Road Salt, Lake Chloride and the Madison Driveway

What the lakes are telling the county

Public Health Madison and Dane County's 2022 road salt report measured chloride in the area's five major lakes and in the Yahara River. Lake Wingra came in at 129.0 mg/L, Monona at 75.9, Waubesa at 75.7, Kegonsa at 71.6, Mendota at 57.7 and the Yahara River at 53.0. For scale, Wisconsin Salt Wise puts naturally occurring chloride under 10 mg/L in most of the state's water, and the chronic toxicity level cited in the report is 395 mg/L. None of the lakes is near that toxic line, but every one sits well above where nature left it.

The groundwater points the same way. The report compares city wells in two groups. Deeply cased wells that draw only from the lower aquifer rose from about 4 mg/L of chloride in 1982 to 18 mg/L in 2022, and wells with short casings that draw from both aquifers rose from 5 to 46 mg/L over the same span. All remain below EPA limits, but the direction is the point: once salt dissolves into water it does not break down, so every winter adds to what is already there.

Where the chloride comes from

Wisconsin Salt Wise lists several sources of chloride: winter salt on roads, parking lots and sidewalks, which it calls one of the most common, water softeners that send brine through treatment plants and septic systems, and smaller amounts from dust control and fertilizer. Salt has been used as a road de-icer in Wisconsin since the late 1950s, and chloride levels in water peak in winter and spring, when snowmelt carries the season's salt into storm sewers and streams.

The city has cut back. Its total road salt use dropped from 16,351 tons in 2013 to 6,124 tons in 2022, and the report credits both mild winters and the loss of school salt routes during virtual classes, even though the per-mile application rate rose. Salt routes, which include major streets, bus routes and streets near schools, hospitals and police and fire stations, cover about half of all city streets. The city also pre-treats with salt brine, which it says can cut salt use by up to 70 percent.

Municipal trucks are only part of the picture. The same report points out that besides the city, eight townships, two villages, three cities, Dane County and a multitude of private applicators put chloride into the watershed. Private applicators include the contractors who salt apartment lots and office parking, and every homeowner with a bag in the garage. That is why the City of Madison's Winter Salt Certification program, started in 2017 with support from Wisconsin Salt Wise, has trained more than 300 municipal and private applicators in Dane County.

What salt does to a driveway

Salt melts ice by turning it into brine, and brine is water. On a driveway, that water goes where water always goes: into open cracks and into the porous patches of a surface that has lost its seal. The Great Lakes Integrated Sciences and Assessments program notes that water expands when it freezes, so repeated freezing, thawing and refreezing damages pavement and forms potholes. At Dane County Regional Airport, NOAA's 1991-2020 normals count about 87 days a year that cross the freezing line, and March alone averages about 19.

Wisconsin Salt Wise tells property managers plainly that salt damages pavement, along with landscaping and the outsides and insides of buildings. Part of the problem is volume. Salt spread on a bitter morning below 15 degrees does not work at all, so the extra just sits on the drive until a warmer afternoon turns it into a larger dose of brine. The local ground adds to the effect: silt loam soils common in the county, such as the Dodge series, are frost susceptible, and brine that reaches the base through a crack gives them the water that frost heave needs.

A sealed, crack-free surface changes the equation. Water and brine run off a tight coat instead of soaking in, and the filler in a sealed crack keeps meltwater out of the base. That is the practical case for driveway sealcoating in Madison in a salt-heavy winter: not that a coat stops salt, but that it stops the water salt produces from getting inside the pavement.

Salting less without slipping

The local advice fits on an index card. Shovel before the snow packs into ice, since anything shoveled does not need melting. Then scatter rather than pour: Wisconsin Salt Wise reckons that one coffee mug's worth covers a whole 20-foot driveway, and the City of Madison's guidance puts a 12-ounce mug at 10 sidewalk squares. Below 15 degrees, switch to sand for traction or to a melter made for lower temperatures. After the storm, sweep any dry salt still lying on the drive so the next rain does not carry it to the gutter.

Those steps save money, and they protect the drive. Less salt means less brine in the cracks each night and less white residue baked into the surface by spring. Neighbors in Verona hear the same from their public works department, which points out how much melting power salt loses between 30 degrees and 20. Anyone hiring out snow removal can ask whether the company has been through the Winter Salt Certification training.

• Shovel first, before snow packs into ice
• A coffee mug of salt is enough for a 20-foot drive
• Below 15 degrees, use sand or a low-temperature melter
• Sweep up dry leftover salt after a storm

A spring routine before the next seal coat

By April the snow is gone, but the salt season leaves traces: sand in the surface texture, a white film along the edges, and new cracks from March's freeze-thaw run. Before a crew can seal, the drive has to be swept and, where residue is heavy, washed and dried, because sealer bonds to whatever is on the surface. That is also the moment to count cracks. Anything a quarter inch or wider needs crack filling before it takes on another winter of brine.

Timing follows the thermometer. The median last freeze at the airport station falls on May 2, and a sealer coat wants a full day at 50 degrees or warmer, so crack work can start in dry April weather while sealing waits for late May or June. A drive that reaches November with filled cracks and a fresh coat meets the salt season on its own terms.

More from the blog

• Sealing a Driveway on the Isthmus: Parking, Permits and Timing