Road Salt Pollution – Moving from Monitoring to Action

It's winter again, and the salt trucks are a familiar sight as they rumble along our roads to melt snow and ice.  If you walk across a parking lot, you feel that familiar "crunch" underfoot from rock salt that has been spread. Many people think it is harmless because it's just salt and will dissolve and disappear after the snow melts.  Unfortunately, we now know that salt pollution has a profound effect on the environment, drinking water, soil, and infrastructure. This article is the first of a series of three where we will examine road salt pollution, its impacts, how to monitor it, and what can be done to reduce it.

Causes of Salt Pollution

Every year in the US, we use 15-32 million metric tons of road salt, and that amount has doubled since 1975. This is due to several reasons: first, increased development has produced more roads and parking lots that need to be kept clear of snow in winter.  Also, the public's expectations have changed over the past few decades, and people today expect roads and parking lots to be kept completely free of snow.  As a result, municipal employees and contractors often spread more chemicals than is necessary to avoid any possible accidents or injuries.

The main deicers used are sodium chloride (rock salt), magnesium chloride, and calcium chloride. They work by reducing the freezing point of water, and while they have slightly different properties, they all contain chloride, which travels very easily through the environment.

After they are spread, deicers can bounce and scatter and end up on the adjacent ground. Subsequent rain events wash some of these chemicals directly into streams, lakes, and ponds as part of stormwater runoff; some get washed down storm drains and travel to waterways, and many infiltrate the ground where they soak deep down into the groundwater. Studies indicate that approximately 45% of deicing chemicals end up in groundwater. Once they are there, they gradually build up over time.

Groundwater pollution is a serious issue in Pennsylvania because our streams are classified as "gaining" streams, which are fed year-round by the groundwater. They also receive water from rain events, but that only results in temporary increases in water levels. The level of flow that exists when there is a period of no rain is called "base flow." If we test the water at these times, we can get a good indication of the level of salt pollution in the groundwater.

Impacts of Chloride Pollution on Stream Ecosystems

Chloride is problematic for stream organisms because it reduces oxygen levels and creates osmotic stress. "EPT" organisms: Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies) are a vital part of the food web and are used as bioindicators of stream health. Research has shown that their numbers decrease significantly as chloride levels increase. Current research indicates that the threshold for long-term toxicity for these creatures for chloride is 50 mg/l.

Chloride also has an impact on other aquatic life. Studies have shown that chloride affects frogs and salamanders during their larval stages in vernal pools, causing deformities and increased mortality. Fish have been found to have reduced hatching rates and reduced growth overall, and aquatic plants are also affected because chloride affects their ability to absorb water through their roots. In ponds and lakes, salty water is denser and forms a layer of oxygen-deficient water at the bottom, which is harmful to wildlife.

Soil

Road salt ends up in soil as a result of runoff, plowing, bouncing, and splashing. This effect can be as far as 10 meters from the road edge. Accumulation of chloride can reduce soil permeability and fertility and increase alkalinity and density, causing chemical changes and a reduction in the soil's ability to retain water. Soil microbes are also impacted, and studies show a reduction in their activity, biomass, and community structure.

Drinking Water 

Pollution of rivers and groundwater also has the potential to affect our drinking water supply. Water with more than 20 mg/l of sodium is unhealthy for people with high blood pressure. Sodium levels are not typically measured, but if we measure the quantity of chloride in water, 31 mg/l  of chloride indicates that it will have 20 mg/l of sodium.

Here in the Lehigh Valley, our local water supplier draws water from the Little Lehigh Creek in Allentown and has been measuring the chloride levels since 2001. (See graph below) This data reflects the levels year-round. During that time, the amount of chloride has more than doubled.  Since 2007, it has consistently exceeded the 31 mg/l level for hypertension. Also, note that it has been above the 50 mg/l levels of toxicity for EPT organisms since 2012.

Effects on Infrastructure

Chloride corrodes vehicles, bridges, and roads at a cost of approximately $5 billion a year. It causes chemical changes and freeze-thaw effects in concrete structures, causing cracks, crumbles, and discoloration.  It also accelerates the corrosion of water pipes, which can lead to them leaching harmful metals like lead into the water supply.

Green Infrastructure

Green infrastructure like riparian buffers and rain gardens slow chloride infiltration, but unfortunately, they do not fully mitigate it. This is because chloride anions are negatively charged, which are repelled by soil particles and move very easily down into the ground. As it passes through, chloride can damage plants and soil and reduce the ability of microorganisms to degrade or alter other pollutants.

In the next issue of Watershed Winds, we will discuss monitoring salt pollution.

References

Road Salt. (n.d.). Cary Institute of Ecosystem Studies. 

Kelly, V.R., Findlay, S.E.G., Weathers, K.C. 2019. Road Salt: The Problem, The Solution, and How To Get There. Cary Institute of Ecosystem Studies.

EPA. (2021, March 8) Winter is Coming! And with it, tons of salt on our roads. 

Minnesota Stormwater Manual. (2011).  Environmental Impacts of Road Salt and Other de-icing Chemicals - Minnesota Stormwater Manual.

‌ Collins, S. J., & Russell, R. W. (2009). Toxicity of road salt to Nova Scotia amphibians. Environmental Pollution, 157(1), 320–324. doi.org/10.1016/j.envpol.2008.06.032 

Sanzo, D., & Hecnar, S. J. (2006). Effects of road de-icing salt (NaCl) on larval wood frogs (Rana sylvatica). Environmental Pollution, 140(2), 247–256. doi.org/10.1016/j.envpol.2005.07.013 

Hintz, W. D., & Relyea, R. A. (2017). Impacts of road deicing salts on the early-life growth and development of a stream salmonid: Salt type matters. Environmental Pollution (Barking, Essex : 1987), 223, 409–415. doi.org/10.1016/j.envpol.2017.01.040 

Norrstrom, A., & Bergstedt, E. (2001). The Impacts of Road De-Icing Salts (NaCl) on Colloid Dispersion and Base Cation Pools in Roadside Soils, Water, Air, & Salt Pollution [Review of The Impacts of Road De-Icing Salts (NaCl) on Colloid Dispersion and Base Cation Pools in Roadside Soils, Water, Air, & Salt Pollution]. Springer Nature Link, 27, 281–289. 

Yan, N., Marschner, P., Cao, W., Zuo, C., & Qin, W. (2015). Influence of salinity and water content on soil microorganisms. International Soil and Water Conservation Research, 3(4), 316–323. doi.org/10.1016/j.iswcr.2015.11.003 

Dingle, A. (2016, December). The Flint Water Crisis: What’s Really Going On? - American Chemical Society. American Chemical Society. 

Zhou, J., Wang, G., Liu, P., Guo, X., & Xu, J. (2022). Concrete Durability after Load Damage and Salt Freeze–Thaw Cycles. Materials, 15(13), 4380–4380. doi.org/10.3390/ma15134380

‌Lancaster, N. A., Bushey, J. T., Tobias, C. R., Song, B., & Vadas, T. M. (2016). Impact of chloride on denitrification potential in roadside wetlands. Environmental Pollution, 212, 216–223. doi.org/10.1016/j.envpol.2016.01.068 

Snodgrass, J. W., Moore, J., Lev, S. M., Casey, R. E., Ownby, D. R., Flora, R. F., & Izzo, G. (2017). Influence of Modern Stormwater Management Practices on Transport of Road Salt to Surface Waters. Environmental Science & Technology, 51(8), 4165–4172. doi.org/10.1021/acs.est.6b03107