The double-edged sword of salting roads in winter

The problem of road salt working its way into local waters is nicely explained in a StarTribune article:

This winter, the Pollution Control Agency (PCA) started a four-year project to figure out which Twin Cities' lakes hold too much chloride, a primary ingredient in salt, and what it will take to keep urban waters healthy...

Shingle Creek, which flows under highways and behind shopping malls in the western suburbs, is a case in point. It's in one of the few watersheds where the salt load has been calculated. In theory, to get it back to healthy levels, the nine communities along its banks would have to cut salt use by a whopping 71 percent... "When your tradeoff is public safety, it's very difficult."

Road salt use has been rising steadily for more than 20 years. Now, in an average winter, some 350,000 tons of salt are dropped on roads, sidewalks and parking lots in the Twin Cities metropolitan area... Eric Novotny, one of the researchers on the university chloride study, found that 78 percent of the salt applied to roads stays in the water. Unlike some other pollutants, it does not flow to the Mississippi and down to the Gulf of Mexico. Instead, the denser, salty water sinks to the bottom of lakes and into groundwater, accumulating year after year...

Brownie Lake, a small, deep lake in Minneapolis near Cedar Lake Parkway and Interstate 394, contains so much chloride that its top and bottom layers no longer mix, which is vital to a healthy lake. In part, it doesn't turn over because the lake is so deep relative to its surface area, and pollutants of all kinds flow down its steep banks. But salt has been collecting in its depths for years. Two meters below the surface, the water in Brownie contains virtually no oxygen, said Rachael Crabb, a water quality expert for the Minneapolis Parks Department, indicating there's not much life at the bottom. "Whatever chloride has come into Brownie from 394 is still there, and it's going to say there," she said...

In recent years, the city bought new, high-tech salt trucks, and began making its own brine to apply to roads before a snowstorm. It also uses GPS units to precisely regulate exactly how much salt the trucks put down in any location. Since 2007, the amount of salt used per mile per snowstorm has been cut in half, Albrecht said. It took awhile to educate Prior Lake citizens about the project, and to convince them that roads were still safe, even if they were not cleared down to bare pavement...

More at the link. The layering of lake waters, with brine in the depths, is reminiscent of the situation in the Black Sea, where there is a pycnocline.

Addendum:  New word for the day:  A lake whose layers do not mix is called a meromictic lake, in contrast to a normal holomictic lake.

Image source (showing thermal layering, but the principles are also applicable re salinity)