Via Gizmodo, this National Geographic video is a nice little introduction to how we get from undersea volcanoes to Streets and San trucks over the course of geologic time.
That's where it comes from, but where does the 350,000 metric tons of salt—an average Chicagoland winter—go after it's spread? Lake Michigan, for starters.
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The concentrations are much more intense in Chicago's rivers and canals, which have also been increasing over the past few decades (the CSSC is the Chicago Sanitary and Shipping Canal).
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No one's panicking just yet, but there is concern for its impact on fish and amphibians as similar trends can be found throughout the Midwest. In the future, if cities decide to curtail the salinization of waterways—and in some cases, the water supply—the streets could run with beet juice, or smell faintly of cheese.