During the next few months, we’ll spread 20 million tons of salt on roads and sidewalks across the northern tier of the U.S. Mostly, this is rock salt, also known as sodium chloride or table salt when it is refined for human consumption. Salt lowers the freezing point of water, due to what we call the colligative properties of water—dissolved salts interfere with the ability of water molecules to bind together to form a solid. For example, seawater doesn't’t freeze until it cools to 28 F. Although other compounds could work just as well (for example, calcium chloride or potassium acetate), rock salt is by far the least-cost alternative for de-icing roads.
Salt is, of course, very easily dissolved in water, so road salt is quickly transported in surface runoff and percolates downward to groundwater. The latter leads to contamination of wells, so that some waters are unfit for consumption by people who suffer from high-blood pressure (hypertension). The massive and widespread use of road salt in the snowbelt has changed the chemical composition of runoff in many areas. About 1/3 of the waters draining the northern areas of the continental United States carry higher concentrations of sodium than a century ago. The chloride ion (Cl-) is now the dominant negatively-charged ion in some areas. Increasingly, lakes in this region are becoming more saline.
Where does increasing salinity of freshwater join the list of other human perturbations of the earth? Certainly, we should take note of how the effluents of modern society have changed the chemistry of freshwater, which is consumed by all. Excessive salt affects the survival and reproduction of a number of small freshwater species that are important to the productivity of species higher on the food chain—for example fishes that we value for human consumption. As a result of their high density, salty waters can prevent the spring mixing of lake waters that supplies oxygen for deep-water fishes, such as lake trout. Those who pass along ice-free roads in the winter may recall their childhood, when there were trout in the adjacent ponds and lakes—never linking the cumulative runoff of sodium chloride to the disappearance of lake trout today.
It is hard to imagine that we could dump 20 million tons/year of anything on the landscape and not expect some effects. Combined with the growing suite of other chemicals of human origin that find their way to freshwaters, we should be most concerned about the quality of every drop of water we drink.
Dugan, H.A. and 14 others. 2017. Salting our freshwater lakes. Proceedings of the National Academy of Sciences 114: 4453-4458.
Findlay, S.E.G. and V.R. Kelly. 2011. Emerging indirect and long-term road salt effects on ecosystems. Annals of the New York Academy of Sciences 1223: 58-68.
Kaushal, S.S., P.M. Groffman, G.E. Likens, K.T. Belt, W.P. Stack, V.R. Kelly, L.E. Band, and G.T. Fisher. 2005. Increasing salinization of fresh water in the northeastern United States. Proceedings of the National Academy of Sciences 102: 13517-13520.
Kaushal, S.S., G.E. Likens, M.L. Pace, R.M. Utz, S. Haq, J. Gorman and M. Grese. 2018. Freshwater salinization syndrome on a continental scale. Proceedings of the National Academy of Sciences doi: 10.1073/pnas.1711234115
Kelly, V. and 4 others. 2010. Road Salt: Moving Toward the Solution. Cary Institute of Ecosystem Studies. https://www.caryinstitute.org/sites/default/files/public/downloads/report_road_salt.pdf