Chloride concentrations in Lake Simcoe have increased significantly (P < 0.001) over a 36-year period during which the human population in the watershed has grown. Lake concentrations are now between 36 and 40 mg/L, having increased more than three-fold at the lake's outflow since 1971. Concentrations increased significantly in eight tributaries of the lake from 1993 to 2007 (P < 0.05), and were highest in those rivers draining the greatest percentage of urban land and roads, and in a river close to a major highway. The cumulative chloride load estimated at the mouths of seven rivers flowing into Lake Simcoe ranged from 11,563 to 32,107 tonnes/year from 1998 to 2007, and increased significantly over this period (P < 0.05). The fluxes or unit area loads of chloride, averaged from 2004 to 2007 for each of 10 tributaries, were positively correlated with the proportion of urban land and roads drained (P = 0.005, r = 0.80). Although Lake Simcoe is a large lake and only 12% of its watershed drains urban land and roads, evidence of road salt application can already be seen. This indicates that inputs must be reduced to preclude future ecological impacts on the lake.

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