Atrazine, a member of the triazine class of herbicides, is primarily used in the United States to control broadleaf weeds and annual grasses in crops such as corn, sorghum, and sugarcane. It functions by inhibiting photosynthesis in plants, making it highly effective for pre-emergence applications. Despite being one of the most widely used herbicides, it is also one of the most frequently detected pesticides in U.S. surface water and groundwater sources. The continued presence of this chemical presents a persistent environmental challenge.
Primary Pathways of Contamination
The vast majority of Atrazine contamination originates from non-point source agricultural runoff, where it is applied to fields during the early spring planting season. Farmers typically apply the herbicide pre-emergence, meaning it is sprayed directly onto the soil before the crop or weeds sprout. This application timing is immediately followed by the period of highest risk for heavy rainfall and intense storm events.
When heavy rainfall occurs shortly after application, the herbicide is dislodged from the soil surface and transported via surface runoff into nearby streams, rivers, and drainage ditches. This mechanism bypasses the soil’s natural capacity to degrade the chemical, leading to a rapid influx of Atrazine into surface water systems. Non-agricultural uses, such as on golf courses or along rights-of-way, can also contribute to localized contamination.
Environmental Movement and Persistence
Once Atrazine enters the environment, its fate is governed by its relatively long half-life and its chemical properties. In surface soil, the half-life—the time required for half the applied chemical to dissipate—typically ranges from 14 to 109 days. This persistence allows the chemical to remain active long enough to be transported off-site, especially under cool, dry conditions where microbial degradation is slow.
Atrazine does not bind strongly to soil particles and possesses a moderate solubility in water, enabling it to move downward through the soil profile into aquifers. This process, known as leaching, is the primary source of groundwater contamination, where the chemical is far more persistent due to the lack of sunlight and low-oxygen conditions. In deep, anaerobic groundwater environments, the half-life can extend to nearly 578 days.
During its slow breakdown, Atrazine forms several metabolites, the most common of which is de-ethylatrazine (DEA). This breakdown product is often detected alongside the parent compound in both surface and groundwater. For regulatory purposes, the Environmental Protection Agency (EPA) treats DEA as having equivalent toxicity to Atrazine, meaning both compounds contribute to the total regulatory limit.
Geographical Hotspots and Prevalence
The prevalence of Atrazine in water is directly correlated with its use, making the Midwestern Corn Belt the primary geographical hotspot. States like Illinois, Iowa, and Nebraska, where corn is extensively cultivated, exhibit the highest concentrations and frequency of detection. The chemical’s presence is pervasive, being one of the most commonly detected pesticides in both the surface and groundwater across these agricultural regions.
Surface water systems, such as rivers and reservoirs in these areas, experience distinct seasonal spikes in concentration. These peaks occur consistently in the late spring and early summer, immediately following the peak application period and subsequent rainfall events. These short-term spikes in surface water can often exceed the federal drinking water standard before dilution and degradation reduce the levels. Groundwater contamination, in contrast, is more stable and persists year-round due to the slow movement and breakdown in underground aquifers.
Regulatory Standards and Monitoring
The presence of Atrazine in public water systems is governed by the U.S. EPA under the Safe Drinking Water Act. The Maximum Contaminant Level (MCL) for Atrazine in drinking water is set at 3 parts per billion (ppb). This standard is based on a running annual average, a calculation that can effectively mask the short-term, high-concentration spikes that occur seasonally in surface water sources.
Public water systems that draw from surface water in high-use areas are required to monitor for Atrazine, especially during the spring and summer application window. To mitigate the risk of runoff, the EPA has implemented various use restrictions, including mandatory application setbacks. Product labels require a 66-foot buffer zone where Atrazine cannot be applied near the points where field surface water enters perennial or intermittent streams and rivers. These regulatory measures are intended to reduce the initial load entering the water supply and ensure that the annual average remains below the federal limit.