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The map to the left illustrates the USGS and Lake Michigan Mass Balance Study sampling stations in the watershed. In 1995, extensive sampling was conducted around Lake Michigan. This included tributary and atmospheric sampling utilized to calculate a mass balance of pollutants affecting Lake Michigan. The St. Joseph River was identified as the largest tributary source of atrazine to Lake Michigan. The graph below illustrates the concentrations measured at the mouth of the St. Joseph River. The table below illustrates the concentrations and daily loads measured at the USGS sampling stations. See the bottom of this page for more atrazine information.

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  • Atrazine is the most widely used herbicide in the U.S., primarily in corn production.
  • Atrazine has a mild solubility and low Henry's Law constant, which makes it very susceptible to leaching by runoff.
  • Atrazine is applied in the field late April through May.
  • Atrazine degradation is slow in the water (only 0.8% per year in Lake Michigan).
  • Lake Michigan LaMP 2000 listed atrazine, a possible carcinogen, as an "Emerging Pollutant".
  • Lake Michigan LaMP 2002 continues to put atrazine on its "Watch List".
  • The Lake Michigan Mass Balance (LMMB) study, lead by EPA in 1994-1995, estimated that the St. Joseph River Watershed is the largest atrazine contributor to Lake Michigan at 602 kg/yr. However, (1) this estimate needs to be updated; (2) more monitoring data are needed to support the estimate (only 11 data points at the mouth of the St. Joe were collected in 1995 for the estimate), and; (3) no sub-basin level estimates are available.
  • The LMMB study, when modeling atrazine for Lake Michigan, used a "removal rate" (% of land application atrazine transported to the lake) approach to estimate tributary contributions. These removal rates were obtained from the literature were not calibrated in the St. Joe River Basin.
  • In the LMMB study, EPA also indicated that more analysis at the watershed level is required.
  • A USDA-ERS research report pointed out that the most cost-effective strategy to control atrazine leaching to surface and ground waters is to target atrazine management restrictions and alternatives (including partial atrazine ban, runoff control BMPs, and crop management, etc.) to meet water quality standards. This strategy requires the identification of those watersheds with atrazine water quality problems and hydrological conditions best suited for these management restrictions and alternatives.
  • The USGS, with funding from the State of Michigan, is conducting a study to look into the transport of atrazine in groundwater .