Six Persistent Herbicides Found in Wells Across USA

Results for one of the first national studies on the presence of herbicides in groundwater is published by the U.S. Geological Survey in the September-October issue of the "Journal of Environmental Quality."

One goal of the study was to track the retention of various types herbicides and pesticides used over the years.

"The results of this study are encouraging for the future state of the nation’s groundwater quality with respect to pesticides," said hydrologist Laura Bexfield of the U.S. Geological Survey's New Mexico Water Science Center, who conducted the data analysis.

The study is a part of the National Water Quality Assessment Program, federally funded and conducted by the USGS. It aims to provide an understanding of water quality conditions and how those conditions may vary locally, regionally, and nationally; whether conditions are getting better or worse over time; and how natural features and human activities affect those conditions.

Artesian well in Missouri (Photo by Alaina Culbertson)

Changes in pesticide detection frequency and concentrations in groundwater might be expected to have occurred on a regional or national basis within the United States during at least the past 10 to 20 years as a result of targeted use restrictions and chemical bans and the introduction of new pesticide compounds.

Bexfield's data analysis evaluated samples collected from a total of 362 wells located in 12 local well networks characterizing shallow groundwater in agricultural areas and six local well networks characterizing the drinking water resource in areas of variable land use.

Each well network was sampled once during 1993-1995 and once during 2001-2003. The networks provide an overview of conditions across a wide range of hydrogeologic settings and in major agricultural areas that vary in dominant crop type and pesticide or herbicide use.

Laboratory analysis was performed using methods that allowed detection of pesticide compounds at concentrations as small as 1,000 times below EPA drinking water standards.

Of about 80 pesticide compounds analyzed, only six compounds were detected in groundwater from at least 10 wells during both sampling events.

These six pesticide compounds were the triazine herbicides atrazine, simazine, and prometon; the acetanilide herbicide metolachlor; the urea herbicide tebuthiuron; and an atrazine degradate, deethylatrazine.

Concentrations of these compounds generally were less than 0.12 parts per billion, or more than 10 times lower than applicable EPA drinking water standards.

But exposure to pesticides at doses below EPA standards does make a difference to human health, says Alex Formuzis, communications director of the Environmental Working Group. The Washington, DC research and lobbying organization has focused on contaminants in U.S. drinking water.

"Toxic pesticides or pharmaceuticals in drinking water are unacceptable at any level," he told ENS in an interview. "Low dose exposures do matter. People continue to buy the industry spin that low doses don't matter. The opposite is true."

"The therapeutic level of prescriptions specified by doctors is at parts per billion, and that low concentration has the desired effect. Low doses matter," said Formuzis.

"People are drinking this water every day, using it to prepare food, and every time they use it, they're being exposed to pesticides," he said.

One of the chemicals found to persist in the test wells is atrazine, the most commonly used herbicide in the United States.

Bexfield says in her paper that because national use of atrazine has not changed substantially in the Unites States during the past 10 to 20 years, the decrease in atrazine and DEA concentrations could reflect the reduction in atrazine application rates starting in the early 1990s.

To decrease the presence of atrazine in surface and groundwater, atrazine application rates were reduced in the early 1990s. Bans have been placed on atrazine in some areas.

Sprayed on farm crops, atrazine may wash from soil into streams or groundwater where it will stay for a long time, because breakdown of the chemical is slow in water, according to the federal Agency for Toxic Substances.

Birth defects and liver, kidney, and heart damage have been seen in animals exposed to high levels of atrazine. There are limited human and animal data that suggest that there may be a link between atrazine exposure and various types of cancer, the agency says.

"Atrazine was banned in Europe in 2004 because of persistent groundwater contamination," said Formuzis. "In the United States, 80 million pounds are applied every year."

"A number of top scientists in this country have called for its ban has not happened," he said.

Bexfield said, "Despite sustained use of many popular pesticides and the introduction of new ones, results as a whole did not indicate increasing detection rates or concentrations in shallow or drinking water resources over the 10 years studied."

Characterization of large-scale trends could help determine the effectiveness of efforts aimed at minimizing groundwater contamination, identify compounds that are of increasing concern, and estimate time scales over which changes might affect water quality, Bexfield wrote in her study.

Learning more about these trends is important in determining how quickly ground water systems respond to changes in pesticide use and land management practices, Bexfield wrote, and in identifying compounds that may pose a threat to water quality before large-scale problems occur.

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