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 Grea t Lakes to estimate in situ transformation rates and the relative impo rtance 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. Annua l inputs were <10% of lakewide atrazine inventories in all lakes excep t Erie. Tributaries plus connecting channel inputs account for >75% of the total load to Lakes Erie, Ontario, Huron, and Michigan. Atmospher ic inputs account for 95% of the inputs to Lake Superior. Internal tra nsformation rates are similar throughout the Great Lakes, ranging from similar to 0.05 to 0.13 yr(-1). Internal transformation and outflow p roduce an atrazine water column residence time of similar to 2 yr in L ake Erie and >5 yr in the other lakes. Approximately 5-10% of the lake wide atrazine inventory is lost annually through internal transformati on.