- 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
It is estimated that atrazine is the most heavily used herbicide in the United States. Application to corn crops accounts for approximately 86% of total U.S. usage. Atrazine has been banned in the European Union since October 4, 2003 when the herbicide was not granted re-registration. The Lake Michigan Mass Balance Project (LMMBP) provided an opportunity to improve our understanding of atrazine transport and fate in a large freshwater lake, Lake Michigan. The modeling analysis also provided some information on the potential impact of the chemical to aquatic organisms in the lake. A rigorous, quality-assured, large supporting data set derived from samples collected in 1994-1995 was used to establish atmospheric and tributary loads, estimate initial conditions, and perform model calibration and confirmation exercises. Historical data collected outside of the LMMBP were also used to support the modeling effort.
The primary models developed at the United States Environmental Protection Agency’s Large Lakes Research Station to assess atrazine transport and fate in Lake Michigan included LM2-Atrazine and LM3-Atrazine. Both of these mass balance models utilized results from a hydrodynamic model to describe the lake’s physics. The period of 1982 to 1983 was used to calibrate the hydrodynamic models. During this time period, hydrodynamic conditions were not at any extreme. This is also true for the period of 1994 and 1995 when the water quality models were applied. Results from air and tributary models were used to provide atrazine loadings to the lake. Additional tributary loading estimates of atrazine to the lake were determined based on historical usage of the chemical in the basin and a literature-derived Watershed Export Percentage (WEP) of 0.6%.
LM2-Atrazine is a sophisticated and state-of-the-art toxic chemical fate and transport model for Lake Michigan. It is a revision of the USEPA-supported WASP4 water quality modeling framework. The processes modeled included advection, dispersion, decay, absorption, and volatilization. The transport fields that were output from the hydrodynamic 19-layered 5 km x 5 km gridded Princeton Ocean Model for the Great Lakes (POMGL) were aggregated and used by LM2-Atrazine. The LM2-Atrazine model had 10 surface water segments with at total of 41 water segments that included the vertical layers.
The results of the LM2-Atrazine system mass balance model show that the largest atrazine load to the lake is from the watershed. For the year 1994, it was estimated that 5,264 kg of atrazine were discharged to the lake via the tributaries. The second major load to the lake was from atmospheric wet deposition with a loading estimate of 2,493 kg. The greatest loss of atrazine from the lake was through transport to Lake Huron (2,531 kg) via the Straits of Mackinac. Loss due to internal decay (1,648 kg) was the second largest loss mechanism. The total inventory of atrazine in the lake was determined to be 182,779 kg in 1994. In this large, cold northern lake, the model suggests that in situ atrazine decay is very slow (0.009/year). This translates into an estimated atrazine half-life of 77 years.
The calibrated model was used to predict alternative futures that start in the year 2005. If constant loading and boundary conditions were imposed on model forecasts that are based on measurements made in the mid to late 1990’s, lake concentrations would be expected to increase until reaching a value of approximately 66 ng/L in 2263 (scenario 3). Eliminating all loadings to the lake (scenario 4) resulted in the largest decline in model predictions. A total loading reduction of approximately 35% (scenario 8), if implemented on January 1, 2005, would be needed in order to prevent atrazine concentrations from increasing further than what was estimated in the lake on January 1, 2005. If only the atmospheric loadings ceased (scenario 6), then concentrations in the lake would not be expected to change much after January 1, 2005, and the model-predicted concentrations in the lake would be expected to be only slightly higher than that predicted by scenario 8. However, if only the tributary loadings ceased (scenario 5), then atrazine concentrations in the lake would decline relative to scenario 3 predictions. Maintaining the vapor phase concentration at 0 ng/l (scenario 7) has very little effect compared to the constant condition scenario 3. This is intuitive because scenario 3 vapor phase concentrations are set to one-half the detection limit of atrazine in the vapor phase. One can conclude from the model that net volatilization of atrazine is negligible in Lake Michigan.
LM3-Atrazine is a high-resolution (44,042 cells and 19 sigma layers) model that provides a better spatial resolution of areas such as near and offshore zones, bays, river confluences, and the thermocline. The transport fields are provided by output from the Princeton Ocean hydrodynamics Model. Our high-resolution model, LM3-Atrazine, was primarily used to evaluate environmental exposure concentrations of atrazine in 5km x 5km model cells receiving loadings from the major tributaries to the lake where lake concentrations are expected to be the greatest. The modeled processes included advection, dispersion, decay, absorption, and volatilization. The atrazine decay (0.009/year) used in LM3-Atrazine was taken from the results derived from the hindcast run using LM2-Toxic.
Based on 1994-1995 measured loadings, the highest simulated concentration was 0.10 µg/L in a model cell near the mouth of the St. Joseph River. winter. Other model predictions are all lower. These predicted exposure concentrations in the lake are all below selected toxicological endpoints, including the most sensitive, phytoplankton primary production reduction. A water sample analyzed directly from the St. Joseph River (not a model result) in 1995 did exceed the phytoplankton primary production reduction criteria.
