By Matilde Urrutia and and Allison Madison
Many local administrations in Dane County are emphasizing the need to reduce road salt use in winter. (City of Madison, Dane County, Wisconsin Saltwise). Salt (usually sodium chloride) has been used to reduce ice formation on roads, sidewalks and driveways during cold weather in northern states since the 1950’s. Why is this “northern tradition” being looked down upon as of late?
When road salt—sodium chloride—dissolves, it releases sodium, a cation, and chloride, an anion. These ions will move with the rainfall into the street drain and from there into streams, rivers, and lakes, thus increasing the amount of ions in these water bodies. Of the two ions that make up road salt, chloride stays in the water, and sodium is mostly bound to soils through which the water moves or to the solids in the bottoms of lakes and rivers. We will explain later on why this is the case.
Chloride stays in solution, over time reaching concentrations that are higher than what is “normal” for most freshwater environments. A “small” increase may not be a problem, but consistent use of salt over years and decades can become a problem. This is one of the major concerns behind the push to reduce road salt use. Chloride has been shown to be progressively increasing in many local streams, rivers, wetlands, and lakes, as is shown in the graph that follows (Dane Co Public Health Report, 2022).
Excessive chloride may be toxic to aquatic organisms that are not adapted to high salinity levels. The Wisconsin DNR has set the chronic toxicity level for chloride in freshwaters at a concentration of 395 mg/L. The US EPA water quality criterion “chronic” level to protect freshwater aquatic organisms is 230 mg/L, substantially lower than the WIDNR limit. Our local lakes and the Yahara River are on average below those values, meaning their waters are not “toxic” to aquatic organisms yet. But we want to slow down the progressive increase illustrated in the 2022 report from Public Health Madison and Dane County.
Also, levels may be much higher at specific times, as shown by the Gebert et al. USGS report in 2012 during the winters of 2007 and 2008 in Middleton. This research showed concentrations of chloride in the Pheasant Branch stream that exceeded the EPA chronic criterion of 230 mg/L for 10 days during the 2007 winter and more than 45 days in 2008 (figure 21 from Gilbert et al 2012). So, very elevated concentrations are occurring, mostly during times of low flow. Higher concentrations of chloride also increase corrosion of engineered structures like bridge supports, concrete roadways, and vehicles, since water with more chloride is more corrosive.
Removing chloride from water is not a viable idea. It is technologically and energetically very demanding to remove salts from water. It can be done, but at a tremendous cost. Prevention is the best solution in this case.
And what about sodium? Sodium ions released from road salt mostly bind to soils or sediments in lakes and rivers, as well as stormwater retention ponds and basins. Soils and sediments contain solid particles of very small size (<0.1 mm) called colloids that have a strong negative charge on their surface. Since opposites attract, these particles hold onto cations like sodium but let the anions like chloride go free.
The problem comes when soils have a lot of bound sodium under certain conditions (low salinity), since then the colloidal materials clog soil pores. As a result, water movement is slowed down, sometimes so much that water ponds at the surface instead of infiltrating. So, if an infiltration basin in a neighborhood—designed by engineers to reduce runoff and promote slow infiltration—is overloaded with sodium, it stops being an infiltration basin. The soils have become “impermeable” and the infiltration basin (or rain garden) has failed. This situation is also hard to reverse.
So, let’s use less salt in our driveways and sidewalks. You may follow the recommendations included in the Wisconsin Saltwise website. It will benefit our urban infrastructure, our streams and lakes, and all the aquatic organisms that call those water bodies home.
About our authors: Matilde Urrutia serves on the Friends of Pheasant Branch Conservancy Watershed Committee and retired from the UW-Madison Teaching Faculty at the Department of Soil Science. Allison Madison is the WI Salt Wise Program Manager at Capital Area Regional Planning Commission.