Mass balance model to quantify atrazine sources, transformation rates, and trends in the Great Lakes

A dynamic mass balance model for atrazine was constructed for the Great Lakes to estimate in situ transformation rates and the relative importance of sources. Annual atrazine inputs ranged from similar to 1 ton/yr in Lakes Superior to 10-25 ton/yr in Lakes Erie and Ontario. Annual inputs were <10% of lakewide atrazine inventories in all lakes except Erie. Tributaries plus connecting channel inputs account for >75% of the total load to Lakes Erie, Ontario, Huron, and Michigan. Atmospheric inputs account for 95% of the inputs to Lake Superior. Internal transformation rates are similar throughout the Great Lakes, ranging from similar to 0.05 to 0.13 yr(-1). Internal transformation and outflow produce an atrazine water column residence time of similar to 2 yr in Lake Erie and >5 yr in the other lakes. Approximately 5-10% of the lakewide atrazine inventory is lost annually through internal transformation.