Chloride

The amount of road salt used in winter maintenance across Wisconsin has increased since the 1960's. The combination of an increase in miles of roads that need to be maintained and an increase in the amount of salt applied per mile results in more than a total of 550,000 tons or 16 tons of salt per mile (annual average over past five years) applied each winter to Wisconsin's Trunk Highway system. These total salt volumes only include the amount reported to the Wisconsin Department of Transportation by county highway departments which does not include the amount used by local municipalities, contractors, or private citizens.

• Hover over each point to see increase from 1960

Wisconsin Salt Use on State Trunk Highways

Data Source: WisDOT Annual Winter Maintenance Report 2016-2017 History of Salt Use on State Trunk Highways. Data submitted through Salt Inventory Reporting System.

New development results in additional impervious surfaces - roads, parking lots, and sidewalks - that require winter maintenance and lead to increased salt use. Compared to the surrounding region, Dane County has the most miles of State Trunk Highways. The amount of salt applied each winter depends on the winter's severity (an index is calculated incorporating the number of snow events, the number of freezing rain events, and amount of snow). Typically a lower severity index results in less salt application and lower winter costs.  In the winter of  2017-2018, Vernon County had the highest winter severity index in the region, but the most salt applied per lane mile was in Columbia County. In terms of total costs for salt, however, Dane County leads the region in the amount spent on winter salt and the total amount applied (over 35,000 tons in the winter of 2017-2018).

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Winter 2017-2018: Regional Salt Use on State Trunk Highways

Data Source: WisDOT Annual Winter Maintenance Report 2017-2018 History of Salt Use on State Trunk Highways. Data submitted through Salt Inventory Reporting System.

With dense development, Dane County is particularly impacted by increasing chloride use, and long term monitoring has documented how salt application reaches our surface waters.

Chloride levels can become toxic to aquatic life as snowmelt runs into streams. Wisconsin has developed a chloride chronic toxicity threshold for aquatic life of 395 mg/L and an acute threshold of 757 mg/L. Prolonged exposure to chloride concentrations above chronic levels adversely effects organisms, and exposure at acute levels can be lethal. Short-term pulses may exceed chronic or acute levels during snowmelt conditions, causing this to be an area of increasing concern. At Spring Harbor, winter and spring monitoring has documented toxic chloride levels following salt application and snowmelt (see graph below). During the summer, chloride in snowmelt that has seeped into groundwater recharges streams and contributes to increasing chloride concentrations year round. USGS monitoring has documented increasing baseflow summer concentrations in Pheasant Branch and the west branch of Starkweather Creek, both listed as impaired by WDNR for chloride. While chloride concentrations are lower in the summer than they are in the winter, increasing summer concentrations demonstrate that winter chloride application has year round effects. At Pheasant Branch, summer chloride concentrations increased from 20 mg/L in the 1970's to 100 mg/L in 2013.

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Seasonal USGS Monitoring

Data Source: Water quality field samples collected by USGS at Spring Harbor, West Branch Starkweather Creek, Pheasant Branch.
See more about chloride monitoring in Starkweather Creek here. Data collected by Public Health Madison & Dane County and presented in 2017 Road Salt Report

In addition to winter road salt application, chloride in surface water can increase as the result of resident, business and industrial uses. Chloride used in water softeners for treating hard water leaves houses and businesses and flows to regional wastewater treatment plants. Click here to see chloride and hardness levels in water from Madison's municipal wells (all wells exceed the 180 mg/L of calcium carbonate that constitutes very hard water). Groundwater already contains some chloride and water softeners add more which means that wastewater leaving a home contains half a pound of chloride daily (study conducted by Madison Metropolitan Sewerage District). Since treatment plants are not designed to remove chloride, and it is very expensive to upgrade systems to do so, chloride passes through the plant and is discharged in effluent. Many regional plants have variances for the amount of chloride they can discharge and must implement source reduction strategies to comply with their wastewater permits. A study from the Madison Metropolitan Sewerage District (MMSD) found that water softeners were the source of 57% of their chloride, road de-icing contributed 7%, and the background amount of chloride in municipal wells accounted for 8%. Chloride levels decrease downstream of effluent discharges as effluent is diluted with other baseflow. The graph below show results of chloride monitoring downstream of effluent discharges from MMSD, the Village of Oregon, and the City of Stoughton in Badfish Creek and the Yahara River. Notice that the three tributaries that flow into Badfish Creek (shown in shades of blue) generally have chloride levels below 20 mg/L.

