Atrazine and nutrients in precipitation: Results from the Lake Michigan mass balance study

Atmospheric deposition of anthropogenic air pollutants contributes to degradation of water quality in the Great Lakes and other water bodies and is indicative of atmospheric pollution. In this paper, we discuss deposition of three air pollutants: atrazine; total phosphorus; and nitrogen (total Kjeldahl nitrogen and nitrate) to Lake Michigan. Throughout 18 months in 1994-1995, over 600 atmospheric samples (gas, particulate, and precipitation combined) were collected and analyzed for persistent organic pollutants and nutrients. Here the measurements and modeled deposition estimates are presented for atrazine, phosphorus, and nitrogen. Results indicate that concentrations of atrazine in precipitation have remained constant over 5 years (0.10-0.40 μg L^-1), consistent with the nearly constant sales rate of the herbicide over that time period. Actual loading of atrazine to the lake was less in 1994- 1995 (1.04 x 10³ kg y^-1) than in 1990-1991 (2.6 x 10³ kg yr^{- 1}). This difference in loading is due to lower overall precipitation in 1994-1995. Phosphorus concentrations in precipitation, on the other hand, have decreased from an average of 57 μg as P L^-1 in 1976 to an average of 6.36 μg as P L^-1 for 1994-1995. Nitrate deposition has decreased by a small, but not statistically significant, amount since the late 1970s.