to 90 percent from 53 percent recorded during the 20-year course of the study. Two insecticides, fipronil and dichlorvos, were responsible for most of the climb during the later years of the study, according to researchers.

Due to the great variety of insecticides in the environment, researchers studied just those which easily dissolve in water.

During the two decades of the study, pesticide levels rarely exceeded safe levels for human health. During the years 2002 to 2011, such high concentrations were only recorded once, in a single stream, draining an agricultural area.

Fipronil was the most common insecticide recorded at levels at or above levels considered safe for animals in the wild.

"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," Wesley Stone, hydrologist at the USGS, said.

Development of new insecticides, scientific research, and changing regulations constantly alter levels of the various chemicals in the environment.

"The information gained through this important research is critical to the evaluation of the risks associated with existing levels of pesticides," William Werkheiser, associate director for water at the USGS, told the press.

Fipronil is a broad-use insecticide that destroys the central nervous systems of insect pests. Dichlorvos, based on the atom phosphorus, is also used to protect stored food from the tiny pests.

Details of the 20-year study by the USGS investigating cocentrations of insecticides in American waterways was published in the journal Environmental Science and Technology.

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