Model Predicts Groundwater Atrazine Levels Unlikely to Exceed EPA Standards

USGS study used 20 years of data to create new statistical models

A new model predicts that atrazine and its breakdown product deethylatrazine have less than a 10% chance of exceeding the U.S. Environmental Protection Agency’s (EPA) standard for public drinking water supplies in shallow groundwater in about 95% of the nation’s agricultural areas. Atrazine is a commonly used herbicide for weed control in corn and sorghum production.

These findings are based on new statistical models developed from almost 20 years of nationwide water quality monitoring data collected by the USGS National Water Quality Assessment Program (NAWQA).

EPA’s Maximum Contaminant Level (MCL) of 3.0 µg/L for atrazine in public drinking water supplies is not a regulatory standard for shallow groundwater or domestic supplies, but serves as a benchmark for potential human health concerns. Predicted concentrations are compared to the MCL for atrazine in order to provide a perspective on potential significance to human health.

Additional findings from the study include:

• Concentrations of atrazine residues (atrazine plus deethylatrazine) in groundwater are strongly influenced by the history of atrazine use in relation to the time period that the sampled groundwater infiltrated through the soil and replenished groundwater supplies.
• The highest concentrations of atrazine residues were predicted for recently recharged groundwater in agricultural areas where substantial atrazine use is combined with natural conditions of permeable soils and high groundwater recharge. These conditions readily move water from the land surface to groundwater. Because of these factors, the largest area where elevated concentrations are predicted in shallow groundwater is in eastern Nebraska.
• Concentrations of atrazine residues are predicted to be lower across much of the Corn Belt, even in parts of Iowa, Illinois and Indiana, where atrazine is known to be applied in the greatest quantities. Soils in these areas tend to be poorly drained and often require artificial drainage through trenches and tile drains that capture soil water and divert it from groundwater to nearby streams.


Results of the USGS study, "Regression models for estimating concentrations of atrazine plus deethylatrazine in shallow groundwater in agricultural areas of the United States," are published in the Journal of Environmental Quality and are currently available online.

This study is part of the NAWQA Pesticide National Synthesis Project, which is a national-scale assessment of the occurrence and behavior of pesticides in streams and groundwater of the U.S. and the potential for pesticides to adversely affect drinking-water supplies or aquatic ecosystems.

U.S. Geological Survey

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