In comparing the results from the LMMBP atrazine models to other models in the literature, it is apparent that atrazine decays very slowly in large lakes that stratify in the summer months. The literature suggests that degradation of atrazine in small lakes and streams that are shallow and well-mixed can be significant. A hypothesis can be formulated that the decay in surface water is likely dominated by photolytic processes, either directly or indirectly. In lakes that stratify in the summer, atrazine in the hypolimnion is isolated from the intense solar radiation during the peak time of the year. Hence, atrazine in this layer of the lake receives little degradation. The LMMBP atrazine models differ from two other atrazine models recently applied to Lake Michigan. The main reason for the differences appears to be based on how they estimated tributary loadings – both used higher estimates of tributary loadings. Consequently, these other models predicted much faster in situ decay. Since tributary loadings are the major source atrazine to the lake, detailed assessments of these loads are very important.
Rygwelski, K.R., W.L. Richardson, and D.D. Endicott. 1999. A Screening-Level Model Evaluation of Atrazine in the Lake Michigan Basin. Journal of Great Lakes Research, 25(1):94-106.
Melendez, W., M. Settles, J. J. Pauer, and K. R. Rygwelski. 2009. LM3: A High-Resolution Lake Michigan Mass Balance Water Quality Model. U.S. Environmental Protection Agency, Office of Research and Development, National Health and Environmental Effects Research Laboratory, Mid-Continent Ecology Division, Large Lakes Research Station, Grosse Ile, Michigan. EPA/600/R-09/020, 329 pp.
Rygwelski, K. R. (ed.). 2008. Results of the Lake Michigan Mass Balance Project: Atrazine modeling report, EPA/600/R-08/111, 140 pp.
Feist, T.J., K.R. Rygwelski, W. Melendez, and W.L. Richardson. May 2005. Development and Application of a High-Resolution Atrazine Mass Balance Model for Lake Michigan. Presented at the 48th Conference on Great Lakes Research, International Association for Great Lakes Research, The University of Michigan, Ann Arbor, Michigan. May 23-27, 2005.
Rygwelski, K.R., T.J. Feist, and X. Zhang. November 2005. Forecasting the Transport and Fate of the Herbicide Atrazine in Lake Michigan Using Mass Balance Models. Presented at the Lake Michigan State of the Lake and Great Lakes Beach Association Conference, Green Bay, Wisconsin. November 2-3, 2005.
Rygwelski, K.R. and X. Zhang. May 2005. Forecasting the Concentration of the Herbicide Atrazine in Lake Michigan Under Various Loading Scenarios. Presented at the 48th Conference on Great Lakes Research, International Association for Great Lakes Research, The University of Michigan, Ann Arbor, Michigan. May 23-27, 2005.
Rygwelski, K.R. and R.G. Kreis, Jr. June 2003. An Overview of the Lake Michigan Mass Balance Project: Background, Accomplishments, and Future Work. Presented at the 46th Conference on Great Lakes Research, International Association for Great Lakes Research, DePaul University, Chicago, Illinois. June 22-26, 2003.
Kreis, R.G., Jr., K.R. Rygwelski, and W.L. Richardson. June 2002. Lake Michigan Mass Balance Project: Atrazine Modeling Results. Presented at the 45th Conference on Great Lakes Research, International Association for Great Lakes Research, University of Manitoba, Winnipeg, Manitoba, Canada. June 2-6, 2002.
Rygwelski, K.R., R. Rossmann, R.G. Kreis, Jr., and W.L. Richardson. May 2002. Linking Great Watersheds and Rivers to Forecast the Impact of Stressors on Large Receiving Waters. Presented at the U.S. Environmental Protection Agency Science Forum 2002: Meeting the Challenges, Washington, D.C. May 1-2, 2002.
Richardson, W.L., K.R. Rygwelski, X. Zhang, J.J. Pauer, and W. Melendez. June 2001. Models of Chloride and Atrazine in Lake Michigan at Two Spatial Resolutions. Presented at the 44th Conference on Great Lakes Research, International Association for Great Lakes Research, University of Wisconsin, Green Bay, Wisconsin. June 10-14, 2001.
Rygwelski, K.R. and W.L. Richardson. June 2001. The Loadings, Transport, and Fate of Atrazine in Lake Michigan. Presented at the 44th Conference on Great Lakes Research, International Association for Great Lakes Research, University of Wisconsin, Green Bay, Wisconsin. June 10-14, 2001.Rygwelski, K.R., W.L. Richardson, and D.D. Endicott. June 1997. A Screening-Level Model Evaluation of Atrazine in the Lake Michigan Basin. Presented at the 40th Conference on Great Lakes Research, International Association for Great Lakes Research, Great Lakes Center for Environmental Research and Education, Buffalo State College, Buffalo, New York. June 1-5, 1997.