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• Hover over each mapped site to see data highlighted on the graph

Stream Monitoring Sites Downstream of Effluent Discharges

Data Source: Water quality samples collected by MMSD and USGS. Samples collected during the growing season under baseflow conditions. Median result for each year is shown.  

Even in streams that do not have direct effluent discharges, regional chloride concentrations have been increasing at some monitoring sites. While average concentrations remain much lower than what is observed in effluent, these significant increases mean that stream conditions are changing. Average chloride concentrations in the Yahara River near Stoughton have risen from 42.32 mg/L in 1997 to 64.7 mg/L twenty years later (show in red below). This linear increase suggests that average chloride concentrations will continue to rise. This graph shows seven sites with significant increases in chloride concentrations over the past twenty years. Most of these sites are not downstream from wastewater discharges, pointing to winter salt application as a likely cause of baseflow increases. Chloride applied during the winter can seep into groundwater and then increase surface water concentrations as groundwater recharges streams throughout the summer.

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• Hover over the data point to see the location, average chloride concentration, and the number of samples collected

Monitoring Sites Showing Chloride Increases

Chloride concentrations are also increasing in local lakes. Water with salt in it is more dense and sinks to the bottom, which prevents lake mixing in the spring and fall. Without this mixing, oxygen-depleted water at the bottom of a lake is not replaced with oxygen-rich water from the surface. Without replenished oxygen from the surface, conditions for aquatic life towards the bottom of the lake degrade. Therefore, steadily increasing chloride concentrations is a concern for the lake ecosystems.

Since 1970, chloride concentrations have increased approximately 1 mg/l per year in the Yahara chain of lakes. This has not occurred in Fish Lake, located in rural northwestern Dane County. Only 6% of the Fish Lake watershed is developed, meaning less salt use compared to the urbanized Madison watersheds.

The highest chloride concentrations are detected in Lake Wingra which has the most urbanized watershed (65% developed) of the six Yaraha lakes. Chloride accumulation in Lake Wingra illustrates the biggest impact of long-term winter salt application. Lake Mendota, with only 27% of its watershed developed, has the lowest chloride concentrations within the chain of lakes. The other three (Monona, Waubesa and Kegonsa) all have similar chloride concentration trends and have watersheds that are roughly 34% developed.

The interactive display below shows changes in chloride concentrations over time, as well as depth profiles of chloride concentrations in Fish, Mendota, Wingra, and Monona. Click Lake Wingra on the map to display high winter chloride concentrations towards the bottom of the lake.

Data Source: Average annual surface chloride concentrations for Lake Wingra, Lake Mendota, Lake Monona, Lake Waubesa, and Lake Kegonsa are from the Public Health Madison & Dane County 2019 Road Salt Report. Data for Fish Lake and chloride concentrations at various depths for Lake Wingra, Lake Monona, and Lake Mendota are from North Temperate Lakes Long-Term Ecological Research Network. 

The UW - Madison Arboretum, upstream of Lake Wingra, uses chloride sparingly by applying a sand and salt mix on roads and manually removing snow and ice before applying salt in pedestrian areas. Conscious efforts by winter salt applicators can reduce overall salt use, while still maintaining safe winter conditions. The combination of these efforts will help to slow the chloride increases in these lakes.

Agencies across the region seek to reduce chloride use and continue to monitor and research changing chloride concentrations.