The rapid expansion of urban land cover is associated with negative impacts on stream ecosystems. Elevated specific conductance (SC) and major ion concentrations are increasingly documented in urban streams. However, the degree to which nonpoint sources contribute to elevated ion concentrations is unclear. We characterized SC and major ion concentrations in five small watersheds along a forested-tourban gradient with impervious surface cover (ISC) ranging from 0−25%, no major point sources such as wastewater treatment plants, and similar bedrock chemistry. Ion concentrations increase by an average of 27 times along the forested-tourban gradient, including an increase of 10 times in dissolved inorganic carbon (DIC). Inputs from road salt caused large increases in Na + and Cl − concentrations as well as high seasonal variability. Significant increases in Ca 2+ , Mg 2+ , DIC, and SO 4 2− concentrations in watersheds with high ISC provide the best evidence to date that concrete is a substantial nonpoint source contributor to urban stream chemistry. Elevated Ca 2+ , Mg 2+ , DIC, and SO 4 2− concentrations in urban watersheds due to nonpoint source contributions have wide ranging implications, including potential reduction of Cl − toxicity, changes in metal speciation and toxicity, a shift to waters saturated with respect to calcium carbonate, and altered carbon and sulfur cycling.

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