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Lake Michigan Mass Balance Study - Atrazine
LMMB Study
Overview- The Issue: Are sediments, air, land, and water sources or pathways of contamination that affect the integrity of the ecosystem?
- What is the Lake Michigan Mass Balance Study?
- Sample Design and Sample Collection
- PCBs
- Atrazine
- Mercury
- Nutrients-Eutrophication
Lake Michigan Atrazine
Atrazine is one of the chloro-triazines, which also include simazine and cyanazine. Atrazine is a widely used herbicide for control of broadleaf and grassy weeds in corn, sorghum, rangeland, sugarcane, macadamia orchards, pineapple, turf grass sod, forestry, grasslands, grass crops, and roses. In the Lake Michigan basin, atrazine is used primarily on corn crops and is usually applied in the spring before or after emergence of the crop. Trade names for atrazine include Aatrex, Alazine, Crisazina, Malermais, Primatol, and Zeapos. Atrazine has been widely used in the agricultural regions of the Great Lakes basin since 1959 when it was registered for commercial use in the United States (USEPA 2001). Atrazine was estimated to be the most heavily used herbicide in the United States in 1987 to 1989 with heavy use in Illinois, Indiana, Iowa, Kansas, Missouri, Nebraska, Ohio, Texas, and Wisconsin (Figure 7 ). Peak total annual U.S. usage of atrazine occurred in 1984 at 39.9 million kilograms. Usage has been dropping since then and was estimated at 33.8 million kilograms in 1995.
Unlike PCBs, the herbicide atrazine does not bioaccumulate in organisms but does remain in the water column. The two most important atrazine loads to Lake Michigan are tributaries and wet deposition (rain and snow). Historical loading estimates of atrazine from both tributaries and wet deposition to Lake Michigan are depicted in Figure 8 . Decreases in loadings from the tributaries are evident starting in 1985. A decreasing trend of loadings from the atmosphere in the form of wet deposition is not as evident. All of the estimates of tributary loadings assumed that 0.6% of the applied active ingredient (atrazine) reached Lake Michigan. This 0.6% is often referred to as the Watershed Export Percentage (WEP). Tributary loadings for 1989, 1992, 1993, 1994, 1995, and 1998 were based on actual records of amounts applied per each county in the basin, and calculating what portions of the amount applied in those counties falls within a Lake Michigan Hydrologic Unit Code area that eventually drains into the lake. Tributary loading estimates for other years depicted were based on total annual U.S. usage for those years. For 1991, 1994, and 1995 wet deposition load estimates were based on actual precipitation data collected in the basin. Wet deposition loading estimates for other years were based on total annual U.S. usage for those years. Atmospheric loadings to the lake are higher in the southern portions than in the northern areas. The higher loadings in the south are likely due to the close proximity of this area to corn growing regions in the southern basin (Rygwelski et al 1999).
Tributaries are the most significant source of atrazine to the lake. Figure 9 illustrates atrazine loadings from the eleven major rivers monitored from the LMMB Study (Hall 2000). The largest load of atrazine to the lake in 1994 and 1995 was the St. Joseph River followed by the Grand River.
In order to understand the impact of the atrazine loadings to Lake Michigan, a modeled mass balance was developed from the LM2 model (Figure 10). From these model results (Rygwelski et al. 1999; Rygwelski et al 2006), one can note that the largest load to the lake is from the watersheds, followed by wet atmospheric deposition. Dry deposition to the lake is negligible. Input from Lake Huron and atmospheric absorption to the lake’s surface are modest. The largest flux out of the system is the gross export to Lake Huron through the Straits of Mackinac. Export through the Chicago diversion and loss to the atmosphere through volatilization are small. In water, atrazine is primarily in the dissolved state and, therefore, any processes that involve sediment or suspended particle interactions are of minor significance.
The results from the modeling effort indicate the primary sources and pathways of atrazine within Lake Michigan. It also indicates that atrazine in water is decaying only at an estimated rate of less than 1% of the total water column inventory. The literature suggests that atrazine decay is moderately rapid on soils and is can be at a moderately fast rate in shallow, warm freshwater systems that have high suspended solids, high dissolved organic carbon, low pH, and high concentrations of nitrate ions. The cold, deep, high pH, oligotrophic waters of Lake Michigan, together with a long retention time, do not appear to support considerable decay of atrazine.
Long-term simulations under various loading scenarios from LM2 are depicted in Figure 11. The constant load scenario (all loadings set at the 1998 loading level) indicates that the lake wide concentration continues to increase fairly rapidly through the end of the century and levels to 66 ng/L after the year 2200. This scenario can be regarded as the no action scenario. To maintain the lake concentration observed at the present (no further degradation), the second scenario indicates that a total load reduction (tributary and atmospheric) of 35% would be required. Two additional scenarios are also provided which show the response of Lake Michigan to 100% reductions in tributary and total loads, respectively. These scenario concur with the previous finding of tributary and atmospheric load importance.
Results of LMMB atrazine measured data and modeling forecasts is compared to effects thresholds in Figure 12. Note that the thresholds are on a logarithmic scale and that additional effects thresholds are known but are at greater values than those presented in the comparison (USEPA 2005) . The comparisons indicate that measured and forecasted lakewide concentrations of atrazine all fall below the presently known effects thresholds. However, one measured concentration in the St. Joseph River in 1995 was greater than the threshold for phytoplankton production.
LMMB Major Findings: Atrazine
- Observed and forecast lake-averaged concentrations of atrazine are well below EPA biological effects thresholds.
- Tributaries are the major source of atrazine to the lake.
- Atrazine is very persistent in Lake Michigan – decay is estimated at less than 1% per year.
- Atrazine concentrations are forecasted to increase in the lake under present loads (1994-1995 constant load).
Figure 7. Atrazine use - kilograms per square kilometer (U.S. Geological Survey, 1991) (more information about this image).
Figure 8. Historical tributary and wet deposition loadings of atrazine to Lake Michigan (more information about this image).
Figure 9. Atrazine loads (kg/yr) to Lake Michigan from major monitored and unmonitored tributaries, 1994-1995 (more information about this image).
Figure 10. Lake Michigan atrazine mass balance (including Green Bay), 1994 (more information about this image).
Figure 11. Lake Michigan atrazine forecasts (LM2-Toxic Model) (more information about this image).
Figure 12. Atrazine effects thresholds compared to observations and model predictions. (more information about this image).