Road salt pollution in many US lakes may stabilize at or below thresholds set by the EPA

Since de-icing with road salt began in the 1930s, the salinity of lakes in most of the US has been steadily increasing, posing a potential threat to aquatic life and drinking water supplies.

However, a cautiously optimistic new study of Limnology and Oceanography Letters concluded that if we can keep constant or reduce the use of road salt, the level of many lakes can stabilize below the thresholds set by the US Environmental Protection Agency (EPA).

“For most lakes in the US, road salt pollution can be a solvable problem, if we put our minds to it,” said lead author Chris Solomon, who studies lake ecology at the Cary Institute of Ecosystem Studies. However, he cautioned that more research is needed to better understand what exactly is a safe salt level in a freshwater ecosystem.

The US applies an estimated 24.5 million tons of road salt to its roads each winter—mostly in the form of sodium chloride. Rain and melting snow carry this salt into local waterways and aquifers, where it can cause freshwater salinization syndrome. This salt not only harms many organisms, but it leaches toxic metals and radioactive materials from the soil and water pipes.

Solomon saw upward trends in salt concentrations in US lakes and wanted to know where they were going. Will road salt levels continue to rise, or will they stabilize? With colleagues, he developed a model to assess road salt concentration controls in lakes to reveal the concentration at which they can level up.

The model looks at road density in lake watersheds, the amount of road salt applied per mile of road, and rainfall. Hydrologic fluxes are considered to predict how salt pollution flows into and out of lakes. The model calculated the levels at which road salt would be expected to stabilize if salt use was maintained at the amount reported in 2010–2015, for all 461,000 lakes and reservoirs larger than 2.5 acres in the contiguous US.

For lakes in areas with light to moderate road density, the authors found that holding the road salt application rate constant helps the lakes stabilize below 230 mg/l of chloride per liter of water, the threshold designated by the EPA to protect aquatic life. The reduced application can provide additional environmental and economic benefits without threatening road safety.

The authors note that more research is needed to determine whether the EPA’s 230 mg/l chloride threshold is too high. Solomon explained, “The EPA’s chronic toxicity threshold for chloride was developed with limited data, and there is growing evidence that negative effects can occur at concentrations as low as 230 mg/l.” Even less is known about how mixing salt from multiple sources affects aquatic life.

Some areas set lower chloride guidelines, including 150 mg/L in Michigan and 120 mg/L in Canada. The model predicts that chloride concentrations will eventually exceed the 120 mg/L threshold in more than 9,000 US lakes, even if road density and salt use rates remain at current levels.

Not surprisingly, lakes with predicted salt concentrations exceeding the EPA’s 230 mg/l threshold are common in the Northeast and Midwest. The most vulnerable are lakes with high road density and high road salt use in their watersheds. This includes about 9-10% of lakes in Illinois and Ohio, as well as a small percentage (<0.1 to 1%) of lakes in Indiana, Iowa, Kansas, Michigan, Minnesota, New York, Pennsylvania, and Wisconsin.

Achieving safe salt levels in these lakes will require a reduction in salt use. This can be done safely by adopting best management practices and new technologies.

As a test of the model’s accuracy, the predictions were compared to measurements taken at Mirror Lake in New Hampshire, a site monitored since 1967 by Cary Institute founder Gene E. Likens. After plugging in local data, the model predicted maximum and minimum salt levels, and real-world measurements fell within the predicted range. “It gives us confidence that we’re in the right ballpark,” Solomon said.

“We don’t think the model is perfect. It’s a simple model meant as a tool for thinking about the problem,” Solomon said. Among other things, the model ignores salt inputs from natural rock weathering and from human activities such as agriculture and industry, and does not consider temporary seasonal spikes in chloride. “We hope that others will detail the method and make better predictions. But in the meantime our results suggest that efforts to prevent salt use can make a big difference, and may help prioritize efforts,” concluded Solomon.

The next steps include comparing the model’s predictions with observed data in other areas where salt use and lake chloride levels have been documented for many years, and using the model to assess how other forms of global change—such as land use or climate change—change rainfall and the need for road salt application.