To salt or not to salt

Biology and geology students and their professors who monitor ground water pollutants ruefully acknowledge the compromise between protecting the water supply and keeping the population safe.

Fact: Snow and ice on the roads and walkways are treacherous to pedestrians and drivers.

Fact: Road salt melts snow and ice and is relatively inexpensive, effective and easy to use.

Fact: Road salt reduces crashes by up to 88 percent, injuries by up to 85 percent and accident costs by up to 85 percent, according to the American Highway Users Alliance, a trade advocacy group in Washington, D.C.

Fact: One teaspoon of salt will contaminate five gallons of water, creating a toxic effect to people, animals, fishes and plants.

Yet U.S. road crews annually spray, spread and pour more than 22 million tons of salt, 137 pounds per person. After it dissolves, it is split into sodium and chloride ions and deposited into streams, lakes and rivers and the groundwater under our feet.

Consequently, an estimated 40 percent of the country’s urban streams have chloride levels that exceed safe guidelines. Furthermore, road salt causes approximately $50 billion annually in damage due to corrosion of vehicles, bridges and infrastructures of all kinds.

In Boone and on the Appalachian State University campus, Department of Geology Professor and Chair Dr. William Anderson said “the amount of salt distributed varies over time and season. And, there are options, of course” – but none are as effective, easy or inexpensive.

“The Physical Plant folks are willing to reduce the amount [of salt] they use,” he said. “However, they cannot do it because they have legal obligations in case someone falls. They are between a rock and a hard place. And, even if the university uses less, there is also the Town of Boone to consider and the Department of Transportation – all three apply salt in this area.”

A plethora of data, fewer solutions

Research and data around salt toxicity, impacts on water quality, destruction of plant life and salinity levels in the High Country creeks and streams is plentiful. But, solutions to the problems associated with salt are not so readily at hand.

More than a decade ago, the Department of Geology started monitoring stream temperatures and salinity levels along Boone Creek. Over time, the monitoring network has increased to seven electrical conductivity sensors along the length of Boone Creek, through the Town of Boone and the Appalachian campus. (Electrical conductivity is an indicator of saline levels – the higher the measured conductivity, the higher the saline concentration.)

In the simplest terms, data recorded by the Water Resources Research Group in the Department of Geology shows the concentration of salt in the groundwater has more than doubled since the collection of the first data in 2007. Unlike stream temperature spike, which can be limited by storing water in well-shaded retention ponds until the temperatures cool or by reducing the amount of runoff through the construction of rain gardens, Anderson said there is no way to remove salt from runoff. So, over time, the concentration of salt in the groundwater increases and is released during a storm event or flooding, contaminating waters and plant life downstream.

Dr. Chuanhui Gu, an associate geology professor specializing in environmental hydrology and geochemistry, and his students also are studying salt concentrations in the Boone Creek watershed. Conclusions derived from that research, as illustrated in the figures below, are:

  • the annual salt application for the Boone Creek watershed ranges from 200 tons to 2,000 tons per year
  • stream salt concentration as of 2015 was six times higher than the salt concentration in 1964
  • the only effective way to reduce stream salt concentration is to stop or reduce using salt on the roads

Future climate change such as dryer climate and snowy winters, Gu explained, might exacerbate stream salt contamination. Furthermore, urban sprawl will lead to increased road mileage, and subsequently, increase stream salt contamination, he said.

Graduate student Matt Fleetwood ’13, mentored by Biology Professor Dr. Shea Tuberty, is studying environmental toxicology. A native of Bostic, Fleetwood graduated from Appalachian with a Bachelor of Science degree in biology, with a concentration in cell/molecular biology. According to Fleetwood, “urbanization of the watershed, a process that involves the removal of riparian vegetation and replaces the natural landscape with impervious surfaces like concrete, ultimately increases the amount of storm water runoff and snowmelt runoff – which increases flooding, which pushes more contaminants into the groundwater where it is stored.”

His research has focused mostly on conductivity levels from two streams within the Upper South Fork New River basin (USFNR) – Boone Creek, which runs through urbanized areas and is heavily impacted by storm water runoff and development, and the South Fork of the New River that runs through the Boone Greenway and represents a moderately impacted stream.

“Many basins in the Appalachian mountain ranges experience increases in specific conductivity,” Fleetwood explained, but the USFNR is one of the few basins that experience this increase solely due to road salt pollution. For his research, he recreated in the lab scenarios based on data gathered by AppAqua. In his thesis, which studied the impact on mayflies, he concluded “chronic salinity toxicity in the streams of the USFNR have an impact on the intolerant aquatic insects and that toxicity is killing aquatic wildlife at alarming levels.”

Professor of Biology Dr. Howard S. "Howie" Neufeld’s research at Appalachian is focused on the physiological ecology of plants. “Salt can be very damaging to plants,” he said, “and here in the High Country, salt spray that is kicked up by cars on highways can drift on to plants along the roads causing injury, or be added to the soil, where it mixes with rainwater, creating a salty brine that can injure plant roots.”

Road salt is often sodium chloride (NaCl) which can damage cells in plants, “mainly by causing water to be withdrawn through the process of osmosis. Some de-icing salts are composed of different salts, thought to be less toxic, but even these can cause injury to plants,” he said.

Conifers, which are evergreen, are particularly susceptible to roadside salt spray, he said, notable “along U.S. Highway 321 between Boone and Blowing Rock, where the needles of pine trees near the road turn brown and die after the DOT puts out deicing materials. Deciduous trees are less damaged because they usually don’t have leaves when de-icing agents are used. Still, they can suffer latent damage in the summer if the soils get too salty from runoff in the winter.”

The conclusions, across the board, are salt levels in our groundwater are reaching chronic stages and unless measures are taken, the conditions will worsen, negatively impacting the environment and quality of life in the High Country and beyond.

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Figure 1: Record of the 2004 through 2013 road salt purchase orders by Appalachian State University Physical Plant compared to the modeled input of road salt for the Boone Creek watershed. Note that the salt application rate of 2004~ 2013 ranged from 200 tons to 2200 tons each year. Image courtesy of Dr. Chuanhui Gu

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Figure 2: Comparison of observed (circles) and simulated (line) annual stream salt concentrations. A significant increase in stream salt concentrations occurred in 1975 in concurrence with a population growth in Boone, North Carolina. The observed salt concentrations were measured at Steam Plant (SP), Varsity Gym (VG), and Durham Park (DP) stations. Image courtesy of Dr. Chuanhui Gu

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Figure 3: Projected stream salt concentrations under six scenarios. Current condition unchanged (Baseline), 25% decrease in precipitation, 25% increase in snow days, no salt applied, 25% increase in road mileage, and 25% decrease in salt application for the next 50 years. Image courtesy of Dr. Chuanhui Gu

What’s to be done?

Both Gu and Anderson said the use of shovels over salt is the simple solution, but both professors concur the safety of drivers and pedestrians trumps not salting the roads.

Fleetwood said new methods – chemical, organic and technological – for snow and ice management have been extensively tested and “there is evidence that they are inherently more efficient and affordable. But, small towns, such as Boone, will not be willing to make the investment until it is economically feasible.”

Moving forward, he suggested, reactive solutions would include allocating “resources toward restoring the stream/groundwater interaction back to a natural state from the effects of urbanization, and making an attempt to capture or retain salts once applied to reduce the impact on the adjacent environment.”

Proactively, he said, “we can focus on green infrastructure during construction around aquatic ecosystems, keeping the percentage of impervious surface low and thereby reducing road salt runoff into surface waters.”

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